Look at the Pacific Ocean from space, and you can trace a line around almost the whole basin: down from Alaska, through Japan and the Philippines, past New Zealand, and back up the coasts of Chile, Mexico and the Pacific Northwest. Together they form a horseshoe roughly 40,000 kilometers long.
0:18Along that line sit most of the world's active volcanoes, by common estimates about three quarters, and roughly 90% of its earthquakes. Geologists call it the Ring of Fire.
0:29In this episode, we'll find out why the Ring exists, and what it has done. We'll look at the largest earthquake ever measured, which struck on the Ring, at the eruption that cooled the entire planet, and at one stretch of it that is quiet for now, but far from finished.
0:47Why a Ring?
The explanation begins with plate tectonics. The floor of the Pacific is mostly one enormous slab, the Pacific plate, with smaller plates along its edges, such as the Nazca, Cocos, Juan de Fuca and Philippine Sea plates. All of them are topped by oceanic crust, made largely of a dark volcanic rock called basalt, which is denser than the rock of the continents.
1:03When a dense ocean plate meets a continent, or a younger and lighter ocean plate, it bends and slides down beneath it, back into the mantle. Geologists call this process subduction. Off the coast of Chile, for example, the Nazca plate is sinking beneath South America at about 7 centimeters a year.
1:22Where a plate bends down, the seafloor sags into a long, deep trench. The deepest water on Earth, in the Mariana Trench, lies about 11 kilometers down, and it formed in exactly this way. In that sense, the Ring of Fire is really a ring of trenches.
1:35The fire comes from water. The sinking plate carries seawater down with it, bound up in its minerals. At a depth of about 100 kilometers, that water is released into the hot rock above, and it lowers the rock's melting point. Part of the rock melts, the molten rock rises, and a chain of volcanoes grows about 100 to 300 kilometers inland from the trench.
1:55Every arc of the Ring is built by the same process: a trench offshore, where the ocean floor descends, and a line of volcanoes behind it, where melted rock reaches the surface. Between them, earthquakes are common, because the two plates do not slide past each other smoothly. They lock together, store up strain for decades or centuries, and then slip all at once.
2:13The best known exception is California. From the Gulf of California north to Cape Mendocino, the plates slide past each other sideways, on the San Andreas fault system, instead of one sinking beneath the other. That is why this stretch of coast has large earthquakes, but no volcanic arc. North of Cape Mendocino, subduction begins again, and so do the volcanoes, such as Mount Shasta and Lassen Peak. Our episode on the Loma Prieta earthquake tells the story of the San Andreas.
Southern Chile, on May 22, 1960, just after three in the afternoon. The Nazca plate had been locked against South America for more than a century, and that afternoon, the fault between them gave way.
2:47The rupture tore along the coast for about 1,000 kilometers, from just south of Concepción, past Valdivia and the island of Chiloé, almost to the Taitao Peninsula, and by many accounts the shaking lasted about ten minutes. Seismographs around the world recorded it. Its magnitude, later calculated at 9.5, makes it the largest earthquake ever measured.
3:07In Valdivia, the ground itself moved. Along parts of the coast, the land sank by 2 meters or more, and the sea flowed in permanently, turning farmland into marsh and leaving streets that now end in water.
3:21The tsunami came next. Offshore, the seafloor had lurched upward, lifting the entire column of water above it. Within minutes, the sea surged ashore along the Chilean coast, and in one place it reached 25 meters above sea level.
3:34The wave did not stop there. In the open ocean, a tsunami travels about as fast as a jet airliner. Fifteen hours after the earthquake, it reached Hawaii, and 22 hours after it, Japan, some 17,000 kilometers from Chile.
3:51In Hilo, on the island of Hawaii, warning sirens had sounded hours before the wave arrived, but many people went down to the waterfront to watch. The third wave came in at about 35 feet, and 61 people died.
4:04Japan had no warning at all, and there the wave killed about 140 people. In total, the earthquake and its tsunami killed an estimated 1,600 or more people in Chile, and more than 200 others across the Pacific.
