5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
OCR A-Level Geography Revision Notes 2025/2026 | 730
Questions & Answers on Plate Tectonics, Earth Structure,
Volcanoes, Earthquakes, Sea-Floor Spreading & Tectonic
Hazards
What are the 3 layers of the Earth? Crust
Mantle
Core (inner and outer)
What separates the crust and the mantle? Moho
What 2 layers does the upper mantle consist of? Lithosphere (semi-core)
Varies in thickness
Boundary with asthenosphere difficult to define (melts and becomes
incorporated in asthenosphere)
Asthenosphere extends 100-300km
Semi-molten/viscous - Allows rock to move due to high pressure in
mantle. Flowing slowly
What are continental and oceanic plates made up Lithosphere and crust
of?
Where do convection currents Asthenosphere
exist? What do these do?
Caused by vast amounts of heat generated in mantle.
Semi-molten asthenosphere flows carrying with it the solid lithosphere
and crust.
What are the properties of the continental crust? Thickness:
35km average (<30-70km)
Density:
2.6-2.7
Mineral composition:
Mainly granitic, silicon, aluminium
What are the properties of the oceanic crust? Thickness:
5-10km
Density:
3.0
Mineral composition:
Mainly basaltic, silicon and magnesium
What are the properties of the mantle? Thickness:
To a depth of 2900km
Density:
3.3 at Moho
5.6 at core
Mineral composition:
Rich in magnesium and iron
1/75
,5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Explain the movement of the crust caused by Hot rock rises from lower to upper
convection currents in the mantle.
asthenosphere. Hot rock spreads and
cools, pushing plates apart. Cool rock sinks
back down towards core.
As (oceanic) plate subducts at ocean trenches, gravity pulls it under
(slab pull).
Rising mantle pushes crust upwards at mid-ocean trenches, while
gravity pulls it back down (ridge push).
What was Alfred Wegener's big idea? 250 million years (Carboniferous period), all the Earth's continents fit
together
(Pangea).
Over time, continents have moved apart through continental drift.
Explain 3 pieces of (Wegener's) geological (rocks) Mountain chains and rock sequences on opposite sides of oceans show
close
evidence for continental drift. similarities (e.g. northeast Canada and northern Scotland).
- These mountains are likely to have been created together and then
split apart as the continents moved.
Continents seem to fit together (particularly South America and Africa).
- Suggests that continents once fit all together and continental drift
has separated them.
- (Erosion wouldn't've made them this shape).
Evidence of glaciations 290 million years ago in southern Africa, Australia,
South America, India and Antarctica.
- Suggests these land masses were joined during this time, located
close to the South Pole.
- (India has a tropical climate now, and due to it's current location it
wouldn't have been glaciated in an ice age.
- Suggests India was one at a higher latitude further from the equator).
Explain 3 pieces of (Wegener's) biological (living) Similar fossils of marine shellfish (e.g. brachiopods) were found in
Australian
evidence for continental drift. and Indian limestones.
- Brachiopods are small shellfish that would be unlikely to cross open
stretches of ocean between Australia and India.
- Suggests India and Australia were once much closer together
meaning the brachiopods have been separated by continents
moving apart.
Similar reptile fossils found in South American and South Africa.
- Reptiles wouldn't've been able to cross the ocean suggesting all
land was once joined together.
- They are unlikely to have evolved the same in separate areas.
*Fossil from rocks younger than Carboniferous period show fewer
similarities between animal, suggesting they followed different
evolutionary paths.
- These animals originally evolved together (during carboniferous period).
- Once the continents split the animals were separated.
- They evolved as their environments changed, becoming more
different.
What is paleomagnetism? Changes in the Earth's polarity.
Occurs every 400,000-500,000 years
2/75
,5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Explain why there are different bands of rock on the ocean It has been found that there are very small variations in the
Earth's magnetic field. floor.
(Paleomagnetism) These can be explained by changes in the Earth's polarity.
Describe the locations of the different bands of rock (old Oceanic crust and the ocean floor is made up of igneous rock which
originated from and new rocks). lava flows and contains iron particles.
