5.2.2 Flashcards
What is the theory of plate tectonics?
The tectonic theory of crustal evolution is a scientific theory that revolutionised people’s understanding and study of geological processes and the Earth
It helps to explain geological phenomena such as:
the occurrence of hazards such as earthquakes and volcanic eruptions
the formation of mountain ranges
the movement of continents
the distribution of some mineral resources such as diamonds
Provide an overview of continental drift theory
The theory emerged from earlier work by the German researcher, Alfred Wegener, who gathered evidence (Table 5.2, page 192) to suggest that just one giant continent (Pangaea) existed about 300 million years ago. He believed that it later split into two smaller continents (Laurasia and Gondwanaland). Today’s continents were then formed from further splitting
of these two masses. Wegener called his ideas, the theory of continental drift.
Outline how convection currents caused the movement of tectonic plates
In the past the theory of convection currents was used on its own to explain tectonic plate movement
Heat from radioactive decay in the core moves upwards in the mantle
It creates convection currents which rise towards the surface before spreading in the asthenosphere, cooling and sinking
As they reach the asthenosphere they carry the lithospheric plates above with them
Outline the process of slub pull
A subduction zone is formed when two plates move towards each other
The heavier, denser oceanic plate subducts under the lighter, less dense plate
As the plate sinks, gravity pulls the plate down into the mantle
This is known as slab pull
Outline the process of gravitational slinging( ridge push)
New oceanic crust is formed at mid-ocean ridges and stands 2-3km above the ocean floor
As the crust moves away from a mid-ocean ridge, it cools and becomes denser and thicker
This causes the lithosphere to slope away from the ridge and gravity pulls the lithosphere down this slope, pushing it forwards
This is known as gravitational sliding (or ridge push)
Ridge push is much less important than slab pull in causing the movement of tectonic plates
Outline the process of sea floor spreading
Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge
Lava cools and solidifies with the minerals lining up with the magnetic field
The direction of the minerals on either side is a mirror image
What are the 3 main types of plate boundaries?
Constructive (divergent) - plates move apart or diverge
Destructive (convergent) - plates move together or converge
Conservative (transform) - plates sliding past each other
What can occur at a constructive plate boundary?
At a constructive (divergent) boundary the plates are moving apart
Both volcanic eruptions and earthquakes can occur at this type of plate boundary
Outline of oceanic ridge is created at a constructive plate boundary?
An ocean ridge forms when the diverging plates are under the ocean
As the plates move apart, magma rises up to fill the gap and this accumulates over time to become taller and wider
The Mid Atlantic Ridge is an example of an ocean ridge
Outline how a rift valley is formed at a constructive plate boundary?
A rift valley forms when the diverging plates are beneath the land
As the plates move apart, the crust stretches and fractures
Areas of crust drop down between faults to create a valley
The East African Rift Valley is an example of a rift valley
What happens at a destructive plate margin?
At a destructive (convergent) plate boundary, the plates are moving towards each other
What happens when oceanic and continental plate meet?
Deep sea trenches are long, narrow depressions in the ocean floor with depths of over 6km and up to 11km
Trenches are found adjacent to land areas and associated with island arcs
The trench is usually asymmetric, with the steep side towards the land mass
The boundary between the Nazca plate and the South American plate is an example
The zone of earthquakes along the subduction slab is known as the Benioff Zone
The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
This forms deep ocean trenches in the subduction zone
Both violent volcanic eruptions and earthquakes occur at this type of plate boundary
Volcanoes tend to be steep-sided, composite volcanoes with andesitic and rhyolitic lava flows
Oceanic and continental convergent plate boundaries are also responsible for fold mountains
The Andes are an example of fold mountains at an oceanic–continental destructive boundary
How deep oceanic trenches are formed at a destructive plate boundary?
Deep sea trenches are formed as the oceanic plate bends and subducts
An example of a deep sea trench is the Peru-Chile trench
How fold mountains are formed at a destructive plate boundary?
Fold mountains are formed when the continental plate is compressed and deformed, buckling and folding due to pressure
Fold mountains form the highest of the world’s mountain ranges
They are long, relatively narrow belts of mountains, with parallel ridges and valleys
The main fold range is made up of a series of smaller ranges, with flatter areas forming plateaus in the mountains
What happens when 2 oceanic plates meet and what landforms are formed?
The older and denser oceanic plate subducts beneath the younger less dense plate
Both volcanic eruptions and earthquakes occur at this type of plate boundary
The convergence of two oceanic plates creates two distinctive landforms:
Deep sea trenches form due to subduction
An example of a deep-sea trench at an oceanic–oceanic boundary is the Mariana Trench
Island arcs may form where submarine volcanic eruptions lead to crust building up and rising above sea level
Japan is an example of an island arc
