EQ1- Why Are Some Locations More At Risk From Tectonic Hazards

Question 1

Define tectonic hazards

Answer 1

Include earthquakes and volcanic eruptions, as well as secondary hazards such as Tsunami’s. Can be classified as seismic or volcanic.

Question 2

Describe the distribution pattern of earthquakes and volcanoes in relation to plate boundaries

Answer 2

Most earthquakes and volcanoes occur on the border of plate boundaries.
Most of the worlds volcanoes and earthquakes occur on the pacific plate, the ring of fire. 70% of earthquakes take place there.
The most powerful earthquakes are at convergent or conservative boundaries.
Rare intra-plate volcanoes and earthquakes can happen in the middle of plates, due to ancient faults or hotspot volcanoes.

Question 3

Define the Oceanic fracture zone (OFC)

Answer 3

Is a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, The Red Sea and California.

Question 4

Define the continental fracture zone (CFZ)

Answer 4

Is a belt following the mountain ranges from Spain, via The Alps to the Middle East and the Himalayas

Question 5

Give an example of major, minor and micro plates

Answer 5

Major- Eurasian, African, Antarctic, Pacific
Minor- Nazca, Cocos, Caribbean, Scotia
Micro- Burma, Banda Sea Plate

Question 6

Explain 3 examples of Intra Plate locations and explain how they occurred

Answer 6

Rhine Rift valley- localised divergence, crust cracked and fell downwards causing an earthquake. Found between France and Germany
African Rift Valley- localised divergence, magma rises and drives plates apart. Found in East Africa, will separate Somalia from mainland Africa.
Tibetan Plateau- faults in crust happen thousands of km behind plate boundary. Found in Central Asia

Question 7

Explain and draw the formation of Hotspot Volcanoes which cause intra- plate volcanoes

Answer 7

Mantle plumes occur in the middle of a plate away from the boundary.
Give a constant source of heat called a hotspot, which rises through the mantle and melts the asthenosphere and base of the lithosphere.
Magma rises through the weakness in the crust, it erupts onto the sea floor and forms a seamount.
Continuous eruptions occur causing the seamount to break through the surface of the water and form an Island Volcano.
Shield volcanoes form in oceanic hotspots due to the magma being basaltic, at continental hotspots lava is viscous with the potential to erupt explosively.
As the plate moves the Island is carried away from the hotspot, cutting it off from the magma and stoping volcanic activity. The plate continues to move and the process continues leaving Island Chains such as Hawaii.

Question 8

Define mantle plumes as a cause of intra-plate activity

Answer 8

Are especially hot areas of the mantle that move upwards underneath the crust and push it up, can cause weak points in the crust which can become hotspots

Question 9

Define a hotspot as a cause of intra plate activity

Answer 9

Is magma from the mantle that has broken through a weak point in the crust

Question 10

Define intra-plate earthquakes

Answer 10

Occur in the middle of tectonic plates and are much rarer than boundary earthquakes

Question 11

Define plate tectonics

Answer 11

Refers to a set of concepts and theories that explain the formation and distribution of the earth’s major structural features and seismic and volcanic events.

Involves the structural layers of the earth, existence of major lithospheric players and the movement of plates by various mechanisms

Question 12

State the layers of the earth and how its movement takes place

Answer 12

Crust- movement occurs from plate boundaries
Mantle- movement occurs from convection currents
Outer core- movement occurs from the spinning of the axis
Inner core- movement occurs due to rotations

Question 13

State the processes which can be used to explain plate tectonic theory (plate movement)

Answer 13

Mantle convection
Slab pull
Subduction
Seafloor spreading
Paleomagnetism

Question 14

Explain mantle convection as a process explaining plate tectonic theory (plate movement)

Answer 14

Occurs where heat is produced by the decay of radioactive elements in the core and heats the lower mantle, creating convection currents. These hot liquid magma currents move in circles in the asthenosphere, causing plates to move.

Question 15

Explain slab pull as a process explaining plate tectonic theory (plate movement)

Answer 15

Occurs where newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools, causing it to sink into the mantle under its own weight and pull down the rest of the plate with it.

Question 16

Explain subduction as a process explaining plate tectonic theory (plate movement)

Answer 16

Destroys new crust being created. As two oceanic plates or one oceanic and one continental plate move towards each other, the denser plate slides under the other into the mantle and melts in the subduction zone.

