Images

Schematic illustrations of the formation of inverted topography. Top graphic: Lava flows travel in topographic lows and valleys below surrounding topographic highs. Middle graphic: The hard volcanic rock protects the underlying country rock from erosion, eroding more slowly than the surrounding topography.

This lava flow is one of five flows of the same composition erupted over a short period of time in the High Valley area. The Round Mountain cinder cone in this area is related to these flows, and all are basaltic andesite, a rock with slightly more silica than basalt and slightly less than andesite.

The rhyolite outcrop shown here is found along Bottle Rock Rd., which gets its name from the volcanic glass that’s common within this rock unit. Rhyolite is the most silica-rich rock in the Clear Lake Volcanic Field, and Thurston Creek Rhyolite, show here, is around half a million years old.

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

Kyle R. Anderson is a Volcano Geophysicist who works at the USGS California Volcano Observatory.

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.

Phil Dawson, a Volcano Seismologist with the California Volcano Observatory, stands next to a volcanic rock on the flank of Mount Shasta, California.

Joshua Crozier is a Mendenhall Postdoctoral Fellow working at the USGS California Volcano Observatory

Devils Kitchen in the Lassen Volcanic Region (within the bounds of the Lassen Volcanic National Park) is the second largest of Lassen's hydrothermal areas, after Bumpass Hell. Located in the Warner Valley in the southeast corner of the Park, Devils Kitchen contains boiling springs, mudpots, hot streams, and steam vents. USGS photo courtesy of Deb Bergfeld

Eruption episode 1 contains the oldest rocks in the Clear Lake Volcanic Field. The rock on the ridge in the background., located off Butts Canyon Road, is a lava flow that erupted around 2 million years ago.

Morro Rock is the westernmost peak in a chain of hills known locally as the “Morros” or the “Nine Sisters.”. It is a volcanic neck or plug composed of dacite which crystallized in the vent of a volcano and was later exposed by erosion. USGS photo by J. Ball

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

Ash-flow tuff (ignimbrite) on the fire road leading to the summit of Mount St. Helena, mapped as part of the intracaldera Sonoma volcanics. 

The Moraga Volcanics, one of several volcanic formations found in the Berkeley Hills Volcanic Field, frame a view of Mount Diablo to the east. The basaltic lava flows and their associated lapilli tuffs can be found in the Sibley Volcanic Regional Preserve. 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.