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Title: Bruneau-Jarbidge  
Author: World Heritage Encyclopedia
Language: English
Subject: Supervolcano
Publisher: World Heritage Encyclopedia


The Bruneau-Jarbidge caldera[1] (sometimes called a supervolcano) is located in present-day southwest Idaho. The volcano erupted during the Miocene, between ten and twelve million years ago, spreading a thick blanket of ash in the Bruneau-Jarbidge event and forming a caldera. Animals were suffocated and burned in pyroclastic flows within a hundred miles of the event, and died of slow suffocation and starvation much farther away, notably at Ashfall Fossil Beds, located 1000 miles downwind in northeastern Nebraska, where up to two meters of ash was deposited. At the time, the caldera was above the Yellowstone hotspot.

The existence of the event was discovered in 1971 by Prof. Mike Voorhies, a paleontologist at the University of Nebraska State Museum, at a lagerstätte near Royal, Nebraska, the Ashfall Fossil Beds; there, two hundred fossilized rhinoceros remained at a single site, together with the prehistoric skeletons of camels and lizards, horses and turtles, a death assemblage preserved in two meters of volcanic ash.

By its uniquely characteristic chemical "fingerprint" and the distinctive size and shape of its crystals and glass shards, the volcano stood out among dozens of prominent ashfall horizons[2] laid down in the Cretaceous, Paleogene, and Neogene periods of central North America. The event responsible for this fall of volcanic ash was identified at Bruneau-Jarbidge, 1600 kilometers west in Idaho. Prevailing westerlies deposited distal ashfall over a vast area of the Great Plains.

The evolving composition of the erupted material indicates that while it is derived in large part from melted material from the middle or upper crust, it also incorporated a young basaltic component.[3]



  • BBC: "Supervolcanoes" Program transcript, 3 February 2000
  • W. I. Rose, C. M. Riley, and S. Dartevelle, "Sizes and Shapes of 10-Ma Distal Fall Pyroclasts in the Ogallala Group, Nebraska" (pdf file) Includes bibliography.
  • Izett, G. A. 1981. "Volcanic ash beds: recorders of Upper Cenozoic silicic pyroclastic volcanism in the western United States." "Journ. Geophysical Res. 86:10, 200–10, 222.

Coordinates: 42°18′N 115°12′W / 42.3°N 115.2°W / 42.3; -115.2

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