As lava erupts and cools, the magnetic orientation of iron particles is
locked into rock, depending on the Earth's polarity at the time.
Different bands of rock on the ocean floor have been found which are
explained by the changes in the Earth's magnetic polarity - rock
changes direction and type with each change of polarity.
Oldest rock would be located furthest from the point it is created
(nearer land).
Because new rock is being created from the centre point which forces
older rock outwards (towards land).
What is sea-floor spreading? Lateral movement of new oceanic crust away from a mid-ocean ridge
(constructive plate boundary).
Explain the process of sea-floor spreading. Width of each strip of the ocean bed with the same magnetic
orientation
corresponded with the age of time scale of the magnetic reversals.
(Link to paleomagnetism)
Indicates that, as fresh molten rock from asthenosphere reaches ocean
bed, 'older' rock is pushed away from the mid-ocean ridge.
Sea-floor spreading moves material across ocean floors.
Eventually, the sea floor reaches an ocean trench where material is
subjected into the asthenosphere and becomes semi-molten (caused
by convection currents).
What is the evidence for sea-floor spreading? Newest rock is found in the middle (at mid-ocean ridge), therefore new
rock is being
created here.
Rocks are not the same age as the earth (4.5 billion years) meaning
oldest rock is being destroyed and replaced by newer rocks.
Global patterns of plate boundaries Earthquakes are concentrated in narrow bands at plate
boundaries (rigid lithosphere and crust broken up into tectonic
plates that are moving).
What are the 3 types of plate boundary? Divergent
(constructive)
Convergent
(destructive)
Conservative
Example of divergent (constructive) plate boundaries Iceland (Eurasion and North American plate)
Mid-atlantic ridge (South American and African plate)
Great African rift valley (African plate)
What happens at divergent plate boundaries? Plates move apart.
Convection currents rise and spread
apart. New crust is created.
3/75
, 5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Describe the movement of tectonic plates at Hot rock rises, cools and spreads.
a divergent boundary Convection currents push plates apart.
Gap releases pressure.
Mantle rocks can melt, forming
magma. Magma erupts effusively.
New crust is created (basic lava -
basaltic). Ridge created.
Ridge push accelerates plate movement.
How is magma formed at divergent boundaries? Asthenosphere (semi-molten rock) rises, pressure decreases towards
crust so rock
melts.
Plates moving apart creates a gap that relieves the pressure (less
pressure).
Features of lava at divergent boundaries Type: Basalt
(runny)
Acidic/basic:
Basic
Silica content: Low (thin, milk-like)
Temperature: High
Viscosity: Low (runny)
Frequency: High
Duration: Often
Why are volcanoes at divergent boundaries effusive? Basic lava is runny (has low
viscosity) (smooth, gentle flows of lava - when lava has low viscosity)
How does volcanic activity affect landforms Runny basaltic rock creates pillow lava formations (round igneous rocks)
- eruption on divergent boundaries? of magma occurs mostly underwater. Magma erupting directly onto sea
bed is
rapidly cooled which forms pillow lava.
Lava plateaux, e.g. Giant's causeway.
Mid-ocean ridge formed by new crust (long chain of underwater
mountains). Low, flat shield volcanoes formed by runny lava.
Rift valley
What is a mid-ocean ridge? Very long chains of mountains on the sea
floor. Altogether have a combined length
of 60,000km.
At intervals, ridges are broken by transform faults.
Vary in shape depending on rate of spreading (determined by amount
and rate of magma rising).
How does the spreading of plates cause earthquakes? Transform faults
Small, shallow-focus earthquakes occur along their lengths as they slip.
What are transform faults? Large-scale faults in the crust at right angles to a mid-ocean ridge,
which range from tens to hundreds of km.
Why do divergent boundaries have shield volcanoes? Lave flows far which forms low, gently sloping volcanoes.
Runny basic lava erupts gently.
Less viscous lava flows far (flows
easily). Creates wide, gently sloping
volcanoes.
