Earthquakes & volcanoes

Earthquakes are rock that has been bent too far and snaps. Volcanoes are rock that has melted and found a way out. Most of both happen where plates meet, though hotspots such as Hawaii build volcanoes far from any boundary.

By the end of this module, you will be able to
  • Explain why earthquakes happen, using the idea of elastic rebound, and why they can't be predicted
  • Tell the focus from the epicentre, and magnitude from intensity, and locate an epicentre from three seismograms
  • Give the three ways rock melts, and why some volcanoes explode while others ooze
  • Put volcanic CO2 in perspective against human emissions

Why the ground shakes

A fault is a crack in the crust where the rock on either side can move. Plates push past each other steadily, a few centimetres a year, but the fault itself is held shut by friction. So the rock near it bends instead, storing energy like a drawn bow. When the stress finally overcomes the friction, the fault slips in seconds, and the bent rock springs back. This is elastic rebound, and the stored energy is released as an earthquake.

map view · dashed line = fault 1 Locked 2 Stress builds offset (metres) 3 Rupture
  1. Locked. A road is built straight across the fault. The two sides are already moving past each other, but friction holds the fault shut.
  2. Stress builds. Over decades to centuries the rock either side bends, and the road bends with it. The fault stays stuck.
  3. Rupture. The fault slips in seconds and the rock springs back. The road is straight again, but offset, by several metres in a large earthquake.
The 1906 San Francisco earthquake offset fences and roads by several metres in exactly this way. It's how the elastic rebound idea was first worked out.

The point underground where the rupture starts is the focus (or hypocentre). The point on the surface directly above it is the epicentre, which is the location you see on news maps. From the focus, energy radiates outwards as seismic waves.

Epicentre directly above the focus Focus where slip begins Fault planethrust, ~30° dip ~10 km Rupture rock slips along it Seismic waves spread out in every direction
A typical shallow earthquake, with its focus about 10 km down. Most earthquakes start in the top 20 km or so of the crust, though some at subduction zones are hundreds of kilometres deep. The rupture then spreads along the fault, much faster than you could drive. In the 2015 Nepal earthquake it unzipped about 150 km of fault in under a minute.
Arrive first

P waves

Push-pull waves, about 6 km/s in the crust. They feel like a sudden jolt or bang.

Arrive second

S waves

Side-to-side shear waves, about 3.5 km/s. Stronger shaking than P waves.

Arrive last

Surface waves

Rolling and swaying waves that travel along the surface. Slowest, but usually the most destructive.

Worked example

Because P waves outrun S waves, the gap between them grows with distance: roughly 1 second for every 8 km. If a seismometer records P waves 10 seconds before S waves, the earthquake was about 80 km away. That gives a distance but not a direction, so the earthquake lies somewhere on a circle around the station. Do the same at three stations and the circles cross at the epicentre.

Try it · find the epicentre

Three seismometers, A, B and C, recorded the same earthquake. On each seismogram, read the gap between the P and S arrivals, then set that station's slider. Each gap becomes a circle of possible locations. Where all three cross is the epicentre.

Simplified: distance = S − P gap × 8.4 km, which assumes P waves at 6 km/s and S waves at 3.5 km/s all the way, on a flat map. Real networks use many stations and models of how wave speed changes with depth, which also gives the depth of the focus.

How big was it?

Two different scales get confused. Magnitude measures the energy the earthquake released at its source, so each earthquake has one magnitude. Today seismologists use moment magnitude (Mw), which is based on the area of fault that slipped and how far it moved. The scale is logarithmic: each whole number up means about 10 times more ground motion on a seismogram and about 32 times more energy. The largest earthquake ever recorded was M9.5, in Chile in 1960.

Intensity describes how strongly the shaking was felt and what damage it did at a particular place, on a scale of I to XII. One earthquake has many intensities: high near the epicentre and on soft, wet ground, lower far away and on solid rock.

Try it · compare two earthquakes

Energy ratio = 101.5 × difference in magnitude. Rupture lengths are typical values from a worldwide fit (about 10 km for M6, 200 km for M8, 1,000 km for M9); real earthquakes vary around them by a factor of two or more.

Where earthquakes happen

Most earthquakes happen along plate boundaries, where plates grind, collide or pull apart (see Plate tectonics). Volcanoes follow many of the same lines, but not all of them, and the differences are telling.

