Earth system
Earthquakes and volcanoes
Two of the most visible expressions of plate tectonics — and two of the most commonly misexplained. Nothing collides in an earthquake, and volcanoes do not all form the same way.
An earthquake is the sudden slip of two blocks past one another on a fault. Strain accumulates while the surface is locked and is released as seismic waves when friction is overcome. Most earthquakes occur on the faults that make up plate boundaries.[1, 2, 3]
Magnitude and intensity are different quantities: one earthquake has a single magnitude, but the shaking it causes varies from place to place. Magnitude scales are logarithmic, and moment magnitude has replaced the Richter scale for all but small local events.[1, 2, 3]
Volcanoes form at divergent boundaries, above subduction zones, and in plate interiors far from any boundary. Whether an eruption flows or explodes is governed by magma composition, decided long before the surface.[1, 2, 3]
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Why earthquakes happen
Nothing collides at the moment of an earthquake. A locked surface slips, and elastic energy stored over years is released in seconds.
How magma forms and why volcanoes erupt
Magma and lava are the same material in different places. Whether an eruption pours or explodes is decided long before the surface, by what the magma is made of — and volcanoes do not all form the same way.
Earthquake processes
How stress becomes rupture, and rupture becomes a recorded wave.
Geological process
Faulting
The fracturing of rock and displacement of the blocks on either side. The sense of that displacement — normal, reverse or strike-slip — records the orientation of the stresses that produced it.
Geological process
Elastic rebound
The mechanism of earthquakes: strain accumulates while a fault is locked, and is released suddenly when friction is overcome, the rock springing back and radiating the stored energy as seismic waves.
Geological process
Seismic wave propagation
The travel of elastic waves through the Earth. Compressional (P) and shear (S) waves behave differently in solids and liquids, and it is that difference which reveals the state of Earth’s interior.
Fault
San Andreas Fault
A transform fault zone slicing through two thirds of the length of California, connecting divergent boundaries to its north and south; one of the few transform faults exposed on land.
Volcanic processes
Melt generation, transport and eruption — plus the intraplate case that plate boundaries alone cannot explain.
Geological process
Partial melting
The production of liquid from only part of a solid rock, leaving a residue behind. Almost all magma on Earth is generated this way; the conditions that trigger it differ from setting to setting.
Geological process
Volcanism
The transport of magma to the surface and its eruption as lava, tephra and gas. Volcanoes occur at divergent boundaries, above subduction zones and in plate interiors, and these are not the same process.
Geological process
Hotspot volcanism
Long-lived volcanism in plate interiors, far from plate boundaries. Its explanation in terms of deep mantle plumes is an active scientific argument rather than settled fact.
Geological process
Magmatic crystallisation
The solidification of molten rock into an igneous rock. How fast it happens sets the grain size: slow cooling at depth gives coarse crystals, rapid cooling at the surface gives fine-grained or glassy rock.
Volcanoes
Seeded from monitoring-agency profiles, with coordinates and elevations taken from those profiles rather than inferred from maps.
Volcano
Kīlauea
One of the world’s most active volcanoes, on the Island of Hawaiʻi. As a basaltic shield volcano far from any plate boundary, it is the type example of intraplate volcanism.
Volcano
Mauna Loa
The largest volcano of the Island of Hawaiʻi, a basaltic shield rising more than four kilometres above sea level and far more above the sea floor.
Volcano
Mount St. Helens
A stratovolcano of the Cascade Range in Washington State, and the standard illustration of how magma composition governs whether an eruption flows or explodes.
Volcano
Krafla
A volcano in northeastern Iceland astride the Mid-Atlantic Ridge, where rifting episodes between 1975 and 1984 opened ground cracks that could be measured directly.
Where they sit
The structures that place these processes in the tectonic system.
Geological structure
Hawaiian–Emperor seamount chain
A long trail of volcanic islands and seamounts across the Pacific floor. Its linear form and age progression are the classic observation that hotspot volcanism must be explained.
Mountain range
Cascade Range
A volcanic mountain range of the Pacific Northwest of North America, one of the classic expressions of arc volcanism above a subduction zone.
Trench
Mariana Trench
An oceanic trench paralleling the Mariana Islands, marking where the Pacific Plate converges against the Philippine Plate. Its southern end holds the Challenger Deep, the deepest known point of the ocean floor.
Trench
Peru–Chile Trench
The trench marking where the oceanic Nazca Plate descends beneath the continental margin of South America — the convergent boundary responsible for the Andes and for the region’s great earthquakes.
Hub last updated 2026-09-06. Every entry above is a structured record; the bracketed references link to the entities whose claims support each statement.