Mount Chiginagak, Stratovolcano in Lake and Peninsula Borough, United States

Mount Chiginagak is a stratovolcano on the Alaska Peninsula, about 15 km northwest of Chiginagak Bay. Its summit area holds a 400 m wide, 105 m deep acid crater lake that formed between November 2004 and May 2005. Fumaroles vent at 1,600 m on the northeast flank, and hot-spring travertine deposits lie at the northwest base near Volcano Creek.

Mount Chiginagak's most recent products are an unglaciated lava flow and an overlying pyroclastic-flow deposit extending east from the summit, probably from a lava dome at 1,687 m on the southeast flank. Brief ash eruptions were reported in July 1971 and August 1998. The volcano lies about 600 km from Anchorage and is not permanently monitored.

The volcano lies about 600 km from Anchorage, so no permanent monitoring network is installed on it; the 2005 acid flood was reconstructed afterwards from satellite images and field evidence. About 15 other volcanoes worldwide are known to hold acid crater lakes, among them Poás and Rincón de la Vieja in Costa Rica.

The volcano lies about 600 km from Anchorage and is not permanently monitored, so check conditions before any approach. The summit crater lake drains through an outlet cave into Indecision Creek, which carries a strong sulphur smell, yellow colour and pH of 1.3; avoid its water.

A crater lake formed between 2004 and 2005 from snowmelt and rainwater, eventually reaching about 400 meters across with highly acidic water. When this water drained away, it significantly altered local salmon migration patterns, revealing how volcanic activity directly affects wildlife below.