Plate Tectonics - Theory And Outcomes

Question 1

1. Plate Tectonics: Margins and Landforms

Answer 1

Students should be able to:

(i) demonstrate knowledge and understanding of the evidence for and the theory of plate tectonics

(ii) demonstrate knowledge and understanding of plate and sub-plate processes at constructive, destructive and collision margins

(iii) demonstrate knowledge and understanding of resultant landforms - ocean ridges, rift valleys, deep sea trenches, island arcs and fold mountains

Question 2

The Earth’s Layers

Answer 2

The Earth’s core has a diameter of 7000km with two distinct sections: an inner solid core surrounded by an outer liquid core. The inner core is largely made of iron and along with the outer core, creates the planet’s magnetic field.

Surrounding the core is the mantlewhich is a 2900km thick layer that contains 80% of the Earth’s volume. The mantle is mainly solid rock but beneath the crust it can flow and deform (viscoelasticity). This layer of the upper mantle is termed the asthenosphere.

Above the upper mantle is the Earth’s crust, which averages at only 20km in thickness. Ranging from 60km in continental crust to 5km in oceanic crust. Continental crust is composed of rocks that are light with a more granitic nature, while oceanic crust is composed of denser basaltic rocks. This layer is termed the lithosphere.

Question 3

The Early Theory of Plate Tectonics

The Idea of Continental Drift
- Alfred Wegener, 1915

Answer 3

The theory of plate tectonics suggests that our current world map is merely a point along a continuum of change.

In 1915 Alfred Wegener proposed that the world’s continents were once one single land mass. Wegener named this Pangea, which he said gradually broke apart over the past 200 million years.

Question 4

Wegener’s Idea of Continental Drift
- Evidence

Answer 4

Parallel Nature of Coastlines
•Looking at the coastlines of continents, such as the East coastlines of North America and the NorthWest coastlines of Africa, it appears that they were once joined together. This is due the the similarities in the coastlines and their ability to fit together like ‘jigsaw pieces’ if moved back together.

Distribution of Mountain Chains
•The distribution pattern of rock types and mountain chains make sense if the continents were once joined together. Seen though fragments of similarly aged rock and mountain chains found on the coasts of continents separated by miles of ocean.

Fossil Evidence
•Within these similarly aged rocks, fossil evidence has been found. For example, the ancient Mesosaur reptiles and Glossopteris ferns which have been found in similarly aged rocks in South America, Africa, India and Antarctica that are separated by thousands of miles of ocean water.

Climatic Evidence
•It is also evident from landforms on the surface of these continents that, including tropic hot desert regions, had once been covered in huge ice sheets. Which would not be possible if they were in the positions that they are in today.

Question 5

Modern Plate Tectonic Theory
- Evidence

Answer 5

The Topography of The Oceans
•Accurate maps of the ocean basins were created using sonar techniques developed in WWII. These showed that rather than the ocean basin being the deepest the furthest from land, the ocean floor had huge linear mountain ranges, with deep central valleys running down the centre. These submarine mountain chains stretched i a continuous line for 50,000km around the Earth. These are know as the mid-ocean ridges and the North and South Atlantic Ocean basins provide perfect examples.

The Age and Pattern of Ocean Basin Geology
•An early discovery showed the amount of material lying on the ocean floor was much less than expected. Later scientists developed a method of dating ancient rock using a technique involving the radioactive decay of potassium- argon. It showed that the world’s ocean floor, which form 70% of the surface, are young and recently formed within the last 260 million years. The mountains of the mid-ocean ridges were made of the very youngest rocks and the age of the rocks on the ocean floor increased away from the mid-ocean ridges.

Paleomagnetism and Magnetic Striping
•Scientists steadying paleomagnetism knew that when molten rock solidifies, iron particles in the rock would line up with the Earth’s magnetic field. Every 250,000 years the Earth’s magnetic field reverses. Using magnetometers in ships crossing the Atlantic they revealed a banded and symmetrical pattern in the ocean floor rocks across the mid-ocean ridge.

The Distribution of Earthquakes and Volcanoes
•Distinctive patterns have emerged which shows both volcanic and seismic activity tend to occur in long, narrow, linear bands along plate margins. The most noted distribution is the ‘ring of fire’, a line of volcanoes that mark the circumference of the Pacific basins.

Question 6

Plate Tectonic Theory
-Harry Hess sea floor spreading and subduction

Answer 6

In the 1960s, Harry Hess and others suggested that mid-ocean ridges are weak zones in the crust where the ocean floor is being pulled apart along the ridge crest. New magma from deep in the mantle rises easily through these weaker zones and eventually erupts along the crest of the ridges, creating new oceanic crust. This process, later named sea floor spreading, has operated over many millions of years creating the basaltic rocks of the ocean basin floor. This sea floor spreading hypothesis made good sense of the newly uncovered evidence:

• The existence of submarine mountain chains at the oceans centre

• At the mid-ocean ridges the rocks are very young and become progressively older the further away from the ridge

•Bands of rock parallels to the ridge have alternating magnetic polarity reflecting the reversal of the Earths magnetic field every 250,000 years

• The patterns of submarine volcanoes along the ridge revealed active processes at work

Harry Hess further reasoned that if the Earth’s crust was growing at oceanic ridges but the Earth was not expanding, then somewhere the crust must be shrinking. As new ocean crust forms and spreads away from mid-ocean ridges like a conveyor belt, millions of years later it is destroved at deep ocean trenches. These features were another finding of sea floor mapping along the edge of the Pacific - long narrow deep trenches on the sea floor with associated volcano and earthquake activity.
In effect, the rocks of the ocean floors are continuously recycled, with new lithosphere plate material created at ridges and old oceanic plate melted and destroyed at destructive boundaries. The theory then neatly explains why:

•The earth does not get bigger despite sea floor spreading

• There is so little sediment accumulation on the ocean floor

• The rocks forming the floor of the ocean basins are much younger than continental rocks

Question 7

Sub-plate Processes
Old View

Answer 7

Today, the precise nature of the sub-plate processes that cause the plates to move is debated.

The older view is that the plates forming the lithosphere are driven by slow flows of molten magma in the asthenosphere beneath them. These movements are termed convection currents and represent material rising in the mantle driven by heat originating from radioactive decay processes in the core, in much the same way as warm air rises in the atmosphere. In this model these currents reach the underside of the solid lithosphere, about 80 km below the surface, where they slowly migrate sideways, dragging the plates above along by friction. At mid-ocean ridges the rising part of these convection currents break through into the crust or even through it onto the surface as volcanic activity.

Question 8

Convection Currents Diagram

Answer 8

Question 9

Sub-plate Processes
Modern View

Answer 9

The modern and more complex view of the mechanism involves the slab-pull and ridge-push processes.

Slab-pull is based on studies of subduction zones at deep ocean trenches where oceanic plates are moving down into the mantle. The concept is that the weight of the descending ‘cold’ plate drags itself downwards, deep into the mantle and this is what pulls the plate away from the constructive ridges.

At the same times rising plumes of heat energy from the boundary of the mantle and core stretch plates upwards. At this ocean ridge new oceanic crust is form and the plates move under gravity away from the raised ridge - this is ridge-push.

Question 10

Slab-pull Ridge-Push diagram

Answer 10

Question 11

Plate Margins

Answer 11

In reality, all three processes (convection, ridge-push and slab-pull) may act together or be present at different plate margins.
The outcome is that three distinct types of margin or boundary form where the plates meet:

•Constructive margin- Pulling or tension forces plates apart allowing new material to be formed. Commonly at ocean ridges.

•Conservative margin- Plates slide past each other without forming or destroying plate material

•Destructive margin- Compression forces drives plates towards each other, causing either one to be gradually subducted(oceanic) and destroyed or both to crumple (collision margin)

Question 12

Constructive Plate Margin

Answer 12

The sub-marine mountain chains of the central Atlantic, Indian and Pacific oceans are the products of the process of sea floor spreading.

