The Bruneau-Jarbidge caldera (sometimes called a

supervolcano A supervolcano is a volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than 1,000 cubic kilometers (240 cubic ...

) 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 Wyom ...

. The

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

erupted during the

Miocene The Miocene ( ) is the first 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 means "less recen ...

, 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 bu ...

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 southwe ...

, 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 Ri ...

. 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 th ...

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

, and

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

periods of central

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. The event responsible for this fall of volcanic ash was identified at Bruneau-Jarbidge, 1,600 kilometers west in

. 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 to ...

deposited distal ashfall over a vast area of the

Great Plains The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, an ...

. 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 aphanite, 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 planetary surface, surface of a terrestrial ...

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