Module 3

Question 1

Difference between a Natural Event, Hazard and Disaster

Answer 1

A natural event is a physical process that has an impact on the Earth operating in the lithosphere, hydrosphere, atmosphere and biosphere, which give rise to or cause natural hazards.

A natural hazard are natural events which have the potential to loss of life or damage property

A disaster can be defined as the “realisation of a hazard, which exceeds the band
of tolerance of a population”. It is associated with a large number of lives being lost and injury to people as well as property.

Question 2

Types of Hazards

Answer 2

Tectonic Hazard: earthquakes, tsunamis, volcanic eruptions

Climatic: Drought, Hurricane, Blizzard, Heatwave, Tornado

Geomorphological: Flood, Subsidence, Landslide

Biological: Insect plague, Disease outbreak, Forest fire

Technological: Nuclear accident, Transport accident, Chemical pollution

Question 3

Properties of Hazards

Answer 3

Geographical Location: the area which are associated with hazards. Volcanoes and earthquakes (plate boundaries), Flooding (rivers, sea)

Magnitude: how large a particular event is and is related to the amount of energy emitted and the amount of social disruption that occurred

Frequency: the number of events of a given magnitude that occur over a period of time. Frequency is inversely proportional to magnitude

Question 4

Types of Floods

Answer 4

Riverine
This occurs during periods of heavy rainfall which increases the volume of water in the river, when this is exceeds the channel capacity, the water overflows the banks of the river. It can also occur when the velocity of the river is so high that water flows right out of the channel usually at sharp bends (meanders), Where this water spills out is known as a flood plain

Coastal
This occurs on low-lying land along the coast and it does not encroach far inland and is usually brackish due to the mixing of fresh and salt water. It occurs as a storm surge as water is withdrawn due to the intense low pressure and strong winds during a hurricane or storm and after its passing, the water surges back and floods the coast. Isostatic and Eustatic changes may also play a role

Estuarine
As estuaries are located near the mouth of a river, when river discharge is very high during periods of high tide, water cannot quickly flow into the sea. The river’s water is therefore backed up and the water overtops the river’s banks resulting in flooding

Urban
This occurs in built-up areas where infiltration is limited due to the replacement of permeable surfaces with non-permeable surfaces. Thus water remains on the surface as runoff and becomes a flood. It can also occur due to the blockage of rivers by waste material.

Question 5

Causes of Floods

Answer 5

Meteorological : This has to do with the various types of precipitation and periods of intense and prolonged rainfall.

Hydrological: They arise from conditions and characteristics of the drainage basin (size and shape, drainage density, gradient , surface type) with identical flood generating mechanisms producing different floods depending on the catchment area.

Sea-level change: Rising sea levels(isostatic and eustatic) encroaches onto low-lying land and moves upstream into the river channel. This slows down the river velocity resulting in an increasingly higher river discharge and eventually overtops its banks

Anthropogenic (human): The development of settlements on flood plains. Deforestation. Reduction infiltration. Straightening and covering waterways with concrete. A build-up of waste material in the river

Question 6

Positive impact of flooding

Answer 6

1. It creates an enormous area of flat land known as a flood plain
2. Fertile soil as floods flush out salt from the soil and deposit nutrients and minerals into the soil (Guyana- flood fallowing)
3. Unique species of flora and fauna that have adapted to a riverine habitat

Question 7

Negative effects of floods

Answer 7

Primary effects:

1. Casualties
2. Physical damage of the environment and built environment

Secondary effects:

1. Sickness and death (water-borne diseases - cholera, malaria)
2. Traffic congestion
3. Temporary closing of schools and businesses
4. Soil erosion
5. Waterlogging of vegetation
6. Landslides
7. Pollution of fresh water sources

Tertiary effects:

1. Reducing in land values
2. Cost of rebuilding and cleaning up
3. Food shortages
4. Reduction in production- slowing economic growth- negative impact of country’s debt position

Question 8

Factors that influence flood damage

Answer 8

Magnitude: The size of the flood will influence the depth of water and the area submerged

Speed of onset: Refers to the time taken for the flood to rise and for people to feel the effects of the flood. This is important as it determines the measures used to reduce the impact such as evacuation plans and zoning regulations

Flow velocity: High flows wash away people and have a strong erosive power which can destroy/weaken the foundations of buildings.

Duration: The length of time an area remains submerged has a direct impact on the amount of damage that occurs. When the duration is long there is extra cost in pumping out the water all the while disrupting economic activities

Sediment Load: When the sediment load is large it can destroy buildings and reduces channel capacity

Frequency: Refers to the recurrence interval and seasonality of floods. The frequency of floods will determine what types of construction or agricultural activities should take place in an area.

Question 9

Predicting Floods

Answer 9

This is done by studying weather data (rainfall intensity, duration, volume, location) and the characteristics of the drainage basin.

Recurrence intervals for flood events can be calculated using the formula:

RI = n+1/r

n- number of discharge/storm surge events
r - rank of that discharge/ surge level

Mathematical models have also been developed to determine how a river will react to a rainfall events.

Question 10

Flood prevention methods

Answer 10

Hard engineering techniques

1. Building of embankments
2. Straightening and deepening the channel
3. Constructing dams and river barriers
4. Building sea walls
5. Construct storage reservoirs

These measures may give people a false sense of security and encourage settlement on floodplains which puts more people at risk

Soft engineering techniques

1. The creation of ‘floodways’ on the flood plain that is designed to be outlets for floodwaters
2. Planting trees
3. Terracing
4. Afforestation and reforestation

Zoning, Community Preparedness, Flood resistant design, Flood insurance

Question 11

What is the theory of continental drift ?

Answer 11

It theory was first proposed by Alfred Wegener in 1912 and it suggested that the continents were not always in the relative position that they are currently in. He determined that all the continents were once all joined in a super continent he called Pangea with the Tethys seas before being separated into Laurasia and Gondwanaland.

