November 4, 2013 – Map of the Ash and Tephra fall for Major Volcanic Eruptions in the United States: The map reflects ash and tephra fall for major eruptions from the Long Valley Caldera, Mount St. Helens and Yellowstone. Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff.
The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest and therefore closest to the vent, while smaller fragments travel further—ash can often travel for thousands of miles, even circumglobal, as it can stay in the stratosphere for days to weeks following an eruption. When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions (or from a multitude of smaller eruptions occurring simultaneously), they can reflect light and heat from the sun back through the atmosphere, in some cases causing the temperature to drop, resulting in a climate change: “volcanic winter”. Tephra mixed in with precipitation can also be acidic and cause acid rain and snowfall.
Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years; the third largest was at Long Valley in California and produced the Bishop ash bed. The biggest of the Yellowstone eruptions occurred 2.1 million years ago, depositing the Huckleberry ridge ash bed.
These eruptions left behind huge volcanic depressions called “calderas” and spread volcanic ash over large parts of North America (see map).
The Mesa Falls Tuff is a tuff (pyroclastic rock) formation created by the Mesa Falls eruption that formed the Henry’s Fork Caldera that is located in Idaho west of Yellowstone Park. It is the second most recent caldera forming eruption from the Yellowstone hotspot and ejected of 280 cubic miles of material. This eruption, 1.3 million years B.C., was preceded by the Huckleberry Ridge Tuff and succeeded by the Lava Creek Tuff both of which were created by the Yellowstone hotspot.
Pictured above, the Mesa Falls Tuff exposed at southern rim of both the Island Park Caldera and the Henry’s Fork Caldera near Ashton, Idaho.
The Lava Creek Tuff (pictured above) is a tuff formation in Wyoming, created during the Lava Creek eruption around 640,000 years ago, which formed the Yellowstone Caldera. The Lava Creek Tuff is distributed in a radial pattern around the caldera and is formed of 1,000 cubic kilometers (240 cu mi) of ash in pyroclastic flows. The tuff has been exposed by erosion at Tuff Cliff along the Gibbon River.
Lava Creek Tuff ranges in color from light-gray to pale red in some locales. Rock texture of the tuff ranges from fine-grained to aphanitic and is densely welded. The maximum thickness of the tuff layer is approximately 180–200 meters (590–660 ft).
Pictured above, the northeastern part of Yellowstone Caldera, with the Yellowstone River flowing through Hayden Valley and the caldera rim in the distance.
Pictured above, the Yellowstone Caldera and the eruption history over the last two million years. Note that the West Thumb Caldera erupted 174,000 years ago, young in geological history. Additionally, a major eruption of the Yellowstone Caldera would bury most of the Western United States in ash. Consider the “Ejecta Volume” in the upper left corner. Six hundred cubic miles of material ejected above the U.S. would make Pompeii look like a child’s sand box.
If another large caldera-forming eruption were to occur at Yellowstone, its effects would be worldwide. Thick ash deposits would bury vast areas of the United States and the injection of huge volumes of volcanic gases into the atmosphere would drastically affect global climate. (Credits – Wikipedia and USGS).
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