Lava Creek Tuff
The volcanic rocks of Yellowstone Park are
predominantly rhyolitic in composition and are the result of three
sequential eruptive periods stretching back over 2 Ma (Fig. 1, pg.8341
from Hildreth et al., 1984, JGR). The oldest and largest of these
eruptions occurred 2 Ma, producing the Huckleberry Ridge Tuff and
associated Big Bend Ridge caldera. The Big Bend Ridge caldera covers
the southwestern portion of the park, but the enormous volume erupted
in the Huckleberry Ridge Tuff (2500 km3) extends well outside
Yellowstone, to the southwest over the Snake River Plain. At 1.3 Ma the
Henrys Fork caldera extruded the Mesa Falls Tuff (280 km3),
concentrated just outside the southwestern boundary of the park. The
most recent rhyolitic, caldera forming eruption in the Yellowstone
volcanic plateau peaked 0.6 Ma and resulted in the Lava Creek Tuff.
The Lava Creek Tuff (1000 km3) was dominantly erupted from the Yellowstone caldera as rhyolitic ash flow sheets, but certain sections include air fall pumice and ash as well as occasional basaltic flows. The unit is divided into two members, A and B. The Lava Creek Tuff is found across nearly the entire expanse of the park and is commonly seen overlying the Huckleberry Ridge Tuff. Maximum unit thickness has been measured to be 300 m. In general the ash flow sheets are densely welded, however variations in welding and abundance of phenocrysts allows the distinction of members A and B. In most areas the Lava Creek Tuff is overlain by younger ash flow tuffs and basalts erupted from the Yellowstone caldera. Currently the Yellowstone caldera has two active resurgent domes. The Sour Creek dome to the northeast section of the caldera and the Mallard Lake dome to the southwest appear to alternate activity, which has been measured to be as much as 2 cm/yr of uplift (Pelton and Smith, 1982).
The eruption model for all three of these rhyolitic, caldera forming sequences has long been debated, but it is now commonly accepted that the Yellowstone ignimbrites were erupted from ring fissures. As volcanic activity increases, ring fissures develop which generally delineate the boundary of the caldera. While the ash flow tuffs flow from a series of ring fissures it is likely that the caldera roof simultaneously collapsed. The evacuation of the chamber as a result of the roof collapse resulted in large volume ash flows at the height of the eruptive sequence. The aerial extent of the rhyolitic ash flow tuffs versus caldera filling basalts in the case of the 0.6 Ma Yellowstone caldera event provides evidence for the ring fissure model of ignimbrite eruption.
Pelton, J.R. and R.B. Smith, Contemporary vertical surface displacement in Yellowstone National Park, JGR, 87, 2745-2761, 1982.