OCR GCSE Geography B (J384) — Global Hazards
Did you know tectonic plates move at roughly the same speed your fingernails grow? They travel between 2.5 cm and 15 cm every year. Heat from the Earth's core (reaching up to due to radioactive decay) drives this movement. Earthquakes and volcanoes are largely distributed in narrow belts along the resulting plate margins.
For decades, scientists believed convection currents—where hot, less dense magma rises from the core, cools, and sinks in a circular motion within the semi-molten asthenosphere—were the sole cause of plate movement. However, modern tectonic theory highlights two stronger forces acting on the rigid lithosphere.
Slab pull is now considered the dominant driving force. This occurs when the heavy weight of a dense, sinking oceanic plate pulls the rest of the plate behind it due to gravity. Meanwhile, ridge push occurs when newly formed, elevated crust cools, becomes denser, and slides down away from a mid-ocean ridge under the force of gravity.
Imagine pulling apart a warm chocolate fondant to reveal the molten center. At a constructive boundary, tectonic plates diverge (move apart) driven by ridge push and mantle convection.
As the plates separate, the drop in pressure causes decompression melting in the mantle below, forming hot, runny basaltic magma. This magma rises through fissures to form new oceanic crust.
Because this lava has low viscosity (it is very thin and runny), it spreads out over long distances before cooling, creating broad, gently sloping shield volcanoes. These boundaries experience frequent, gentle eruptions and mostly shallow-focus earthquakes. The resulting landforms are mid-ocean ridges (like the Mid-Atlantic Ridge) and continental rift valleys (like the East African Rift).
The Andes mountains are home to some of the most explosive volcanic eruptions on Earth, all driven by a hidden underground collision. Here, oceanic and continental plates converge. Driven by slab pull, the denser oceanic plate sinks beneath the less dense continental plate in a process called subduction. This downward entry creates a deep ocean trench.
As the oceanic plate descends into the mantle, it reaches the Benioff Zone. Intense friction, pressure, and heat—combined with water carried down by the plate lowering the melting point—cause the rock to melt. The resulting magma is highly viscous (thick and sticky) and pressurized with trapped gases.
When this magma forces its way to the surface, it forms steep-sided composite volcanoes (also known as stratovolcanoes) famous for highly explosive eruptions. These boundaries produce powerful shallow, intermediate, and deep-focus earthquakes as intense friction is suddenly released. They also form massive ranges of fold mountains and volcanic island arcs.
Mount Everest is still growing by a few millimetres every year because two massive landmasses refuse to give way to each other. When two continental plates of similar low density converge, neither is dense enough to subduct into the mantle.
Instead of sinking, the landmasses forcefully collide and compress the sedimentary rocks trapped between them. The immense pressure forces the rock layers to buckle, crumple, and uplift. Over millions of years, this intense folding creates massive ranges of fold mountains with deep crustal roots extending downwards.
Crucially, collision boundaries have absolutely no volcanic activity. This is because there is no subduction to melt the crust, and the continental rock layer is far too thick for magma to penetrate. They do, however, trigger frequent and powerful earthquakes as the plates continuously grind together.
Try sliding two rough blocks of sandpaper past each other—they will catch and resist before suddenly jolting forward. At conservative boundaries, plates slide past one another horizontally (either in opposite directions or in the same direction at different speeds). Like collision zones, these boundaries entirely lack volcanic activity because there is no subduction or rising magma involved.
Instead, they are famous for highly destructive, shallow-focus earthquakes. As the jagged edges of the plates try to slide past each other, friction causes them to snag and become locked together. Tectonic forces continue to push, causing massive tension to build up over decades.
Eventually, the rock reaches its breaking point and snaps. The stored energy is released instantly as seismic waves radiating from the earthquake's underground focus, causing violent shaking at the epicentre directly above.
Not all volcanoes need a plate boundary to form; some punch straight through the middle of a plate. A hotspot is fueled by a mantle plume—a stationary, localized column of superheated rock rising from the core-mantle boundary, driven by radioactive decay.
As this intense heat reaches the crust, it causes the lithosphere to dome, crack, and partially melt. Runny basaltic magma accumulates and erupts through the crust, gradually building up a shield volcano. Because the tectonic plate moves slowly over the stationary mantle plume, the active volcano is eventually carried away from its heat source and becomes extinct.
A new volcano then begins to form above the plume, eventually creating a long chain of volcanic islands (such as Hawaii). Over time, the older, extinct islands erode and sink below the ocean surface, becoming underwater seamounts.
Students often state that volcanoes form at collision boundaries; remember that collision zones (like the Himalayas) ONLY have earthquakes because the crust is too thick for magma to rise and there is no subduction.
For exams from June 2026 onwards, OCR specifically requires you to identify 'slab pull' and 'ridge push' as the main drivers of plate movement, rather than relying solely on 'convection currents'.
In 'Explain' questions about destructive boundaries, marks are specifically awarded for stating that the oceanic plate sinks because it is 'denser', and for using the exact term 'subduction'.
