Internal heating and convection

Earth's rigid outer layer, the lithosphere, consists of the crust and uppermost mantle. It is divided into tectonic plates, which move over the weaker asthenosphere beneath them. Although the mantle is predominantly solid, it can deform and flow over geological time.

Radioactive decay within Earth and heat retained from its formation provide Internal heating. As heat moves towards the surface, it helps produce convection currents: hotter, less dense mantle material rises; cooler, denser material sinks. These slow movements interact with the plates above. The plates aren't simply passengers on a conveyor belt.

Rifting at divergent margins

Where the lithosphere stretches and pulls apart, Rifting occurs. Tensional forces create fractures and faults. Blocks of crust can then drop down, forming a rift valley.

Pressure falls as mantle material rises beneath the thinning lithosphere. Through decompression melting, magma forms without a rise in temperature. It can reach the surface through fissures. At oceanic divergent margins, the magma cools and forms new oceanic crust along a mid-ocean ridge.

Gravity moves the elevated lithosphere at a ridge downslope, contributing to ridge push. Earthquakes here are generally shallow: fracturing is concentrated in the relatively thin, brittle lithosphere.

Subduction at convergent margins

When one tectonic plate descends beneath another into the mantle, the process is called Subduction. Older, colder oceanic lithosphere is dense enough to sink. Continental lithosphere, by contrast, is more buoyant and resists subduction.

As the slab descends, it pulls the rest of its plate towards the trench. This slab pull is an important driver of plate movement. Friction and locking between plates allow stress to build up; earthquakes can also occur within the descending slab.

Water released from the subducting plate lowers the melting temperature of the mantle above it. Partial melting produces magma, which may rise to form a volcanic arc. Where two continental plates collide, the crust shortens and thickens rather than readily subducting.

Mantle plumes and conservative margins

A localized upwelling of unusually hot mantle material is called a mantle plume. As this material rises, it can undergo decompression melting and supply a volcanic hotspot, including one away from a plate margin. A plate moving over a relatively persistent hotspot can produce a chain of volcanoes. The older volcanoes are carried away from the active centre.

At a conservative, or transform, margin, plates slide past one another. No lithosphere is created or destroyed. Frictional locking followed by sudden slip produces earthquakes, but this movement does not normally generate magma.

Why eruption styles differ

Magma is molten rock beneath Earth's surface. Once it erupts, it is called lava. The style of eruption depends particularly on how viscous the magma is and how its dissolved gases behave.

Viscosity is resistance to flow. Hot, silica-poor magma generally has low viscosity: it flows readily, and gases escape relatively easily. This favours effusive eruptions, where lava pours out. By contrast, cooler, silica-rich magma is generally more viscous and traps gases. As magma rises, pressure falls and the gases expand. If they cannot escape, fragmentation can produce an explosive eruption.

These are tendencies, not fixed rules. Gas-rich basaltic magma can erupt explosively too. Even a single volcano can alternate between explosive and effusive activity.

Volcano shape records its eruptions

Shield volcanoes are common at hotspots and divergent margins, whereas composite volcanoes are especially associated with subduction zones. Cinder cones can develop in several tectonic settings, including on the flanks of larger volcanoes.

Associated secondary hazards

A pyroclastic flow is a fast-moving current of hot gas, ash and rock fragments that travels close to the ground. It can form when an eruption column collapses or an unstable lava dome breaks apart. Its high temperature, speed and dense material make it destructive. Valleys often channel the flow.

Water mixed with volcanic debris forms a flowing mixture called a lahar. Heavy rainfall can mobilize loose ash and rock, as can melting snow or ice or the release of stored water. Lahars follow drainage channels and may reach settlements far from the vent. They can occur during an eruption—or long afterwards.

A volcanic landslide occurs when part of a volcanic slope or edifice fails and moves downslope. Steep slopes can destabilize a volcano, as can loose deposits, weathered rock, earthquake shaking and magma intrusion. Large collapses can spread debris over extensive areas. If a collapse enters water, it may also generate waves.

Each hazard has a different footprint. Pyroclastic flows concentrate heat and impact near the volcano and along valleys; lahars can extend far downstream. The footprint of a landslide depends on which slopes fail. A period without eruption does not necessarily mean that unstable slopes or ash-filled valleys are safe.

Stress accumulation and sudden release

An earthquake is a vibration of the ground caused by the sudden release of energy stored within Earth. Friction can lock rocks on either side of a fault even as the plates continue to move. The rocks deform, storing elastic energy. When stress overcomes resistance, they slip suddenly and release seismic waves.