The sequence of events that has created the world’s ocean basins is as follows. Hot spots deep in the mantle cause magma to rise, forcing the solid plates above to warp upwards, stretching the crust and breaking along fault lines.
This zone of weakness is marked by tensional cracks, with uplifted and slumping blocks giving mountain ridges and rift valleys, and rising magma solidifying to create new oceanic plate material.
As this creative process continues, the stretched plate may allow a nearby ocean to spill in and water to flood the rift valley, starting the formation of a new ocean basin.

Shallow earthquakes are also associated with constructive margins, caused by the movement of magma rising towards the surface.

Question 13

Rift Valley to Mid-Ocean Ridge Diagram

Answer 13

Question 14

Examples

Answer 14

The North Atlantic is one of the most recent formation of a mid-ocean ridge, as Europe and North America were firstly separated and then slowly forced apart.

There is one other location on land where a constructive margin may be studied. The Great Rift Valley of East Africa is at the initial stage in the formation of a new ocean, as the land stretches under the rising convection currents of magma from the mantle below. Such processes have already pulled the Arabian Plate away from the African Plate to form the Red Sea. In East Africa, the continental crust has been stretched and the slumping crust formed the Rift Valley which is occupied by many elongated lakes.
Meanwhile, magma rises through the widening cracks, sometimes to erupt and build volcanoes such as Mt Kenya and Kilimanjaro. This could be the site of the Earth’s next major ocean and these features provide scientists with the chance to study, at first hand, the processes that started the birth of the Atlantic Ocean 200 million years ago.
Geologists suggest that if the spreading continues for another 10 million years, the plates will separate completely, allowing the Indian Ocean to flood the Rift Valley through the Afar Lowlands, linking the lakes into a linear sea and leaving the region know as the Horn of Africa as a large island.

Question 15

The East African Rift Valley

Answer 15

Question 16

Destructive Plate Margins

Answer 16

This type of margin, where two plates are forced towards eachother in convergence, produces two possible variations:

A- oceanic plate meeting continental plate

B- two oceanic plates meeting

Question 17

Destructive Margin
A- Oceanic and Continental

Answer 17

The best known example of this lies in the Eastern Pacific Ocean Basin, where the relatively small Nazca Plate, formed at the constructive margin of the East Pacific Rise, moves westward to meet the South American Plate.

The South American Plate not only carries the continent of South America but also the floor of the western section of the South Atlantic Ocean. The eastern edge of the plate is at the Mid-Atlantic Ridge, a constructive margin, but the western edge of the plate marks a destructive boundary. Distinctive landforms and patterns of tectonic activity mark where these two plates meet. On the ocean floor, close to and parallel with the western coast of South America, lies a long, narrow, deep ocean feature - the Atacama Trench. This marks the point at which the denser Nazca Plate, pushing eastwards, meets the South American Plate and is dragged downwards into the asthenosphere beneath. This process is termed subduction. At the trench the leading edge of the continental plate is pulled down towards the sea floor. As the ocean plate subducts, ocean bed sediments are carried down or scraped up against the continent’s edge helping to form the adjacent Andes mountain range. Beneath the surface, as the huge plates slowly grind past each other, earthquakes are frequent. Seismologists can plot each earthquake’s focus with precision and in these regions a clear pattern of shallow to deeper foci is recorded. This gives a clear picture of where the contact plane between the two plates is located as the ocean plate subducts down into the mantle. The region of these seismic events is known as the Wadati-Benioff Zone, after two scientists who identified its significance. Around 200-300 km down in the mantle, the water-laden ocean floor sediment and some rocks of the plate itself start to melt, releasing magma. This new molten material starts to move upwards towards the underside of the continental South American Plate. This means magma may force its way through lines of weakness into the plate or erupt forming volcanoes on the surface.

Ocean Trenches
If the world’s ocean basins were drained, the topography revealed would more than rival the variation seen on land. Along with 9 km high mountains rising from the ocean floor, as at Big Island Hawaii, and the 50,000 km long mountain chains of the mid-ocean ridges, there are also narrow chasms plunging down 10 km - the deep ocean trenches. The deepest of all lies in the Mariana Trench, south of Japan and is nearly 11 km deep. Known as the Challenger Deep, the deepest point was named after the British research vessel that first mapped it in 1951. While 12 people have walked on the surface of the moon, only three have seen the deepest ocean floor.

Ocean trenches are clear evidence of the subduction of an ocean plate at a destructive margin. They are commonly associated with earthquake patterns, increasing in depth with lateral distance from the trench and with parallel lines of volcanic activity often hundreds of kilometres away.

Question 18

Destructive Margins
B- Oceanic and Oceanic

Answer 18

Where convection forces in the mantle cause oceanic plates to collide, a line or arc of volcanoes (submarine or as islands) is often found parallel to a deep ocean trench.
These features, along with a Wadati-Benioff pattern of earthquakes, indicate subduction. Similar to the previous destructive margin, the denser of the two oceanic plates is dragged down into the upper mantle, creating friction earthquakes and eventually melting at depths of up to 600km.
Such margins are common in the western region of the Pacific basin. These include the islands of New Zealand in the south, through those of Tonga, Mariana, Indonesia, the Philippines and Japan, to the Aleutians in the north. Long curving ocean trenches are paralleled by similarly shaped arcs of volcanic islands, known as island arcs.

Tonga Trench cross-section shows the location of earthquake foci beneath the region. Shallow earthquakes occur near the Tonga Trench itself and with increasing distance away the earthquake foci are deeper. The line formed is
interpreted as the contact zone of the two plates along the subduction area, the Wadati-Benioff zone. The islands themselves are the result of ocean crust material melting around 100 km down and erupting onto the ocean floor, eventually building to reach the ocean surface. The plate itself continues to plunge deep into the mantle to depths of over 600 km - slab-pull in action. Over a longer time period, the growth and reworking of rock material can produce more substantial landmasses and islands, such as those of Japan and the Philippines. These are then termed mature island arc systems.

Question 19

Oceanic and Oceanic Diagram

Answer 19

Question 20

Collision Plate Margins

Answer 20

Similar to destructive boundaries, collision margins form where plates are moved towards each other by processes in the asthenosphere. However, in this case both plates carry continents.

Where two continental plates meet there is no subduction of plate material, rather the edges of the plates and any sediments deposited between them are crushed upwards into a mountain belt of folded and faulted mountains (Figure A25). The Himalayas are one example, resulting from the collision of the Indian Sub-continent Plate into the huge Eurasian Plate. In reality, as the plate carrying the Indian sub-continent sped across what is now the Indian Ocean, towards Eurasia, its leading edge was oceanic and subduction occurred. As the two continents drew near, the ocean drained as the sediments on its floor were forced upwards. These sediments continue to rise today as the series of huge ridges that form the mountain kingdom of Nepal and the vast high plateau of Tibet. The summit of Mt Everest (Sagarmatha), at 8850 m, is made of limestone, a rock formed under shallow tropical seas. Another example of a collision boundary, where mountain building, earthquake and volcanic activity continue, is located along the Mediterranean Sea of Southern Europe.

Fold Mountains
During mountain building phases, compression forces horizontal beds of sedimentary and volcanic rock to bend into a series of wavelike forms or folds. Rock folds may be microscopically small or they may involve thousands of metres of rock. Folds may be simple symmetrical waves or, as in the Alps, they may be overturned or recumbent. From the distant geological past there is evidence of several global mountain building periods (orogeny). One, named after the ancient mountains of Scotland, is the Caledonian. It is believed that Ben Nevis, at 1344 m the highest mountain on these islands, is the remnant stump of its 9000 m original height. In the current geological era, across the globe the formation of fold mountains continues. This is the Alpine-Himalayan orogeny and it includes the development of the Rockies and Andes chains, as well as those of Europe and Asia. The formation of fold mountains is closely related to the location of converging destructive and collision plate margins.

Question 21

The Himalayas
Diagram

Answer 21

Question 22

Conservative Plate Margins

Answer 22

Margins where plates slide past each other are extremely common on the Earths surface. Most are under the sea and run at angles across constructive margins.

