3. Causes of Earthquakes & Volcanoes

What are the four main layers of the earth composed of?

• each layer

• diameter

• composure - made of

• temperature

1. Inner core:

• around 1400km in diameter

• composed of iron and nickel

• temperatures of about 5500°C

2. Outer core:

• about 2100km thick

• composed of a semi-molten metal layer

• temperatures of between 5000 - 5500°C

3. Mantle:

• about 2900km thick

• composed of a semi-molten layer but less dense than outer core

4. Crust:

• the thickness varies

• made of 2 types of crusts: continental (25-90km) & Oceanic crust (5-10km)

Describe the Earth’s crust and explain the difference between continental and oceanic crust (4-5 marks)

• The earth has 2 types of crusts: continental and oceanic

• the Continental crust is thicker (25-90km), older and less dense

• the Oceanic crust is thinner (5-10km) and denser/heavier

  • because it is denser, the oceanic crust pushes beneath the continental crust at the plate boundaries.

What is plate tectonics and why do tectonic plates move? (4-marks)

• The Earth’s crust is broken into tectonic plates (e.g. Eurasian, African, Pacific).

• These plates move on top of the semi-molten mantle.

• Plate movement is partly caused by convection currents in the mantle.

• A plate boundary (margin) is where two tectonic plates meet.

What are the types of plate boundaries?

1. divergent (constructive)

2. convergent (destructive)

3. Collision

4. Transform (conservative)

Describe a divergent (constructive) plate boundary and give an example.

• At a divergent (constructive) boundary, tectonic plates move apart.

• Magma rises from the mantle to fill the gap, creating new crust.

• Volcanic eruptions and shallow earthquakes occur at this boundary.

• An example is the Mid-Atlantic Ridge.

Describe a convergent (destructive) plate boundary and give an example.

• At a convergent (destructive) boundary, plates move towards each other.

• The denser oceanic plate subducts beneath the less dense continental plate. → cause its heavier

• This creates volcanoes and powerful earthquakes.

• An example is the boundary between the Nazca Plate and the South American Plate.

Describe a collision plate boundary and name a landform it creates

• At a collision boundary, two plates of similar density move towards each other.

• Neither plate subducts, so the crust is pushed upwards.

• This forms fold mountains, such as the Himalayas.

• Earthquakes are the main hazard at this boundary.

Describe a transform (conservative) plate boundary and state the hazard it causes

• At a transform (conservative) boundary, plates slide past each other.

• They may move in opposite directions or the same direction at different speeds.

• No magma is produced, so no volcanoes form.

• Earthquakes are the only hazard at this boundary.

Where do volcanoes occur and where do they not occur?

• Volcanoes occur at constructive (divergent) boundaries, destructive (convergent), and hot spots.

• Volcanoes do not occur at collision boundaries or transform (conservative) boundaries.

Explain how volcanoes form at constructive (divergent) plate boundaries.

• Plates move apart, usually under the ocean.

• Lava rises through the gap left by separating plates.

• Lava cools and hardens, forming new crust.

• Lava is runny, so eruptions are gentle.

• Forms shield volcanoes with gentle slopes.

Explain how volcanoes form at destructive (convergent) plate boundaries.

• Plates move towards each other.

• Denser oceanic plate subducts beneath the continental plate.

• Friction and heat melt plate material, forming magma.

• Magma rises through cracks in the crust.

• Explosive eruptions occur due to sticky lava.

• Forms composite (strato) volcanoes.

Explain how volcanoes form at hot spots.

• A tectonic plate moves over a stationary magma plume.

• Magma rises through cracks in the crust.

• As the plate moves, a chain of volcanoes/islands forms.

• Example: Hawaii.

Compare lava type and eruption style at constructive and destructive boundaries.

• Constructive boundaries: runny lava → gentle eruptions → shield volcanoes.

• Destructive boundaries: sticky lava → explosive eruptions → composite volcanoes.

Where do earthquakes mainly occur and why?

• Earthquakes can occur anywhere, but mostly at or near plate boundaries.

• This is where plates move, collide, or slide past each other, building up pressure.

Compare earthquake strength at different plate boundaries.

• Constructive (divergent): earthquakes tend to be weaker as plates move apart.

• Destructive, collision, and conservative: earthquakes tend to be stronger due to greater friction and pressure build-up.

Explain why earthquakes at some plate boundaries are more hazardous.

• Plates can lock together, allowing pressure to build up.

• When released suddenly, strong earthquakes occur.

• This is most common at destructive, collision, and conservative boundaries.