12345
EarthquakesVolcanoesPlate boundaries (dashed: mid-ocean ridges)
  1. Ring of Fire. Subduction zones around the Pacific: most of the world's largest earthquakes and explosive volcanoes.
  2. Mid-ocean ridges. Plates pulling apart: frequent small, shallow earthquakes and eruptions, almost all underwater. Iceland is a ridge above sea level.
  3. Himalaya and the Alpine belt. Continents colliding: many large earthquakes, but almost no volcanoes, because no ocean slab is sinking beneath them to release water into the mantle above.
  4. East African Rift. A continent starting to split: earthquakes and volcanoes such as Kilimanjaro.
  5. Hawaii. A hotspot in the middle of a plate: volcanoes, and earthquakes caused by the volcanoes rather than by a plate boundary.
Schematic, not real catalogue data: the dots are spaced along the plate boundaries (from Peter Bird's 2003 model, as on the map in Plate tectonics) to show the pattern, plus a few well-known quakes inside plates, such as Latur and Bhuj in India. The triangles are real volcanoes. A real map of a few years of earthquakes traces exactly these lines.

One of the most active belts on Earth runs along the Himalaya, where India is still pushing into Asia at about 4–5 cm a year. The strain is taken up on a huge, gently sloping fault beneath the mountains, the Main Himalayan Thrust. In April 2015 a section of it beneath Nepal ruptured in the Gorkha earthquake, magnitude 7.8, killing around 9,000 people. Earthquakes also happen inside plates, though less often: the 2001 Bhuj earthquake in Gujarat (M7.7) is an example.

No one can predict the day, or even the year, of a particular earthquake. A fault's strength varies from place to place and from one earthquake to the next, and no instrument can measure it kilometres down. What can be done is to map the hazard: where faults are, how often they have ruptured before, and how the ground will respond. India's building code (IS 1893) divides the country into seismic zones on this basis, from lower to very high hazard, and good building design is what turns that knowledge into lives saved.

Try it · stick and slip

A block held by friction is pulled by a spring whose far end moves steadily, like a plate. Stress builds until it beats the fault's strength, then the block slips and the stress drops: an earthquake. Run it, then turn up how much the fault's strength varies from one earthquake to the next.

A toy model, not a forecast. Stress is shown as a fraction of the fault's average strength. The plate loads the fault at a steady rate, and each earthquake drops the stress back to roughly the same low level. Only the strength varies. Even so, once it varies a little, the gaps between earthquakes become irregular. Average recurrence can be estimated; the date of the next one cannot.

Why rock melts

The mantle is solid, so magma has to be made somewhere special. Rock melts when its temperature rises above its melting point at that depth. Pressure raises the melting point, so deep rock can be very hot and still solid. The geotherm is the actual temperature at each depth. Normally it stays cooler than the melting point of dry mantle rock, which is why the mantle is solid. That gives three ways to make magma: lower the pressure, lower the melting point, or raise the temperature.

Try it · melt a rock

The dot is a parcel of mantle rock, 150 km down and at the normal temperature for that depth. Make it melt in three ways: let it rise, add water, or heat it. You can also drag the dot.

Normal temperature (geotherm)Dry rock meltsWet rock melts

Rough, textbook-style curves for mantle rock (peridotite). A rising parcel cools only slightly, by about 0.4 °C per km, because it expands as the pressure drops; that is far less than the melting point falls. Only a small fraction of the rock melts at first, typically a few to ~20 per cent. Crustal rocks such as granite melt at lower temperatures, about 650–900 °C, so hot magma rising into the crust can melt them.

  1. DecompressionHot mantle rises, the pressure drops, and it crosses its melting curve without being heated. This happens beneath mid-ocean ridges and in plumes.
  2. Adding waterWater lowers the melting point by hundreds of degrees, so hot rock that was solid starts to melt. This is what happens above subduction zones.
  3. Adding heatRising magma heats the crust around it and melts it, making granite-type magmas.

At subduction zones, the sinking plate carries water locked in its minerals. As it heats up, it releases that water into the hot mantle above it, which then starts to melt. This flux melting is why chains of volcanoes line up above subduction zones, such as the Andes, Japan, and Barren Island in the Andaman Sea, India's only active volcano.