Question 17

Explain seafloor spreading as a process explaining plate tectonic theory (plate movement)

Answer 17

Occurs where huge mid-ocean ridges are formed when hot magma us forced up from the asthenosphere and hardens. This new crust pushes the tectonic plates apart in a process known as seafloor spreading

Question 18

Explain Paleomagnetism as a process explaining plate tectonic theory (plate movement)

Answer 18

Is the study of the earth’s magnetic field where solidified magma aligns with the North Pole and moves plates sideways away from the mid- ocean ridge

Question 19

State the Seismic activities found at Divergent plate boundaries (4)

Answer 19

Earthquakes
Volcanoes
Ocean ridges
Rift valley

Question 20

State the seismic activities found at conservative plate boundaries

Answer 20

Eathquakes

Question 21

State the seismic activities found at convergent plate boundaries (5)

Answer 21

Earthquakes 
Volcanoes
Fold mountains 
Ocean trench 
Island arcs

Question 22

Explain the process of a divergent plate movement

Answer 22

Divergent boundaries are often found in the sea, if the eruption is strong magma can build up high enough to form an island.

Convection currents rise and then diverge creating high temperatures that cause the uplifting of the crust. Tensional forces pull the playes apart to from rift valleys.

As the two plates move away from each other a gap appears between the two plates, filled by magma rising from the asthenosphere. Reduced pressure allows the magma to rise.

The magma bubbles gently to the surface to from low volcanoes, where eruptions are frequent but relatively gentle, and shallow focus earthquakes

Question 23

Explain the process of a convergent (subduction) plate boundary

Answer 23

Occurs where the oceanic and continental plates meet and the denser oceanic plate is forced under the lighter continental plate.

As it is forced down it rubs against the continental crust, this friction causes great heat and melts the oceanic crust into magma.

As more and more magma is created it puts great pressure on the crust above it. As the oceanic crust moves down the rates of movement can trigger major earthquakes along the line of the subducting plate.

Composite volcanoes are formed with explosive eruptions and a high silica level

Question 24

Explain the process of a convergent (collision) plate boundary

Give an example

Answer 24

Continental plates are lower in density than the asthenosphere beneath, meaning the subduction does not occur and no volcanic activity takes place

As the colliding plates move together they become uplifted and buckle to form high fold mountains

Occurred in India where seafloor spreading of the indo-australian plate in a north east direction led to a collision with the Eurasian plate, forming the Himalayas

Question 25

Explain the process of a conservative plate boundary

Answer 25

Along boundaries two plates slide past each other horizontally forming a conservative plate margin

This results in a major break in the lithosphere as a fault. Occurs where two plates stick as they move past each other, causing stress and pressure to build up, which is suddenly released as a strong magnitude and shallow focus earthquake.

There is no volcanic activity. Is tectonically very active and leads to powerful earthquakes.

Question 26

Define an earthquake

Answer 26

Is the ground shaking caused by shockwaves emanating from a sudden slip on a fault, due to pressure building up and fracturing the rock along cracks, causing faults and the release of energy causing seismic waves

Question 27

Define the focus

Answer 27

Is the point inside the cross from which the strain is released and send seismic waves in all directions

Question 28

Define the epicentre

Answer 28

Is the strongest point of impact on the Earth’s surface, directly above the focus

Question 29

Defined the subduction zone

Answer 29

At a convergent plate boundary with one plate moves down beneath another plate. The contact between the plates is sometimes called a thrust

Question 30

Define a locked fault

Answer 30

Is a fault that his stuck. The frictional resistance on the fault is greater than the stress across the fault. The stored strain is eventually released as a large magnitude earthquake

Question 31

Explain the focus steps at different plate boundaries

Answer 31

The shallowest focus earthquakes tend to be divergent and Conservative as neither of these plates have subduction. The greatest magnitude seismic waves tend to be from shallow focus.

The greatest depth focus are at convergent boundaries. The Benioff zone Is an area of deep seismicity due to a plate being thrust deep down. These are particularly deep at oceanic/oceanic areas such as the Pacific where deep focused earthquakes occur

Question 32

Define magnitude and explain how magnitude affects different plate boundaries

Answer 32

Is the amount of energy released.

