Plate Tectonics

Continental Drift, Tectonic Plate Theory, Earth's Structure, Plate Boundaries, Landforms, Hot Spots (Magma Plumes)

What are plate tectonics?

The process by which heated rock is moved within the earth in vast convection currents, which drag along the underside of the earth’s plates, forcing them to slowly drift across the planet

Define tectonic plate

A huge rigid slab of the earth’s crust

How many tectonic plates are there?

14

What are the 2 types of crust?

Continental and Oceanic

Continental crust:

• Thickness

• Age

• Density

• Composition

• 30-70km

• Over 1500 million years

• 2.6g/cm³

• Mainly granite

Oceanic crust:

• Thickness

• Age

• Density

• Composition

• 6-10km

• Less than 200 million years

• 3.0g/cm³

• Mainly basalt

What are the different layers of the earth called?

1. Lithosphere (crust and rigid upper mantle)

2. Asthenosphere (upper mantle)

3. Mesosphere

4. Outer core

5. Inner core

6. Inner inner core

Lithosphere

• Made of 2 layers, 50-100km thick

• a) crust (14°C)

• b) rigid upper mantle (1500°C at 100km deep)

Asthenosphere

• Upper mantle

• Hotter and semi-molten rock

• More fluid than the lithosphere

• Has a ‘plasticky quality’

• ~200km deep

• ~1700°C

Mesosphere

• Hotter still, but dense and more rigid

• Reaches down to the depth of 2900km

• ~3700°C

Outer core

• Liquid iron and nickel

• 200km thick

• 5000°C

Inner core

• Solid iron core

• 1200km thick

• 6000°C

• 6371km from surface

What are faults and why are they dangerous?

Cracks in the Earth’s crust close to plate boundaries. They are dangerous because its along these faults that the most powerful earthquakes happen.

What happens to solid land when plates collide?

Land gets buckled and folded upwards into mountains (called ‘fold mountains’, e.g. Himalayas)

What are the 3 types of plate boundary?

• Destructive

• Constructive

• Conservative

Converge

To come together

Diverge

To move apart

Destructive plate boundaries: C&O (processes and landforms)

● Denser oceanic plate subducts below the continental.

● The plate subducting leaves a deep ocean trench.

● Fold mountains occur when sediment is pushed upwards during subduction.

● The oceanic crust is melted as it subducts into the asthenosphere.

● The extra magma created causes pressure to build up.

● Pressurised magma forces through weak areas in the continental plate

● Explosive, high pressure volcanoes erupt through the continental plate, known as composite volcanoes.

Benioff Zone

A region of seismic activity tr

Destructive plate boundaries: O&O (processes and landforms)

Heavier plate subducts leaving an ocean trench. Fold mountains will also occur.

● Built up pressure causes underwater volcanoes bursting through oceanic plate.

● Lava cools and creates new land called island arcs.

Destructive plate boundaries: C&C (processes and landforms)

● Both plates are not as dense as oceanic so lots of pressure builds.

● Ancient oceanic crust is subducted slightly, but there is no subduction of continental crust.

● Pile up of continental crust on top of lithosphere due to pressure between plates.

● Fold mountains formed from piles of continental crust.

Constructive plate boundaries: O&O (processes and landforms)

● Magma rises in between the gap left by the two plates separating, forming new land when it cools.

● Less explosive underwater volcanoes formed as magma rises.

● New land forming on the ocean floor by lava filling the gaps is known as sea floor spreading (as the floor spreads and gets wider).

Constructive plate boundaries: C&C (processes and landforms)

● Any land in the middle of the separation is forced apart, causing a rift valley.

● Volcanoes form where the magma rises.

● Eventually the gap will most likely fill with water and separate completely from the main island.

Conservative plate boundaries (processes and landforms)

Between any crust, the parallel plates move in different directions or at different speeds. No plates are destroyed so no landforms are created. When these plates move, a lot of pressure is built up. On oceanic crust, this movement can displace a lot of water. On continental crust, fault lines can occur where the ground is cracked by the movement.

What is the side of a volcano called?

Flank

Hot spots

A rising column of magma which rises through the mantle towards the crust

What is another name for hot spots?

Magma plumes

What percentage of earthquakes and volcanic eruptions occur on plate boundaries compared to above magma plumes?

• 95% of earthquakes and volcanic eruptions occur on plate boundaries

• The other 5% are found above magma plumes, often in the middle of plates

What happens at hot spots?

Hot magma plumes from the mantle rise and burn through weaker parts of the crust. This can create volcanoes and islands.

How are magma plumes formed?

• Radioactive decay in the Earth’s core generates very hot temperatures

• If the decay is concentrated, hot spots will form around the core

• These hot spots heat the lower mantle creating particularly hot plumes of magma which rise vertically

Do magma plumes/hot spots or the plates move?

The plume stays in the same place but the plates continue to move

How many hot spots are there?

125

Where are most hot spots NOT located?

Most magma plumes are not found at tectonic plate boundaries

Which well-known hot spot IS located at a plate boundary?

The Hawaiian Islands

How many islands make up the Hawaiian state?

137

Explain how the hot spot theory works

As tectonic plates move over magma plumes, the heat and pressure from the plume can cause the overlying crust to melt, resulting in volcanic eruptions. Over time, as the plate continues to shift, the volcanic activity moves, creating a series of islands or volcanic features.

Which scientist came up with the hot spot theory and when?

John Tuzo Wilson in 1963

Why is there much debate and uncertainty around the hot spot theory?

The hot spot theory challenges the theory of plate tectonics showing that the theory of plate tectonics has limitations and doesn’t fully explain tectonic activity

What is an example of an oceanic hot spot?

The Hawaiian Islands

What type of lava do oceanic hot spots typically produce?

Less viscous basaltic lava

What type of euptions do oceanic hot spots typically produce?

Less explosive, gentle eruptions and more fluid

What is an example of a continental hot spot?

Yellowstone National Park

What type of lava do continental hot spots typically produce?

More viscous rhyolitic lava

What type of volcanic eruptions do continental hot spots typically produce?

Explosive eruptions

What are the advantages of hot spot volcanism for the residents of Iceland?

• Geothermal energy provides sustainable power

• Volcanic ash creates fertile soil for agriculture

• Unique landscapes boost tourism

What are the disadvantages of hot spot volcanism for the residents of Iceland?

• Eruptions pose risks to lives and properties

• Ash clouds can disrupt air travel

• Environmental changes can affect ecosystems

What is Mt Everest doing each year?

Growing 5mm/year

How hot is the lava lake at Mount Nyiragongo?

800°C on the surface

How fast can the lava from Nyiragongo travel?

60mph

Explain why the earth’s tectonic plates are moving

Convection currents spreads out the tectonic plates and drags against the undersides of the plates, causing them to slowly move apart

How old is our planet?

4.5 billion years

How long have modern humans been around?

200,000 years