The Ring of Fire

About nine in ten of the world's earthquakes happen along a single horseshoe. The Ring of Fire is the belt of subduction zones tracing the rim of the Pacific for some 40,000 kilometers — up the Andes and the American cordillera, out along the Aleutians, down through Kamchatka, Japan, the Philippines and Indonesia, and on to New Guinea, the Solomons, Tonga and New Zealand. Wherever oceanic crust dives beneath a neighboring plate it drags water down with it, lowering the melting point of the rock above and sending magma to the surface. That one mechanism explains roughly three-quarters of the planet's active volcanoes, its deepest trenches, and the tsunamis that have killed hundreds of thousands within living memory.

The ring is not a single fault but a chain of plate boundaries, and they behave differently. Off Chile and Alaska, great thrust faults lock for centuries and then rupture in the largest earthquakes ever recorded, up to magnitude 9.5. In the western Pacific, older and colder crust sinks steeply and carves trenches, including the Mariana Trench, the deepest place in any ocean. Volcanoes along the belt include Fuji, Pinatubo, Krakatoa, Mount St. Helens, Popocatepetl and, in the Andes, Ojos del Salado at 6,893 meters, the highest volcano on Earth. Hawaii sits inside the ring but is not part of it: those islands are a hotspot, a different mechanism entirely, which is worth knowing before drawing the line.

Hundreds of millions of people live on the ring, and many live there because of it: volcanic ash weathers into extraordinarily fertile soil, which is why Java, one of the most crowded places on the planet, farms a landscape of active cones. The trade-offs arrive periodically and brutally. The 2004 Indian Ocean tsunami killed some 230,000 people; the 2011 Tohoku earthquake shifted Japan's coastline and flooded Fukushima. Nations along the belt have become the world's laboratory for living with that risk — Japanese building codes, Chilean seismic design, Pacific tsunami warning networks — and the rest of the world borrows the lessons. Geothermal power, which supplies a large share of the electricity in New Zealand, the Philippines and Indonesia, is the belt's consolation prize.