Inside the Earth

The Earth is made of layers, hottest and densest at the centre:

• Inner core — solid iron and nickel, extremely hot, kept solid by immense pressure.
• Outer core — liquid iron and nickel; its movement helps generate Earth's magnetic field.
• Mantle — the thickest layer, semi-molten rock that flows very slowly.
• Crust — the thin, solid outer shell we live on, broken into tectonic plates. Oceanic crust is thin and dense; continental crust is thick and less dense.

Convection currents

Heat from the core (partly from radioactive decay) warms the base of the mantle. Hot, less dense rock rises; as it nears the crust it cools, becomes denser and sinks again. These looping convection currents act like a slow conveyor belt, dragging the tectonic plates above them. Where currents rise, plates are pushed apart; where they sink, plates are pulled together. The lines where plates meet are called plate margins (or boundaries).

What moves the plates?

The four plate margins

There are four ways plates can meet:

• Constructive — plates move apart. Magma rises to fill the gap and forms new crust, building a mid-ocean ridge (e.g. the Mid-Atlantic Ridge).
• Destructive — an oceanic plate is forced down beneath a continental plate at a subduction zone, forming an ocean trench; the melting plate feeds explosive volcanoes.
• Collision — two continental plates push together; neither sinks, so the crust crumples upward into fold mountains (e.g. the Himalayas).
• Conservative — plates slide past each other along a fault line; crust is neither made nor destroyed, but friction causes earthquakes (e.g. the San Andreas Fault).

Sort the feature to its margin

Tap a feature, then tap the plate margin that produces it.

Naming a margin

The three rock types

Rocks are grouped by how they form:

• Igneous — formed when molten magma or lava cools and crystallises. Fast-cooling lava gives fine-grained basalt; slow-cooling magma underground gives coarse granite.
• Sedimentary — formed when sediment is compacted and cemented in layers over time. Examples: limestone (from shells) and sandstone (from sand grains).
• Metamorphic — existing rocks changed by heat and pressure. Mudstone becomes slate; limestone becomes marble.

Local link: the Giant's Causeway in County Antrim is made of basalt columns — an igneous rock formed as lava cooled.

Match each rock to its type

Tap a rock on the left, then its matching type on the right.

How earthquakes work

Most earthquakes happen along plate margins, where plates lock together and stress builds until it is suddenly released as shockwaves. Key terms:

• Focus — the exact point underground where the earthquake starts.
• Epicentre — the point on the surface directly above the focus, where shaking is usually strongest.
• Magnitude — the energy released, measured on the Richter scale using a seismograph (seismometer). The scale is logarithmic, so each whole number is far more powerful than the last.

Focus or epicentre?

Liquefaction & tsunami

Earthquakes trigger further hazards:

• Liquefaction — violent shaking makes saturated, loose soil behave like a liquid, so buildings sink or tilt.
• Tsunami — an undersea earthquake displaces a huge volume of water, sending fast, low waves across the ocean that rise into a destructive wall as they reach shallow coasts.

Case study flag: CCEA asks you to compare an earthquake in an MEDC with one in an LEDC (short- and long-term impacts, and how well each prepared and responded). Your teacher will name the two events — check death tolls, magnitudes and costs against your class notes rather than quoting them from memory.

Your turn — the Richter scale

Your turn — focus depth

Three types of volcano

The kind of volcano depends on the lava that builds it:

• Shield volcano — runny, low-silica basaltic lava flows far before setting, building a wide, gently sloping cone. Eruptions are frequent but gentle (found at constructive margins/hotspots).
• Composite (stratovolcano) — thick, sticky lava and ash build steep-sided layers. Eruptions are violent and explosive (found at destructive margins).
• Supervolcano — a vast magma chamber that erupts only rarely but on an enormous scale, with global consequences.

Global impact of a supervolcano

A supervolcano such as the one beneath Yellowstone (USA) is thousands of times larger than an ordinary volcano. A major eruption could bury huge areas in ash, block sunlight and cool the planet for years (a "volcanic winter"), devastating harvests worldwide.

Case study flag: Yellowstone is the standard CCEA example, but figures for eruption size, ash volume and time between eruptions vary between sources — verify any specific numbers against your class notes before using them in an exam.

Which volcano?

The big ideas to know

You've covered all of CCEA Theme D — The Restless Earth. Press Finish to see your score.