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October 24, 2023

Though volcanoes like Mount Shasta or Lassen Peak may dominate their skylines, California also has shield volcanoes, named because their shape is reminiscent of a round warriors' shield resting flat. Several of these are found in the northern part of the state, including Medicine Lake (a huge shield and caldera) as well as the volcano pictured here, Ash Creek Butte. 

In front of an outcrop of matte grey rock splotched with lichens, a young man in an orange safety vest gives a happy thumbs up and an older man sits writing notes in a small notebook and smiles at the camera. The rocks are perched on a moderate slope covered in brushy manzanita and scraggly pine trees.
Andy Calvert and Tony Pivarunas drill into the face of a lava flow on Ash Creek Butte, a Pleistocene shield volcano northeast of Mount Shasta. USGS photo by Dawnika Blatter

Located just to the northeast of Mount Shasta, Ash Creek Butte is an 11 km³ (2.6 mi³) basaltic andesite to andesite shield volcano. Erupted in the middle Pleistocene (227 thousand years ago), this shield volcano was thought to have been formed relatively quickly, with little time passing between the emplacement its many layers of lava flows. To prove this, CalVO geologists used paleomagnetic methods to sample both early (first erupted) and late (last erupted) lava flows. Paleomagnetism is the study of the intensity and orientation of the Earth's geomagnetic field as recorded in natural materials, which changes on the scale of decades to centuries. This can help constrain eruption duration within a few hundreds or even tens of years, a much more precise timescale than radiometric dating (like Ar/Ar) can pinpoint.

The sampling shown in this photo of CalVO scientists Tony Pivarunas and Andy Calvert led to the conclusion that Ash Creek Butte was probably erupted in a geologic and paleomagnetic "instant" - over a period of around 50-200 years. While long in human terms, that's quite fast on a geologic timescale! These short but intense periods of volcanism have begun to change our understanding of the hazards of eruptions in the Cascades Volcanic Arc, from focusing on large stratovolcanoes to also considering the smaller distributed edifices scattered throughout the region.

To read more about the study on the tempo of volcanism at California's shield volcanoes, check out

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