Eruptions, explosions, magma help form volcano craters

  • A plume of ash rises from the Pu'u 'O'o crater on Hawaii's Kilauea Volcano, in Hawaii Volcanoes National Park.

    A plume of ash rises from the Pu'u 'O'o crater on Hawaii's Kilauea Volcano, in Hawaii Volcanoes National Park. U.S. Geological Survey, 2018

Updated 6/10/2019 6:33 AM

"Why do volcanoes have cool craters?" asked a young patron at the Grayslake Area Public Library.

Most of the 150-plus volcanoes in the U.S. are along the Pacific coast where there's energetic earthquake and volcano activity.


Only last year, Kilauea, the most recent volcano to launch clouds and ash in the U.S., splashed fiery lava across Hawaiian neighborhoods and oozed boiling hot rocks and ash into the ocean.

When a volcano erupts, it's been agitated by moving tectonic plates that propel magma, lava, ash and rocks from deep inside the Earth through the magma chamber, spraying or trickling debris out of the volcano's vents.

Kilauea is the world's biggest shield volcano rising 4,000 feet above ground and extending below the ocean more than two miles. Hawaiian legend places the goddess Pele in the crater, the bowl at the top of the volcano. The crater sits in a caldera, an even larger bowl caused by the collapse of the magma chamber that funnels the lava explosion.

Kilauea and nearby Mauna Loa, the world's oldest volcano, share the same magma hot spot, and at times eruptions from one have acted like a pressure release valve for the other. As shield volcanoes, they gently rise from the Earth's surface, sloping up to cone-like peaks.

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There are five major types of volcanoes -- cinder cone, composite, lava dome, stratovolcano and shield. These terms refer to the ways the layers of magma either drip or rocket from the inner Earth and then harden to create the various volcano shapes.

A 6-mile-wide caldera is most of what's left of Mount Mazama, a steep-sloped stratovolcano that erupted more than 6,000 years ago, covering Oregon, Washington, Idaho and parts of Canada in its explosions. The eruptions were so explosive that the mountain was destroyed, and all that remains is a very deep blue lake at Crater Lake National Park dotted by a small island formed from the volcano cone.

Mt. St. Helens in Washington is a cinder cone volcano that once reached almost 10,000 feet in height. The volcano lost 1,300 feet in a violent eruption and landslide nearly 40 years ago. Mt. St. Helens now is crowned by a two-mile-wide horseshoe-shaped crater. Ash from the explosion sprinkled hundreds of miles across the U.S., reaching the northeastern states. Like many volcanoes, the eruptions were not solo but occurred over months or even years. More recent ash and smoke plumes pushing from Mt. St. Helen's have built a lava dome inside the crater.

More than 90 percent of continents and ocean basins we live on have been crafted by volcanic explosions. Not all that comes from volcanoes is destruction. The ash and rocks are full of minerals and make excellent agricultural soil.

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