Why the Ring of Fire Never Stops Shaking

Plot every strong earthquake on a world map, and something strange appears. The dots aren't scattered randomly. Most of them crowd into one giant horseshoe wrapped around the Pacific Ocean: up the coast of South America, through Central America and the US West Coast, across Alaska, and down through Japan, the Philippines, Indonesia and New Zealand.

That horseshoe is the Ring of Fire. It's about 25,000 miles (40,000 kilometers) long. Roughly nine out of ten of the world's earthquakes happen along it, and about three out of four of its active volcanoes.

Why a ring? Why there? And what does it mean for the parts of America that sit on it? Let's work through three hard questions.

Hard question 1Why do earthquakes and volcanoes line up in a ring?

Earth's outer shell is cracked into giant moving pieces called tectonic plates. Most earthquakes and volcanoes happen where plates meet, so the real question is: why are there so many plate edges around the Pacific?

The answer is that the Pacific Ocean is surrounded on almost every side by places where plates collide. And when an ocean plate collides with another plate, something specific happens: the heavier ocean plate dives underneath and sinks back down into Earth. Geologists call this subduction, and the deep trench where it begins can be miles deep.

That sinking plate explains both halves of the Ring of Fire.

The earthquakes: the two plates don't slide past each other smoothly. They grind and stick. Stress builds for years or centuries, then releases all at once.

The volcanoes: as the ocean plate sinks deeper, it heats up and releases water trapped in its rocks. That water rises into the hot rock above it and makes it easier to melt, the same way salt lowers the freezing point of ice. Melted rock, called magma, rises and builds a line of volcanoes parallel to the trench. That's why the volcanoes form chains along the coasts: the Andes, the Cascades, the Aleutians, Japan.

The takeaway: the Pacific is ringed by subduction zones. Sinking ocean plates cause the earthquakes and feed the volcanoes, so both trace the same ring.

Hard question 2Why do the biggest earthquakes strike there?

Plates also meet in the middle of oceans, along undersea mountain ranges where plates pull apart. Earthquakes happen there too. So why do the true giants, magnitude 9 and above, almost all strike along the Ring of Fire?

It comes down to how big a piece of fault can lock and then break.

Where plates pull apart, the rock is hot and thin. It breaks in small pieces, often, so stress never builds very high.

In a subduction zone, the two plates press together along an enormous slanted surface, hundreds of miles long and tens of miles deep. That whole surface can stay locked for centuries while the plates keep moving, storing energy like a bent spring. When it finally breaks, it can break all at once.

How much bigger is a bigger quake? Each step up the magnitude scale releases about 32 times more energy. Let's work it out:

The largest earthquake ever recorded struck Chile in 1960, at magnitude 9.5, on the Ring of Fire. The largest ever recorded in the United States struck Alaska in 1964, at magnitude 9.2, also on the Ring.

The takeaway: subduction zones lock over huge areas for centuries, and the bigger the area that breaks, the bigger the earthquake. That's why the giants cluster around the Pacific.

Hard question 3What's next for America's part of the ring?

The United States sits on the Ring of Fire in three places: Alaska and the Aleutian Islands, the volcanic Cascade Range of the Pacific Northwest, and the plate boundary along California.

Alaska has already shown what the Ring can do: the 1964 earthquake, magnitude 9.2. The Cascades have too: Mount St. Helens erupted in 1980 and buried the land around it in ash.

But the place geologists watch most closely is quieter, and that's exactly why. Off the coasts of northern California, Oregon, Washington and British Columbia lies the Cascadia Subduction Zone, where an ocean plate is sinking beneath North America.

No one alive has felt it break. But we know when it last did, thanks to a remarkable piece of detective work. On January 26, 1700, a tsunami struck the coast of Japan with no earthquake felt there to explain it. Japanese records preserved the date of this "orphan" tsunami. Centuries later, scientists matched it to evidence in the Pacific Northwest: coastal forests that suddenly dropped and drowned in seawater, leaving "ghost forests" of dead trees, and layers of sediment shaken loose on the seafloor. The orphan tsunami had a parent: a giant earthquake in Cascadia.

So what's next? Past gaps between Cascadia's big earthquakes have ranged from a few hundred years to more than a thousand. That makes exact predictions impossible. But scientists estimate the chances for the next 50 years: about 10 percent for a magnitude 9, and about 37 percent for a magnitude 8 to 8.4.

The takeaway: America's part of the ring is active, and the biggest risk is the long-quiet Cascadia zone. Its next big earthquake is a matter of when, not if, which is exactly why preparing makes sense.

Try it yourself

Open the USGS's free online earthquake map and set it to show the past 30 days. Watch the dots gather around the Pacific. Then, if you live near the West Coast or in Alaska, look up your local tsunami and earthquake hazard maps, and learn the three words that matter most when the ground shakes: drop, cover, and hold on.

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