Schematic of the hotspot and the Yellowstone Caldera
| |
Past locations of the hotspot in millions of years
| |
| Country | United States |
|---|---|
| State | Idaho/Wyoming |
| Region | Rocky Mountains |
| Coordinates | 44.43°N 110.67°WCoordinates: 44.43°N 110.67°W |
The Yellowstone hotspot is a volcanic hotspot in the United States responsible for large scale volcanism in Idaho, Montana, Nevada, Oregon, and Wyoming as the North American tectonic plate moved over it. It formed the eastern Snake River Plain through a succession of caldera-forming eruptions. The resulting calderas include the Island Park Caldera, the Henry's Fork Caldera, and the Bruneau-Jarbidge caldera. The hotspot currently lies under the Yellowstone Caldera. The hotspot's most recent caldera-forming supereruption, known as the Lava Creek eruption, took place 640,000 years ago and created the Lava Creek Tuff, and the most recent Yellowstone Caldera. The Yellowstone hotspot is one of a few volcanic hotspots underlying the North American tectonic plate; others include the Anahim and Raton hotspots.
Snake River Plain
The eastern Snake River Plain is a topographic depression that cuts across Basin and Range Mountain structures, more or less parallel to North American plate motion. Beneath more recent basalts are rhyolite lavas and ignimbrites that erupted as the lithosphere passed over the hotspot. Younger volcanoes that erupted after passing over the hotspot covered the plain with young basalt lava flows in places, including Craters of the Moon National Monument and Preserve.
The central Snake River plain is similar to the eastern plain, but differs by having thick sections of interbedded lacustrine (lake) and fluvial (stream) sediments, including the Hagerman Fossil Beds.
Nevada–Oregon calderas
Although
the McDermitt volcanic field on the Nevada–Oregon border is frequently
shown as the site of the initial impingement of the Yellowstone Hotspot,
new geochronology and mapping demonstrates that the area affected by
this mid-Miocene volcanism is significantly larger than previously appreciated.
Three silicic calderas have been newly identified in northwest Nevada,
west of the McDermitt volcanic field as well as the Virgin Valley
Caldera. These calderas, along with the Virgin Valley Caldera and McDermitt Caldera,
are interpreted to have formed during a short interval 16.5–15.5
million years ago, in the waning stage of the Steens flood basalt
volcanism.
The northwest Nevada calderas have diameters ranging from 15–26 km and
deposited high temperature rhyolite ignimbrites over approximately
5000 km2.
The Bruneau-Jarbidge caldera erupted between ten and twelve million years ago, spreading a thick blanket of ash in the Bruneau-Jarbidge event and forming a wide 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 a foot of ash was deposited. There, two hundred fossilized rhinoceros
and many other animals were preserved in two meters of volcanic ash. By
its characteristic chemical fingerprint and the distinctive size and
shape of its crystals and glass shards, the volcano stands out among
dozens of prominent ashfall horizons laid down in the Cretaceous, Paleogene, and Neogene
periods of central North America. The event responsible for this fall
of volcanic ash was identified as Bruneau-Jarbidge. Prevailing
westerlies deposited distal ashfall over a vast area of the Great Plains.
Volcanic fields
Twin Falls and Picabo volcanic fields
The
Twin Falls and Picabo volcanic fields were active about 10 million
years ago. The Picabo Caldera was notable for producing the Arbon Valley Tuff 10.2 million years ago.
Heise volcanic field
The
Heise volcanic field of eastern Idaho produced explosive
caldera-forming eruptions which began 6.6 million years ago and lasted
for more than 2 million years, sequentially producing four large-volume
rhyolitic eruptions. The first three caldera-forming rhyolites —
Blacktail Tuff, Walcott Tuff and Conant Creek Tuff — totaled at least
2250 km3 of erupted magma. The final, extremely voluminous, caldera-forming eruption — the Kilgore Tuff — which erupted 1800 km3 of ash, occurred 4.5 million years ago.
Yellowstone Plateau
Yellowstone sits on top of three overlapping calderas.