4:18The Ring then showed the other half of its nature. Thirty-eight hours after the earthquake, Cordón Caulle, a volcano in the Andes, began to erupt, most likely because the shaking had disturbed the magma system beneath it. Within two days, southern Chile had seen the whole process at work: the trench, the earthquake, and the volcano.
4:38The Volcanoes
That brings us to the volcanoes themselves. Twenty years after the Chilean earthquake, the Ring erupted in the lower 48 states.
4:46On May 18, 1980, at 8:32 in the morning, a magnitude 5.1 earthquake shook Mount St. Helens, in Washington State. The entire north face of the mountain slid away, in the largest landslide ever recorded, and the volcano behind it exploded sideways.
5:04The blast flattened forests up to 17 miles away, ash fell across 11 states, and the mountain lost 1,300 feet of its summit. Fifty-seven people died. Among them was a geologist, David Johnston, who was watching from a ridge about 6 miles away. His last words on the radio were: Vancouver, Vancouver, this is it.
5:27Scientists had been watching the mountain for two months, and the authorities had closed the area around it. Without those closures, the death toll would almost certainly have been far higher. The lesson applies to the whole Ring: volcanoes usually give warning signs, if someone is there to read them.
5:42Eleven years later, that lesson saved thousands of lives. Mount Pinatubo, in the Philippines, had been quiet for about 500 years when, in the spring of 1991, it began to stir.
5:55Scientists from the Philippines and the U.S. Geological Survey recognized the warning signs and persuaded the authorities to evacuate. On June 15, Pinatubo produced the second largest eruption of the twentieth century, with an ash column that rose 35 kilometers into the stratosphere.
6:14Hundreds of people died when rain-soaked ash collapsed their roofs, and hundreds more in the months that followed, from mudflows and from disease in crowded evacuation camps. Yet more than 60,000 people had been moved out of danger in time, and the evacuation is still studied as a model of how to manage a volcanic crisis.
6:32Pinatubo also affected the entire planet. It injected about 20 million tons of sulfur dioxide into the stratosphere, where it formed a thin haze of droplets that spread around the Earth and reflected sunlight back into space. Over the following two years, the average temperature of the globe fell by about half a degree Celsius.
6:54The Next One
Now consider the stretch of the Ring between Northern California and Vancouver Island, known as the Cascadia subduction zone. Like the rest of the Ring, it has a subduction zone offshore, its trench buried under sediment, and a line of volcanoes inland, yet in two centuries of written records it has produced no great earthquake. That silence is exactly what worries geologists.
7:11For a long time, nobody knew that Cascadia could produce a great earthquake. Then, in the 1980s, geologists found ghost forests along the Washington coast: stands of dead cedar, killed when the land suddenly dropped and salt water reached their roots. Tree rings later dated their deaths to the winter of 1699 to 1700.
7:34The decisive evidence came from across the ocean. Japanese records describe a tsunami that arrived on the night of January 27, 1700, although no earthquake had been felt in Japan. Scientists call it an orphan tsunami. By working the travel time backwards, scientists traced it to a magnitude 9 earthquake off Cascadia, at about nine in the evening on January 26.
7:59That earthquake struck more than three centuries ago, and the geologic record shows that great Cascadia earthquakes recur irregularly, about every 500 years on average along its whole length, and more often in the south. The next one will shake Seattle, Portland and Vancouver, and its tsunami will reach parts of the coast in as little as 15 minutes. That is why the region holds earthquake drills, and why tsunami sirens stand along the Oregon coast.
8:22Still Moving
The Ring of Fire is never quiet for long. Hundreds of active volcanoes stand along it. Japan alone feels around 2,000 earthquakes in a typical year, and the Pacific plate continues to move, 7 to 10 centimeters a year, into the trenches around its rim.
8:40So, three things to remember. First, the Ring of Fire marks the places where the Pacific's ocean plates sink beneath their neighbors, at the deep trenches around its edge. Second, that sinking causes most of the world's great earthquakes, including the largest ever measured, in Chile in 1960. And third, the water those plates carry down lets the rock above them melt, and that builds the volcanoes that line the Pacific.
8:54All along those 40,000 kilometers of coastline, the floor of the Pacific is slowly sinking back into the Earth, and the Ring will go on reshaping the lands around it.