Conservative margins are so called because they do not involve the creation or destruction of plate material and therefore no significant volcanic activity. They are, however, frequently the cause of earthquakes. One conservative margin that appears on land runs through the state of California. This margin is marked by a series of faults, the most famous being the San Andreas fault. Since 1906, when a powerful earthquake along the fault line destroyed the city of San Francisco, the San Andreas has been the focus of intensive study into the causes and possible prediction of earthquakes.

At conservative margins plates are normally moving in opposite directions but in this case the plates are both moving in the same north-west direction but at different speeds.
The two plates involved are the North America Plate and the Pacific Plate. The first is moving at an average rate of 6 cm per year, the second at 2 cm per year, giving a relative difference of 4 cm per year (Figure A29). In the short-term, the people of California, especially around San Francisco, fear the Big One, a quake of the scale experienced in 1906 (8.3 on the Richter scale). It is estimated that this would cause in the region of
10,000 deaths and over 50,000 injuries. In the longer term, if the plates continue on their present course, Los Angeles will move past San Francisco and south California will become an island off the west coast, in around 8 million years.

Question 23

The Distribution of Tectonic Activity

Answer 23

The Distribution of Tectonic Activity
The mapping of the worldwide distribution of earthquakes and volcanoes played a key role in the development of plate tectonic theory. By identifying the linear zones of these activities, the boundaries of the lithosphere’s plates were marked out. The presence of both earthquakes and volcanic activity at both destructive and constructive zones, though different in nature, helped shape the concepts.
At conservative margins, the tension that caused earthquakes but not volcanic action confirmed their nature.
Plate margins and related fault lines account for the vast majority of tectonic activity but there are some processes found well away from these boundaries. Among these are the highly active and huge volcanoes of the Hawaiian Islands.. Located in the centre of the Pacific Ocean these islands are about as far from an active plate boundary as it is possible to be - so why all the activity?

Hot Spots
Sub-lithosphere thermal anomalies or hot spots are seen as a driving force behind plate tectonic movement and the creation of lines of construction and destruction at plate margins, but isolated hot spots beneath unbroken plates may also cause local volcanic and earthquake activity. A common picture used is the idea of a piece of paper being moved across the tip of a Bunsen burner flame. The paper would show a scorched or burn line. The paper represents an oceanic plate being pushed across a fixed plume of heat and magma, rising from a hot spot originating deep in the mantle near the edge of the outer core. Some magma rises through the plate to form a submarine volcano that might just grow to reach the surface as a volcanic island. The outcome is a conveyor-belt sequence of volcanoes that pass from active (over the hot spot) to dormant and eventually extinct as they move away.

Question 24

Hawaii Hotspot Diagram

Answer 24

Question 25

The Hawaiian Island

Answer 25

In the central Pacific, a chain of 50 volcanic islands, coral atolls and submarine mountains (seamounts) stretches over 6,000 km from Hawaii to the Aleutian Trench. Starting with the Hawaiian Island Chain itself, this merges with a series of atolls (coral reefs on the remains of islands), including Midway and then a string of mountains on the ocean floor - the Emperor Seamounts. It is believed that all these features are the result of magma rising to the ocean floor above a fixed hot spot beneath the Pacific Plate.

As the volcanic islands are moved away from the rising magma plume they become extinct.
Weathering and erosion gradually reduces their size until they are worn down to below sea level, firstly forming platforms for coral reefs to grow on and finally becoming mere stumps of their former size on the sea floor. The only active volcanoes today are found on the main and most southerly island of Hawaii. Although offshore, there is already evidence of volcanic activity starting to build a new Hawaiian island (Lo’ihi).

It appears that the Pacific Plate has moved at an average rate of 8.2 cm per year for the past 70 million years. About 43 million years ago, the direction of plate movement changed from northerly to north-westerly, as shown by the dog-leg in the line near the Yuryaku seamount.

Question 26

Hawaii Chain of Islands Diagram

Answer 26

Question 27

2. Volcanic Activity and It’s Management

Answer 27

Students should be able to:

(i) demonstrate knowledge and understanding of volcanic activity at constructive and destructive plate margins and at hot spots

(ii) demonstrate knowledge and understanding of the socio-economic and environmental hazards and benefits of volcanic activity

(iii) evaluate how a country prepares for and responds to volcanic activity

Question 28

Volcanoes - General Info

Answer 28

The term volcanic is used to describe all the activity associated with the extrusion of magma onto the Earth’s surface.
This includes a range of material from huge boulders through lava flows to emissions of steam and gases. Specifically a volcano is where molten rock (lava) and gas erupts and solidifies through a rift or vent in the crust.
Volcanoes come in many shapes and sizes and the type of eruption is equally varied.
Eruptions may come from a central single vent or along a line of weakness known as a fissure.
Volcanoes are normally classified as being active, dormant or extinct.
Dormant suggests that while there is no historic record of eruption the volcano cannot vet be regarded as extinct. Each year about 50 of the world’s active volcanoes actually erupt.
For some this is for the first time in many centuries (Mt Pinatubo in 1991) while for others it is a regular event (Mt Etna on Sicily has some crater or vent activity most years). Some volcanoes are in almost continual activity such as Kilauea in Hawaii.

Question 29

Plate Margins and Hot Spots

Answer 29

Volcanic activity is associated with three distinct locations: constructive margins, destructive margins and at individual hot spots.

Constructive Margin
While 75% of volcanic material is created at constructive margins, in general eruptions are less violent and extrude very hot, freely flowing basaltic (low silica content) lavas to form volcanoes with gentle slopes. Most constructive margin volcanic activity is hidden beneath the sea where magma forms the mountains of the mid-ocean ridges. As magma extrudes onto the ocean floor it cools rapidly forming bulbous shapes known as pillow lavas, composed of fine textured igneous basalt or gabbro rocks. This is where and how the ocean plates are created and so over 65% of the Earth’s crust is the result of this volcanic action. On land the basalt rocks of the Antrim plateau, including those eroded by the sea at the Giant’s Causeway,are the produce of constructive margin activity some 55 million years ago. Today, the island of Iceland gives us easier access to these processes and landforms. Along with frequent eruptions from central volcanoes and fissures creating wide lava flows, thermal lakes and geysers are also common.

Destructive Margins
At destructive margins, including the Pacific ‘Ring of Fire’, eruptions tend to be more violent and the less fluid, acidic, silica rich lavas form steeper, cone shaped volcanoes.
The magma produced by former ocean plate and ocean floor sediment material melting deep in subduction zones rises to penetrate and move through the plate above, leading to violent volcanic activity in the mountains or along island arcs. Around 80% of the world’s active volcanoes are at destructive margins.

Mt Pinatubo Eruption
The eruption of Mt Pinatubo (Luzon, Philippines) in 1991 was an example of the violent volcanic activity associated with destructive plate margins. This eruption sent a vast cloud of debris, 16 km wide, more than 30 km up into the atmosphere. Volcanic ash, 10 cm deep, covered the landscape for a 60 km radius. The event released more material (a total of over 20 million tonnes) into the atmosphere than any other eruption in the last 100 years.
The sheer weight of debris caused thousands of buildings to collapse, including schools, hospitals, children’s homes and thousands of houses in the city of Olongapo, 56 km south-west of Pinatubo. Much more damage was caused by rivers of water mixed with hot volcanic ash, known as lahars, which inundated the low lying farms and villages.
The final death toll was around 900 people and but for a well co-ordinated prediction and evacuation plan, would have been thousands more. The damage and costs due to the eruption were estimated at over €10 billion.

Hot Spots
Hot spot volcanoes are among the most active and predictable. In Hawaii they produce lava capable of flowing rapidly and for long distances.
Aa is a blocky lava while pahoehoe lava is a liquid lava ** with a surface skin that resembles coiled rope. Kilauea on Big Island, Hawaii has been erupting lava for decades and tourists can routinely view the material flow across the island surface eventually reaching the sea. Such fluid lava means the volcanoes have very wide gentle slopes. The dormant volcano of Mauna Kea in Hawaii stretches over 6 km up from the floor of the Pacific Ocean and 4 km above sea level, making it higher than Everest and the island is the largest single volume of volcanic material in the world.

