A few features present in the Yellowstone thermal areas display evidence
that extremely violent thermal explosions occurred in the past,
particularly during Pinedale Glaciation, about 15,000 years ago. Such
explosion features, of which Pocket Basin in Lower Geyser Basin is a
good example, appear as craterlike depressions a few tens of feet to as
much as 5,000 feet across surrounded by rims of rock fragments that were
blown out of the craters. The underground mechanism causing the
explosions was similar to that of geysers, but in these special cases
the energy remained bottled-up until a very critical explosive stage was
reached.
The best explanation for Pocket Basin and related features is that the
ground above the sites of the explosions was weighted down by the water
of small lakes which had formed in melted-out pockets of glacial ice.
Such localized melting of the glaciers would occur where the ice was in
direct contact with underlying thermal features. A rapid draining of the
lake waters would then produce a sudden release of pressure over the hot
area, resulting in an unusually violent thermal eruption.
[Illustration: MUD VOLCANO near Pocket Basin in the Lower Geyser
Basin. The mud is formed by chemical decomposition of the rocks
chiefly by the action of carbon dioxide and sulfuric acid. The
splatter, 5-6 feet high, is caused by the escaping gases. (Fig. 53)]