There is less friction at conservative plate boundaries so magnitude are usually lower. At a subduction zone that is more likely to be locked pleats and more friction stress, leading to a mega thrust earthquake. At a convergent boundary, frictional resistance is greater than she is stress across the benoiff zone

Question 33

State the seismic waves caused by the sudden movement of plates

Which ones are the most destructive

Answer 33

Primary- (fast)
Secondary (slower)
Love (surface)

Secondary and Love waves are the most destructive

Question 34

Define a seismometer

Answer 34

Measures seismic waves and detects ground movement by knowing the speed of different waves, can calculate time, location and magnitude

Question 35

Define the Moment Magnitude Scale as a measure of tectonic activity

Answer 35

Measures the total amount of energy released by all shockwaves at the moment it occurs using the amplitude of the seismic waves including the amount of slippage and area of the fault surface broken

Question 36

Define the richter scale as a measure of tectonic activity

Answer 36

Only uses primary and secondary waves and the amplitude of secondary waves snd distance from epicentre

Question 37

Define the modified mercalli intensity scale as a measure of tectonic activity

Answer 37

Takes observations from people who experienced the earthquake and rates them from hardly noticed (1) to catastrophic (12)

Question 38

Explain primary waves

Answer 38

Are from the focus. Caused by compression and are the fastest and first to reach the surface. Can travel through solids and liquids and are only damaging in very powerful earthquakes

Question 39

Explain secondary waves

Answer 39

Are slower waves from the focus, due to the longer wavelength. Can only travel through solids and do more damage than primary waves

Question 40

Explain surface waves

Love and Rayleigh

Answer 40

Both shake the earths crust away from the epicentre.

A love wave only travels through the surface of the crust and moves from side to side as it moves forward

A Rayleigh wave travels through the surface of the crust in a rolling motion up and down

Question 41

Define and explain the primary hazards from seismic hazards

Answer 41

Ground shaking causes buildings, bridges, roads and infrastructure to collapse

Crustal fracturing cause the earths crust to crack, when energy is released during an earthquake, leaving gaps

Question 42

Define and explain the secondary hazards from seismic hazards

Give an example

Answer 42

Liquefaction happens because soft sediment often behaves like quicksand during an earthquake. The shaking brings water to the surface. As a result buildings can often topple over or sink into the ground.

Land slides describe the movement of rock, debris and soil down the slope of land, due to the shaking off the ground, loosening the hillside and making it collapse.

Avalanches a car where earthquakes triggered a downward movement of snow downhill

The 1989 Loma prieta earthquake in San Francisco, saw the buildings collapsing due to liquefication, because of the areas soft and sandy soil. 42 deaths were caused by a landslide where people were crushed on the freeway

Question 43

Define a volcano

Where do volcanoes occur

Answer 43

Is an extrusive feature found on the earth’s surface. No volcano is the same in it’s cause , characteristics or the hazard it creates.

Over 90% of the world’s volcanoes occur on plate margins. Volcanic areas are usually densely populated

Question 44

What does the volcanic hazard depend on

Answer 44

Magma type - silica/gas level
Magma viscosity
Plate boundary type
Explosiveness

Question 45

Explain the volcanic explosivity index (VEI) as a measure of volcanic eruptions

Answer 45

Measures the size of the eruptions based upon the volume of ejecta (gas, ash and lava) erupted and how it is thrown into the atmosphere

Question 46

Describe the dangers of volcanoes

Answer 46

In comparison to other hazards, volcanoes have killed far fewer people due to the greater ability to predict an eruption

Up to 1 million people have been affected by volcanic activity

Question 47

Explain the process of Basaltic lava effusive eruptions of lava (shield volcanoes at hot spots or fissures)

Exemplification

Answer 47

Basaltic lava is formed by the melting of mantle materials from the core-mantle boundary
Most common along spreading ridges but also found at hotspots.
Basaltic lava has low silica levels and has a low viscosity. Most eruptions are fluid and free flowing so prevent sudden explosive activity, most are shield volcanoes.