4/75
PM
OCR A-Level Geography Revision Notes 2025/2026 | 730
Questions & Answers on Plate Tectonics, Earth Structure,
Volcanoes, Earthquakes, Sea-Floor Spreading & Tectonic
Hazards
What are the 3 layers of the Earth? Crust
Mantle
Core (inner and outer)
What separates the crust and the mantle? Moho
What 2 layers does the upper mantle consist of? Lithosphere (semi-core)
Varies in thickness
Boundary with asthenosphere difficult to define (melts and becomes
incorporated in asthenosphere)
Asthenosphere extends 100-300km
Semi-molten/viscous - Allows rock to move due to high pressure in
mantle. Flowing slowly
What are continental and oceanic plates made up Lithosphere and crust
of?
Where do convection currents Asthenosphere
exist? What do these do?
Caused by vast amounts of heat generated in mantle.
Semi-molten asthenosphere flows carrying with it the solid lithosphere
and crust.
What are the properties of the continental crust? Thickness:
35km average (<30-70km)
Density:
2.6-2.7
Mineral composition:
Mainly granitic, silicon, aluminium
What are the properties of the oceanic crust? Thickness:
5-10km
Density:
3.0
Mineral composition:
Mainly basaltic, silicon and magnesium
What are the properties of the mantle? Thickness:
To a depth of 2900km
Density:
3.3 at Moho
5.6 at core
Mineral composition:
Rich in magnesium and iron
1/75
,5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Explain the movement of the crust caused by Hot rock rises from lower to upper
convection currents in the mantle.
asthenosphere. Hot rock spreads and
cools, pushing plates apart. Cool rock sinks
back down towards core.
As (oceanic) plate subducts at ocean trenches, gravity pulls it under
(slab pull).
Rising mantle pushes crust upwards at mid-ocean trenches, while
gravity pulls it back down (ridge push).
What was Alfred Wegener's big idea? 250 million years (Carboniferous period), all the Earth's continents fit
together
(Pangea).
Over time, continents have moved apart through continental drift.
Explain 3 pieces of (Wegener's) geological (rocks) Mountain chains and rock sequences on opposite sides of oceans show
close
evidence for continental drift. similarities (e.g. northeast Canada and northern Scotland).
- These mountains are likely to have been created together and then
split apart as the continents moved.
Continents seem to fit together (particularly South America and Africa).
- Suggests that continents once fit all together and continental drift
has separated them.
- (Erosion wouldn't've made them this shape).
Evidence of glaciations 290 million years ago in southern Africa, Australia,
South America, India and Antarctica.
- Suggests these land masses were joined during this time, located
close to the South Pole.
- (India has a tropical climate now, and due to it's current location it
wouldn't have been glaciated in an ice age.
- Suggests India was one at a higher latitude further from the equator).
Explain 3 pieces of (Wegener's) biological (living) Similar fossils of marine shellfish (e.g. brachiopods) were found in
Australian
evidence for continental drift. and Indian limestones.
- Brachiopods are small shellfish that would be unlikely to cross open
stretches of ocean between Australia and India.
- Suggests India and Australia were once much closer together
meaning the brachiopods have been separated by continents
moving apart.
Similar reptile fossils found in South American and South Africa.
- Reptiles wouldn't've been able to cross the ocean suggesting all
land was once joined together.
- They are unlikely to have evolved the same in separate areas.
*Fossil from rocks younger than Carboniferous period show fewer
similarities between animal, suggesting they followed different
evolutionary paths.
- These animals originally evolved together (during carboniferous period).
- Once the continents split the animals were separated.
- They evolved as their environments changed, becoming more
different.
What is paleomagnetism? Changes in the Earth's polarity.
Occurs every 400,000-500,000 years
2/75
,5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Explain why there are different bands of rock on the ocean It has been found that there are very small variations in the
Earth's magnetic field. floor.
(Paleomagnetism) These can be explained by changes in the Earth's polarity.
Describe the locations of the different bands of rock (old Oceanic crust and the ocean floor is made up of igneous rock which
originated from and new rocks). lava flows and contains iron particles.