The Yellowstone Plateau volcanic field is composed of four adjacent calderas. West Thumb Lake is itself formed by a smaller caldera which erupted 174,000 years ago. The Henry's Fork Caldera in Idaho was formed in an eruption of more than 280 km3 (67 cu mi) 1.3 million years ago, and is the source of the Mesa Falls Tuff. The Henry's Fork Caldera is nested inside of the Island Park Caldera
and the calderas share a rim on the western side. The earlier Island
Park Caldera is much larger and more oval and extends well into Yellowstone Park.
Although much smaller than the Island Park Caldera, the Henry's Fork
Caldera is still sizeable at 18 miles (29 km) long and 23 miles (37 km)
wide and its curved rim is plainly visible from many locations in the
Island Park area.
Of the many calderas formed by the Yellowstone Hotspot, including
the later Yellowstone Caldera, the Henry's Fork Caldera is the only one
that is currently clearly visible. The Henry's Fork of the Snake River
flows through the Henry's Fork Caldera and drops out at Upper and Lower
Mesa Falls. The caldera is bounded by the Ashton Hill on the south, Big
Bend Ridge and Bishop Mountain on the west, by Thurburn Ridge on the
North and by Black Mountain and the Madison Plateau on the east. The
Henry's Fork caldera is in an area called Island Park. Harriman State Park is situated in the caldera.
The Island Park Caldera is older and much larger than the Henry's
Fork Caldera with approximate dimensions of 58 miles (93 km) by 40
miles (64 km). It is the source of the Huckleberry Ridge Tuff that is found from southern California to the Mississippi River near St. Louis. This supereruption occurred 2.1 million years BP and produced 2500 km3
of ash. The Island Park Caldera is sometimes referred to as the First
Phase Yellowstone Caldera or the Huckleberry Ridge Caldera. The youngest
of the hotspot calderas, the Yellowstone Caldera, formed 640,000 years
ago and is about 34 miles (55 km) by 45 miles (72 km) wide.
Non-explosive eruptions of lava and less-violent explosive eruptions
have occurred in and near the Yellowstone Caldera since the last super
eruption. The most recent lava flow occurred about 70,000 years ago,
while the largest violent eruption excavated the West Thumb of Lake Yellowstone
around 150,000 years ago. Smaller steam explosions occur as well – an
explosion 13,800 years ago left a 5 kilometer diameter crater at Mary
Bay on the edge of Yellowstone Lake.
Both the Heise and Yellowstone volcanic fields produced a series
of caldera-forming eruptions characterised by magmas with so-called
"normal" oxygen isotope signatures (with heavy oxygen-18 isotopes)
and a series of predominantly post-caldera magmas with so-called
"light" oxygen isotope signatures (characterised as low in heavy
oxygen-18 isotopes). The final stage of volcanism at Heise was marked by
"light" magma eruptions. If Heise is any indication, this could mean
that the Yellowstone Caldera has entered its final stage, but the
volcano might still exit with a climactic fourth caldera event analogous
to the fourth and final caldera-forming eruption of Heise (the Kilgore
Tuff) – which was also made up of so-called "light" magmas. The
appearance of "light" magmas would seem to indicate that the uppermost
portion of the continental crust has largely been consumed by the
earlier caldera- forming events, exhausting the melting potential of the
crust above the mantle plume. In this case Yellowstone could be expiring. It could be another 1–2 million years (as the North American Plate
moves across the Yellowstone hotspot) before a new supervolcano is born
to the northeast, and the Yellowstone Plateau volcanic field joins the
ranks of its deceased ancestors in the Snake River Plain. (References
to be added: Kathryn Watts (Nov 2007) GeoTimes "Yellowstone and Heise: Supervolcanoes that Lighten Up": Kathryn E. Watts, Ilya N. Bindeman and Axel K. Schmitt (2011) Petrology, Vol. 52, No. 5,
"Large-volume Rhyolite Genesis in Caldera Complexes of the Snake River
Plain: Insights from the Kilgore Tuff of the Heise Volcanic Field,
Idaho, with Comparison to Yellowstone and Bruneau-Jarbidge Rhyolites"
pp. 857–890).
