Explore Geology
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National Monument & Preserve


cover of park brochure

park geology subheading
Photo of landscape at Aniakchak Nationa Monument & Preserve
Aniakchak National Monument & Preserve, Alaska

Aniakchak is part of "the Ring of Fire". In this active volcanic area along the pacific Ocean rim, volcanoes occur as the Pacific plate is thrust, or subducted, beneath continental plates. Very explosive, these subduction volcanoes produce large volumes of ash. They build island arcs and high mountain chains. Aniakchak is cousin to America's Katmai, Redoubt, and Mount Saint Helens, and to Japan's Fuji, and Indonesia's Krakatau.

Remote and Recent Volcanic Wildlands
Midway down the wild and roadless Alaska Peninsula lies one of the nation's most fascinating recent volcanic features. Aniakchak is a 6-mile-wide, 2,000-foot-deep caldera formed by the collapse of a 7,000-foot mountain. Lying inland in a region of frequent clouds and stormy weather, Aniakchak remained unknown to all but native inhabitants until the 1920s. Then, geographers remotely plotting mountain locations along the caldera's rim noticed their circular configuration. Eventually, in 1922, a geologic field party gazed down into the caldera. They brought back news of Aniakchak's immense proportions.

Although a dozen calderas stand on the Alaska Peninsula, Aniakchak ranks among the largest. Its fascinating volcanic history can be read from its exposed internal plumbing. About 3,500 years ago, a dramatic explosion caused the loss of some 3,000 feet of the upper mountain. The remainder of the mountain next collapsed, leaving a relatively flat-floored, ash-filled bowl. Since the caldera first formed, many lesser eruptions have created the small cinder cones, lava flows, and explosion pits dotting its floor today. Because some explosion features seem to have been created underwater, the caldera probably once contained a deep snow-fed lake, much like Crater Lake in southern Oregon. Eventually, however, at a low place along the rim, lake waters began to spill out. Over time, this fast-flowing stream created the great breach in the rim known as The Gates. The Gates now allow the Aniakchak River to begin its tumultuous 27-mile course southeastward to the Pacific Ocean.

Aniakchak's most recent volcanic activity came in 1931. A small but impressive explosion pit was added to the pockmarked caldera floor that year. Many thousands of tons of ash lay strewn within the caldera and scattered up to 40 miles away over the small villages. Fortunately, this volcanic episode was documented both before and after by an indomitable geologist and Jesuit priest, Father Bernard Hubbard. His photographs and descriptions provide an important benchmark for judging the likely rate of recovery of vegetation to the devastated caldera. Mosses, grasses, and more complex flowering plants have invaded sheltered spots. Brown bear and caribou have returned. Spawning runs of sockeye salmon now fight their way up the Aniakchak River and into Surprise Lake, the river's shallow headwater lake inside the caldera.

In creating Aniakchak National Monument and Preserve, Congress recognized the unique geological significance of the caldera and also acknowledged the outstanding wildlife and recreational values of the Aniakchak River by designating it a wild river within the National Wild and Scenic Rivers System.

Exploring the Caldera
Prehistoric and historic peoples hunted, trapped, and fished the Aniakchak area, but the caldera's modern exploration began only 70 years ago with geologists in the 1920s. The area's most dramatic visits were those of "the Glacier Priest", Father Bernard Hubbard. He was flown into the caldera itself in 1930, where he witnessed "a world within a mountain" and called it Paradise Found.

A year later Aniakchak erupted and Hubbard returned, this time hiking some 30 miles from a base camp on Kujulik Bay with three companions. En route across the coastal-barrier mountains and Aniakchak River valley their footsteps sent up clouds of ash that made their hair like wire brushes. Desert-like dust whirlwinds everywhere raised miniature tornadoes. They made 30 miles to the volcano rim the first day and climbed it the next morning.

"Black were its snowfields and black its glaciered head... Climbing to the crater rim,
we were going through a valley of death in which not a blade of grass or a flower or a
bunch of moss broke through the thick covering of deposited ash. Black cinders clinked
under our feet and slid away. It was like walking on wheat in a huge bin, and equally

At the rim, they "were to see Paradise Lost after having lived in Paradise Found the previous year."

The reaction was silence. "There was the new Aniakchak, but it was the abomination of desolation, it was the prelude of hell. Black walls, black floor, black water, deep black holes and black vents; it fairly agonized the eye to look at it."

A new sight since 1930 greeted them: "There was a huge black crater built out from the wall, and from its black maw yellow and brown gases were pouring, and clouds of escaping steam." Here, too dust whirlwinds danced like mad ghosts, and deadly gases and smoke created a "vision of hell in Aniakchak."

The only familiar sight was those hot springs whose flow sufficed to wash away the ashfall. In an explosion pit, like a huge paint pot, "Yellow sulphurs seethed and boiled around the edge of broken blocks of red lava... Colored fumes too heavy to rise rolled about like waves on a stormy sea."

Hubbard's party stuck a glass tube in the ground and lowered a thermometer eight inches. "It burst in three minutes at 200 degrees centigrade!"

They verified a new theory that chlorine gas can be found in volcanoes. They witnessed a bird being overcome by lethal gases. Lava bombs had made living room-sized craters on the caldera floor. The party climbed to the new subcrater's steaming rim: "We stood awe stricken on the edge," wrote the priest, "looking, like Dantes, into a real inferno."

park maps subheading



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A geology photo album for this park can be found here.


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Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6



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NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

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updated on 01/04/2005  I   http://nature.nps.gov/geology/parks/ania/index.cfm   I  Email: Webmaster
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