Geology and History

The Geysers-Clear Lake area has been one of the most productive in the United States for mercury, and gold was mined in the late 1800s. Many of the deposits are directly associated with outcrops of early Clear Lake volcanic rocks.

The Clear Lake Volcanics are the northernmost and youngest of several volcanic fields in the California Coast Ranges, which grow progressively older to the south. Clear Lake rocks date from 2.1 Ma to about 8.5 ka and have an estimated erupted volume of 100 square kilometers. Silicic lavas are the dominant type, with the most voluminous rock type in the volcanic field being rhyodacite, but basaltic...

Despite the name of the steam field, no natural geysers exist anywhere in the Geysers-Clear Lake area. The name was suggested to early explorers by the fumarolic activity and steam rising from hot springs in what later became the earliest developed part of the steam field.

Mount Konocti is a 1,312 m (4,305 ft) mountain that dominates the view of the eastern shore of Clear Lake.

The complex eruptive history over the past 2 million years and the Holocene age of the youngest eruptions indicate that the Clear Lake magmatic system is not extinct and that future eruptions are possible.

The Clear Lake Volcanics erupted during four periods of time beginning at about 2 million years ago. There is a general decrease in age northward from 2 million years in the south to about 10,000 years in the north. Geophysical data suggests there is currently a spherical to cylindrical magma chamber about 14 km (8.7 mi) diameter and about 7 km (4.3 mi) from the surface. Seismic studies indicate...