However his theory lacked a mechanism to drive the movement of the continents

Question 12

Evidence used by Alfred Wegener to support his theory

Answer 12

1. Identical fossils in rocks in continents widely separated by oceans.
2. Rocks, fold belts and mountain belts which would be contiguous if the continents were fitted together.
3. Coal and evidence of glaciations in areas whose distribution could not be explained by current climatic conditions.
4. The geographical fit of the continents

Question 13

Convection currents

Answer 13

Arthur Holmes contributed to the Continental drift theory as a he suggested that convention currents in the mantle led was the driving force for the movement of the plates.

Question 14

Theory of Plate Tectonics

Answer 14

The theory of plate tectonics states that the Earth’s surface is made up of rigid plates that form the lithosphere. They consist of crustal rock and the top layer of the mantle. These plates ‘float’ on the asthenosphere, powered by convection currents that derive their heat from the Earth’s interior.

Question 15

Sea-floor spreading

Answer 15

This is driven by deep-mantle convection caused by temperature differences in the rock.

1. Hot mantle rock rises under the mid-oceanic ridge with the circulation splitting and diverging near the surface.
2. The mantle rocks then moves horizontally away from the ridge crest to either side of the ridge .
3. This results in ridge push where the rising magma become hydrated, cools and solidifies and expands and pushing the crest apart.
4. Basaltic eruptions magma produce new crustal rocks which are forced to the surface
5. As the rock moves away from the mid-oceanic ridge they cool and become thicker and denser causing them to sink at subduct in subduction zones where they form trenches

Question 16

Evidence of sea-floor spreading

Answer 16

1. Marine geomagnetic polarity reversals:
   During the cooling of rocks , minerals such as iron align themselves with the Earth’s magnetic field thus storing this in themselves when they harden. When Navy scientists examined the data, they found bands of alternating strong and weak magnetism in the rocks of the seafloor. This made it possible to estimate the age of the seafloor rocks and measure the rate of movement.

It was determined that the rocks on either side of the mid-Atlantic ridge got progressively older the further away they are from the ridge.

2. In 1963, Fred Vine and Drummond Matthews, determined that there was symmetrical pattern of magnetic reversals on either side of the mid-oceanic divergences
3. Earthquakes epicenters outlining the edges of plates and therefore their location, size and shape

Question 17

Earth’s Internal Structure

Answer 17

1. Lithosphere- a relatively inflexible and buoyant layer which floats on the underlying mantle. As the lithosphere moves, it carries the plates. It comprises the crust and the top layer of the mantle.
2. Asthenosphere - occurs below the lithosphere contains middle mantle
3. Mesosphere - contains lower mantle
4. Liquid outer core - made up of iron
5. Solid inner core- made up of iron and nickel

Question 18

How do the plates move?

Answer 18

These rigid plates of crust “float” above the asthenosphere and are transported by the currents of convection cells.

Question 19

What are Hot Spots?

Answer 19

They are strong, localized, rising currents of magma (plumes) which, on reaching the crust causes fracturing which allows the magma to rise to the surface forming volcanoes. Responsible for the formation of the Hawaiian volcanic chain

Question 20

Divergent Boundaries

Answer 20

Where two plates move apart, a gap is formed, through which magma (molten rock) oozes up. On contact with the water, the magma cools and forms rock formations. It also warms the water and enriches it with sulphur, producing a rich diversity of marine life. These plate boundaries are sometimes called constructive margins because new crust is being created. An example of this is the Mid-Atlantic Ridge. As as result of this divergence, the country of Iceland which is found on the ridge is splitting up. This has also created the volcano of Krafla and the formation of fissures on surface.

Landforms: New oceanic crust, volcanoes

Question 21

Convergent Plate Boundaries

Answer 21

1. Oceanic- Continental
   When a continental and an oceanic plate converge, the lighter, less dense continental plate rides up over the heavier, denser oceanic plate which is forced down into the heat of the mantle. It then cracks and melts creating gases and molten rock that move upwards in weakness in the continental crust to form volcanoes.
   Landforms: Fold mountains, volcanoes, deep-sea trenches
2. Oceanic-Oceanic
   Where two oceanic plates meet, the denser plate subducts beneath the younger. The destroyed material rises to the surface as magma which eventually solidifies to form a chain of volcanic islands. Sediment is also scraped off the top of the descending plate to form an accretionary prism such as the island of Barbados.

Landforms: volcanic island arcs, deep sea trenches, accretionary prism

3. Collision Margin
   When two plates formed of continental crust collide, fold mountains are formed as neither crust is dense enough to sink beneath the other. Sediments are squeezed into folds and slowly pushed up by the steady advance of the two plates, eventually forming thick layers in the process of orogeny.
   Himalayas Formation
   This collision margin was responsible for the formation of the Himalayas as the Indo-Australian plate and the Eurasian plates collided. The sea between the two closed up and its sediments such as limestone and marine fossils were forced upwards into fold mountains.

Landforms: Fold mountains (Himalayas, Tibet Plateau)

Question 22

Conservative Boundaries

Answer 22

This boundary is found where plates move parallel to each other without creating or destroying crust. Earthquakes at shallow depths can be caused by friction and the build-up of pressure between the moving plates whose rough edges lock together then suddenly jerk. An example of this boundary is the San Andreas Fault formed by the North American and the Pacific plate sliding past each other.

Landforms: Faults

Question 23

What is a Fault?

Answer 23

A fault is a break in the Earth’s crust along which there is an observable amount of displacement. It occurs when rocks have subjected to stress beyond the limit of their resistance.

Question 24

Types of Faults

Answer 24

1. Normal: They occur due to tensional stresses that results in the lowering of the hanging wall in relation to the footwall.

Features: cliffs

2. Reverse: They occur due to compressional stresses that results in the raising of the hanging wall relative to the footwall

Features: escarpments

3. Strike slip/ Transform: Blocks of rock slide laterally past each other horizontally.

Features: faults

Question 25

Horsts/ Tilt Blocks & Rift Valleys

Answer 25

The crust can be divided into rectangular shaped blocks. Uplifted blocks may either be titled (tilt blocks) or horizontal forming horst/ block mountains. The depressed blocks between parallel faults often form rift valleys.

Rift Valleys:
They are elongated troughs that can be formed through tensional forces which causes a block to sink between parallel faults or compressional forces which causes fault blocks to rise up towards each other and over a central block

Block Mountains:
It is a raised block between two parallel faults. They can be developed through compression or through tensional forces where blocks sink on either side of parallel faults leaving a central block standing

Question 26

What is a Fold?

