Plate Tectonics, Earthquakes and Volcanos

Question 1

What is the structure of the Earth?

Answer 1

• Earth’s surface is constantly being churned up, replaced and reused (hence why there are no craters like on the moon).
• Convection currents drive this inside the mantle where lighter materials expand, becoming less dense and rising. This occurs all around the mantle forming a pattern of many different convection cells.

Question 2

What features occur at a mid-ocean ridge?

Answer 2

• A pattern of high and low intensity magnetic fields occur symmetrically either side of the ridge.
• This suggests that plates are moving sideways and that new crust is being formed, allowing us to date the ocean floor.

Question 3

What are the different types of plate boundary?

Answer 3

Divergent = when plates move apart.

Convergent = when plates move together.

• Oceanic / Oceanic crust = a destructive boundary where one plate is subducted, forming an ocean ridge.
• Oceanic / Continental crust = oceanic crust sinks below the continental crust as it is denser.
• Continental / Continental crust = both crusts get compressed together forming a large mountain range.

Question 4

How is the gravity inside the Earth affected?

Answer 4

• We can map out densities/temperatures within the mantle, showing patterns of hot and cold materials at different depths.
• This difference in density means gravity is stronger in some places rather than other causing the Earth to bulge in certain areas.
• Therefore, gravity doesn’t always point towards the centre of the Earth.

Question 5

What is a geoid?

Answer 5

A geoid is a diagram illustrating how far the ocean surface would be from the centre of the Earth depending on gravity.

Question 6

What is an accretionary wedge?

Answer 6

An accretionary wedge is made up of ocean sediment which is scraped onto the continental crust as the oceanic crust is subducted underneath.

Question 7

How are earthquakes produced at a subduction zone?

Answer 7

• Friction between the two plates produces to earthquakes.
• As the crust moves deeper, it becomes very hot / ductile meaning there are no more earthquakes.

Question 8

What is slab pull?

Answer 8

As the oceanic crust at a subduction zone sinks, it pulls the following slab along with it.

Question 9

What is the magma chamber?

Answer 9

• The magma chamber is where liquid rock accumulates beforebreaking through to the surface.
• The behaviour of this chamber and the consequent lifting and subsiding of the volcano above are good indicators as to whether an eruption is due or not.

Question 10

What are the different types of lava?

Answer 10

Basaltic lava:

• 1000^oc to 1200^oc
• Mafic
• High temperature / Low viscosity

Rhyolitic lava:

• 800^oc to 1000^oc
• Felsic
• Low temperature / High viscosity

Question 11

What are the different features of lava?

Answer 11

• Lava may contain dissolved gasses such as CO₂ which produces lots of bubbles inside the lava. The release of these gasses can kill animals.
• The smaller the ejector, the farther it will travel.

Question 12

Pyroclastic Flows

Answer 12

Ash is blown away from the volcano due to a strong wind where it falls down as there is no longer an upward force (previously produced from the volcano below). Pyroclastic flows are dangerous because:

• They move unpredictably as they are driven by wind.
• Temperatures are very high (up to 800^oc).
• They can move very quickly (up to 200km/h).

Question 13

What are the different types of volcano?

Answer 13

Shield volcano:

• Gently sloping sides, as the basaltic lava flows very easily, as so spreads a large distance.

Volcanic dome:

• Steep sloping sides, as the felsic magma is very viscous and so doesn’t spreadas far before cooling.

Cinder-cone volcano:

• Made up of alternating layers of ejector made up offelsic lava.

Composite volcano:

• Made up of alternating layers of pyroclastic materials and lava flows.
• These volcanos are brittle and so crack easily, filling with lava.
• These cracks are most likely to occur as the magma chamber fills up and forces the volcano upwards.

Hot spot volcano:

1. A hot spot produces a mantle plume between the mantle and outer core which is very long lasting.
2. This produces a series of volcanos as the crust moves over the mantle plume.

Question 14

What are the features of mantle plumes?

Answer 14

1. On the boundary between the core and mantle, a chemical reaction occurs which produces a material with a lower density than the mantle above it causing it to rise.
2. When this reaches the Earth’s crust, it breaks through allowing a large volume of low density, hot material to break through - forming flood basalts.

Question 15

What are the different features of volcanos?

Answer 15

Crater / vent:

• The material on the edge of the crater falls in meaning the crater is larger than the ‘pipe’ supplying the lava.

Feeder pipe:

• The main pipe which connects the vent to the mantle.

Volcanic ejecta:

• Water vapour and other gases which are released during an eruption produce explosive eruptions.
• These re more likely to occur in rhyolitic lava.

Question 16

How are caldera formed?

