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Eruptive history of Clear Lake Volcanic Field

The Clear Lake Volcanics erupted during four periods of time beginning at about 2 Ma. There is a general decrease in age northward from 2 Ma 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 that the vertical extent is approximately 30 km (18.6 mi) deep.

Group 4: 2.1 to 1.3 Million Years Ago

The earliest eruptive period (group 4) lasted from about 2.1 to 1.3 Ma and was by far the most widespread, with dominantly mafic lavas scattered over an area about 50 by 60 km (31 by 37 mi). These early lavas included basaltic eruptions in the Wilbur Springs district and at the site of the McLaughlin mine, both areas of mercury and gold formation. There is no evidence of a major magma chamber for this, oldest, group of the Clear Lake Volcanics. The basalt to basaltic-andesite eruptive vents were widespread but concentrated in a northwest-trending zone of deep magma access, possibly controlled by the regional right-lateral stress system.

Group 3: 1.1 to 0.8 Million Years Ago

Early magma chamber evolution may have initiated by deep injection of basaltic magma from 1.3 to 1.6 Ma, which may have spurred the Group 3 phase of eruptive activity from 1.1 to 0.8 Ma. The 5 km3 (1.2 km3) rhyolite eruption of Alder Creek about 1.12 Ma was likely the first eruption spurred by this new chamber development, and down-faulting of part of this rhyolite has been interpreted as the result of possible collapse above a local magma chamber. The 1.02 Ma rhyolite of Bonanza Springs and most of the andesites and dacites of group 3 age were vented in an elongate east-west 7 by 14 km (4.3 by 8.7 mi) area, the west part of which is directly above the current magma chamber. This series of eruptions culminated in the Mount Hannah edifice of several dacite bodies erupted within less than 5,000 years, possibly within 100 years.

Group 2: 0.65 to 0.3 Million Years Ago

Nearly all silicic eruptions of group 2 (0.65 to 0.3 Ma) age occurred to the north of the group 3 eruptions. Dacites of 0.5 to 0.6 Ma erupted from widely distributed vents as much as 19 km (11.8 mi) apart. Their disperse pattern suggests that (1) the magma chamber was much larger than the present one, or (2) several magma reservoirs existed, or (3) magma migrated significantly from the storage region before eruption. Dacites of 0.35 to 0.5 Ma age vented in a west-northwest trending zone 5 by 20 km (3 to 12 mi), which lies mainly to the north of the 0.5 to 0.6 Ma pattern. These eruptions culminated at the northwest end of the zone in the voluminous edifice of Mount Konocti. This elongate zone of dacite vents suggests that shallow ascent of magma was controlled by regional northwest-trending structures rather than by structures generated by a subsurface magma chamber body.

Several dacite flows and a rhyolite flow from about 0.6 Ma indicate that a magma chamber, or chambers, were in existence by that time. The locations of group 2 silicic vents do not support a simple pattern of distribution above a single magma chamber. The 4 km3 (1 mi 3) rhyolite of Thurston Creek, an early group 2 flow, erupted from vents along a 13-km (8-mi) northward-concave arc and may have initiated the formation of part of the Clear Lake basin. If the arc is an expression of a ring-fracture zone, it has no northern counterpart and has no simple relation to the dacite vents of similar age or to the current inferred magma chamber.

Group 1: 0.1 Million to 10,000 Years Ago

During the most recent eruptive period, 0.1 Ma to 10,000 years ago, igneous activity shifted from the area near the southeastern arms of Clear Lake and northeast of the lake. Most of the lavas and tephra were basalt, basaltic andesite, or andesite, rocks that suggest a new cycle of deep-source magma generation, heating of the crust, and possible development of other magma chambers.

The types of minerals and chemistry present within the rhyolite of Borax Lake (0.9 Ma) suggest that it may be the only eruptive unit in the youngest age group that may have evolved in a magma chamber. Whether the Borax Lake flow was fed laterally from a larger, main, magma chamber or derived from its own chamber is unknown. Existing geophysical data are insufficient to determine whether one or more small magma chambers are present near Borax Lake or farther northeast of Crater Lake.