Most of the time the ground feels like the one thing we can count on. Earthquakes and volcanoes are reminders that it is part of a moving system. Both are concentrated where plates meet, and both can be understood well enough to save lives, even though neither can be tamed.

Why the ground shakes

After the San Francisco earthquake of 1906, the American geologist Harry Fielding Reid studied how fences and roads had been offset along the San Andreas Fault. He proposed the theory of elastic rebound. Rocks on either side of a locked fault slowly bend as strain builds up, then slip suddenly and spring back, releasing the stored energy as an earthquake. It is like bending a stick until it snaps. The rupture begins at one point and spreads along the fault. The hypocenter, or focus, is the point underground where the rupture begins; the epicenter is the spot on the surface directly above it. Most earthquakes happen at plate boundaries, but not all: the series of large quakes near New Madrid, Missouri, in 1811 and 1812 struck in the middle of a plate.

Measuring earthquakes

Two different ideas are often confused. Magnitude measures the energy released at the source, while intensity describes how strongly the shaking was felt and how much damage it did at a particular place. One earthquake has one magnitude but many intensities, strongest near the epicenter and fading with distance. Charles Richter devised his famous magnitude scale in 1935; today seismologists mainly use the moment magnitude scale, which works better for very large quakes. Each scale steps up by whole numbers, but the steps are not equal in energy. Each step up in magnitude means about 32 times more energy released, so a magnitude 7 releases about 32 times the energy of a magnitude 6 and about a thousand times that of a magnitude 5. The largest earthquake ever recorded struck Chile in 1960, at magnitude 9.5.

Where volcanoes form

Volcanoes appear in three main settings. Along divergent boundaries, as in Iceland, rising mantle melts as the pressure on it drops. Along subduction zones, which ring much of the Pacific Ocean in the so-called Ring of Fire, the sinking plate carries water down with it. Water carried down by the sinking plate lowers the melting point of the mantle above it, producing magma that rises to feed chains of volcanoes such as the Cascades and the Andes. The third setting is the strangest. Hawaii lies thousands of kilometers from any plate boundary, above a hotspot, a plume of unusually hot mantle. In 1963 the Canadian geophysicist J. Tuzo Wilson explained the island chain: as the Pacific plate moves over the hotspot, it carries older volcanoes away and new ones form, leaving a line of islands that get older and more eroded to the northwest.

Why some volcanoes explode

The key is how thick, or viscous, the magma is, which depends mostly on its silica content. Silica-rich magma is thick and sticky, so it traps gas until the pressure bursts out explosively; low-silica basalt magma is runny, so gas escapes easily and lava flows more gently. Hawaiian eruptions usually send rivers of lava downhill that people can often outwalk. Mount St. Helens in Washington State, fed by stickier magma, blew out its side in 1980, killing 57 people.

Lava is rarely the deadliest danger. Pyroclastic flows, avalanches of hot gas, ash and rock that race downhill faster than anyone can run, are among the deadliest volcanic hazards. They buried Pompeii and Herculaneum when Vesuvius erupted in 79 CE. Pliny the Younger watched from across the Bay of Naples and wrote two letters describing a towering cloud spreading out at the top like the branches of a pine tree, along with the death of his uncle, Pliny the Elder, who had sailed toward the eruption to help. Volcanoes also give: they build fertile soils, new land and the geothermal heat that warms most homes in Iceland.

Worked example: Haiti and Chile, 2010

On 12 January 2010 a magnitude 7.0 earthquake struck close to Port-au-Prince, the crowded capital of Haiti. Estimates of the dead vary widely, from around 100,000 to more than 300,000. Six weeks later, a magnitude 8.8 earthquake struck off central Chile. It released roughly 500 times more energy, yet killed about 500 people. The difference was not in the ground but in the cities. Chile, shaken repeatedly since the 1960 quake, had strict building codes that were largely enforced, and much of its housing was designed to flex. In Haiti, poverty, weak oversight and heavy unreinforced concrete meant that buildings collapsed onto the people inside them. The magnitude told how much energy the fault released; the intensity of the human loss depended on choices made long before. The next level looks at that link between hazard and harm more closely.