Hi All. There are some suggestions that the past volcanic activity in and around Clear Lake, California is not really understood, or at the very least not agreed upon. Take Care…Don in creepy town

“Some researchers (e.g. Christiansen and Lipman, 1972) have suggested that Sutter Buttes and the Sonoma and Clear Lake volcanics, south and southwest of Lassen, are older extensions of subduction-related Cascade volcanism. This however does seem unlikely. If Sutter Buttes were part of a series of older Cascade stratovolcanoes abandoned due to the northward migration of the south end of Juan de Fuca Plate, the "last Cascade volcano" hypothesis would be tenable. But northward, arc volcanoes are young and active. In fact, why do the Cascades have an abrupt southern termination?”

“The Clear Lake and Sonoma volcanics are the less-than-5-million-year old components of a northwesterly younging line of volcanic fields of Tertiary to Holocene age (Hearn, et.al., 1981). All these volcanics lie within the San Andreas fault system, which appears to have provided magma access to the surface. (I have read that the San Andreas fault cuts down to the mantle north of San Francisco) Hearn et.al. point out that the timing of the volcanism suggests that it follows termination of subduction, as the Mendocino triple junction migrated northward. They also propose that the volcano alignment reflects an underlying hot spot. That suggestion seems inconsistent with the northward movement of the Pacific Plate which most of the volcanics ride. These volcanics are among the closest to a subduction plate boundary of any in the world and will repay closer tectonic investigation. Similarly, a tiny sliver of basalt dated at 3.57 million years (Prowell, 1974, quoted in Luedke and Smith, 1981) occurs 45 kilometers east of Santa Cruz, California near the Calaveras and Hayward faults. Apparently leakage of basalts along the San Andreas fault system has occurred repeatedly.”

“The Clear Lake volcanic field (late Pliocene to Holocene) lies in a tectonically active, complex geologic setting within the San Andreas transform fault system in northern Coast Ranges of California. Clear Lake and the volcanic field are located within a fault-bounded, locally extensional basin. The lake is the largest freshwater lake entirely within California; it is probably volcano-tectonic in origin, but is not a calderas lake. The volcanic field is the northernmost of a series of young Cenozoic volcanic fields in the Coast Ranges. Within the field, eruptive loci have migrated northward through the last 2.1 million years. Eruptive centers are lacking. Volcanism appears to be related to extension in a pull-apart basin within the San Andreas fault system and is not directly related to subduction, which ceased off the California coast at this latitude around 3 million years ago.”

“The Clear Lake volcanics range from basalt through rhyolite in composition. Basalt is rare, and the dominant composition is dacite. Different compositional ranges characterized four eruptive episodes separated by time gaps of 0.15 - 0.2 million years.”