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Voluminous Pleistocene basalt flows in northern California erupted within a few hundred years of each other

Scientists at the volcano observatories of the U.S. Geological Survey pay close attention to volcanoes of the Cascade Range, Alaskan Peninsula and Aleutian Arc.

These areas have young and frequent volcanic eruptions, form conspicuous large edifices, and can produce high-silica magmas that are sometimes very explosive. These volcanoes threaten life and property, even including jet aircraft that might fly over them.

A different group of volcanoes are located to the east of the Cascade Range from northern California to central Oregon. In this region, the extensional tectonics of the Basin-and-Range province impinge on the subduction process that created the Cascade Range. These "rear-arc" volcanoes are not explosive, but instead erupt fluid magmas, akin to Hawaiian basalts, at intervals of 10s to 100s of thousands of years. Their high effusion rates produce eruptions that can inundate scores of km2 in just months to years with volumes 6 to 8 km3of lava. The broad lava flow fields from these eruptions are gently inclined, and fill the topographic basins between older, eroded volcanoes. Through time, the basins are covered by sediments, and form marshy, and grassy meadows in the dry environments of northern California and central Oregon. Because the basalt lava in these flows shrinks upon cooling and forms cracks, they hold and convey groundwater from zones of higher rainfall to areas that are semi-arid. This abundant groundwater resource is very important to the economy of NE California and central Oregon.

Scientists at the California Volcano Observatory are studying an interesting aspect of 3 of these voluminous rear-arc basalt eruptions around 300,000 years ago. While rear-arc eruptions are usually separated in time, these 3 eruptions share nearly identical whole-rock chemistry values, and have identical characteristic remanent magnetic directions, "locked in" when they erupted and cooled. This suggests that all 3 eruptions occurred in no more than a century or two. Vents for the Tennant and Dry Lake basalt fields are only 9 km apart, separated by a ridge of older volcanic rocks, whereas the vent for the Hammond Crossing basalt field is farther SSE and 56 km from the Dry Lake vent. The dike(s) that fed the common eruptive episode which created these lava fields may have been very long, breaking the surface at very separate locations. The study's authors are examining other, older voluminous rear-arc lava fields to see if additional, common eruptive episodes can be identified.

Rear-arc basalt lava outcrop
A 3-4m (10-12 ft) high outcrop of basalt lava, part of a broader lava flow. These flows were erupted about 300,000 years ago following basalt dike intrusions in a rear-arc region of northern California. The eruptions of three of these flows are thought to have taken place within several hundred years of each other.