Geography

Plate Tectonics

Plate Tectonics

Key Words:

Term	Definition
Plate	Large rigid sections of the Earth's crust that move due to convection currents in the mantle.
Plate Tectonics	The movement of plates.
Convection Currents	Movement of Magma in the Mantle that results in Plate Movement.
Plate Boundary	The edges of plates – separating them from each other.
Lithosphere	The part of the Earth's internal structure that is made up of solid rock. It includes the Earth's crust and rigid upper Mantle. It is 150km deep.

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Geography

Plate Tectonics

Inside the Earth

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Geography

Plate Tectonics

• Folding has a South West-North East trend therefore mountains were created in North West and Leinster region.

Continental crust:

Lies under the continents and is thick – 30km to 70km. Made of light rock e.g. Granite.

Oceanic crust:

Lies under oceans and is thinner – 3km to 10km. Made of heavy rock e.g. Basalt.

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Plate Tectonics

Geography

Figure 1: Lithosphere and Asthenosphere

Figure 2: Plate Boundaries

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Plate Tectonics

How do the plates move?

• The movement of plates is caused by heat escaping from the core and the lower mantle

• Heat escapes from the extremely hot core, heating up rocks in the lower mantle

• These heated rocks rise slowly towards the earth's surface, creating currents of magma

• As the magma reaches the asthenosphere, it has cooled down, so it starts to move sideways before sinking back down towards the core

• This cycle repeats itself and has led to the formation of approximately 20 currents within the Earth

• Friction exists between the plates and convection currents meaning the plates are moved

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Plate Tectonics

Theories of Plate Tectonics

• Continental Drift by Alfred Wegener.
• Sea Floor Spreading by Harry Hess.

Continental Drift:

• Alfred Wegener.
• Wegener noted that the continents could fit together like a Jigsaw puzzle.
• Suggested that all continents were once joined together in a huge supercontinent called Pangaea.
• Suggests that the continents are transported across the planet by convection currents
• Drifting of the continents is still occurring today

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Geography

Plate Tectonics

• Continents were once all joined together in one single landmass known as Pangaea. It was effectively a supercontinent

• Approximately 200 million years ago Pangaea began to break apart

• The continents were fueled by the convection currents and so they began to drift apart

Figure 4: Continental Drift

Proof:

• Continents fit together

• Fossil Distribution: fossils found in South America were also found in Africa. These fossils were of freshwater creatures and could not have travelled across oceans.

• Common rock types: rocks of similar age and structure have been found in different continents; e.g. Mountains in North America and Ireland.

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Plate Tectonics

• Glacial deposits in Australia, Africa, India and South America are the same age.

Sea Floor Spreading:

• Harry Hess in the 1960's.

• New rock is formed where plates were split apart

• Ocean floor widens; magma rises from within the mantle and fills the opening

• Magma cools; new ocean floor formed

• New ocean floor is youngest at the mid-ocean ridge

• Older crust moves away towards continents

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Plate Tectonics

Proof:

• Rock samples from the ocean floor show that the crust is youngest where the magma cools at the Mid-Atlantic ridge. (10 million years)

• Crust gets older as you move towards the continent (200 million years).

• Young volcanic islands are found closer to the Mid-Atlantic ridge where the sea floor is separating (at divergent plate boundaries)

Plate Boundaries

Tectonic activity occurs at plate boundaries as Endogenic forces (forces from inside the Earth) are active here.

3 types of plate boundaries:

• Divergent (also known as constructive) – plates are separating.

• Convergent (also known as destructive) – plates are colliding.

  • Oceanic-Oceanic boundaries
  • Continental-Continental boundaries
  • Oceanic-Continental boundaries.

• Transform (also known as passive/conservative) – plates slide passed each other.

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Plate Tectonics

Divergent Plate Boundaries:

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Plate Tectonics

Geography

Key Info:

Term	Definition
Process	Sea-floor spreading
Landforms	Rift valleys, mid-ocean ridges, volcanic islands.
Examples	Mid-Atlantic Ridge, Iceland

• Divergent plate boundaries occur when plates pull apart.

• Convection Currents are moving in the opposite direction.

• This causes the earth's crust to crack creating a fissure.

• Magma leaks out through this fissure and solidifies to create new land.

• If it occurs on continental crust then it creates volcanoes, e.g. Iceland.

