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Tectonic extension causes volcanic activity at Coso volcanic field

The Coso volcanic field is located just east of the Sierra Nevada Mountains along the western edge of the Basin and Range province.

Geologic map schematic of the Coso volcanic field.
Geologic map schematic of the Coso volcanid field. White lines are heat flux values from Combs, 1980 (values 3, 5, 10, 15 HFU). Yellow lines are fault positions; blue diamonds represent rhyolite vent locationss and red squares represent basaltic vent locations younger than 0.3 Ma. HFU = 41.87 mW/m2.

The Garlock fault, which runs perpendicular to the San Andreas fault, is only 60 km (37 mi) to the south, and the field itself sits within a step-over zone of extension (spreading) in a strike-slip fault system. This complex tectonic environment relates directly to the volcanic activity of the Coso field.

As the process of extension, or spreading, occurs in this region, fractures occur allowing magma to fill in the openings. These are magma filled fractures are called dikes, and with greater extension, more dikes will be emplaced. Dikes can then feed volcanic vents, and therefore eruptions, on the surface. So, the greater the extensional strain in the Coso region, the greater number of dikes, and the greater number of vents. However, it is important to note that although extension causes intrusion of magma into dikes, the rate of intrusion must exceed the rate of extension by 200 times in order to cause a widespread eruption.

The pattern of volcanic activity in the past can be related to the build up of tectonic extensional strain in the Coso region. Based upon research conducted by Bacon (1982), it is possible to predict when Coso may erupt again in a devastating manner. Assuming a constant extension rate, a constant rate of magma production, and no change in the depth of the magma source, eruption of rhyolite magma may occur within the next 60,000 ∓ 35,000 years, and eruption of basaltic magma should take place within the next 55,000 years.

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