..Geologic History (pg. 1)

A COMPLEX VOLCANIC PLATEAU

…..Yellowstone is at the northeast tip of a smooth U-shaped curve through the mountains, which is now the Snake River Plain. This curved plain was created as the North American continent drifted across a stationary volcanic hotspot beneath the Earth’s crust. This hot spot used to be near what is now Boise, Idaho, but North America has drifted at a rate of 45 mm a year in a southwestern direction, shifting the hot spot to its present location.

Yellowstone Caldera RimYellowstone Caldera is the largest volcanic system in North America. It has been termed a “supervolcano” because the caldera was formed by exceptionally large explosive eruptions. It was created by a cataclysmic eruption that occurred 640,000 years ago that released 1,000 cubic kilometers of ash, rock and pyroclastic materials (this was 450 times larger than Mount St. Helens’ 1980 eruption), forming a crater nearly a kilometre deep and 30 by 70 kilometres in area (18 by 43 mi) (the size of the caldera has been modified a bit since this time and has mostly been filled in, however). The welded tuff geologic formation created by this eruption is called the Lava Creek Tuff. In addition to the last great eruptive cycle there were two other previous ones in the Yellowstone area.

Exploding VolcanoEach eruption is in fact a part of an eruptive cycle that climaxes with the collapse of the roof of a partially emptied magma chamber. This creates a crater, called a caldera, and releases vast amounts of volcanic material (usually through fissures that ring the caldera). The time between the last three cataclysmic eruptions in the Yellowstone area has ranged from 600,000 to 900,000 years, but the small number of such climax eruptions can not be used to make a prediction for the time range for the next climax eruption.

The first and largest eruption climaxed to the southwest of the current park boundaries 2.2 million years ago and formed a caldera about 50 by 80 kilometres in area (30 by 50 mi) and hundreds of meters deep after releasing 2,500 cubic kilometers of material (mostly ash, pumice and other pyroclastics). This caldera has been filled in by subsequent eruptions, and the geologic formation created by this eruption is called the Huckleberry Ridge Tuff.

The second eruption, at 280 km³ of material ejected, climaxed 1.2 million years ago and formed the much smaller Island Park Caldera and the geologic formation called the Mesa Falls Tuff. All three climax eruptions released vast amounts of ash that blanketed much of central North America and fell many hundreds of miles away (as far as California to the southwest; see Lake Tecopa). The amount of ash and gases released into the atmosphere probably caused significant impacts to world weather patterns and led to the extinction of many species in at least North America. About 160,000 years ago a much smaller climax eruption occurred which formed a relatively small caldera that is now filled in with the West Thumb of Yellowstone Lake.

Steamboat Geyser, currently the world’s tallest active geyser, during a minor eruption. Major eruptions can reach as high as 300 feet (90 m).
Enlarge
Steamboat Geyser, currently the world’s tallest active geyser, during a minor eruption. Major eruptions can reach as high as 300 feet (90 m).

Lava strata is most easily seen at the Grand Canyon of the Yellowstone where the Yellowstone River continues to carve into the ancient lava flows. According to Ken Pierce, U.S. Geological Survey geologist, at the end of the last glacial period, about 14,000 to 18,000 years ago, ice dams formed at the mouth of Yellowstone Lake. When the ice dams melted, a great volume of water was released downstream causing massive flash floods and immediate and catastrophic erosion of the present-day canyon. These flash floods probably happened more than once. The canyon is a classic V-shaped valley, indicative of river-type erosion rather than glaciation. Today the canyon is still being eroded by the Yellowstone River.

Columnar BasaltAfter the last major climax eruption 630,000 years ago until about 70,000 years ago, Yellowstone Caldera was nearly filled in with periodic eruptions of rhyolitic lavas (example at Obsidian Cliffs) and basaltic lavas (example at Sheepeaters Cliff). But 150,000 years ago the floor of the plateau began to bulge up again. Two areas in particular at the foci of the elliptically shaped caldera are rising faster than the rest of the plateau. This differential in uplift has created two resurgent domes (Sour Creek dome and Mallard Lake dome) which are uplifting at 15 millimeters a year while the rest of the caldera area of the plateau is uplifting at 12.5 millimeters a year.

Grand Prismatic SpringPreserved within Yellowstone are many geothermal features and some 10,000 hot springs and geysers, 62% of the planet’s known total. The superheated water that sustains these features comes from the same hot spot described above.

The most famous geyser in the park, and perhaps the world, is Old Faithful Geyser (located in Upper Geyser Basin), but the park also contains the largest active geyser in the world, Steamboat Geyser in the Norris Geyser Basin.

In 2003 changes at the Norris Geyser Basin resulted in the temporary closure of some trails in the basin. This coincided with the release of reports about a multiple year USGS research project mapping the bottom of Yellowstone Lake that identified a structural dome that had uplifted at some time in past beneath Yellowstone Lake. On March 10, 2004, a biologist discovered 5 dead bison which apparently had inhaled toxic geothermal gases trapped in the Norris Geyser Basin by a seasonal atmospheric inversion. Shortly after, in April 2004, the park experienced an upsurge of earthquake activity. These events inspired a great deal of media attention and speculation about the geologic future of the region. The United States government responded by allocating more resources to monitor the volcano and reminding visitors to remain on designated safe trails. The intervals between the historic large, caldera-forming explosions suggest that another such explosion may be “due,” if not overdue.

From Wikipedia

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