Question 30

Mt Pinatubo Eruption 1991
Diagram

Answer 30

Question 31

Shield Volcano Lava Flow, Hawaii 2009
Diagram

Answer 31

Question 32

The Hazards Created by Volcanic Activity

Answer 32

Hazards Created by Volcanic Activity
The variety of potential hazards following a volcanic eruption. Some hazards are confined to the immediate area, including lava flows. Others, such as lahars, may travel many kilometres from the source, while ash falls can settle over hundreds of square kilometres or enter the upper troposphere and circle the globe.

Explosion
Some volcanoes erupt without significant violence, such as the volcanoes of Hawaii, but elsewhere the force of a volcanic eruption can be enormous. When the Indonesian volcanic island of Krakatoa erupted in 1883, the explosion was heard 4000 km away in Australia. The blast destroyed the island itself and 36,000 people drowned in the 40m tsunami that swept the coasts of the neighbouring islands. On one island a ship was washed 20 km inland along a river valley.
Tsunamis are huge waves generated by either volcanic eruptions or earthquakes, which can travel across oceans at great speed resulting in the devastation of coastal regions.
The scale of volcanic eruptions can be measured in a similar way to earthquakes and the Richter scale.

The Volcanic Explosivity Index (VEI), while open-ended, varies from 0 (a non-explosive eruption) , through 4 (a cataclysmic event) to the very rare 8 (a mega-colossal eruption), such as the Yellowstone supervolcano event of two million years ago.

Materials
Numerous types of material are ejected by volcanoes such as lava, pyroclastic material and gases.
Lava rarely threatens life as its flow is relatively predictable but it does destroy property by swamping buildings or starting fires. The frequent flows of lava down the slopes of Mt Etna (Italy) in recent decades have destroyed cable car stations and overwhelmed houses, hotels and restaurants. These also buried rich farmland, burning vines and orchards.

Proclastic material is the term used to describe a wide variety of solid material ejected by volcanic activity other than lava. Nuée ardentes, (literally ‘glowing cloud’) are spectacular, potentially lethal mixtures of superheated gases, hot ash and rock fragments that flow at enormous speed down the side of some volcanoes. Over the USA volcanic ash from the Mt St Helens eruption of 1980 entered the upper atmosphere and circled the globe helping create spectacular sunsets for months.

Volcanic gases are often hot and toxic. One August night in 1986 at Lake Nos, a crater lake in Cameroon, West Africa over 1700 people died of carbon dioxide poisoning. A heavier than air cloud, rich in carbon dioxide, was expelled from the volcanic lake and swept down adjacent valleys. Up to 23 km away people died in their sleep as the cloud replaced the air. Farmers livestock and the local wildlife were similarly impacted.

Landslides
Volcanoes often bulge as magmatic pressure builds up beneath them. This deformation of steep slopes may cause landslides. The devastating eruption of Mt St Helens in 1980 followed the collapse of the north side of the mountain, in the largest landslide ever recorded on film. Currently it is feared that volcanic activity in the Canary Islands might cause a huge landslide to generate an enormous tsunami, with devastating consequences especially on the densely populated eastern seaboard of the USA.

Lahars

These are volcanic mud flows. When hot ash mixes with river water or with heavy rain, which can be triggered by eruptions, it can flow as a thick hot mixture at great speed, flooding valleys, burying the environment and drowning people. In 1985, the eruption Of Nevado del Ruiz in Colombia resulted in a lahar flowing at 100 km per hour through the town of Armero, some 50 km from the volcano. In one night over 20,000 of the lorris 23,00 inhabitants perished, buried by hot mud. At both Mt St Helens (1980) and Mt Pinatbo (1991) volcanic lahars extended the area of impact and deaths tens of kilometres from the eruption crater.

Jokulhlaups
(Glacial outburst floods)
These are floods caused by volcanic eruptions beneath ice sheets or glaciers. Small events happen almost every year in Iceland, where over 20 volcanoes lie beneath the ice. Large-scale events threaten farmland and transport networks, in particular the island’s vital coastal highway. Despite its propensity for regular earthquakes and volcanic eruptions, Iceland has not had a death caused directly by such events in over 100 years.

Question 33

Potential Hazards Diagram

Answer 33

Question 34

Volcanic Explosion Diagram

Answer 34

Question 35

Material Ejected by Volcanic Activity
Diagram

Answer 35

Question 36

The Socio-Economic and Environmental Hazards of Volcanic Activity

Answer 36

Socio-economic
Volcanic hazards affect both individuals and communities, causing costly damage to infrastructure, agriculture, industry and governments.

Volcanic eruptions are often fatal but the estimated total of 200,000 deaths in the last 500 years is much lower than other natural (earthquakes and floods) or human (wars and traffic accidents) disasters. Sometimes **signs or precursor events* allow prediction and evacuation of an area but this is not always the case. It has been possible to divert or steer some small lava flows but more often the red hot lava will destroy anything in its path regardless of planning.
The preceding section identifies a number of examples of fatal volcanic hazards:
-lahars,
-gas emissions,
-tsunamis,
-pyroclastic flows
-floods

Fatalities may be caused by secondary impacts such as the Icelandic famine after the Skaftar Fires eruption of 1783, in which one quarter of the island’s population died.
Other social impacts include homelessness and refugee movements.
The 1991 Mt Pinatubo eruption displaced over 100,000 people, many of whom could not return to their homes for years, if at all. This was the result of a huge ashfall that collapsed their houses and buried their farmland.
Repeated lahars swept the region’s river valleys in the months and years after the eruption had died away. The aboriginal, Aeta tribal people had to abandon their mountain forest homes on the slopes of Pinatubo and in lowland refugee camps hundreds of their young children and elderly died of illness. This was a result of overcrowding and inadequate supplies of water and medicine. Any traumatic event caused by volcanic activity will have a huge psychological impact on the communities involved and people may simply refuse to return to the region, even when it is possible to do so.
Closely linked to social impacts are the negative effects on a region’s economy. Lava flows can destroy any built structure, such as houses, factories, roads, bridges or farms.
Lahars, ash falls and jokulhlaups can extend this destruction over a much wider area.
The damage caused to farmland, commercial forestry or tourist amenity may take decades of restoration to re-establish the economy.
The blast of the 1980 Mt St Helens eruption flattened several million trees over an area of 600 km.
Between 1995 and 1997, the Caribbean island of Montserrat, a British dependency, suffered because of an erupting volcano called Soufriere Hills. The net outcome was that most of the island was uninhabitable and over 7000 of its 11,000 population had be to be evacuated and resetiled for several years at least. Dealing with the impacts and this reass relocation program cost the British government over £100 million. Even in cases where the nature of the activity is less destructive, any disruption to people and their employment will be expensive in economic terms.

Environmental
The impacts of volcanic hazards on the landscape, the ecosystem and the climate.
Volcanoes are capable of re-writing landscapes. People frequently describe the scene of recent eruptions as lunar - barren and desolate. In 1883, the enormous eruption on the island of Krakatoa replaced a 300 m high mountain with a 300 m deep submarine crater. At Mt St Helens the landslide and explosive eruption on its northern flank eventually reduced the near 3000 m summit by 400 m. Such explosions and movements of lava and ash often kill all vegetation and animal life in the region.
We know, for example, that over one million farm animals died at Pinatubo in 1991 but no accurate figure is known for its impact of the natural ecosystem (Figure A34).
Major volcanic events, or a series of them, can impact the global climate. In 1815, the cold summer and consequent worldwide crop failures and famine, in which millions died, has been linked to the eruption of Mt Tambora in Indonesia. Scientists speculate that worldwide mass extinction of species in the past may be the outcome of a series of volcanic eruptions. These could fill the upper atmosphere with dust and reduce the level of insolation entering the Earths energy system.

Question 37

The Socio-Economic and EnvironmentalBenefits of Volcanic Activity

Answer 37

Despite their destructive image, volcanic activity is not only hazardous, it can prove beneficial to both people and the environment. In fact, volcanoes may have been the seed bed of all life on planet Earth, as the one location where the necessary physical and chemical conditions existed.

