Volcanoes of Southcentral Alaska

Holocene Volcanic Activity in Alaska

Most volcanic activity in Alaska is associated with the subduction of the Pacific Plate under the continent. A little bit of activity occurs in other parts of the state. A volcano erupts almost every year in the Aleutian Islands.

Southcentral Alaska is a geologically active area, not only with earthquakes, but also with volcanic activity. The volcanoes of Southcentral Alaska occur in two regions. In the west are the Cook Inlet volcanoes, and in the east are the volcanoes of the Wrangell Mountains. Curiously, there is a 275-mile gap between these two volcanically active regions, which has not been recently volcanically active. Geologists have not yet explained this gap.

Most Alaskan volcanoes are a result of the subduction of the Pacific Plate underneath the North American Plate. When the relatively heavy oceanic plate descends deeply enough underneath the lighter continental rock, it begins to melt. As it melts, the material decreases in density and flows upwards as a great plume of magma, melting other rocks on its way up through joints and fissures. Occasionally these plumes of magma reach the surface and explode as great amounts of pressure are released. Ash and lava from these eruptions accumulate to form volcanoes.

Cook Inlet Volcanoes

The volcanoes of Cook Inlet are the easternmost part of the Aleutian volcanic arc. While the angle of subduction in the Aleutian Trench is relatively steep along the Aleutian Islands, the angle decreases to the north and east. Because of this, the Pacific Plate must travel a long distance underneath the North American continent before it melts. The result is that the Cook Inlet volcanoes are more than 200 miles from the subduction zone. Some of the material that makes up these volcanoes has traveled beneath the entire Kenai Peninsula and Cook Inlet before rising again to the surface.

Other volcanoes can be formed in spreading centers or over hot spots, neither of which occur in Alaska. Spreading centers are rift zones, where two plates are spreading apart such as in East Africa, Iceland, or some extinct volcanoes in the southwestern states. Hot spots occur over superheated areas of the mantle, which melt the crust as it slides over them, such as in the Hawaiian Islands or at Yellowstone National Park.

Cook Inlet Volcanoes

The Cook Inlet volcanoes are a result of subduction in the Aleutian Trench, to the west of the Kenai Peninsula. The subducting Pacific Plate reaches its melting point under the Alaska Range. The molten material rises to create these active volcanoes.

The volcanoes from north to south are Hayes Volcano (not to be confused with Mount Hayes, in the northern Alaska Range), Mount Spurr, Mount Redoubt, Mount Iliamna and Mount Augustine. Hayes Volcano was largely destroyed in its last eruption about 3500 years ago and is difficult to detect, even from the air. The rest are clearly visible from the populated east side of Cook Inlet. Mount Spurr lies due west of Anchorage at the south end of the Tordrillo Range. Mount Redoubt is due west of Soldotna while Mount Iliamna is due west of Ninilchik. They are both in the Chigmit Mountains. The towering, somewhat conical shapes of Mts. Redoubt and Iliamna make them easy to discern from the surrounding non-volcanic mountains. Mount Augustine is an almost perfect cone-shaped island easily seen from Homer. A good mnemonic for these volcanoes from south to north is ‘AIRS’.

The volcanoes are geologically young: all are less than 2 million years old, and Mount Augustine, the youngest, is no more than 19,000 years old. This is in contrast to the underlying granite batholith on which they sit, which dates from the Jurassic Period, more than 120 million years ago.

All of the volcanoes in the Cook Inlet region, with the exception of Hayes, are active. Mount Augustine is the most active, having erupted five times this century, most recently in 1986. Mount Redoubt erupted in 1989 and the Crater Peak vent of Mount Spurr in 1992. Only Iliamna has not had an historic eruption, but nearby volcanic mudflows that are likely associated with eruptions have been dated to less than 300 years old and strong seismic activity was recorded beneath the volcano in 1996.

Most of the damage from these volcanic eruptions is local devastation from superheated pyroclastic flows of volcanic rock, ash and gases that rush down the flanks of the volcanoes into the surrounding valleys. These flows are often compounded by water from the glaciers that they melt on the way down. Massive debris flows and flooding result in the rivers below. Due to the explosive nature of the eruptions, not much actual lava is produced.

The population centers in the Cook Inlet region are usually subjected to ashfall, which can disrupt oil production and transportation for the whole region. The 1989 eruption of Mount Redoubt almost turned deadly when a 747 airplane flew through the ash cloud and lost power in all four engines. The plane fell several miles out of the sky before the pilots managed to restart the engines and made a safe landing in Anchorage. This near tragic event has particular significance to the author, since his dear grandmother happened to be on that very plane.

Volcano	Height (ft)	Last Eruption
Hayes Volcano	5543	1500 BC
Mount Iliamna	10,014	1700+
Mount Redoubt	10,194	1989
Mount Augustine	4130	1986
Mount Spurr	11,066	1992

The Wrangell Volcanoes

The Wrangell Volcanoes all lie to the east of Glennallen. From Glennallen 12,000-foot high Mount Drum looms large over the Copper River Valley. Behind it and to the north lies Mount Sanford at 16,000 feet. Capital Mountain is the peak just north of Mount Sanford and about half of its height. Behind Mount Drum and to the south is 13,000-foot high dome of Mount Wrangell. Tanada Peak, Mount Jarvis and Mount Gordon are visible from the Nabesna Road and Mount Blackburn lies along the Edgerton Highway. Mount Churchill and Mount Bona lie east of McCarthy, near the Canadian border.

Wrangell Volcanoes

The Wrangell volcanoes are presently not very active. They were created as the Yakutat Terrane subducted underneath Alaska. The mountains are enormous, and spectacular volcanic features can be seen throughout the region.

The Wrangell volcanoes are considerably different from the Cook Inlet volcanoes. They originated with the subduction of the Yakutat Terrane underneath Alaska, but since subduction is apparently not occurring now, modern volcanism in the region is not fully understood. The Wrangell volcanoes are not nearly as active as those in the Cook Inlet region. The most recent eruption was from the vicinity of Mount Churchill about 1500 years ago. This eruption was large enough to deposit more than an inch of ash as far away as the Northwest Territories in Canada, more than 400 miles from the source. Any modern activity seems to have been limited to steam vents and very minor eruptions within the crater. The Wrangell Volcanoes are also the oldest active volcanoes in Alaska, dating to 5 or 10 million years ago in the Miocene Epoch. This is still very young compared to the Paleozoic Wrangellia rocks upon which they lie.

The nature of volcanic activity in the Wrangell Mountains was different than that in Cook Inlet as well. While powerful, explosive eruptions have occurred in the past, the more recent eruptions in the Wrangells were gentler, characterized by fluid lava emerging from their craters. This lava cooled slowly and flowed down the mountain slopes to build up dome-shaped volcanoes. Often so much lava poured out that it could flow 30 or more miles across the landscape. In some places this lava accumulated more than 300 feet thick. Some of this lava lies high on the slopes of the Saint Elias Mountains to the south, which at the time of the eruptions were still a coastal plain that had not yet been uplifted.

Volcano	Height (ft)	Last Eruption
Mount Blackburn	16,387	3-4 mya
Mount Bona	16,416	holocene?
Capital Mountain	7728	1 mya
Mount Churchill	15,639	holocene
Mount Drum	12,005	200,000 ya
Mount Gordon	9036	pleistocene
Mount Jarvis	13,418	1-2 mya
Mount Sanford	16,236	100,000 ya
Tanada Peak	9236	1-2 mya
Mount Wrangell	14,160	50,000+ ya