Bruneau-Jarbidge caldera
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The Bruneau-Jarbidge caldera (sometimes called a supervolcano) is located in present-day southwest
Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...
. The
volcano A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Ear ...
erupted during the
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
, between ten and twelve million years ago, spreading a thick blanket of ash in the Bruneau-Jarbidge event and forming a
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
. Animals were suffocated and burned in
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s within a hundred miles of the event, and died of slow suffocation and starvation much farther away, notably at
Ashfall Fossil Beds The Ashfall Fossil Beds of Antelope County in northeastern Nebraska are rare fossil sites of the type called lagerstätten that, due to extraordinary local conditions, capture an ecological "snapshot" in time of a range of well-preserved fossil ...
, located 1,000 miles downwind in northeastern
Nebraska Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the sout ...
, where up to two meters of ash were deposited. At the time, the caldera was above the
Yellowstone hotspot The Yellowstone hotspot is a volcanic hotspot in the United States responsible for large scale volcanism in Idaho, Montana, Nevada, Oregon, and Wyoming, formed as the North American tectonic plate moved over it. It formed the eastern Snake Riv ...
. By its uniquely characteristic chemical composition and the distinctive size and shape of its crystals and glass shards, the volcano stood out among dozens of prominent ashfall horizons laid down in the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, Paleogene, and Neogene periods of central North America. The event responsible for this fall of volcanic ash was identified at Bruneau-Jarbidge, 1,600 kilometers west in
Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...
. Prevailing
westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend t ...
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 molten material from the middle or upper crust, it also incorporated a young
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
ic 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


References


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. * {{DEFAULTSORT:Bruneau-Jarbidge Caldera Volcanoes of Idaho Calderas of Idaho Volcanism of Idaho Yellowstone hotspot Hotspot volcanoes Landforms of Owyhee County, Idaho Miocene calderas Miocene United States Neogene Idaho Supervolcanoes Extinct volcanoes


External links


Cenozoic Geology of Idaho: "The Bruneau-Jarbidge Eruptive Center, Southwestern Idaho" by Bill Bonnichsen (pages 251-268)
Idaho Bureau of Mines and Geology