The Lava Creek Tuff is a very large layer of ash-flow tuff found in Wyoming, Montana, and Idaho in the United States. It formed during the Lava Creek eruption about 630,000 years ago, which created the Yellowstone Caldera. This eruption was the most important event in Yellowstone's third volcanic cycle. The Lava Creek Tuff covers an area larger than 7,500 km² (2,900 mi²), centered around the caldera, and is estimated to contain 1,000 km³ (240 cu mi) of magma.
The eruption spread volcanic material across much of North America, forming one of the most widespread layers of ash known as the Pearlette type O ash bed in the United States and the Wascana Creek ash in Canada.
The thick tuff layer from this eruption is clearly visible in several places within Yellowstone National Park, such as Tuff Cliff along the Gibbon River, Virginia Cascade, and along U.S. Highway 20.
The Lava Creek Tuff varies in color from light gray to pale red in some areas. The texture of the tuff ranges from fine-grained to very fine-grained and is tightly fused together. The thickest part of the tuff layer is about 180–200 meters (590–660 feet).
Chronology of tuff
The ash flows of the Lava Creek Tuff are divided into six parts, given temporary names: unit 1, unit 2, member A, and member B, from bottom to top. Units 3 and 4 are not clearly placed in the rock layers. The Lava Creek Tuff did not form all at once in a single event. Instead, it formed in stages with breaks between eruptions, and the different parts were erupted at different times.
To determine when each part was erupted, scientists use two dating methods: Ar/Ar on sanidine and U–Pb on zircon. These methods work differently because zircon crystals form early and continue to form as magma changes over time. This means U–Pb ages are older than the exact time of eruption, which is recorded by sanidine.
Two samples from ignimbrite layers that look similar to unit 1 or 2, the oldest parts of the Lava Creek Tuff, have Ar/Ar ages of 634.5 ± 6.8 thousand years and 630.9 ± 4.1 thousand years. Ar/Ar dating of sanidine from member B shows eruption ages of 627.0 ± 1.7 thousand years, 631.3 ± 4.3 thousand years, and 630.9 ± 2.7 thousand years.
U–Pb dating of zircon crystals from member A and member B gives an age of 626.5 ± 5.8 thousand years, which matches closely with the Ar/Ar dates from sanidine. Another group reported U–Pb ages of 626.0 ± 2.6 thousand years for zircon from member A and 629.2 ± 4.3 thousand years for zircon from member B.
Petrography
The ignimbrite sheet was formed from rhyolite magma and contains large crystals of quartz, sanidine, and less common sodic plagioclase, along with small amounts of magnetite, ilmenite, ferroaugite, fayalite, iron-rich hornblende, zircon, chevkinite, and allanite. However, the number of large crystals varies between the members. Hornblende is more common in member A but rare in other members. Unit 3 is different from units 1 and 2 because it has more crystals and more plagioclase. Member A is different from member B mainly because amphibole is present in member A but not in member B.
The edges of zircon and large crystals show that the magma of the Lava Creek Tuff came from a mix of mantle material, Archean crust, and shallow rocks inside the volcano that were changed by hot water. Member A and member B came from separate magma reservoirs before the eruption, located at depths between 3–6 km (1.9–3.7 mi) and temperatures between 790–815 °C (1,454–1,499 °F). The eruption of member B was likely caused by a combination of new silicic magma entering the reservoir and the release of gases from magma as it cooled and solidified.
Eruption
Scientists have studied the deposits left by the Lava Creek Tuff eruption to understand how it happened. Units A and B, which are close to where the eruption occurred, have been carefully examined and matched with ash found far from the site. New units, numbered 1 through 4, have only been found in a few places, but they show that the Lava Creek eruption was more complicated than earlier believed. These ash and rock layers represent the first known events of the Lava Creek eruption.