How volcanoes work

Kilauea's dramatic eruption has shaken Hawaii's Big Island. Could other U.S. volcanoes blow? Here's everything you need to know:

What's happening in Hawaii?

Kilauea, one of the most active volcanoes in the world, is in the midst of the most destructive eruption in its recent history. More than 2,000 people living along the volcano's eastern slope have been forced to flee their homes since early May, when a series of fissures belching lava and noxious gases began opening in the earth. Lava flows have destroyed more than 600 buildings so far, boiled away Hawaii's largest freshwater lake, and filled in Kapoho Bay. Toxic clouds of lava haze, or "laze" — a combination of steam, hydrochloric acid, and microscopic fragments of volcanic glass — have formed where lava is pouring into the ocean. Plumes of ash have risen more than 30,000 feet in the air. There have been no deaths so far, and one serious injury. Depending on how much magma (underground lava) has accumulated beneath Kilauea's surface, the eruption "could last days, weeks, years," said U.S. National Oceanic and Atmospheric Administration volcanologist Bill Chadwick. "We really can't peer through the ground and see it exactly in all its details and intricacies."

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Why is Kilauea erupting now?

The volcano has been steadily oozing lava since 1983, and erupting off and on since it was born between 300,000 and 600,000 years ago. It's the youngest of the five volcanoes that created the Big Island through successive lava flows. All the Hawaiian Islands, in fact, are peaks of undersea mountains located over a "hot spot" in the Earth's partially molten mantle. As the Pacific tectonic plate slowly slides over the hot spot, magma flows through cracks to the surface and new volcanoes are formed, creating a chain of islands. Iceland and the Galápagos Islands were also formed this way. The internal plumbing of volcanoes isn't well understood. But the current eruptions were probably caused by a new injection of magma from deep in the mantle, causing pressure to build inside and around Kilauea until the system burst like a pipe.

What other volcanoes are active?

There are 169 potentially active volcanoes in the U.S., according the United States Geological Survey. The USGS defines as "potentially active" those that have erupted within the last 10,000 years. Outside of Hawaii, almost all of these volcanoes are scattered throughout the American West and Alaska as part of the "Ring of Fire," a string of volcanoes circling the Pacific Ocean along the boundary of the Pacific plate.

Are they dangerous?

Potentially. About 50 volcanoes located in six states — Hawaii, Alaska, California, Oregon, Washington, and Wyoming — are rated high priority or highest priority by the USGS for monitoring for possible eruptions. They include Mount Rainier and Mount St. Helens in Washington, Mount Hood in Oregon, and Lassen Peak and Mount Shasta in California. Mount St. Helens is considered the most likely volcano in the 48 contiguous states to erupt again. Fifty-seven people were killed during its last explosive eruption, in 1980. Many of these volcanoes could become far more dangerous than Kilauea.

Why is that?

They're more explosive. Oceanic plates are denser than continental plates, and create less gaseous magma when they melt. The result is the kind of red, runny lava that Hawaiian volcanoes are famous for. Although Kilauea can explode if enough pressure builds up underneath the surface — for example, from molten rock turning groundwater to steam — most of the time lava just seeps out in what's known as an effusive eruption. These relatively gentle flows give the volcano the convex shape of a warrior's shield sitting on the ground, which is why they're called shield volcanoes. In the Ring of Fire, continental plates rich in silica create a stickier, slower moving magma that degasses explosively when it escapes, creating stratovolcanoes with tall, steep sides of hardened ash and volcanic rock. Mount St. Helens was one such volcano; when it erupted, so much energy was released that it blew off the top 1,300 feet of the mountain.

Can scientists predict eruptions?

With the right tools, researchers can forecast the likelihood of an impending eruption — but only several days to several weeks before it happens. They do this by measuring seismic activity created by magma moving up through the Earth, as well as by checking for temperature changes and the presence of volcanic gases. Only half of the country's active volcanoes are equipped with in-ground tools to measure for changes. Spurred by the eruptions in Hawaii, the U.S. Senate recently passed a bill that would authorize $55 million over the next five years to create a "volcano watch office" that would provide round-the-clock monitoring of the nation's active volcanoes, although the measure still has to pass the House. But even the best monitoring won't reveal all of a volcano's secrets. Mount Rainier has been dormant since it last erupted 1,100 years ago, but is under constant surveillance due to its proximity to Seattle and Tacoma. "You can have a volcano that's super well-monitored, but if nothing's happening, you're not getting the opportunity to understand how it works," said Natalia Deligne, a volcano hazard and risk modeler in New Zealand. "It's always the quiet ones, you know?"

How volcanoes kill

Of the more than 278,000 people who have died in volcanic eruptions since 1500, fewer than 1,000 were killed by flowing lava. The biggest danger is from pyroclastic flows — clouds of hot gas, ash, rocks, and other volcanic material that rush from the summit at speeds of more than 50 miles per hour. Temperatures inside this cloud can reach 1,300 degrees. It was a pyroclastic flow that killed an estimated 16,000 people after the eruption of Mount Vesuvius in A.D. 79, burying the Roman cities of Pompeii and Herculaneum. More than 110 people were killed and some 200 still missing after a pyroclastic flow buried a village following an eruption in Guatemala several weeks ago. Tsunamis triggered by eruptions are the second-biggest cause of volcano-related deaths. "Hawaiian volcanoes can be extremely deadly," says geophysicist Mika McKinnon. "But it's a hazard you can walk away from."