Extremities:  Geology and Life in Yellowstone and Implications for Other Worlds


The largest volcanic eruptions are not the familiar basalts or andesites (like Mt. St. Helens), but ash-flows from granitic magma. Ash-flows are huge hot clouds (also called nuee ardents) of shards of volcanic glass and fragments of volcanic crystals, suspended and carried in hot turbulent gas. The main eruption of Mt. St. Helens was a small ash-flow -- those from Yellowstone were hundreds of times bigger.
Ash-flow eruptions start when a water-rich magma in the earth is suddenly released to the surface. The magma's water flashes to steam, turning the magma into foam and then ripping the foam into small shards. The shards and pumice and gas flow together as if they were a liquid, at 600°C+ and runnier than water.

Huckleberry Ridge Tuff
Lava Creek Distribution

Cliff and roadcut in ash-flow tuff along the Gardner river near Osprey falls, above Mammoth. This is the Huckleberry Ridge tuff, which erupted from the Island Park Caldera (SW of Yellowstone) about 2.1 million years ago. The Huckleberry Ridge tuff here was several separate ash-flows, one on another while they were still very hot, with some layers of falling ash in between. These layers cooled together and welded into a thick dense rock. The different individual ash-flows now appear as the cliffs, and the ash-fall as the thin layers forming slopes between the cliffs. Much of the Huckleberry Ridge tuff has been eroded away since it erupted, but its original volume is estimated at ~3,000 cubic kilometers!
Reconstructed distribution of ash-flow tuffs at Yellowstone after eruption of the Lava Creek ash-flow tuff, 640,000 years ago (modified from Fig. 20 of Christiansen, 2001). Remnants of the the Huckleberry Ridge tuff (2.0 million years old) are in pink —. The Lava Creek ash-flow tuff, in yellow, erupted with the collapse of the Yellowstone caldera. All ash-flow tuffs at Yellowstone look very much like the Huckleberrry Ridge, and can be difficult to sort out. The Lava Creek tuff has a total volume of ~ 1,000 cubic kilometers (a cube 10 km on a side), and thicknesses up to hundreds of meters (filling pre-existing canyons). There have been no eruptions like this in recorded history.
Welded Pumice
Cliff in Welded Tuff

Broken piece of ash-flow tuff, from the roadcut. Tony Irving's finger points at the white spots, which were pieces of pumice in the ash. The pumice was flattened (along with the ash shards) by the weight of the hot ash over them.

Geologists in their element - a roadcut on a busy highway. Tony Irving explains how little bits of volcanic ash get compacted and cemented into massive rock, with the proof of it as a backdrop.

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Updated 11/19/02.
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