Answer 26

It is a bend in the rock strata caused by compression forces. This can lead to the formation of raised arches called anticlines and depressed areas called synclines

Question 27

Types of Folds

Answer 27

Symmetrical: Both limbs are the same- equal compression

Asymmetrical: One limb is steeper- unequal compression

Overfold: Both limbs face the same direction

Recumbent: The center of the fold moves from being vertical to a horizontal position

Overthrust: The fold fractures

Nappe: The fold breaks off along the fractures

Question 28

Value of faulted and folded landscapes

Answer 28

1. Source of Materials: They provided minerals such as copper, silver and gold as they experience intense heat and pressure . Fractured rocks are pervious which allows for the formation of aquifers which can be drilled for fresh water
2. Tourism: Uplifted areas attract activities such as hiking, rock climbing and skiing
3. Biodiversity: Areas of high relief have unique flora and fauna which have adapted to this environment which some areas being designated as protected areas/national parks
4. Watershed: Upland areas results in orographic rainfall as such they are the source of rivers and springs which provide fresh water for settlements

Question 29

What is an Earthquake?

Answer 29

It is the sudden movement of the Earth’s tectonic plates. Most earthquakes occur at plate boundaries with the deepest earthquakes occurring at subduction zones.

Question 30

What causes Earthquakes?

Answer 30

They can either be formed by tectonic or volcanic activity. Tectonic quakes result from the movement of the earth’s plate as stress is built up and released causing vibrations. When the plates converge or move past each other, the zone between them is under great stress. The ground first bends, then, upon reaching a certain limit, breaks and ‘snaps’ to a new position. In the process of breaking or ‘faulting’, vibrations are produced that are the earthquakes. Volcanic quakes are generated from changes of pressure under the volcano caused by the injection or removal of magma.

Question 31

The Focus and Epicenter of an Earthquake

Answer 31

The focus of the earthquake refers to the point of weakness on the fault line where movement takes place while the epicenter refers to the point the earth’s surface directly above the earthquake’s focus.

Question 32

Seismic Gap

Answer 32

According to the US Geological Survey, a seismic gap is a “section of a fault that has produced earthquakes in the past but is now quiet.” while no earthquakes have been observed historically, it is believed that the fault segment is still capable of producing earthquakes, based on plate-motion information or strain measurements.

Question 33

Body Waves

Answer 33

Following an earthquake, two types of body waves occur:

1. P-waves (also known as primary or pressure waves)
2. S-waves (also known as secondary or shear waves)

P-waves travel by compression and expansion and are able to pass through solids, liquids and gases. They are fastest waves, reaching the surface first, travelling at rates of 6-13km per second.

S-waves are a series of oscillations at right angles to the direction of the movement and travel with a side to side motion. They are only able to travel through solids. They are much slower than P-waves, moving at 3-7km per second.

Question 34

Surface Wave

Answer 34

1. Love waves (L-waves) cause horizontal motion on the earth’s surface, moving from side to side.
2. Rayleigh waves (R-waves) cause horizontal and vertical motion the earth’s surface causing a ‘rolling sea’ effect on the surface. These waves cause the most damage to buildings and other structures.

Question 35

*Mohorovicic Discontinuity

Answer 35

By observing the speed at which different seismic waves travel, Mohorovicic deduced that the slower waves had travelled from the focus of the earthquake through the upper layer of the crust. Therefore suggesting that there was as change in the density between the earth’s crust and the mantle. This boundary is known as the MOHOROVICIC DISCONTINUITY

Question 36

The Magnitude and Intensity of an Earthquake

Answer 36

The earthquake’s magnitude refers to the amount of energy that is released. This is then measured through the use of a seismometer and the Richter scale with each whole number represents ground motion ten times greater than the next lowest number.

The intensity of an earthquake refers to the destructive power of the event and and is measured on the Mercalli scale. This is a qualitative measurement as it is based on the impact of the earthquake (change in natural surroundings and buildings)

Question 37

Earthquake Hazards

Answer 37

Primary:
Ground shaking which can lead to the collapse of buildings and damage to infrastructure such as roads and bridges

Secondary:

1. Liquefaction
2. Landslides
3. Tsunamis
4. Fires
5. Floods
6. Disease

Tertiary:

1. Permanent evacuation
2. resettlement of new areas
3. Aid
4. Landscape changes
5. Debt forgiveness

Question 38

Factors affecting damage done by an earthquake

Answer 38

1. Strength of the earthquake
2. Distance from the epicentre
3. Geology
4. Relief
5. Geographic location/Distance from coast
6. Building construction
7. Time of day
8. Population density

Question 39

Positive impact of earthquakes

Answer 39

1. Allows scientist insight into the structure of the earth
2. The threat of earthquakes may result in improved building codes
3. Positive community spirit as people work together during times of crisis
4. Debt relief and large amounts of external aid

Question 40

Methods of predicting earthquakes

Answer 40

1. Statistical assessment (probability) of earthquake occurring- while it may provide information on the chances of earthquake occurring in a particular region, it fails to determine the exact location of focus/epicentre
2. Animal behaviour- while some animals exhibit strange behaviour before an earthquake, there may be other reasons other than earthquakes to explain such behaviours. Also, persons need to be regularly monitoring animals to make predictions
   .
3. Monitoring foreshocks- these can indicate an impending quake but the foreshocks may occur at various locations and not indicate the specific location. Further, these ‘foreshocks’ may not necessarily mean a major quake is imminent.
4. Monitoring gaseous emissions and chemical changes in groundwater

Question 41

Human Responses to Earthquakes

Answer 41

Modify (Prevent) the Event
1. Controlling the physical variables. This method is unrealistic as it is near impossible to try to control the actual earthquake event and the other physical aspects of an earthquake

2. Hazard-Resistance Design
   The incorporation of earthquake resistant (aseismic) design features can reduce the impact of hazards as the majority of deaths are caused by the collapse of buildings in squatter developments and older housing. In MEDCs, authorities are able to introduce and enforce strict building codes, which can reduce death rates whereas in citizens in LEDCs may be unable to afford the high costs associated with aseismic designs,

Modify Human Vulnerability

1. Prediction and warning
2. Community preparedness
3. Land-use planning.

Modify the Loss
1. Aid
Disaster aid may help to distribute financial losses on an international level. This aid is usually in the short term to provide medical services and relief goods as such this does not help in the long term reconstruction process.

