The planet's engine
Inside the Earth
Earthquakes and volcanoes aren't random — they're the surface symptoms of a vast heat engine below your feet. Spin the cutaway, then see how the core, mantle and crust shake the ground and feed volcanoes.
Layers
5
crust to inner core
Earth's radius
6,371
km to the centre
Inner core
~5,200 °C
as hot as the Sun's surface
Plate speed
cm / yr
fingernail pace
The layers
From the crust to the core
Earth is built from concentric shells, each with its own composition, temperature and behaviour. The 'lava layer' people picture is really the hot, ductile mantle — mostly solid rock that only melts under the right conditions.
Crust & lithosphere
0–100 km · 0–1,000 °CThe thin, rigid outer shell — 5–10 km thick under oceans, up to ~70 km under continents. Together with the brittle uppermost mantle it forms the lithosphere, broken into the tectonic plates that carry continents and ocean floor. Nearly all earthquakes happen here.
Upper mantle & asthenosphere
100–660 km · 1,000–2,000 °CSolid rock that behaves like extremely stiff putty over long timescales. The asthenosphere is hot and partially molten enough to flow — it's where most magma originates and the slippery layer the plates slide over.
Lower mantle
660–2,890 km · ~3,000 °CThe thickest layer — hot, dense, solid silicate rock under immense pressure. It convects in slow-motion currents (centimetres per year) that, over millions of years, drag the plates above.
Outer core
2,890–5,150 km · 4,000–5,000 °CA churning ocean of liquid iron and nickel. Its convection — stirred by Earth's rotation — generates the planet's protective magnetic field through the geodynamo.
Inner core
5,150–6,371 km · ~5,200 °CA solid ball of iron–nickel as hot as the Sun's surface, kept solid by crushing pressure. As it slowly freezes it releases heat that powers the outer core's convection — the engine at the planet's heart.
Model layer thicknesses are exaggerated for clarity; depths and temperatures are approximate values from seismology and geophysics.
How it all works
A heat engine that moves continents
Leftover heat from Earth's formation and the decay of radioactive elements keeps the interior scorching. That heat has to escape — and the way it does shapes the surface.
Heat rises
The core heats the base of the mantle. Hot rock is slightly less dense, so it slowly rises; cooler rock sinks. These convection cells creep along at the speed your fingernails grow.
Plates ride along
The rigid plates floating on the asthenosphere are dragged and pushed by this convection — plus the pull of cold, dense slabs sinking back into the mantle ('slab pull'). Plates move a few centimetres a year.
Boundaries do the work
Where plates meet — pulling apart, colliding, or grinding past — stress and heat concentrate. That's where almost all earthquakes and volcanoes occur.
What causes earthquakes
Stuck plates, then sudden slip
Plate boundaries don't slide smoothly. Friction locks them while tectonic forces keep pushing, storing elastic energy in the rock like a bent spring — until it snaps.
Elastic rebound
As a locked fault is loaded, rock deforms and stores strain energy. When the stress finally exceeds the fault's strength, the two sides lurch past each other in seconds. The stored energy radiates outward as seismic waves — the shaking we feel. The bigger the slipped area and the further it moves, the larger the magnitude.
Three kinds of boundary
- Convergent — plates collide; one dives beneath the other (subduction). These produce the largest quakes (e.g. M9 megathrusts) and deep events.
- Transform — plates grind past sideways (e.g. the San Andreas). Shallow, often destructive.
- Divergent — plates pull apart at mid-ocean ridges; frequent but usually smaller quakes.
Quake depthis a clue to mechanism: shallow (<70 km) events dominate at transform and ridge boundaries, while subduction zones produce intermediate and deep (down to ~700 km) earthquakes within the sinking slab. See the live picture on the 3D globe or browse active fault lines.
What causes volcanic activity
Where rock melts — and finds a way up
The mantle is mostly solid. Magma forms only where conditions tip rock past its melting point — and three settings do exactly that.
Subduction (flux melting)
A sinking ocean plate carries water down. Water lowers the mantle's melting point, generating magma that rises to build volcanic arcs — like the Pacific Ring of Fire and the Philippine volcanoes.
Mid-ocean ridges (decompression)
Where plates pull apart, mantle rises and the drop in pressure lets it melt. This erupts continuously along the ocean floor, paving the sea bed with new crust.
Hotspots (mantle plumes)
Narrow columns of extra-hot mantle rise from deep below, burning through the plate moving over them — building chains like Hawaii, far from any boundary.
Once formed, magma is buoyant and gas-rich, so it rises and collects in chambers. As it nears the surface, dissolved gases expand — gently for runny basalt, explosively for sticky, silica-rich magma. We turn this physics into per-volcano odds on the eruption-forecast model and track every volcano on the volcano monitor.
FAQ
Earthquakes & volcanoes: your questions answered
What causes earthquakes?+
Earthquakes are caused by the sudden release of built-up stress along faults where tectonic plates meet. Friction locks the plates while they keep pushing, storing elastic energy until the rock ruptures and slips in seconds — radiating the seismic waves we feel as shaking.
What causes volcanoes to erupt?+
Volcanoes erupt when magma — molten rock formed in the mantle — rises to the surface. Magma forms mainly where a sinking plate releases water (subduction), where plates pull apart and pressure drops (mid-ocean ridges), or above hotspots. Dissolved gases expand as it nears the surface and drive the eruption.
What are the layers of the Earth?+
From the centre outward: the solid iron–nickel inner core (~5,200 °C); the liquid-iron outer core, which generates Earth's magnetic field; the thick rocky mantle (mostly solid but slowly convecting); and the thin, rigid crust and lithosphere that form the tectonic plates.
Is the Earth's mantle made of lava?+
No. The mantle is almost entirely solid rock. It is hot and ductile — it flows slowly over millions of years — but only melts into magma under specific conditions. 'Lava' is simply magma that has already reached the surface.
Why do earthquakes and volcanoes happen in the same places?+
Both are surface expressions of plate-tectonic motion driven by Earth's internal heat, so they cluster along plate boundaries — most dramatically the Pacific Ring of Fire, home to roughly 75% of the world's volcanoes and 90% of its earthquakes.
Two symptoms, one cause
Why earthquakes and volcanoes travel together
Both are surface expressions of the same plate motion driven by the interior's heat. That's why they cluster along the same lines — most dramatically the Pacific Ring of Fire, where subducting plates ring the ocean with ~75% of the world's volcanoes and ~90% of its earthquakes. The Philippines sits squarely within it, which is why it has both frequent quakes and active volcanoes.