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About 3,660 years ago a volcano in the middle of the Alaska Peninsula blew itself apart in one of the largest eruptions of the last ten thousand years, and fell into the hole it had emptied. The result is Aniakchak Crater, a caldera six and a half miles across and half a mile deep — and the heart of Aniakchak National Monument and Preserve, the least-visited unit of the National Park System.
The remotest park
| Size | about 603,000 acres of volcanoes, glaciated mountains and coast |
| Distance | about 120 miles from the Park Service's base at King Salmon, 400 from Anchorage |
| Visitors | fewer than 300 a year for the past decade |
| Getting in | usually by floatplane to Surprise Lake inside the caldera, then by raft down the Aniakchak River to the Pacific; increasingly by hiking up from Port Heiden and packrafting out |
The caldera
| Diameter | 10.5 km (6.6 miles) |
| Average depth | 610 m (2,000 ft) |
| Highest point | Aniakchak Peak, 1,340 m (4,400 ft) |
| Lowest point | the bed of Surprise Lake, 302 m (991 ft) above sea level; the lake is 19.5 m deep |
The eruption that made it, called Aniakchak II, rated 6 on the volcanic explosivity index. As a huge volume of magma poured out, the top of the volcano collapsed into the emptied chamber. It was the second of two possible caldera-forming eruptions here in the last 10,000 years.
- Its deposits. About 70 cubic kilometres of material: first a blanket of pumice, then a thick sheet of ignimbrite — rock from a ground-hugging flow of hot ash and gas — that reached the coasts on both sides of the peninsula and covers more than 2,500 square kilometres. Fine ash fell 1,000 km north on the Seward Peninsula.
- A tsunami. Where the flow ran into Bristol Bay it seems to have raised a tsunami: along the bay's north shore, layers of pumice sand lie in peat up to 12 metres above high tide, sometimes on top of ash that fell first.
Since the collapse
The caldera has kept erupting — most recently in 1931. Inside it are lava domes (Vulcan, West, Pumice and Bolshoi), cinder cones (Surprise, Windy and Breezy), Vent Mountain and Half Cone, two maars, and the lake. The lake once burst through the caldera wall in a great outburst flood, and in 2010 a maar lake broke out too.
Heat is still there. Fifteen warm springs, 18–22 °C, feed Surprise Lake along its northwest shore — iron- and soda-rich, low in oxygen, mildly acidic. The ground near Half Cone reaches 85 °C just 25 cm down. The largest spring gives off carbon dioxide and helium from magma, though not at harmful levels; the Geothermal Steam Act requires the features to be monitored.
Older rock and fossils
The young volcanic deposits lie on Tertiary volcanic, intrusive and sedimentary rock and on Late Mesozoic sediments, a 161-million-year record of volcanism and of the plate movements that assembled southern Alaska.
- Jurassic and Cretaceous layers hold marine fossils — Buchia clams and ammonites in the Naknek Formation — a little petrified wood with tree rings, and three hadrosaur tracks in the Chignik Formation.
- Tertiary layers hold well-preserved plant fossils; those of the Tolstoi Formation show the climate was once subtropical.
The Park Service's Geologic Resources Inventory has published geologic maps and a report for Aniakchak, free on the NPS DataStore. Fossils in parks are protected by the Paleontological Resources Preservation Act of 2009.
Sources
Written from "NPS Geodiversity Atlas—Aniakchak National Monument & Preserve, Alaska", National Park Service Geologic Resources Division, a work of the United States government in the public domain, which cites Miller and Smith (1987), Riehle and others (1987), Waythomas and Neal (1998) and others.
Лицензия: CC0 1.0 (общественное достояние) · По материалам www.nps.gov
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