Volcanoes and earthquakes

Earth's outer shell is broken into tectonic plates that move a few centimetres a year. Most earthquakes and volcanoes occur along plate boundaries, where plates collide, separate or slide past each other.

The Pacific Ring of Fire, a horseshoe of boundaries around the Pacific Ocean, hosts most of the world's active volcanoes and a large majority of its largest earthquakes.

Plates do not slide smoothly. Along faults, friction locks the rocks together while stress builds over years or centuries. When the stress exceeds the strength of the rock, it slips suddenly, releasing energy as seismic waves.

The point underground where slipping begins is the focus or hypocentre, and the point on the surface above it is the epicentre. Fast primary waves arrive first, followed by slower secondary waves and then surface waves, which often cause the most damage.

Scientists measure earthquakes with seismometers. The Richter scale, devised in 1935, has been largely replaced by the moment magnitude scale, which better measures very large quakes. Each whole-number step represents about 32 times more energy released.

The largest earthquake ever recorded struck Chile in 1960, with a magnitude of about 9.5. The 2004 Indian Ocean earthquake, around magnitude 9.1, triggered tsunamis that killed more than 200,000 people across many countries, and the 2011 Tōhoku earthquake in Japan caused a tsunami and the Fukushima nuclear disaster.

Volcanoes form where molten rock, called magma, rises to the surface. This happens at subduction zones, where a sinking plate releases water that lowers the melting point of rock above it; at spreading ridges, where plates pull apart; and over hotspots, such as Hawaii.

How a volcano erupts depends mostly on its magma. Runny, low-silica basalt lets gas escape easily, producing gentle lava flows, as in Hawaii and Iceland. Sticky, high-silica magma traps gas until pressure builds and it explodes violently, as at Mount St Helens in 1980 and Vesuvius, which buried Pompeii in 79 CE.

• Pyroclastic flows: fast avalanches of hot gas, ash and rock, the deadliest volcanic hazard.
• Ash fall: collapses roofs, damages crops and grounds aircraft.
• Lahars: volcanic mudflows that can travel far down river valleys.
• Volcanic gases: can be toxic and, in large eruptions, can cool the global climate.
• The 1815 eruption of Tambora in Indonesia led to 1816 being called the year without a summer in Europe and North America.

Earthquakes cannot yet be predicted precisely, but scientists can map faults and estimate long-term risk. Buildings designed to flex, with reinforced frames and base isolation systems, survive far better, and building codes are the main reason that similar earthquakes cause very different death tolls in different countries. Early warning systems in Japan, Mexico and elsewhere can give seconds of notice before shaking arrives.

Volcanoes often give warning signs, such as small earthquakes, swelling ground and changes in gas emissions, allowing evacuations. Volcanic activity also benefits people: volcanic soils are very fertile, and geothermal energy provides much of Iceland's heating and electricity.