Images

This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite.

Rhyodacite has more silica than dacite and less than rhyolite. This flow-banded rhyodacite is gorgeous in outcrop and is around a half million years old.

The cinder cone in the distance is Round Mountain, which is being actively quarried for road rock. Eruption episode 4 is the most recent in the Clear Lake Volcanic Field, and is characterized primarily by lava flows, cinder cones (like the one pictured), and maar deposits.

Eruption episode 2 rocks are higher in silica than those characterizing Episode 1, and instead of being primarily lava flows that filled existing valleys, Episode 2 rocks commonly erupted as volcanic domes. Cobb Mountain erupted during this episode, as did Boggs Mountain.

This view of Mt. Konocti from within the Clearlake Riviera neighborhood shows the topographic outline of the many discrete volcanic domes that comprise the mountain. Volcanic domes ooze from the ground to form steep-sided domes which can coalesce, forming mountains with multiple peaks. Mt.

The layered deposits shown in this picture were created by a maar eruption, when magma and volcanic gasses interact with groundwater to create an explosive eruption. Maar eruptions like these commonly generate craters, some of which can be seen along the western shore of Clear Lake. Soda Bay, near where this photo was taken, is a maar crater.

This lava flow of basaltic andesite composition erupted in the High Valley area. The blocky appearance of this flow top is termed 'A'ā (pronounced "ah-ah"), which is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. USGS photo by Dawnika Blatter

This outcropping of rocks along Morgan Valley Road is comprised of dacite, the most common composition in the Clear Lake Volcanic Field. Dacite contains more silica than basalt but less than rhyolite, so it’s commonly referred to as an “intermediate” composition.

A southwesterly view of Mount Konocti, which is one of the largest volcanic features in the Clear Lake Volcanic Field. Buckingham Peninsula can also be seen stretching from the base of Mount Konocti into the foreground toward the eastern shore of the lake.  USGS photo by J.Ball

These freshly excavated boulders along Spruce Grove Road are basalt, which is an iron and magnesium-rich rock that’s fairly uncommon in the Clear Lake Volcanic Field.

Eruption episode 3 is the most voluminous in the history of the Clear Lake Volcanic Field. Episode 3 rocks are of intermediate composition, primarily dacite, and make up many of the topographic high points in the field, including Mt. Konocti, where this photo is taken.

Cinder cones (or scoria cones) are relatively small, steep-sided volcanoes that are constructed around a single vent when lava is ejected into the air, breaks into smaller pieces, and builds up around the sides of the vent. This photo is looking across the quarry which exposes the interior of a cinder cone east of Clearlake Oaks in the Clear Lake Volcanic Field.

Located at the midpoint of Clear Lake in Lake County, California, Buckingham Peninsula shows evidence of young explosive volcanism in the Clear Lake Volcanic Field. Little Borax Lake, enclosed within the Buckingham golf course, is a water-filled maar crater, as is Horseshoe Bay to its northeast. USGS photo by J. Ball.

Mount Konocti as seen from the town of Clear Lake.

Thurston Lake and Mount Konocti, Clear Lake Volcanic Field, California.

Clear Lake Volcanic Field, California. Tuff deposit outcrop.

Mt. Konocti erupted during the most recent stage of volcanism (01. Ma to 10,000 years ago), and a future eruption from the same site would be devastating to the nearby inhabitants.

Mount Konocti, the most distinct volcanic feature of the Clear Lake Volcanics as viewed to the west from Cinder Cone.

Mount Konocti, a mostly dacitic composite cone, on the western shore of Clear Lake, California. Thurston Lake is in the foreground.