2. Insurance
   Insurance is mainly available in the economically more developed seismically active nations and serves as a loss-sharing mechanism. However, the greater majority of persons in the world at risk from earthquakes have no realistic access to insurance and even in the richer countries, it is mainly the commercial and industrial properties which are insured.

Question 42

What is a volcano

Answer 42

A volcano is an opening through the Earth’s surface through which magma, molten
rock and ash are erupted onto the land. A volcano is formed when the Earth’s crust is fractured or weakened and magma reaches the surface, either gently oozing out or violently erupting.

The shape of
the volcano depends on:
 The nature of the material erupted
 The type of eruption
 The amount of change since the eruption

Question 43

Distribution of Volcanoes

Answer 43

Most of them are found in the Pacific Rim or Ring of Fire. Most volcanoes are located near plate boundaries, either at destructive plate margins or at constructive plate margins. Some also occur away from plate margins, in the interior of plates, at locations known as ‘hot spots’.

Question 44

Volcanoes at Destructive Plate Margins

Answer 44

At destructive plate margins, oceanic plate is forced down into the mantle. Where the oceanic crust is forced down into the mantle, it is called a subduction
zone and there is partial melting of the subducted plate. The melted rock is less dense than the surrounding rock so it rises upwards and forms a volcano if it reaches the surface. It melts the continental crustal rocks as it passes through, mixing to form an acidic magma. The viscous (sticky) nature of this magma makes it difficult for gases to escape, thereby creating bubbles and pockets of hot gas under great pressure. A rock commonly formed from this type of lava is andesite (named after the Andes).Due to the build-up of gases under high pressure within this type of magma, these volcanoes often explode when they erupt. They tend to cause much more violent eruptions than those elsewhere.

They can produce pyroclastic flows (avalanches of hot rock, gas and ash moving at 300 km per hour).The intense heat during a volcanic eruption causes air to rise, frequently resulting in heavy rain. The rain mixes with the ash, forming mud flows known as lahars. Some volcanoes release both pyroclastic material and lava. They are known as composite volcanoes, as they are formed of layers of both ash and lava.

Question 45

Volcanoes at Constructive Plate Margins

Answer 45

Where tectonic plates move apart, under the oceans, magma rises to the surface at mid-ocean ridges and oozes out to create new sea floor. When the magma reaches the Earth’s surface, it is called lava (forming mainly basalt rock). Volcanoes at these margins tend to erupt as gentle lava flows, often from fissures (long narrow cracks in the crust).

Question 46

Volcanoes at Hot Spots

Answer 46

Hot spots are areas of volcanic activity not associated with the edges of crustal
plates. They are cause by mantle plumes, which are isolated, long, slender columns of hot magma that rise from within the mantle. When oceanic plates move over the hot spot, the magma breaks through and a volcano forms. If the hot spot lies beneath continental crust, there may be hot springs, as underground water is heated by the magma below (Yellowstone, USA). Over a hot spot in the ocean, the volcano might form an island. Volcanic activity on the island diminishes as the plate moves from the hot spot but, behind it, the hot spot remains and creates a new volcanic island. Eventually, an island chain may form (Hawaii Islands). The lava in these cases is basic and highly fluid thus it produces low, gently sloping shield volcanoes (Mount Loa).

Question 47

Lava Viscosity

Answer 47

It refers to the resistance of molten rock to flow. The higher the viscosity, the greater the resistance to flow the faster the molten rock will solidify. It depends on temperature, silica content and incorporated gases.

Question 48

Types of Lava

Answer 48

Basaltic (Basic) Lava:

1. Has approximately 50% SiO2
2. Low-viscosity (readily flows across the surface)
3. Found mainly close to divergent boundaries
4. Forms basalt when it solidifies

Andesitic Lava:

1. Contains 60% SiO2
2. Has a substantial content of incorporated gas
3. Moderate viscosity

Rhyolitic Lava (Acid Lava):

1. 70% SiO2
2. Contains the largest amount of gas
3. Highest viscosity lava
4. Responsible for violent eruptions
5. Found mainly close to subduction zones

Question 49

Types & Nature of Lava Flow

Answer 49

Lavas, particularly basaltic ones, come in two primary types:
1. Aa
Aa lava is a few metres thick and consists of an upper rubbly part and a lower part of solid lava that cools quickly. It surface consist of free chunks of very angular pieces of lava (irregularly shaped, cindery blocks with sharp sides). Aa forms when lava flows rapidly. Under these circumstances, there is rapid heat
loss and a resulting increase in viscosity. When the solid surface crust is torn by
differential flow, the underlying lava is unable to move sufficiently rapidly to ‘heal
the tear’. Bits of the crust are then tumbled in and coated by still liquid lava, forming the chunks. Sometimes the crust breaks in large plates, forming a platy aa.

2. Pahoehoe Lava
   The pahoehoe flow has a comparatively cool, smooth or “ropy” surface, with flows occurring underneath. It is the least viscous of all lavas. Pahoehoe forms when lava flows move slowly. Under these circumstances, a well- developed ‘skin’ can form which inhibits heat loss. When a tear in the skin does form, it is readily healed.

Both magma discharge rate and the steepness of the slope over which the lava flows affect the flow rate. Thus aa lavas are associated with high discharge rates and steep slopes while pahoehoe flows are associated with lower discharge rates and gentle slopes. Flows which begin as pahoehoe can convert to aa when a steep slope is encountered.

Another type , pillow lava, forms during submarine eruptions.

Question 50

Types of Lava Eruptions

Answer 50

Lava that is released where the oceans meet the continents absorbs silica-rich
sediments. This causes it to become more viscous and block the vents until enough pressure has built up to break them open.