Runny or sticky: why some volcanoes explode

How a volcano erupts depends mostly on how runny its magma is. That runniness is its viscosity, and it is controlled by silica. Silica-rich magma is full of linked chains of silicon and oxygen that tangle together, making it thick and sticky. It is also cooler. Dissolved gas (mostly water vapour and CO2) is the other ingredient. As magma rises and pressure drops, the gas comes out of solution like the fizz in an opened bottle. In runny magma the bubbles escape easily. In sticky magma they are trapped until the magma blows apart.

gas escapes lava flows far bubbles rise through the melt and escape gas overpressure stays low Runny basalt ~50% silica · ~1,150 °C ash + gas froth shatters bubbles trapped: pressure builds gas overpressure high Sticky rhyolite ~70% silica · ~750 °C
The same gas, two outcomes. In runny basalt (left), bubbles rise through the magma faster than it flows and escape at the vent. Pressure stays low, and lava pours out quietly and flows far, building a broad, gentle shield. In sticky rhyolite (right), bubbles cannot escape the melt; they rise with it, and the sticky melt holds back their growth, so the gas inside ends up at a higher pressure than the magma around it. This overpressure peaks near the top of the conduit, and when it exceeds the strength of the melt, the froth shatters (dashed line) into ash. The expanding gas blasts the ash up the vent into a column that spreads downwind and rains ash, and the debris piles into a steep cone. Andesite sits in between.
MagmaSilicaTemperatureBehaviour
Basalt~50%~1,100–1,200 °CRunny. Gas escapes; lava flows quietly (effusive). Hawaii, Iceland, the Deccan Traps.
Andesite~60%~900–1,000 °CSticky. Builds pressure, often explosive. Typical above subduction zones.
Rhyolite~70%~700–850 °CVery sticky. The most violent eruptions, throwing out ash and pumice.

Big explosive eruptions can change the climate. In 1991 Mount Pinatubo in the Philippines blasted about 15–20 million tonnes of sulphur dioxide into the stratosphere. It formed a haze of tiny sulphate droplets that reflected sunlight, and average global temperature fell by about 0.5 °C for a year or two.

Volcano shapes

Viscosity also sets the shape of a volcano. Runny lava travels far before it freezes and builds wide, gentle slopes. Sticky lava and ash pile up close to the vent into steep cones, layer upon layer, each draped over the slopes of the one before.

Shield volcano runny basalt gentle slopes, ~5–10° e.g. Mauna Loa, Hawaii Stratovolcano sticky lava + ash steep slopes, ~30° e.g. Mount St Helens Flood basalt runny basalt, fissures flat, very wide plateau e.g. Deccan Traps, India
Lava flowsAshMagmaWeathered soil between flows
Not to the same scale. Mauna Loa in Hawaii is about 120 km long, while a stratovolcano such as Mount St Helens or Pinatubo is typically 10–20 km across. Flood basalts are bigger still: the Deccan Traps still cover about 500,000 km² and are more than 2 km thick in the Western Ghats. The flood basalt is shown while erupting; today, erosion has cut the edges of the Deccan into stepped hillsides, and "trap" comes from the Swedish word for stairs.

Volcanoes and CO2

Volcanoes release CO2 from the mantle, and over geological time they are the main natural source of carbon to the atmosphere. But their output today is small next to ours. All the world's volcanoes, on land and under the sea, release roughly 0.3–0.4 billion tonnes of CO2 a year. Burning fossil fuels releases about 37 billion tonnes: about 100 times more.

Volcanoes ~0.3–0.4 Gt a year Fossil fuels · ~37 Gt a year
Each square is about 0.37 billion tonnes of CO2 a year, roughly everything the world's volcanoes emit.
Why this matters at Alt Earth

Flood basalts like the Deccan Traps are the feedstock geology for enhanced rock weathering: enormous volumes of calcium- and magnesium-rich rock, right at the surface in western and central India. Their runny, effusive eruptions are why they spread so far.

The carbon cycle also gives useful perspective. Over millions of years, volcanic CO2 is roughly balanced by the slow weathering of silicate rocks, which locks carbon away (see Weathering, erosion & soil). Today humans emit about 100 times more CO2 than volcanoes, far faster than natural weathering can absorb. Enhanced weathering aims to speed up that natural sink.

Volcanoes also make ore. The magma chambers beneath arc volcanoes release hot, metal-rich fluids that form porphyry copper and epithermal gold deposits. That is why explorers search old volcanic arcs, where erosion has cut down to the level where those deposits formed.