• If it occurs on Oceanic crust then it is sea-floor spreading. E.g. Mid Atlantic ridge.

• Successive eruptions over time build up to create underwater mountain ranges and volcanic mountains. If these mountains build up and appear above water, then they are known as volcanic islands. E.g. Iceland.

• The Mid-Atlantic ridge is 40,000km long and runs from the North Pole to the South Pole.

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Geography

Plate Tectonics

• American and Eurasian plates are pulling away at 2.5cm per year.

Convergent Plate Boundaries

Three types:

Oceanic-Oceanic

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Plate Tectonics

• When two plates collide the heavier, older plate is subducted beneath the lighter younger plate.

• At the point of subduction, a deep, narrow sea trench can develop.

• The plate that has been subducted melts as it sinks into the Asthenosphere.

• Volcanic activity occurs at this type of plate boundary.

• The descending plate is melted quickly.

• Overtime magma rises from this point on the ocean floor and forms islands that may appear above sea level (an island arc).

• Active volcanoes can be found here, and earthquakes can also occur.

• E.g. Japan

Oceanic-Continental:

Figure 6: Oceanic-Continental plate boundary

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Plate Tectonics

When two plates collide the heavier, older plate is subducted beneath the lighter younger plate.

Term	Definition
Process	Subduction
Landforms	Fold Mountains, Volcanoes, Ocean Trenches.
Examples	Andes Mountains, Rocky Mountains

Heavier oceanic plate is subducted into the Mantle.

As this oceanic plate is dragged underneath the continental plate, a deep oceanic trench is created.

The immense pressure of the collision of these plates forces the edge of the continental plate to buckle upwards creating fold mountain ranges.

E.g: The Andes in South America formed as the Nazca Plate was subducted beneath the South America Plate.

The edge of the Oceanic plate is melted in the hot Asthenosphere. The molten magma forces its way to the surface creating explosive volcanoes.

E.g. There are over 2,000 active volcanoes in the Andes.

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Plate Tectonics

• Plates can get 'stuck' and a sudden movement by either plate can cause earthquakes.

Continental-Continental boundaries:

Figure 7: Continental-Continental plate boundary

Key Info:

Term	Definition
Process	Subduction
Landforms	Fold Mountains
Examples	Himalayas and the Alps

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Geography

Plate Tectonics

• When two continental plates meet neither plate will sink.

• The continental crust will fracture and buckle upwards leading to the formation of Fold Mountains.

• These mountain ranges are formed in the interior of continents. E.g. Himalayas.

• Himalayas were formed from the collision of the Indian plate and the Eurasian plate.

• The Indian and Eurasian plates are still pushing together, causing the Himalayas to grow at a rate of approximately 3cm per year.

• Stresses and pressure can build up when plates are colliding. When this pressure is released it can cause an earthquake – e.g. Nepal earthquake in 2015.

Transform Plate Boundaries

Figure 8: Transform/Conservative/Passive Plate Boundary

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Plate Tectonics

Geography

Term	Definition
Process	Seismic Activity
Landforms	Fault Lines (create earthquakes)
Examples	San Andreas Fault, California

• Plates slide past each other or move in the same direction at different speeds.

• Rock is not created or destroyed.

• The line/plate boundary along which plates slide is known as a fissure or a fault line.

• These faults are known as transform/transverse faults or tear faults.

• Most faults occur under the ocean, but some are located within continents. E.g. San Andreas fault which is 1,300km long.

• At this fault line the plates are moving in the same direction but at different speeds. The Pacific plate is moving six times faster than the North American plate. (5cm per year)

• Slippage is not smooth and friction between the moving plates causes massive pressure to build up.

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Plate Tectonics

• When this pressure is released seismic activity/earthquakes occur. E.g waiting on the 'big one' in San Francisco.

• If the North American and Pacific plates continue to move at the rate that they are today, then in 10 million years San Francisco and LA could be located beside each other (see diagram above for locations).

Ireland's geological past

• 850 MYA (Million Years Ago) divided into two sections
• 400 MYA plates collided, forming Ireland
• 380 MYA Ireland covered in a hot desert south of the equator
• 350 MYA Ireland under warm tropical sea, limestone laid down
• 250 MYA Armorican fold mountain period
• 200-140 MYA continental drift, Ireland moving northwards
• 65 MYA Atlantic Ocean formed