Land Creation
While ash and lava may bury useful land, the same activity can create new land. In 1963, a small fishing fleet off the south coast of Iceland saw a column of smoke rise from the sea. They immediately shipped their nets and made way to what they assumed was another boat in distress.
On arrival at the scene, they witnessed the summit of an underwater volcano breach the surface to form a new island, later named Surtsey.
The new island has provided a golden opportunity for scientists to study not only volcanic processes but also the development of a prisere and ecological succession. On another Icelandic island, Heimaey, the eruption of Eldfell destroyed hundreds of houses buried by ash fall or burnt by lava. However, the lava flows that had threatened to block off the harbour entrance were stopped and actually enhanced the shelter provided for the local fishing fleet.

Fertile Soils
Benefits from volcanic activity include the fact that some, but not all, lava flows and ash falls can be weathered into rich, fertile soils. Soils based on basic lavas or ash deposits rich in potassium or phosphorus are highly valued. It is no coincidence that over 20% of the population of Sicily lives and depends on the fertile slopes of Mt Etna, an active volcano. Here the high yield from olive and orange groves, and wine produced from local vineyards supports a thriving agricultural community. This is at once both an economic and a social benefit. The natural environment benefits in a similar way. In the three years following an eruption of Katmai, Alaska, in 1912, the ash fall resulted in the tallest grass and largest berry production ever known.

Mineral Deposits - Industrial Resources
Volcanic deposits provide a wide variety of industrial materials and chemicals including sulphur, pumice, arsenic and boric acid. Beneath the surface in active volcanic areas, mineral-rich gas from lava cools, forming veins of minerals and metal ores. The oldest written reference to our islands off Western Europe is from Greek sources, naming them the ‘Tin islands’ due to the copper and tin deposits in old volcanic rocks of Cornwall and southern Ireland. Today in Indonesia, at the lien volcano, local workers climb into the 200 m deep crater at the top of the mountain to mine and carry out blocks of sulphur that is deposited around the perimeter of the crater lake. They undertake this back-breaking task on alternate days, as the conditions within the crater are hazardous to their respiration. Diamonds are formed deep in volcanic zones along narrow channels called kimberlite pipes.

Energy
In Iceland, New Zealand, Italy and he USA naturaly produced volcanic steam is harnessed to generate electricity. The largest such plant is The Geysers in California, generating 1000 MW of electricity, said to be enough energy to supply the needs of San Francisco, a city of 800,000 people.
Reykjavik, Iceland’s capital also gets most of its heating from geothermal water derived from volcanic springs.
Over 50.000 homes receive water heated naturally to 87°C from this environmentally friendly system. As a natural and renewable energy source, geothermal energy is
both economicallv and environmentall beneficial.

Tourism
Volcanoes, especially those that are currently or recently active, are strong magnets for adventurers and tourists alike.
The 2001 eruption of Mt Etna in Sicily coincided with the holiday season and companies flew, coached and sailed thousands of visitors in to witness the event, which was particularly spectacular at night.
The mud pools and geysers of Yellowstone National Park, Wyoming USA, are the key attraction for tens of thousands of visitors annually. The most famous feature being Old Faithful, a geyser that regularly (around every 95 minutes) sends a column of hot water and steam into the air.
In the Canary Islands or the volcanic islands in the Caribbean, a volcanic barbeque is often part of the tourist itinerary.
The sheer beauty of volcanoes such as Mt Fuji in Japan is a priceless asset and Crater Lake in Oregon is regarded as one of the world’s most beautiful landscapes. Even ancient volcanic activity can be of economic benefit.
Northern Ireland’s leading tourist attraction is the remnant of the outpouring of millions of tonnes of lava during the Tertiary era, which solidified into the regularly shaped columns of the Giant’s Causeway. The economic spin-off from volcanic attractions is common to all tourism: jobs and income from guides, accommodation, catering, transport, ancillary services and the selling of souvenirs.

Question 38

The Management of Volcanic Activity

Answer 38

In common with many other natural hazards, the management of volcanic activity involves three areas:

1. Prediction: when, where and exactly what is going to happen.
2. Protection: developing physical structures to improve safety.
3. Preparation: raising awareness and educating people in how to be ready for, and react to, the event.

In addition, because volcanic activity does create potential socio-economic benefits, such as fertile soils and tourist attractions, there is an important positive aspect to its management.

Prediction of Volcanic Activity
Some volcanoes are highly predictable, such as the huge crater of Hawaiis Mauna Loa or Mt Etna in Sicily - currently Europe’s most active volcano. Others are much less readily anticipated. Of the 700 active (as opposed to dormant or extinct) volcanoes, only around 70 are continuously surveyed. Not surprisingly, volcanoes in the developed nations of Japan, New Zealand, Iceland and the USA are studied more intensely and prediction has improved. Prediction has several aspects; in the case of volcanic activity it is necessary to predict not only the time and length of an eruption but also its scale and the nature of its impacts. Inaccuracy on any one of these factors could prove even more disastrous than no prediction at all. Technological advance means that volcanic activity can now be monitored from the air and also from space by satellite.

Monitoring the Warning Signs
The obvious way to predict volcanic activity is to monitor the likely precursors or warning signs of eruptions. These include local seismic events, the deformation of the ground surface and any steam or gas release. Other observations that might be made are the melting of snow caps, changes in levels in crater lakes or the death of local vegetation. Volcanoes by definition involve the release of magma or gas, so beneath them material must be moving upwards, causing earth tremors and bulging of the surface. Seismic activity does not guarantee eruption: For example, Vesuvius in south Italy has shown strong activity several times without a subsequent volcanic event. It is not uncommon for prediction to be accurate in terms of timing but inaccurate in terms of scale and even direction. The 1980, Mt St Helens’ eruption was closely monitored and, with respect to timing, well predicted. A 5 km wide exclusion zone was set up and if the volcano had erupted vertically then it is possible no lives would have been lost. In the event, the bulging northern slope of Mt St Helens collapsed in a huge landslide, creating an outlet for the pressure from which an enormous blast of ash, debris and superheated gas erupted laterally, devastating the landscape well beyond the 5 km zone in that direction.
Sometimes it all goes wrong. In 1985, a Colombian volcano, Nevado del Ruiz, was monitored by scientists following signs of activity. After several weeks they declared that a major eruption was not imminent. The next day it erupted and as mentioned earlier, a lahar swept down an adjacent valley burying the town of Armero. It was of little comfort to the scientists that they had accurately predicted the path of such lahars - only their timing was wrong. By contrast, in 1980, scientists did evacuate many people from a threatening volcano at Mammoth Lake in California. No eruption occurred and the scientists faced the anger of residents over the inconvenience and their economic losses. Bernard Chouet of the United States Geological Service (USGS) believes that the identification of a seismic pattern known as a long-period event is a reliable indicator of volcanic activity. A long-period event is a particular frequency of movement that he suggests links to magma rising towards the surface. Models are now being designed to test these theories under laboratory conditions. Confidence in volcanic prediction based on seismic patterns was undermined when, in September 2014, the Japanese volcano, Mt Ontake erupted without any significant earthquake events being detected.

Tiltmeters are used on the volcano slopes and on craters to monitor rises, falls or bulges in surface levels due to magmatic movement underground. Other physical changes that have been monitored to aid prediction are the temperature of crater lakes and springs, gravity and magnetism. The latter two are based on the idea that new magma moving below a volcano will subtlety alter these values. Satellites routinely use thermal infrared imaging to study the invisible energy radiated by volcanoes and monitor the heat flow patterns of the world’s volcanoes.

Volcanologists also study gas emitted by volcanoes using the geo-chemical profile to inform their predictions. Gas sampling correlation spectrometers and laser monitoring are used to detect small changes in gases across the surface or crater of a volcano. Any changes in the chemical nature or quantity of these may help to forecast the timing, scale or nature of future events. Since 1995, the volcano of Soufriere Hills, on the island of Montserrat, has kept scientists busy assessing the nature and the future of its on-going activity. In this case, remote sensing from satellites has been used to plot lava flows and gas emissions using ultra violet filters on camera shots.