As lava erupts and cools, the magnetic orientation of iron particles is
locked into rock, depending on the Earth's polarity at the time.
Different bands of rock on the ocean floor have been found which are
explained by the changes in the Earth's magnetic polarity - rock
changes direction and type with each change of polarity.
Oldest rock would be located furthest from the point it is created
(nearer land).
Because new rock is being created from the centre point which forces
older rock outwards (towards land).
What is sea-floor spreading? Lateral movement of new oceanic crust away from a mid-ocean ridge
(constructive plate boundary).
Explain the process of sea-floor spreading. Width of each strip of the ocean bed with the same magnetic
orientation
corresponded with the age of time scale of the magnetic reversals.
(Link to paleomagnetism)
Indicates that, as fresh molten rock from asthenosphere reaches ocean
bed, 'older' rock is pushed away from the mid-ocean ridge.
Sea-floor spreading moves material across ocean floors.
Eventually, the sea floor reaches an ocean trench where material is
subjected into the asthenosphere and becomes semi-molten (caused
by convection currents).
What is the evidence for sea-floor spreading? Newest rock is found in the middle (at mid-ocean ridge), therefore new
rock is being
created here.
Rocks are not the same age as the earth (4.5 billion years) meaning
oldest rock is being destroyed and replaced by newer rocks.
Global patterns of plate boundaries Earthquakes are concentrated in narrow bands at plate
boundaries (rigid lithosphere and crust broken up into tectonic
plates that are moving).
What are the 3 types of plate boundary? Divergent
(constructive)
Convergent
(destructive)
Conservative
Example of divergent (constructive) plate boundaries Iceland (Eurasion and North American plate)
Mid-atlantic ridge (South American and African plate)
Great African rift valley (African plate)
What happens at divergent plate boundaries? Plates move apart.
Convection currents rise and spread
apart. New crust is created.
3/75
, 5/12/26, 7:42 OCR A-Level Geography - OCR A Level Geography |
PM
Describe the movement of tectonic plates at Hot rock rises, cools and spreads.
a divergent boundary Convection currents push plates apart.
Gap releases pressure.
Mantle rocks can melt, forming
magma. Magma erupts effusively.
New crust is created (basic lava -
basaltic). Ridge created.
Ridge push accelerates plate movement.
How is magma formed at divergent boundaries? Asthenosphere (semi-molten rock) rises, pressure decreases towards
crust so rock
melts.
Plates moving apart creates a gap that relieves the pressure (less
pressure).
Features of lava at divergent boundaries Type: Basalt
(runny)
Acidic/basic:
Basic
Silica content: Low (thin, milk-like)
Temperature: High
Viscosity: Low (runny)
Frequency: High
Duration: Often
Why are volcanoes at divergent boundaries effusive? Basic lava is runny (has low
viscosity) (smooth, gentle flows of lava - when lava has low viscosity)
How does volcanic activity affect landforms Runny basaltic rock creates pillow lava formations (round igneous rocks)
- eruption on divergent boundaries? of magma occurs mostly underwater. Magma erupting directly onto sea
bed is
rapidly cooled which forms pillow lava.
Lava plateaux, e.g. Giant's causeway.
Mid-ocean ridge formed by new crust (long chain of underwater
mountains). Low, flat shield volcanoes formed by runny lava.
Rift valley
What is a mid-ocean ridge? Very long chains of mountains on the sea
floor. Altogether have a combined length
of 60,000km.
At intervals, ridges are broken by transform faults.
Vary in shape depending on rate of spreading (determined by amount
and rate of magma rising).
How does the spreading of plates cause earthquakes? Transform faults
Small, shallow-focus earthquakes occur along their lengths as they slip.
What are transform faults? Large-scale faults in the crust at right angles to a mid-ocean ridge,
which range from tens to hundreds of km.
Why do divergent boundaries have shield volcanoes? Lave flows far which forms low, gently sloping volcanoes.
Runny basic lava erupts gently.
Less viscous lava flows far (flows
easily). Creates wide, gently sloping
volcanoes.
4/75