Two types of lava flows are:

1. Icelandic Lava eruptions are characterized by persistent fissure eruption. Large quantities of basaltic lava build up massive horizontal plains.
2. Hawaiian eruptions involve more noticeable central activity than the Icelandic type. Occasional pyroclastic activity occurs but this is less important than the lava eruption. Runny basaltic lava flows down the sides of the volcano and gases escape easily.

Question 51

Types of Pyroclastic Eruptions

Answer 51

1. Strombolian eruptions are extremely explosive with large quantities of pyroclastic rock. Eruptions are commonly marked by a white cloud of steam emitted from the crater. Frequent gas explosions blast quantities of runny lava into the air, to form the cone.
2. Vulcanian eruptions are violent and occur when there is very viscous lava that
   solidifies rapidly after an explosion. Often the eruption clears a blocked vent and
   spews large quantities of volcanic ash into the atmosphere. Violent gas explosions blast out plugs of sticky or cooled lava.
3. Vesuvian eruptions are characterized by very powerful blasts of gas that push ash clouds high into the sky. Lava flows also occur and ash falls cover surrounding areas.
4. Plinian eruptions are extremely violent eruptions characterized by huge clouds kilometers thick. Gas rushes up through the sticky lava and blasts ash and fragments into the sky in huge explosions. Gas clouds and lava can also rush down the slopes. Part of the volcano may be blasted away during eruption.
5. Pelean eruptions are ones where large quantities of viscous magma are erupted.

Question 52

Volcano Classification

Answer 52

Volcanic landforms are classified into:

• Explosive landforms- explosive vents, ash and cinder cones
• Effusive landforms- shield volcanoes and lava plateaux
• Mixed landforms- strato volcanoes

Question 53

Explosive Volcanic Landforms

Answer 53

1. Explosion Vent
   An explosion vent is a hole blown through the rock and is surrounded by a low
   crater or ring of rock fragments.
2. Ash & Cinder Cone
   Ash & Cinder Cones are formed by fragments of solid material that accumulate
   around the vent to form a cone. The shape depends on the nature of the material.
   It is usually concave as the material spreads out near the base and has a steep angle as cones are not very high- up to 300m and all ash.

Question 54

Effusive Volcanic Landforms

Answer 54

1. Lava Cones (Shield volcanoes)
   They are formed when there is no explosive activity, therefore no ejective fragments. Volcanic material is purely lava. The shape of the volcano depends on the type of lava. If the lava is basic, it is very fluid and can flow great distances. It builds up shield volcanoes which have a very small angle of slope, a shallow crater and a large circumference. When the lava is acidic, it is very viscous and does not flow far. It builds up a steep dome with convex sides, usually without a visible crater. These volcanoes are often very high in relation to their diameter.
2. Lava Plateaux
   They are formed by lava ejected from a linear or fissure type vent. There is usually no explosive activity. The lava wells up quietly and smothers the landscape under sheets of basalt. Molten material gets to the surface in areas where crustal tension creates cracks. The lava is extruded either along the whole length of the crack or at a series of points. The lava plateau is usually built up by a series of successive flows which extend over a long time. Each flow is not very thick, may be a few metres but can build up to several hundred
   metres.

Question 55

Mixed Volcanic Landforms

Answer 55

Composite Cones (strato volcanoes)
These are the most common type of volcano and are formed by alternating eruptions of fragmental material followed by lava outflows. The highest volcanoes in the world are of this type. The main cone consists of layers of ash and lava that comes from the main pipe. The lava accumulates in the crater. With certain volcanoes, a large explosion may blow the top off the cone and form a much larger crater, within which a secondary cone may develop. Parasitic cones frequently grow on the sides and sometimes a volcano can have a very violent eruption after a long period of being inactive. The pipe becomes plugged with cooled lava, pressure builds up and the result is a violent explosion.

Question 56

Volcanic Features (Intro)

Answer 56

When magma cools it produces igneous rocks. This might be extrusive, from lava and ash on the Earth’s surface or intrusive, from magma that cools and solidifies below the Earth’s surface.

Question 57

Intrusive Landforms

Answer 57

These result from volcanic activity where magma does not reach the surface.
When magma is forced up through the crust it might push along bedding planes and faults or form huge underground domes. They include:

1. Batholith
   These features are formed when large domes of magma cool underground.
   They typically contain granite which, because it has cooled relatively slowly
   underground, is formed from quite large crystals. When magma moves through the crust, it comes into contact with the surrounding rocks. The intense heat and pressure can alter these to form a new rock. These altered rocks in the ‘contact zone’ are called metamorphic. (Limestone-Marble, Mudstones and Clays- Shale and Slate)
2. Sills
   These features are formed when igneous rock intrudes horizontally along bedding planes. They can produce features such as waterfalls and escarpments when they are exposed by weathering.
3. Dykes
   These features are formed when magma intrudes vertically through the crustal
   rocks (sometimes along a fault), and cuts across bedding planes. Sometimes dykes radiate from a batholith to make a ‘dyke swarm’. When they are exposed, they can look like dry stone walls cutting across the landscape. In places where a dyke is formed of rocks less resistant to erosion than the surrounding rock, it can form a long, narrow ditch like depression.
4. Volcanic Plugs
   These features are the remnants of eroded volcanoes and are formed from the more resistant rock of the vent or pipe. They generally form small, steep sided hills.

Question 58

Extrusive Landforms

Answer 58

1. Mountains
   Volcanoes can form hills or mountains which differ greatly in size and shape from steep sided composite cones to gentle sloped shield volcanoes.
2. Vent/Pipe & Parasitic cone
   It is fed from an underground magma chamber (batholith) containing molten rock and dissolved gases. Gas from the chamber might keep the pipe or vent open. If not, the magma sets and subsequent eruptions have to blast this blockage out of the way forming a crater or break through the sides forming a secondary or ‘parasitic’ cone.
3. Lava plateaux/ flood basalts
   These are horizontal layers of basalt that are formed when lava flows from fissures and then solidifies.
4. Fumaroles
   These are ponds of hot mud, often with clouds of steam rising above them. Below the ground surface, the water is above 100°C but the intense pressure
   prevents it from turning into steam. So it becomes ‘superheated’. As it emerges from the ground, the drop in pressure enables it to vaporized and become steam.
5. Solfatara
   These features are emissions of smelly, sulphurous gas.
6. Geysers
   These are intermittent fountains of hot water. Water trickles into the ground from the Earth’s surface and is heated by the hot rocks below. It turns to steam and, as pressure builds up, the water and steam explode to the surface.
7. Hot Springs
   These are remnants of volcanic activity and occur where water trickles through
   warm rocks and becomes heated. It re-emerges, often having absorbed sulphur or other minerals to form hot springs or mineral spas. The springs sometimes become tourist attractions or health resorts.