An important aspect of prediction is the production of accurate hazard mapping. These maps are often based on information from historical eruption, if available, or from survey work on the surrounding landscape. It was this approach by the USGS that produced a hazard map of the potential impact of an eruption of Mt Pinatubo weeks before it happened in 1991. In the longer term, the identification of areas at risk means land-use planning can ensure that vital structures such as transport routes, hospitals and emergency service stations avoid such areas or are, at least, appropriately designed.

Protection and Preparation
Active intervention during a volcanic eruption is rare but as mentioned earlier, when a lava flow threatened the sheltered fishing port on Heimaey, off Iceland’s south coast, fishing trawlers and coast guard boats turned sea water hoses on the flowing lava in an attempt to cool and solidify it. In Sicily, the regular lava flows on the slopes of Mt Etna provide opportunities for attempts to divert and redirect lava away from settlements and valuable property. In 1991-1993, when the settlement of Zafferena was threatened by such a flow, quickly erected earth barriers failed to stop the lava but explosives detonated closer to the flow’s source successfully forced the lava into a new artificial channel that avoided the town.

Preparation concerns people who live with the risk, the emergency services and local government. These agencies act to ensure that information about possible events is communicated accurately and quickly in order for the most significant response to take place - evacuation. Getting out of the way of volcanic activity, whether primary impacts of explosive eruptions, ash fall and lava flows or secondary impacts such as lahars and building collapse, is often the only real response. It takes good pre-planning to move large numbers of people rapidly and to a safe distance and environment. In the 1991 Mt Pinatubo eruption, the world’s largest in the last 100 years, the figure of 900 fatalities is only 5% of the probable total deaths had ample warning not been given.
Alert systems are used for communicating the likelihood of volcanic activity. The detail and style of these vary between countries but they usually have four to six stages and are either numbered or colour coded.

Question 39

Methods of Monitoring Volcanic Activity

Answer 39

Question 40

Hazard Prediction Map of Mt Pinatubo 1991

Answer 40

Question 41

The New Zealand Volcanic Activity Alert System

Answer 41

Question 42

3. Seismic Activity and It’s Management

Answer 42

Students should be able to:

(i) demonstrate knowledge and understanding of the nature of seismic events and their impact - p, s and I waves, seismic shaking, liquefaction and tsunamis

(ii) demonstrate knowledge and understanding of the attempts to predict seismic events - seismic gap theory and dilation

(iii) evaluate how a country prepares for and responds to seismic activity

Question 43

Seismic Events

Answer 43

Contrary to most people’s experience, Earth is not an inactive planet. Seismic events are continuous, occurring every few minutes, with frequent significant hazardous quakes. On average, earthquakes are responsible for up to 10,000 deaths a year. When rocks in the crust are placed under increased stress they deform. Eventually the pressure, tensional (pulled apart) or compressional (pushed together), is released in a sudden movement along a line of weakness or fault. The energy released is an earthquake, a series of focus and seismic waves radiate away from here. The point on the Earth’s surface immediately above the focus is called the epicentre.

Most earthquakes (75%) are shallow, less than 70 km from the Earth’s surface. Intermediate earthquakes are found from 70-300 km below the surface with the focus of deep earthquakes lying 300-700 km down within the upper mantle. The depth of the focus is one important consideration in the impact of earthquakes, as deeper quakes are less damaging.
The two common causes of earthquakes are:

1. The release of stress between rocks moving at plate boundaries.
2. The movement of magma within the crust beneath active volcanoes.

Around 95% of all earthquakes are located at plate boundaries and their global distribution is largely confined to the linear zones of destructive, constructive and conservative margins.

Question 44

Seismic Wave Types

Answer 44

An earthquake releases energy through the surrounding rocks in a number of different forms. There are two sets of body waves that travel through the Earth’s interior: P waves (the primary, pressure waves) and S waves (the secondary, transverse waves).

P waves are compressional in nature with a forward motion of compression and expansion like the pulse along a slinky toy. These waves are readily transferred by rocks, gas and liquid material, and are the first to arrive as they move faster, 5.5 km per second. S waves involve a side-to-side motion at right angles to the direction of travel. Unlike P waves these can only be transferred by rock and not by gas or liquid material, and move at 3 km per second.

On reaching the Earths surface, both P and S body waves transfer energy as L waves (surface waves). These are slower moving waves and include the side-to-side movement of Love waves and the up and down (rolling) motion of Rayleigh waves.

These waves, moving with different speeds and through different materials have helped scientists to create the picture of the Earth’s internal structure that we use today. They also allow the precise location of an earthquake’s focus to be calculated within minutes of an event. In relation to the impact of earthquakes, the geology is important. Unconsolidated sediment, such as sand or clay layers, tends to amplify seismic waves, increasing the motion much as a jelly does. This means structures built on such material are prone to greater levels of damage and destruction in an earthquake than those constructed on solid rock.

The magnitude of earthquakes is commonly recorded by two different scales: the first is the Modified Mercalli scale, a 12 level system based on the impact upon built structures. The second is the Richter scale, which records the energy release and wave size. The Richter scale is open ended and logarithmic in nature. On the Richter scale a magnitude 7 event will have a 10-fold increase in wave size, compared to a magnitude 6 and a 30-fold increase in energy release. In turn, compared to a magnitude 5 event, the 7 has 100 times the wave motion and 900 times the energy. The 2004 Indian Ocean tsunami was the result of an earthquake registered at 9.1 on the Richter scale. This is one of the highest values ever recorded and in energy terms equivalent to 23,000 Hiroshima atomic bombs and enough to move the Earth on its axis so changing the length of a day. Earthquakes normally last for seconds or minutes at most and the main event is often followed by aftershocks, which can cause additional damage to already weakened structures.

Question 45

Types of Seismic Waves Table

Answer 45

Question 46

The Modified Mercalli Scale Based on Observed Impacts

Answer 46

Question 47

The Richter Scale Based on Amplitude

Answer 47

Question 48

The Impacts of Seismic Activity

Answer 48

Apart from the climatic hazards of tropical cyclones, floods and droughts, earthquakes are responsible for more death and destruction than any other natural hazard. They are also a dramatic event that can modify the landscape in seconds, whereas the slow processes of weathering and erosion may take millennia. There are several direct and indirect consequences of an earthquake. Ground deformation is the only direct impact of earthquakes on the surface. Ground deformation is when the ground surface above an earthquake is distorted or displaced. The ground can be moved vertically, horizontally or a combination of these elements. Earthquakes along the transform San Andreas Fault in California often cause lateral deformation, as seen by the offsetting of roads, walls, fences and even furrows in ploughed fields. In other cases, vertical movements can leave cliff-like steps running across the countryside from a few centimetres to several metres in height.

Other earthquake impacts are a consequence of the energy radiating from the focus of an earthquake in the form of seismic waves.

Seismic Shaking
The energy released by an earthquake radiates out from its focus in all directions, like ripples on a pond. These waves shake the crust as they pass through it, and when they reach the Earth’s surface they shake the ground and anything built on it. Most waves produce a lateral, side-to-side motion. This is known as a shear motion, bending structures at right angles. This is why building designs based on triangles are more earthquake resistant than rectangular forms.

Shaking of only 20-30 cm is potentially devastating for buildings made from weak materials and the longer the shaking persists the greater the damage caused. Design is another factor; in the 1995 earthquake in Kobe, Japan, many traditional homes collapsed as they had heavy tiled roofs supported by vertical wooden columns. On the other hand, ancient pagoda buildings have survived many earthquakes as they appear to flex with the seismi motion. A number of high-rise buildings in Kobe were subject to a phenomenon known as pancaking. In this case, shaking causes one floor of a building to crumble bringing the structure above that point collapsing down. Often the missing floor was structurally different and had less support, such as the ground floor shopping are in an office block.

As well as horizontal shaking, earthquakes may also cause violent vertical motion. It has been reported that during some earthquakes, objects and people have been thrown repeatedly into the air.