Examples include those in Hawaii or fissure like in Iceland

Question 48

Explain the process of explosive eruptions from Rhyolitic and Andesitic Lava (convergent margins)

Answer 48

Andesitic and Rhyolitic lava is formed by the subducted oceanic plate being melted by magma

Acid lavas are so rich in silica and so viscous that gas struggles to expand, lava solidifies before reaching the surface, pressure is built up and a violent explosion occurs, with huge lava domes forming

Question 49

Explain how the primary hazard of Lava flow occurs and the nature of this hazard

Exemplification

Answer 49

Occurs when oceanic crust is destroyed and an explosive eruption occurs at a destructive plate boundary. On steep flows it can reach up to 15m a second

Kilauea, Hawaii, 2014

Question 50

Explain how the primary hazard of dome collapse and pyroclastic flow occurs and the nature of this hazard

Exemplification

Answer 50

Results from the frothing of molten magma in the vent, ejects hot gas and pyroclastic materials. Bubbles burst explosively and clouds can be up to 1000 degrees, move at up to 600km/h and travel 20 km from the vent

Mt Sinaburg, Indonesia, 2015

Question 51

Explain how the primary hazard of tephra/bombs occurs and the nature of the hazard

Exemplification

Answer 51

Forms at a subductive plate boundaries, size can vary from bombs (>32mm) to fine dusts (<4mm). Can cause roofs to collapse as well as starting fires.

Mt Asama, Japan

Question 52

Explain how the primary hazard of gas release occurs and the nature of the hazard

Exemplification

Answer 52

Explosions form a gas crater, gas is held at the bottom of the lake by the water layers above, gas escapes from wind. Gas includes water vapour, sulphur dioxide, hydrogen and carbon monoxide

Lake Nyos, 1986

Question 53

Explain how the primary hazard of Flank collapse occurs and the nature of the hazard

Answer 53

Occurs at subducted plate boundaries. Very viscous magma (rhyolitic magma) forms an immovable plug within a volcano. Pressure causes a bulge and a collapse, leads to a sideways eruption.

Question 54

Explain how the secondary hazard of Lahars occurs and the nature of the hazard

Exemplification

Answer 54

Occurs when tephra and water combine and flow downslope. Heavy rain, volcano induced ice and snow melt,or lake outbursts can trigger lahars, moving at speeds of up to 100km/h.

Nevada del ruiz, Colombia, 1995

Question 55

Explain how the secondary hazard of glacial outbursts occurs and the nature of the hazard

Exemplification

Answer 55

Happens anywhere where water accumulates in a subglacial lake, initiated following the failure of an ice or moraine dam. Occur very suddenly with rapid discharge of large volumes of water, ice and debris from a glacial source. Is a hazard to people and infrastructure, causing landform modification.

Jokulhlaup, Iceland 2010

Question 56

Define a Tsunami

Answer 56

Is a sequence of waves with deep troughs in between. It is created by water column displacement, mostly by undersea plate movements where part of the seabed is thrust upwards very quickly or by volcanic explosive eruptions

Question 57

Explain the process of a Tsunami

Answer 57

A Pacific tectonic plate subducts beneath the North American adjoining plate, in Japan.

Between earthquakes the plates slide freely at great depth , where hot and ductile. At shallow depth plates stick together. The North American plate is slowly squeezed, the overriding plate thickens.

During an earthquake the leading edge of the overriding plate breaks free, springing seaward and upward. Behind, the plate stretches, it’s surface falls. The vertical displacements set off a Tsunami.

Question 58

Explain the process of Tsunami waves as they approach the coastline

Answer 58

At first Tsunami’s are difficult to detect by ships due to their small wave height (3m in Japan) and long wave length (150-250m).

As water shallows wave energy is concentrated into a smaller volume of water. In Japan energy is directed upwards, waves slow down and increase in height to produce onshore waves of up to 30m high. Nature of the wave will depend on bathymetry and the shape of the coastline.

Question 59

Explain the physical and human reasons why Tsunamis are so destructive (Japan)

Answer 59

Human:
The total damage from the earthquake and Tsunami was $300 billion
The number of deaths was 15,891
300,000 buildings were destroyed and one million damaged

Physical :
Sendai bay is 30km wide and the focus of the Tsunami went into the Urban area of Sendai
Deep Bathymetry (depth) means wave energy is not diminished, high velocity
A lot of the coastline was below 5m above sea level