The Ring of Fire

About 81 percent of the world's largest earthquakes happen around the rim of the Pacific Ocean.

A horseshoe of trenches

Earthquakes and volcanoes are not scattered at random. Most of them sit on plate boundaries, and the busiest belt of all runs around the Pacific Ocean. USGS calls it the circum-Pacific seismic belt and notes its nickname, the Ring of Fire. It follows boundaries between several plates around the Pacific Ocean. NOAA describes it as a horseshoe nearly 40,250 km long, running from the southern tip of South America up the west coast of the Americas, across the Bering Strait, down through Japan and on to New Zealand, with more than 450 volcanoes along it.

Most of the ring is made of subduction zones, the convergent boundaries from two units ago. USGS describes volcanic arcs and ocean trenches partly encircling the Pacific basin, with each arc parallel to, and always on the landward side of, its trench. The Aleutian Islands are the arc of the Aleutian Trench, Japan and the Mariana Islands sit behind their trenches, and the Andes rise behind the Peru-Chile Trench. The ring is not all subduction, though. Along California the boundary is the San Andreas transform fault, and in the south-east Pacific the East Pacific Rise is spreading.

The numbers

USGS gives about 81 percent of the planet's largest earthquakes to the circum-Pacific belt, including the two largest ever recorded: the magnitude 9.5 Chilean earthquake of 1960 and the magnitude 9.2 Alaska earthquake of 1964. The next belt, the Alpide belt from Java and Sumatra through the Himalayas and the Mediterranean into the Atlantic, accounts for about 17 percent. It includes the magnitude 9.1 earthquake off Sumatra in 2004, whose tsunami killed more than 230,000 people. The third belt follows the Mid-Atlantic Ridge. For volcanoes, USGS counts about 1,350 potentially active volcanoes worldwide, not counting the continuous volcanic belts along the spreading ridges of the ocean floor; about 500 of which have erupted in historical time, and many of them stand on the Pacific Rim.

Why the largest earthquakes happen here

The size of an earthquake depends a great deal on the area of fault that slips at once. At a subduction zone the contact between the two plates is a gently sloping surface that can be hundreds of kilometres long and more than a hundred kilometres wide, and long stretches of it can stay locked for centuries. When such a stretch finally slips, the result is a megathrust earthquake. NOAA notes that most great earthquakes, those of magnitude 8 or more, are megathrust earthquakes at convergent boundaries, and when the sea floor above the fault is pushed up suddenly, the water above it is lifted too and can start a tsunami.

Not every volcano near the Pacific belongs to the ring. Hawaii lies in the middle of the Pacific Plate, more than 3,200 km from the nearest plate boundary, and it is fed by a hotspot instead. That difference is a good test of the idea. Ring volcanoes form lines that follow trenches, and their rocks and eruptions match subduction. Hawaii forms a line that follows the direction the plate moves, with ages that grow older away from the active end. The Volcanoes course looks at hotspots in detail.