Walls made of weak material, such as mud brick (adobe) or poorly mortared brickwork, offer little resistance to shaking and such structures are often the cause of a large proportion of the deaths and injuries sustained. Even in well-constructed buildings, they may have a natural resonance which matches that of the earthquake and this increases the impact of shaking. In 1985, seismic waves from a distant earthquake shook Mexico City and several medium sized tower blocks collapsed with great loss of life. Later scientists showed that the tremors matched the natural resonance of these buildings, causing their destruction, meanwhile nearby smaller and taller buildings survived without significant damage. Action taken to improve earthquake resistance on existing structures is known as retrofitting.

Shaking buildings do not have to fall to create hazards. Glass from skyscrapers, overhanging balconies, parapets and even advertising hoardings may fall onto people and property nearby. Even inside buildings seismic shaking creates hazards; fixtures such as machinery in factories, filing cabinets in offices and large fridges in homes become potential threats to life and limb.

Structures other than buildings, including bridges and flyovers, may also suffer damage from shaking. The 1989 Loma Prieta earthquake, near San Francisco, lasted only 15 seconds but in that time dozens of concrete columns supporting a 1 km long section of the upper tier of the Interstate 880 highway (known as the Nimitz structure) sheared away, causing it to fall onto the roadway below. Dozens of vehicles were crushed and around 50 lives were lost at this site alone.

Seismic shaking also leads to secondary impacts such as landslides, fires and floods.

The term landslide covers the movement of material down slopes and includes rock falls, avalanches and earth slumps. In many cases earthquakes act as the trigger mechanism starting the movement down steep slopes. A disastrous earthquake in central China in 1976 is known to have killed at least 250,000 people. In the mountainous region near the epicentre, most deaths were caused by hundreds of landslides that carried away or buried rural settlements. One of the best documented cases of an earthquake induced avalanche was in 1970, in the mountains of Peru. A quake measuring 7.8 on the Richter scale triggered the movement of 50 million cubic metres of ice, rock and mud. Within minutes the debris fell over 3000 m and travelled 11 km, burying two towns including Yungay, and killing 18,000 residents.

Seismic shaking can also cause flooding in several indirect ways. The earth or concrete walls of dams may be weakened or destroyed, releasing the water in the related reservoir to sweep downstream. River levées are another feature that may collapse during violent earthquake shaking, with the risk that rivers spill onto the adjacent floodplains. Landslides may block river channels causing water to back-up and flood the valleys. Later such a natural dam may itself break or overflow creating another food risk downstream.

Liquefaction
This is the process by which soft or unconsolidated sediments amplify the effect of shaking ground. The effect can occur with either dry or wet sediments but is most clearly seen where there is significant water content. Liquefaction is similar to the effect of standing on sand and wiggling your toes. The sand that supported your weight when standing still, allows you to sink down into it when you move and water rises to the surface. Liquefaction occurs when sediment is shaken loose and starts to act as a liquid, often causing building foundations to sink or subside.

Beach or lake bed sediments, along with reclaimed land, are highly susceptible to liquefaction and it is
wisest not to build on such material in seismically active areas. Not only will liquefaction cause land to fail to support buildings but underground service pipes may bend and fracture. Studies of the major earthquake that helped destroy San Francisco in 1906 showed that the buildings on loose, often reclaimed land were subject to four times as much damage as those founded on bedrock. In 1989, in the same Marina District, the Loma Prieta earthquake induced liquefaction that caused the collapse of buildings and ruptured gas and water pipelines. The resultant fires were eventually brought under control by pumping saltwater from the nearby bay.

Liquefaction may also cause land and anything built upon it to spread laterally so that roads or airport runways can crack open. In 1995, the Japanese port city of Kobe was struck by a magnitude 7.2 quake. The authorities had recently completed building the world’s largest container port terminal on two artificially reclaimed islands in the bay. The loose infill material was water saturated, and during the 20 second quake it suffered widespread liquefaction and lateral settlement. Most of the port was damaged beyond use for many months and Japanese trade was significantly disrupted.

Tsunamis
It is probably true to say that as a result of the events of 26 December 2004 (Indian Ocean) and 11 March
2011 (Japan), the knowledge of the Japanese word tsunami and the perception of its potential threat have become global. Technically known as seismic sea waves, tsunamis are fast moving high waves that radiate away from some large undersea earthquakes.

When a section of oceanic crust rises or falls, a major earthquake follows, displacing a large amount of water and transferring a huge amount of energy

Tsunamis travel across the oceans at an amazing rate of 500-950 km/hr. At the surface, tsunamis spread out like ripples from a splash in a pond. In deep water, the transfer of energy is as a very long, low wave, often less than 1 m high and with a wavelength of 100-700 km. Far from the shore these may go undetected and unnoticed by shipping.
However, a tsunami changes its nature when entering shallower water. As waves feel the seabed, friction slows them down and causes water to pile up. Some can reach heights of 30 m, as high as a 10 storey building. Perhaps surprisingly, the first sign of an approaching tsunami may be the withdrawal of water from the shore, at which point people should quickly move to higher ground. Between 5 and 30 minutes later the first great wave arrives as a surge of water, which may extend hundreds of metres inland, depending on the nature of the shoreline. Several waves may arrive, sometimes with many minutes between. Such events are relatively common in the Pacific, where a good tsunami warning system, based in Honolulu
(Hawaiian Islands, USA) has existed for many years. The Indian Ocean did not have such a warning system and many of the people who lived along its coastline had little or no experience of the phenomenon.

The 2004 Boxing Day tsunami was the direct consequence of one of the largest earthquakes ever recorded. Off the north coast of the island of Sumatra, Indonesia, a 9.1 magnitude earthquake caused a section of seabed over 1500 km long to fall by
15 m. The earthquake itself was felt over a huge region but the tsunami that it generated was the principal cause of the estimated death toll of 245,000, spread over 11 countries
around the Indian Ocean basin.

The impact of the 30 m high tsunami wave was indiscriminate, destroying both luxury tourist resorts and poor fishing settlements. The destruction was most severe in the Indonesian province of Aceh, the island of Phuket in Thailand, along the southern shores of Sri Lanka and the south Indian state of Tamil Nadu. In places the tsunami wave swept up to 3 km inland and it destroyed over 140,000 homes. Even 12 hours after the earthquake the tsunami still had the power to kill, when two people died in South Africa, 5000 km from the epicentre. A warning system has been established for Indian Ocean tsunamis to replicate the one in the Pacific Ocean.

Question 49

Liquefaction

Answer 49

Question 50

Attempts to Predict Seismic Events

Answer 50

While we can determine the regions in which the vast maiority of earthquakes are likely to occur we cannot yet predict when they will happen or their magnitude. Some Tesearchers argue that anything less than a fully accurate prediction in terms of location, time and scale would be a waste of time and effort and potentially dangerous, socially and economically.

The Japanese spend over £80 million each year on earthquake prediction studies, yet they have never successfully predicted a quake and currently say that they will have less that one minute to warn the population of Toyko-Yokahama of an imminent event.
This minute is not really prediction time rather it is the maximum time lapse between an earthquake happening at sea, which they hope to detect, and the first seismic shaking arriving at the coast.

Precursors
One of the main concepts behind earthquake prediction - the precursor: an event or action that signifies that another event, in this case an earthquake, will happen. Many suggestions have been made about things that may happen before an earthquake. Some, such as strange animal behaviour and ‘earthquake weather’, may be based on less than scientific observations; other signs are more measurable and therefore potentially reliable.
If we accept the idea that earthquakes are a sudden release of energy along a fault line, after stress has built up over time then all that is needed is to find and measure the characteristics that indicate growing levels of stress. These would be precursors.
Potential characteristics include ground level uplift, subsidence or tilting. After several Japanese earthquakes, scientists studying records have shown that nearby surface changes have occurred. Sadly, after the event is too late.

Example:
Well water at Kobe
An article in New Scientist in 1995 reported that a well 30 km from the epicentre of the Kobe earthquake had been studied for months before the earthquake. Researchers had continuously monitored the level of radon gas in the water of the well and on 7 January 1995 the level surged to very high levels and dropped to very low levels by 10 January. The earthquake struck one week later.