Question 59

Benefits of Volcanoes

Answer 59

1. Volcanoes provide fertile soil, as lava and ash weather rapidly into deep, rich soils that are ideal form farming.
2. Volcanic activity sometimes produces valuable minerals such as gold, copper, lead and silver. Most mining occurs in areas where volcanoes are extinct or where
   igneous rock has been exposed by erosion.
3. Geysers provide the opportunity to develop geothermal power. Hot water and steam from underground are used to drive turbines or to supple central heating directly.
4. Volcanoes, geyser and hot springs are also tourist attractions.

Question 60

Primary Volcanic Hazards

Answer 60

1. Pyroclastic Flows
   They are rapid streams of high-density mixtures of hot dry pyroclastics (rock fragments produced and ejected by explosive volcanic eruptions), ashes and gases that flow down from volcanic vents following explosive eruptions. They can also occur from effusive eruptions, when parts of volcanic dome collapse and break apart. They destroy almost everything that lies in its path and turn the
   surrounding landscape into a barren land. The flow can burn anything in its path with its extreme temperature and also crush anything with its forceful momentum. They move more rapidly compared to lava flows, with speed ranging from 160-240 km per hour. Pyroclastic flows usually does not provide enough time for persons to evacuate/escape from them. They cause death by suffocation (poisonous gas and asphyxiation) and burning.
2. Lava Flows
   These are the movements of molten rocks that are released from volcanic vent either through effusive or explosive eruptions. Advancing lava flows destroy everything that is on their way. Even with its destructive force, death associated with lava flows is extremely rare (but still very possible) because in most cases, lava flows move very slowly, giving enough time for people to evacuate from the affected area (Figure 2). Lava flows have been known to travel as fast as 64 km/hr. The speed of lava varies depending on several factors such as type
   and viscosity of lava, steepness of the slope it is flowing, flowing pattern and rate of lava production. The area affected by lava flows is also limited since they usually do not move very far from its source. The main effect of lava flows is the total destruction of property. Lava flows completely blanket structures with meters of hardened lava, leaving owners unable to use the land and property after. Lava flows can also cut off the evacuation routes when they surround
   the victims or block roads and highways that serve as the only way out
   of areas.
3. Tephra Falls
   During eruptions, volcanoes release a huge amount of pyroclastic rocks
   or tephra into the atmosphere. These clastic (composed of consolidated sediments formed by the accumulation of fragments) deposits will fall back and accumulate on the surrounding Earth’s surface. Tephra fragments are classified based on their size ranging from ash and lapilli, to blocks and bombs. The distribution of ejected tephra typically involves the larger and more dense blocks and bombs fall down the fastest and closest to the vent while finer ash may stay in the atmosphere for days or weeks and travel thousands of miles away from the vent. Since tephra has the ability to travel at a great distance, the airfalls are the widest range of all direct volcanic hazards. Tephra airfalls pose a number of hazards. First is large falling fragments. Small falling rocks and bombs from eruptions are usually harmless but large falling bombs can give substantial force of impact and may harm victims even if they are protected in a secured shelter. Falling bombs usually affect victims in a close proximity to the vent, but in some cases, the bombs could be thrown up at a considerable distance. The other potential hazard is burial due to accumulation of tephra. This could cause building collapse, destruction in power and communication lines and damage in vegetation. The effect of tephra burial could be worsen if tephra accumulation is immediately followed by rain or snow fall, as the added moisture by the precipitations will greatly increases the density of originally porous and uncompacted tephra. Tephra airfalls can also result in suspension of fine-grained particles in the atmosphere, which may cause serious health problems by
   worsening the air quality and visibility of the affected areas.
4. Ash falls
   Ash falls can cause the collapse of roofs and can affect areas far away from the eruption. Although ash falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow. Ash falls destroy vegetation, including crops and can kill livestock that eat the ash covered vegetation. Ash falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.
5. Poisonous Gas Emissions
   Volcanic gases are gases dissolved in magma that are released to the
   atmosphere during eruptions. Aside from eruptions, gases may also escape continuously into the atmosphere while the magma is still underneath the ground or lies close to the surface. In such cases, gases are released through volcanic vents, fumaroles or hydrothermal systems. Volcanic gases composed of several different types of gas and the composition varies considerably from one volcano to the other. Among the poisonous volcanic gases include: Hydrogen Chloride(HCl), Hydrogen Sulphide (H2S), Hydrogen Fluoride (HF) and Carbon Dioxide (CO2). The chlorine, sulphur and fluorine gases can kill organisms by direct ingestion or by absorption onto plants followed by ingestion by organisms. Sulphur dioxide is a colourless toxic gas that has pungent and irritating odour. It is hazardous to human health as it irritates the eyes, nose and throat even when small amount of the gas coming in contact with the skin or mucous membrane. Inhaling the gas is also harmful for respiratory tract and bronchi. Over exposure to sulphur dioxide may lead to severe respiratory problems, difficulty in breathing, preterm birth and, in some cases, premature death. Sulphur dioxide can also mix with water vapour in the atmosphere to
   form acid rain that is very corrosive to structures and landscape

Carbon dioxide is another colorless gas that is harmful to lives in large amount. It is also odourless and can easily be diluted to low concentrations following an episodic eruptions (series of events) or continuous release from vents or fumaroles. Carbon dioxide poses a great hazard to human, animal and plant when the released gas is not properly diluted; leaving the surrounding area with a concentration level that is lethal to living beings. Additionally, since carbon dioxide is denser than air, the concentrated carbon dioxide subsides to low-lying areas and extremely affects the low-elevation areas. In 1984, CO2 gas escaping from the bottom of Lake Monoun, a crater lake in Cameroon, killed 37 people.