Rupture begins at the focus, or hypocentre, a point within Earth. Directly above it, on Earth's surface, lies the epicentre. The epicentre does not represent the whole area affected: rupture can spread along a large fault.

Depth of focus separates earthquakes into shallow events, less than about 70 km deep; intermediate events, about 70–300 km deep; and deep events, about 300–700 km deep. When events are otherwise comparable, shallow earthquakes often produce stronger surface shaking nearby. Their waves have a shorter distance to travel to the surface.

Seismic wave types

Earthquake magnitude expresses the size of an event through the energy released. A larger rupture area and greater displacement along the fault generally allow more energy to be released. But magnitude alone doesn't determine how much a particular place shakes. Depth and distance matter too, as do local ground conditions.

Plate-margin causes and human triggers

Extension at divergent margins produces normal faulting and predominantly shallow earthquakes. At conservative margins, plates slide past one another along strike-slip faults. If a locked fault suddenly releases, it can produce a powerful shallow earthquake.

Compression at convergent margins causes reverse or thrust faulting. Locked subduction interfaces can rupture across large areas and generate very large earthquakes. Within the descending slab, earthquakes extend to intermediate and deep focuses. Continental collision produces earthquakes too, mainly within the brittle crust.

Human activity can alter stresses or pressures on existing faults. Dam building adds the weight of a reservoir. Water entering fractures can also raise pore-water pressure, reducing frictional resistance. Resource extraction can remove support during mining, while oil and gas withdrawal can alter stresses. Fluid injection associated with extraction can raise fault pressure as well. These processes may trigger slip where faults are already close to failure.

Associated secondary hazards

• Tsunami: a sudden displacement of a large volume of water generates long waves, commonly when the seabed moves vertically during an offshore earthquake. As the waves enter shallow coastal water, they slow and can increase greatly in height, causing inundation. Not every offshore earthquake generates a tsunami.
• Landslides: shaking can dislodge rock or overcome the resistance of unstable slopes. Slope failures may bury settlements or block roads; they may also dam rivers.
• Liquefaction: shaking raises pore-water pressure in loose, saturated sediment. The sediment loses much of its strength and behaves more like a fluid. Buildings may settle or tilt, while the ground may spread sideways.
• Transverse faults: cross-cutting faults may reactivate during an earthquake sequence. If movement reaches the surface, fault rupture can offset roads, pipelines and buildings. Displacement along the fault causes the damage, not simply the presence of the fault.

Gravity, slope resistance and triggers

Mass movement occurs when rock, soil or debris moves downslope under gravity. It begins when the forces pulling material downslope exceed those resisting movement.

Steep gradients and weak or weathered material are physical causes. Rivers or waves can undercut a slope, while earthquake shaking can trigger movement. Heavy rainfall adds weight and can raise pore-water pressure, reducing friction within the slope. Rock loosened by freeze–thaw weathering may fall later.

People can destabilize slopes by excavating or undercutting them. Construction or spoil dumping adds loads; changes to drainage can also cause movement. Removing vegetation can reduce root reinforcement. Causes often work together: a road cutting may weaken a slope that then fails during prolonged rainfall.

Types of movement

Six dimensions of classification

Naming the type doesn't fully describe an event. The guide requires classification using six dimensions:

• Cause: distinguish physical from human influences, then separate long-term weakening from the immediate trigger. Weathering may prepare a slope over years before rainfall triggers failure.
• Liquidity: describe how fluid-like the moving material is. Dry rockfall consists of separate solid blocks; saturated mudflow deforms and flows. Both water content and grain size affect this behaviour.
• Speed of onset: describe how quickly movement begins. Rockfall may start abruptly, while creep develops gradually. Don't confuse onset with travel speed: an instability that develops slowly can culminate in rapid movement.
• Duration: describe how long movement continues. An individual fall may last seconds; a flow may last minutes or hours. Creep can persist for years. Continuing rainfall can produce repeated pulses of movement rather than one uninterrupted event.
• Extent: describe the area, distance or volume involved. Failure may affect a small roadside slope or a large mountainside. A channelled flow may travel far beyond its source.
• Frequency: describe how often movement recurs over a stated period. Small falls may recur frequently from a weathered cliff, while a major slope collapse may be rare. Local conditions affect frequency; movement type alone does not determine it.

These dimensions can vary independently. A rare event can begin suddenly and have a large extent. Frequent creep, by contrast, can be slow and widespread. Classification needs the movement type as well as the characteristics of the particular slope and event.