When explaining both slab pull and ridge push, always explicitly state that they are driven by the force of 'gravity' to secure full marks.
Use specific OCR mark scheme terminology: describe rocks at collision boundaries as 'buckling' or 'crumpling', and plates at conservative boundaries as 'snagging' or getting 'stuck'.
Convection currents
Circular movements of semi-molten rock in the mantle, driven by heat from the Earth's core.
Asthenosphere
The semi-molten, plastic-like upper layer of the Earth's mantle where convection occurs.
Lithosphere
The rigid outer layer of the Earth, consisting of the crust and the uppermost solid mantle.
Slab pull
The primary driving force of plate movement, where the weight of a dense, subducting oceanic plate pulls the rest of the plate behind it due to gravity.
Ridge push
A driving force where newly formed, elevated crust at a mid-ocean ridge cools, becomes denser, and slides away from the ridge under the force of gravity.
Decompression melting
Melting of the mantle that occurs when tectonic plates move apart, causing a drop in pressure.
Basaltic magma
A very hot, runny type of magma with low silica content, typically found at constructive boundaries and hotspots.
Low viscosity
A term describing liquids, such as basaltic lava, that are thin and flow easily over long distances.
Shield volcanoes
Broad, gently sloping volcanoes formed by the frequent, gentle eruption of runny, low-viscosity lava.
Rift valleys
Steep-sided valleys formed on continental crust when tectonic plates pull apart.
Subduction
The process where a denser oceanic plate is forced underneath a less dense continental plate.
Benioff Zone
The inclined zone of intense seismic activity and friction corresponding to the sinking oceanic slab in a subduction zone.
Viscous
A term describing thick, sticky liquids that do not flow easily, such as the high-silica magma created at destructive boundaries.
Composite volcanoes
Steep-sided, explosive volcanoes (stratovolcanoes) formed by highly viscous, gas-rich magma at destructive margins.
Sedimentary rocks
Rock types formed by the accumulation of compressed sediment, which often buckle upwards at collision boundaries.
Fold mountains
Large mountain ranges formed when two continental plates collide, forcing the rock layers to crumple and uplift.
Seismic waves
The energy released from an earthquake that travels through the Earth and causes violent shaking.
Focus
The exact point underground where an earthquake originates and pressure is first released.
Epicentre
The point on the Earth's surface directly above the focus of an earthquake.
Hotspot
A volcanic area situated in the middle of a tectonic plate, fueled by a localized column of rising magma.
Mantle plume
A stationary column of superheated mantle material rising from the core-mantle boundary.
Seamounts
Underwater mountains that were once active hotspot volcanoes but have moved away from the plume, become extinct, and eroded over time.
Put your knowledge into practice — try past paper questions for Geography B
Convection currents
Circular movements of semi-molten rock in the mantle, driven by heat from the Earth's core.
Asthenosphere
The semi-molten, plastic-like upper layer of the Earth's mantle where convection occurs.
Lithosphere
The rigid outer layer of the Earth, consisting of the crust and the uppermost solid mantle.
Slab pull
The primary driving force of plate movement, where the weight of a dense, subducting oceanic plate pulls the rest of the plate behind it due to gravity.
Ridge push
A driving force where newly formed, elevated crust at a mid-ocean ridge cools, becomes denser, and slides away from the ridge under the force of gravity.
Decompression melting
Melting of the mantle that occurs when tectonic plates move apart, causing a drop in pressure.
Basaltic magma
A very hot, runny type of magma with low silica content, typically found at constructive boundaries and hotspots.
Low viscosity
A term describing liquids, such as basaltic lava, that are thin and flow easily over long distances.
Shield volcanoes
Broad, gently sloping volcanoes formed by the frequent, gentle eruption of runny, low-viscosity lava.
Rift valleys
Steep-sided valleys formed on continental crust when tectonic plates pull apart.
Subduction
The process where a denser oceanic plate is forced underneath a less dense continental plate.
Benioff Zone
The inclined zone of intense seismic activity and friction corresponding to the sinking oceanic slab in a subduction zone.
Viscous
A term describing thick, sticky liquids that do not flow easily, such as the high-silica magma created at destructive boundaries.
Composite volcanoes
Steep-sided, explosive volcanoes (stratovolcanoes) formed by highly viscous, gas-rich magma at destructive margins.
Sedimentary rocks
Rock types formed by the accumulation of compressed sediment, which often buckle upwards at collision boundaries.
Fold mountains
Large mountain ranges formed when two continental plates collide, forcing the rock layers to crumple and uplift.
Seismic waves
The energy released from an earthquake that travels through the Earth and causes violent shaking.
Focus
The exact point underground where an earthquake originates and pressure is first released.
Epicentre
The point on the Earth's surface directly above the focus of an earthquake.
Hotspot
A volcanic area situated in the middle of a tectonic plate, fueled by a localized column of rising magma.
Mantle plume
A stationary column of superheated mantle material rising from the core-mantle boundary.
Seamounts
Underwater mountains that were once active hotspot volcanoes but have moved away from the plume, become extinct, and eroded over time.