Question 61

Secondary Volcanic Hazards

Answer 61

1. Lahars
   It refers to a mixture of water and pyroclastic materials flowing down volcanoes or river valleys. The pyroclastic materials that make up lahars consist of volcanic debris of different sizes ranging from fine clay to large boulders. These flows of mud, rocks and water seem to be like a fast moving wet concrete especially when they carry a large amount of volcanic debris. Lahars can form different ways, including ice or snow melting, post-eruption rainfalls, lake failure and landslides. At the beginning of the flows, lahars are usually forceful enough that
   they could rip and drag houses, big boulders and trees for several miles. As they move farther downstream, lahars often lose its intensity and pose a potential threat of burial to the surrounding areas.
2. Volcanic tsunamis
   They are the result of underwater explosions, lahars, pyroclastic flows, collapse or tectonic movement associated with volcanic activities. Due to its massive size, tremendous speed and extensive reach, tsunamis have an enormous destructive power on properties and result in many fatalities even at locations far from the eruptions.
3. Debris Avalanches and Debris Flows
   Volcanic mountains tend to become ‘over-steepened’ as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes. These slopes may become gravitationally unstable, leading to a sudden slope failure that results in landslides, debris slides or debris avalanches.
4. Volcanic Earthquakes and Tremors
   Earthquakes usually precede and accompany volcanic eruptions, as magma intrudes and moves within the volcano.
5. Flooding
   Drainage systems can become blocked by deposition of pyroclastic flows and lava flows. Such blockage may create a temporary dam that could eventually fill with water and fail, resulting in floods downstream from the natural dam. Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods.

Question 62

Tertiary Volcanic Hazards

Answer 62

1. Alterations in river courses
2. Changes in landscape topography
3. Effects on climate (example: blocks sunlight)
4. Addition of nutrients to soil

Question 63

Managing Volcanic Hazards

Answer 63

A- Modify the Event
1. Environmental Control- Unfortunately, very little can be done to control a volcanic eruption. Most attempts, however, have been made to control lava flows with some success. Methods such as water sprays and explosions have been used. In addition, artificial barriers have used to protect against secondary
hazards, like lahars, which tend to follow well-defined routes.

2. Hazard-Resistant Design- Buildings and structures can do very little to resist lava, pyroclastic flows and lahars, as these volcanic hazards will destroy and structures in their paths. The weight of ash on roofs (especially when wet) can cause roof collapse. Hence, roofs need to be strong enough and designed to
   withstand ash, with steep-sloping sides

B- Modify Vulnerability
1. Community Preparedness: Most volcanic events are preceded by warnings of activity from the volcano. If communities at risk are prepared in advanced, many lives can be saved. Evacuation is the most common and important method of hazard management used today. Advanced preparation and management structures to organize the evacuation, temporary housing, food, etc. are needed.
Communications between scientists monitoring volcanoes and government officials must be clear, consistent and accurate.

2. Prediction and Warning: there is incomplete knowledge about volcanic processes. However, great strides have been made in forecasting eruptions.
   Various volcanic physical processes can be monitored for changes which can signal an impending eruption, the record of past eruptions is used to help determine what and where the risks are highest.

Seismometers to detect earthquakes, Temperature rise around the volcano, the release of gas with higher sulphur content indicating that the volcano is close to erupting

Planning for a volcanic eruption includes:

a) creating an exclusion zone around the volcano
b) being ready and able to evacuate residents

Question 64

The Disaster Management Cycle

Answer 64

Disaster management aims to reduce or avoid the potential losses from hazards, assure prompt and appropriate assistance to victims of disaster and achieve rapid and effective recovery. Appropriate actions at all points in the cycle lead to greater preparedness, better warnings, reduced vulnerability or the prevention of disasters during the next iteration of the cycle.

The 4 phases of disaster cycle:

I. Mitigation
This involves implementing measures for preventing future threats of disaster and/or minimizing their damaging effects of unavoidable threat. This phase includes the formulation of public policies and plans that either modify the causes of disasters or mitigate their effects on people, property, and infrastructure such as hazard-proofing homes or having insurance, following safety standards of building materials

II. Disaster preparedness
Preparedness efforts include plans or preparations made in advance of an emergency that help individuals and communities get ready to either respond or to recover. The preparations may include the stocking of reserve food and water, the gathering and screening of willing community volunteers, or citizens education & evacuation plan, holding disaster drills, and installing smoke detectors, mutual aid agreements, development of hospital disaster plans, emergency medical service plans, etc.

III. Disaster response
Disaster response work includes any actions taken in the midst of or immediately following an emergency, including efforts to save lives and to prevent further property damage, evacuation, search and rescue, and sheltering victims. The focus in the response phase is on meeting the basic needs of the victims until sustainable community has been achieved. This phase may still continue even when recovery phase can already be started.

IV. Disaster recovery
Recovery involves restoring, rebuilding, and reshaping the impacted area. It starts after damages have been assessed and adequate response effort is achieved and on-going. It involves actions to return the affected community to its pre-disaster state or better. These measures, both short and long term, aim to return vital life-support systems to minimum operating standards; such as temporary housing, public information, health and safety education; continued health monitor and care, reconstruction of vital facilities; counseling programs;
grants, and it may include economic impact studies.

Question 65

What is risk?

Answer 65

Risk is the extent of exposure of a population to a hazardous event, thereby
presenting a potential threat to people or their possessions, including buildings
and structures.

Generally, people perceive greater risk from events which threaten themselves
than those which threaten their possessions. There are several reasons (identified by Park, 1992) why people may consciously place themselves at risk from natural hazards:

i. Unpredictability of hazards (occurrence or magnitude)
ii. Lack of alternatives (shortage of land, lack of knowledge, etc.)
iii. Changing dangers (e.g. quasi-natural hazards)
iv. ‘Russian Roulette’ (optimistic outlook and turn a ‘blind eye’ to known
risks and accept them as part of the cost of living in a place or as ‘God’s
will’)
v. Costs vs. Benefits

Question 66

What is vulnerability?

Answer 66

Vulnerability is the probability of risk presented or posed by a hazardous event
to a country. Vulnerability may be economic, social, educational/informational and environmental.

