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Bruneau-Jarbidge caldera
The Bruneau-Jarbidge caldera〔Bonnichsen, B., "The Bruneau-Jarbidge eruptive center, southwestern Idaho," in B. Bonnichsen and R.M.. Breckenridge eds., ''Cenozoic Geology of Idaho'' (Bureau of Mines and Geol. Bulletin 26) 1982.〕 (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. The prehistoric skeletons of camels, lizards, horses, turtles and two hundred fossilized rhinoceros were found in two metres of volcanic ash at a lagerstätte near Royal, Nebraska, the Ashfall Fossil Beds.
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〔The increasingly refined science of dating through layers of ash and other tephra thrown out by a volcanic event is called tephrochronology.〕 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.〔Leeman, William P.; Annen, Catherine; and Dufek, Josef (2008). (Snake River Plain – Yellowstone silicic volcanism: implications for magma genesis and magma fluxes ), pp. 235–259 in Annen, Catherine and Zellmer, Georg F., ''Dynamics of Crustal Magma Transfer, Storage and Differentiation'', Geological Society, London, Special Publications, 304. .〕
== Notes ==
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