Question 51

Dilation Theory

Answer 51

The search for earthquake precursors is closely linked with dilation theory (dilatancy).
This states that as the rocks along fault lines become stressed they expand (dilate) and numerous microscopic cracks open up. In turn, such micro cracks may change some characteristics of the rocks which, if measured, can warn of an impending quake. Dilating or expanding rocks are therefore linked to the suggested geophysical changes of rising, tilting or subsiding land. More subtle characteristics have been examined, such as the water levels in wells and the increasing release from the ground of radon gas which is a natural process. Laboratory experiments show that stressed rock samples will show small alterations in their ability to conduct electricity, a value known as electrical resisting. Other measurable changes include the potential decrease in the speed of transfer of seismic waves along with temperature, local gravity or magnetic variation. Ground testing equipment and remote satelite based imagery have been employed to test the ideas but none have yet been successfully used to predict an event with any accuracy.

A second theory of earthquake prediction was based on work by American and Russian scientists and is known as the seismic gap theory. In brief, the concept suggests that in areas where earthquakes are known to happen regularly, but recent records show little activity, then stress must be building and seismic activity becomes more and more likely. One suggested example was that of the 1989 Loma Prieta earthquake along the San Andreas Fault. Before this earthquake, the seismic activity of the previous 20 years showed that three sections of the fault had shown little activity - one around San Francisco in the north, a second to the south near Parkfield and the third around the Santa Cruz Mountains south of San Francisco.

Question 52

The Parkfield Experiment

Answer 52

After the 1989 Loma Prieta earthquake, researchers in California decided to catch a quake. Ironically the greatest difficulty of studying an earthquake to predict is to know when and where it will happen. What Californian scientists knew was that medium size earthquakes seemed to occur with regularity in and around Parkfield on the San Andreas Fault.

At Parkfield, medium sized earthquakes have been recorded at similar intervals - in 1857, 1881, 1901, 1922, 1934 and 1966. Based on this, Californian seismologists invested heavily in monitoring the San Andreas Fault in and around
Parkfield. Laser ranging devices, tiltmeters and seismographs were deployed across the area. Ground deformation, electrical resistivity, magnetism, gravity and temperature were continuously monitored. The experts waited not for the expected 5 years but for 15. Then in 2004 the medium strength earthquake happened. Scientists spent the next year analysing all their collected data. The conclusion was long and technical in its language but essentially they failed to identify any pattern or precursors. One scientist said that in effect, “ it just happened, one moment nothing - the next an earthquake.”

Some researchers suggested that the data needed to be gathered much nearer the focus and so a borehole has now been drilled several kilometres deep down to the San Andreas Fault and instruments are being installed, ready for the next event 20 or 30 years in the future.

Question 53

Preparing for and Responding to Seismic Activity

Answer 53

“Earthquakes do not kill people, falling structures do.”
This statement, while not entirely true, does stress that on many occasions it is the human built environment that contributes to the impact of these seismic events. The social and environmental impacts of earthquake activity are numerous:

Social Impacts
• Death and injury

• Human fear, anxiety and bereavement.

• Buildings collapse, wholly or in part, burying or trapping people.

• Other structures collapsing - bridges, flyovers or elevated route ways.

• Phone, road, rail and other communication links disrupted

• Fracture of underground services - water, gas and sewage pipelines.

•Fires may be started or made worse by gas leaks.

• Homelessness, lack of adequate shelter, refugee camps and out-migration.

• The huge cost or debt in rebuilding infrastructure.

• Loss of jobs, closure of businesses and factories.

Environmental Impacts
• Landslides moving or overwhelming buildings or whole settlements. Alternatively they may disrupt drainage causing flooding.

• Liquefaction and ground failure causing building foundations to sink or subside.
• Tsunamis have a devastating impact on coastlines and coastal settlement.

Any or all of these impacts may follow an earthquake but their intensity and severity may have less to do with the magnitude or nature of the earthquake than with the country or region involved. In general terms, nations that are more scientifically and economically advanced are better placed to prepare for and respond to an earthquake episode. Knowledge and perception are critical aspects concerning earthquake response. Knowledge relates to the scientific understanding of the nature and location of potential earthquake hazards, whereas perception is the broader awareness of these hazards and the degree to which awareness leads to action in preparation and planning.

Management of Earthquake Impact
The impact of an earthquake does not solely depend on its magnitude and duration bur also on the degree to which the region is prepared for such a hazard. The management of earthquake activity starts before any event and can be summarised by the three Ps of: Prediction, Protection and Preparation.

Prediction
In the case of seismic activity the many theories of prediction have not yet provided a working model. However, on the broader scale it is safe to suggest that significant earthquakes are certainly going to be experienced in some places given their tectonic setting: California, Alaska, Japan, New Zealand and Italy are an obvious few.

Protection
the increasing use of information technology and computer modelling in enginering. ind building design. Hit and miss attempts to build earthquake resistant structures have been replaced by scientific design and testing. One example of this approach is Building Information Modelling (BIM). Each new seismic event for which data has been gathered adds to our understanding of their impact and threat. Designs can be been to destruction’ not just on physical shaking tables but on screen by sophisticated simulation modelling.

One popular technique for engineers working in seismically active regions is base isolation. As the name suggests, this approach involves separating a building from its foundation. In one system a building sits on bearings made of rubber and lead. When an earthquake occurs the foundation can move without moving the structure above.

Japanese engineers have developed a new form of base isolation in which the system actually lifts buildings on a cushion of air. Ground sensors in the building automatically detect seismic activity and activate an air compressor system which forces air between the building and its foundation. In less than one second the cushion of air lifts the structure 3 cm above its foundations. After the event the compressor turns off, and the building settles back down to its foundation.

Protection is also about avoidance and reduction of risk. Land-use zoning may play an important role in reducing the impacts of seismic activity. Buildings should ideally avoid low lying coastal plains where tsunamis are a threat and structures should not be placed on unconsolidated ground where liquefaction is likely to occur, amplifying the ground movement and increasing the threat of collapse.

Preparation
Preparation and perception go hand in hand. Regular emergency and evacuation drills can be used to educate the public, as well as the emergency services on how to be ready for a seismic event. Schools in earthquake prone countries have emergency procedures written into their teaching programs including the ‘Duck and Cover procedure used by Japanese and American children from an early age. California runs online simulation events and Japan now has both an earthquake practice event on September Ist and a similar national tsunamis event in November. In the more developed nations the emergency and military forces are required to draw up contingency plans in conjunction with other civil authorities to ensure that the co-ordination after the event is as efficient as possible.

Individual preparation by citizens includes taking steps to reduce the hazards posed by their own homes and having an emergency kit available that allows the family to be independent for up to 72 hours after an event.

Common safety advice tips issued to residents in earthquake prone regions

Have an earthquake response plan ready for homes and families:

• Take professional advice on how to make your home safer, such as bolting bookcases, fridges and water heaters securely to walls, and installing strong latches on cupboards.

• Identify a safe place in each room where nothing is likely to fall on you, where you can go to in an earthquake.

• Open a door as an emergency exit doors may get wedged closed if the frame twists.

• Keep a supply of canned food, an up-to-date first aid kit, 12 litres of water per person, dust masks and goggles, a working battery-operated radio and flashlights, and a bottled gas stove.

•Know how to turn off your gas and water mains in case of leaks.

When the shaking begins:

•Drop down, take cover under a desk or table, protect your head and hold on.

•Stay indoors until the shaking stops and it is safe to exit.

• Stay away from windows. In a high-rise building, expect the fire alarms and sprinklers to go off during an earthquake.

•If you are in bed, stay there, protecting your head with a pillow.

•If you are outdoors, find a clear spot away from buildings, trees, and power lines. Drop to the ground.

•If you are in a car, slow down and drive to a clear place. Stay in the car until the shaking stops.

• Evacuate to a designated safety evacuation zone if a fire or other danger approaches

•Tune in to local radio and do not act on any rumours that you hear.