Vulnerability to a given hazard depends on:
- Proximity to a possible hazardous event
- Population density in the area proximal to the event
- Scientific understanding of the hazard
- Public education and awareness of the hazard
- Existence or non-existence of early-warning systems and lines of
communication
- Availability and readiness of emergency infrastructure
- Construction styles and building codes
- Cultural factors that influence public response to warnings

Human intervention in natural processes can also increase vulnerability by:
- Development and habitation of lands susceptible to hazards. For example,
building on floodplains subject to floods, sea cliffs subject to landslides, coastlines subject to hurricanes and floods, or volcanic slopes subject to volcanic eruptions.

• Increasing the severity or frequency of a natural disaster. For example:
  overgrazing or deforestation leading to more severe erosion (floods, landslides),
  mining groundwater leading to subsidence, construction of roads on unstable
  slopes leading to landslides, or even contributing to global warming, leading to
  more severe storms.

Question 67

Human perception to hazards

Answer 67

People react differently to the threat from natural hazards because we receive,
filter and distort information as part of human perception. This perception, along with the variability of hazardous events, means that we have a selective and partial view of natural hazards. The assessment of the hazard threat and hazard and perception combine to influence the type of human response.

People’s perception are complex and range from those that believe that hazards are due to fate (external control) or are outside their responsibility, to increasing responsibility (internal control) where action can be taken.

The results can be divided into 3 broad groups of perception and response:
Acceptance, Domination and Adaptation.

Question 68

Human response to hazards

Answer 68

There are three human responses to hazards:

1. Modify (Prevent) the Event
   This management strategy aims to control the physical processes involved by
   ‘technological fixes’ to modify or prevent a hazardous event.

Strategies include:

• Hazard Prevention and Environmental Control: while the ideal situation will be
  to prevent hazardous events from occurring, preventing most hazards is
  unrealistic at present levels of understanding and technology. Environmental
  control aims to suppress the actual hazardous event by diffusing releases of
  energy over a greater area or period of time, with the ultimate aim of preventing
  the hazardous event from occurring. An example is flood management (diverting
  flood waters using a wide range of engineering structures, etc.)
• Hazard-Resistant Design (Protection): this aims to protect people and structures from the impacts of hazards, by focusing on building design and construction of engineered defences (hazard-resistant features).

2. Modify Human Vulnerability
   This involves changing human attitudes and behaviour towards hazards, either
   before the event or during it. This can be done using the following strategies:
   - Prediction and Warning
   Prediction is a relatively precise statement of the time, place and the nature and size of an event (precise forecast). They are based on scientific observation such as monitoring of the process in order to identify some kind of precursor event(s) For example, volcanic eruptions are usually preceded by a sudden increase in the number of earthquakes immediately below the volcano and changes in the chemical composition of the gases emitted from a volcanic vent. If these are closely monitored, volcanic eruptions can be often be predicted with reasonable accuracy. However, this is not always possible for all hazards based on the current understanding of geophysical processes and the technology available.

A warning is a statement that a high probability of a hazardous event will occur,
based on a prediction or forecast (a message which informs people at risk of an
approaching or impending hazard) If a warning is issued, it should be taken as a statement that “normal routines of life should be altered to deal with the danger imposed by the imminent event”.

The effectiveness of a warning depends on:
i. The timeliness of the warning
ii. Effective communications and public information systems to inform the
public of the imminent danger
iii. The credibility of the sources from which the warning came

• Community Preparedness
  This involves prearranged measures which aim to reduce the loss of life and
  property damage. It includes measures such as public education and awareness programmes, evacuation procedures and provision of emergency medical, food and shelter supplies.
• Land-Use Planning
  This aims to prevent new developments from occupying hazardous areas. However, a major issue is that it is not applicable to existing developments. Success depends upon knowledge of the nature, location and recurrence
  intervals of hazards.

3. Modify the Loss
   This most passive strategy is to simply accept the losses incurred. However,
   this is rarely accepted, particularly with higher-magnitude events. A more common strategy is to share the losses, mainly through two ways:

i. Aid
This is normally provided at the community, national or international levels for
relief, rehabilitation and reconstruction purposes. High-magnitude events often result in the hazard area being designated a ‘disaster area’ by national governments and the losses shared throughout the tax- paying population.
However, this approach raises issues about what extent of help should be given
to those who have taken no measures to protect themselves or to those who are on high incomes and are able to recover without aid.

ii. Insurance
This is a key strategy in more economically developed countries (MEDCs) and
involves people joining with a financial organization to spread the costs. A person needs to perceive that a hazard exists and be prepared to take action by
purchasing a policy. The insurers need to ensure that the property they cover is spread over a wide geographical area so that, following a disaster, the claims for losses will be less than the premiums paid. Hazards like earthquakes can cause widespread damage and can result in huge insurance claims which can be financially damaging for the insurance companies. As a result, in high-risk areas, insurance will be very expensive or may not even be available. One advantage of insurance as a hazard management strategy is that it can encourage persons to take preventive measures for themselves. For example, the insurance company may only provide cover if the policyholder reduces the risk by using certain building materials and methods. However, there are doubts about insurance as a future response because as claims increase, the costs of premiums will rise or companies will increasingly refuse to provide cover.

Hazard Assessment vs. Risk Assessment
A Hazard Assessment consists of determining the following:
-when and where hazardous processes have occurred in the past
-the severity of the physical effects of past hazardous processes (magnitude)
-the frequency of occurrence of hazardous processes
-the likely effects of a process of a given magnitude if it were to occur now and,
making all this information available in a form useful to planners and public
officials responsible for making decisions in event of a disaster

Risk Assessment involves not only the assessment of hazards from a scientific
point of view, but also the socio-economic impacts of a hazardous event. They involve:
-hazard assessment
-location of buildings, highways, and other infrastructure in the areas
subject to hazards
-potential exposure to the physical effects of a hazardous situation
-the vulnerability of the community when subjected to the physical
effects of the event

Risk assessment aids decision makers and scientists to compare and evaluate
potential hazards, set priorities on what kinds of mitigation are possible and set
priorities on where to focus resources and further study.