Travel Reference
In-Depth Information
In the Yellowstone area there have been three caldera events. Each time, a large magma
chamber was formed, uplifting the land above. Cracks in the overlying rocks reached down
to the magma, and some initial lava flowed out over the land (Figure 6B). The magma had a
great deal of water dissolved in it, so after some time, as the cracks released some of the pres-
sure, this water formed steam bubbles. Water as steam occupies a much larger volume than
when it is dissolved in magma—about a thousand times as much—so suddenly the magma
needed a much larger space. It is the same sort of violent process as when the water below a
geyser overheats, the geyser erupts, and steam and water pour out. All this steam pressure in
the magma caused a huge explosive eruption of tephra , rock fragments and molten globs that
rushed out of the chamber and partially emptied it (Figure 6C). The unsupported roof of the
magma chamber collapsed to create a caldera (Figure 6D). After the collapse, magma flowed
out onto the caldera, partly filling it with rhyolite lava (Figure 6E).
H UCKLEBERRY R IDGE C ALDERA E VENT. This first caldera formed about 2.1 million years ago. The
ejected rock and steam spread up and out as a billowing incandescent cloud, probably trav-
eling at 50 to 100 miles (80-160 km) per hour and being pushed by the material behind it.
As it traveled and expanded, the cloud cooled and deposited tephra over the northern Rock-
ies and the Great Plains. As far as Iowa, for example, the tephra was still as much as 3 inches
(8 cm) thick. The total volume of erupted material was about 600 cubic miles (2500 cu km)!
The explosion probably lasted for a period of days or weeks. An outcrop of this tephra can be
seen in the park at Golden Gate [GEO.7] south of Mammoth Hot Springs, where it's called
the Huckleberry Ridge tuff. In places, it's 550 feet (170 m) thick.
Outcrop of Huckleberry Ridge tuff along the road at Golden Gate [GEO.7].
H ENRYS F ORK C ALDERA E VENT. The center of this caldera is just west of Yellowstone Park near
Island Park, Idaho. This second major explosion happened 1.3 million years ago; the debris
was again mostly tephra. This Mesa Falls tuff can be seen at Mesa Falls [GEO.8] west of the
park boundary. The eruption was smaller than the previous one.
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