Eruption History of the Coso Volcanic Field
Early episode (6.0 to 5.3 Ma)The earliest episode of volcanic activity in the Coso Volcanic Field occurred between about 6.0 and 5.3 Ma in the northeastern Coso Range. Eruptions during this time period produced a wide range of volcanic types including low-silica basalts, high-silica rhyolites, and minor amounts of andesite to dacite lavas (intermediate-composition). These early rocks are geographically separated from the main region of volcanism that erupted during the second episode.
Second episode (4.0 to 3.0 Ma)
The second volcanic episode in this region took place in the eastern part of the Coso Range. During this time period basaltic volcanism was common near the present western margin of the Basin and Range province and within the adjacent Sierra Nevada mountain range. Single eruptive events in the eastern Coso Field produced basaltic features from monogenetic vents.
Dacitic lavas were erupted from multiple vents during multiple eruptions (polygenetic) in this same area, but activity was isolated to about 3.5 to 3.3 Ma. Some dacite lava flows were erupted as late as about 3.0 Ma. During this phase of eruption, the volume of volcanic products produced in the Coso Range reached a maximum, and the proportion of intermediate-lava types (e.g. dacite) was the greatest for the entire second episode. This pattern probably reflects about one million years of continuous supply of mantle-derived basaltic magma beneath the region. Approximately 0.5 Ma after the onset of basaltic volcanism, at the start of the second episode, the crust was sufficiently hot and melted to produce high-silica magma. These silicic magmas mixed to varying degrees with basaltic magma beneath them to form the intermediate-composition (dacite) volcanics erupted from the comparatively long-lived polygenetic centers.
Before the main stage of volcanism in the eastern Coso Range had ended, silicic eruptions began approximately 10 km to the west, near Haiwee Ridge. High-silica rhyolitic explosive eruptions took place at 3.1 Ma followed about 100,000 years later by more voluminous explosive eruption of rhyodacite. Rhyodacitic lava was erupted from several widespread vents at least as late as 2.5 Ma ago. More than 9 km3 (2.1 mi3) of silicic magma erupted from the Haiwee Ridge center, a volume at least twice as great as that of dacite from the eastern center. Additionally, no basaltic or intermediate-composition rocks have been recognized that could have erupted from within the area of silicic vents. These observations suggest that a relatively large crustal magma reservoir may have been active for at least 600,000 years. This reservoir was presumably created and sustained by the eastern-lying basaltic magma supply. Tectonic conditions in this part of the volcanic field evidently allowed this chamber to develop such that it prevented any basaltic or intermediate magma from reaching the surface.
Third (minor) episode (approximately 2 Ma)
After the voluminous episode ended at about 3 Ma, only a few eruptions occurred and they were all basaltic to andesitic. Four basaltic cinder cones and associated lava flows erupted from monogenetic vents about 5 km (3 mi) west of the larger rhyolite field. The youngest of these eruptions has been dated at about 1.8 Ma.
Fourth episode (1.1 Ma to 40 ka)
Thirty eight high-silica, rhyolite domes and lava flows and at least fourteen basaltic eruptive centers were erupted during the fourth episode. The high-silica composition of these rhyolites indicates that they are from the uppermost eruptable magma of a much more voluminous magma reservoir. Dome emplacement was preceded by only minor explosive activity, which indicates that much of the gas within the magma escaped prior to eruption. The basaltic magma that was erupted during the same period originated from vents surrounding the rhyolite field.