Answer 16

1. Fresh magma fills the magma chamber, lifting up the ground above.
2. Eventually, the ground cracks allowing a flow of lava to be released.
3. Then, the pressure decreases, and the lava flow subsides.
4. Next, the entire volcano collapses into the chamber producing large pyroclastic flows and a large release of gas.
5. Finally, a lake forms inside the caldera, and occasionally new volcanos form inside the lake, repeating the cycle.

Question 17

What are phreatic explosions?

Answer 17

Sea water mixes with magma, producing violent eruptions.

Question 18

What is a diatreme?

Answer 18

1. Viscous material with a large gas concentration builds up in pressure.
2. The pressure is then released explosively, producing a rapid flow of material from deep within the Earth’s crust which contain diamonds.
3. These diamonds are then mined.

Question 19

What are the hazards associated with volcanos?

Answer 19

• Lava - predictable, and so not very dangerous.
• Pyroclastic flows - very dangerous.
• Ice and snow - these melt, producing landslides made up of molten rock and ice.
• Ash clouds - reduce the temperatures and therefore food production which could lead to starvation.
• Acid rain - reduces the fertility of the groud.

The ground around the volcano is very fertile, containing lots of nutrients meaning it is good for growing crops which is why people live there.

Question 20

What is a lahar?

Answer 20

A mudflow which contains volcanic debris.

Question 21

What is an aquifer?

Answer 21

An underground store of water, which can then be pumped out and used.

Question 22

What are earthquakes?

Answer 22

• Earthquakes are shockwaves which are transmitted through the body of the Earth.
• They are usually more damaging the closer they are.
• The acceleration of an earthquake is usually measured as a fraction of G.

Question 23

What are the different types of seismic wave?

Answer 23

Compression / pressure wave (longitdinal):

• Direction of movement of particles is parallel to the direction of travel of the wave.
• Needs a medium to be transmitted through.
• Relies upon different pressures.

Shear wave (transverse):

• Direction of movement of particles is perpendicular to the direction of travel of the wave.
• Only able to travel througha solid which has shear strength.
• Relies upon shear strength.

Surface wave (stereotypical ‘sea’ wave):

• Direction of movement of particles follows an elliptical path.
• Occur on a free surface - such as water.

Question 24

Which wave has the fastest speed of transmission?

Answer 24

Question 25

What are the focus and epicentre of an earthquake?

Answer 25

• Focus = The place where an earthquake starts.
• Epicentre = The point on the Earth’s surface which is directly above the focus.

Question 26

How can we detect earthquakes?

Answer 26

• Use a seismograph to detect horizontal movement of rocks.
• A number of seismograph stationsare used in correspondance to work out details about the earthquake.

Question 27

How do we get earthquakes in the UK?

Answer 27

1. We get small earthquakes because when the ice defrosted after the ice age, a lot of weight was removed from thecrust allowing it to rise upwards.
2. This produces a lot of creaking, known as earthquakes.

Question 28

What are the differences between shallow and deep earthquakes?

Answer 28

Shallow earthquakes:

• Cause the most damage.
• Occur during divergence at locations such as mid-ocean ridges.
• Can also occur due to friction created during subduction.

Deep earthquakes:

• Cause little damage.
• Occur just before the subducted tectonic plate becomes ductils.

Question 29

What pattern of pressure waves are produced through the Earth’s interior?

Answer 29

1. The waves refract as they move through different materials inside the Earth.
2. This produces shadow zones where the earthquake cannot be detected.

Question 30

What is seismic tomography?

Answer 30

• You are able to use tomography to work out the speed of waves at any point within the Earth
• The speed of these shockwaves are a function of the temperature insidethe material.
• This allows you to map a profile of temperature within the mantle.

Question 31

How can you measure the intensity of an earthquake?

Answer 31

Use the mercalli intensity scale to record the intensity level of an earthquake at a specific location due to visual clues.

Question 32

What is a tsunami?

Answer 32

• Tsunami are created by vertical acceleration of plates.
• They have a very long wavelength and affect the wholce volume of water, moving across the ocean at very high speeds.
• When they get into shallow water, the wave slows down, meaning the energy is more concentrated (as it is conserved) causing the height of the wave to increase.
• The first sign of a tsunami is the tide going out very quickly.

Question 33

How are tsunami caused?

Answer 33

• Earthquakes.
• Volcanic eruptions (which cause seawater to be displaced).
• Landslides (which cause seawater to be displaced).

Question 34

How are unconformities formed?

Answer 34

1. Beneath the sea, sediments are accumulated in different beds.
2. Tectonic forces cause the uplift of these beds above sea level, exposing them to erosion.
3. Erosion then strips away the uppermost beds, leaving an irregular surface of hills and valleys.
4. The beds eventually fall back below sea level due to subsidence and a new layer is formed through more deposition.
5. The irregular surface of the bed below (due to mountains and valleys) is preserved as an unconformity.
6. This process then repeats.