| The large magma reservoir beneath
Yellowstone may have temperatures higher than 1,475°F (800°C), and the
surrounding rocks are heated by it. Because of this, the average heat flow
from the Earth’s interior at Yellowstone is about 30 times greater than that
typical for areas elsewhere in the northern Rocky Mountains. As snowmelt and
rainfall seep deep into the ground, they can absorb enough of this heat to
raise the temperature of the ground water close to the boiling point. Geyser
basins and other thermal areas in Yellowstone National Park are places where
hot ground water has risen close to the surface. Research drilling at
Yellowstone in the 1960s confirmed that the ground water beneath many of the
park’s thermal areas is very hot. At Norris Geyser Basin, water temperatures
as high as 460°F (238°C) were recorded at depths of only 1,090 feet (332 m).
Because the boiling point of water increases with
increasing pressure and pressure increases with depth, deep water can be
hotter than boiling water near the surface. If the pressure that confines
this deep water is reduced quickly, pockets of water may suddenly boil,
causing an explosion as the water is converted to steam. Such activity
drives the eruptions of geysers, like Old Faithful, which are repetitive
releases of plumes of steam and water. Rarely, steam explosions are more
violent and can hurl water and rock thousands of feet. In Yellowstone’s
geologic past, such violent events, called “hydrothermal explosions,” have
occurred countless times, creating new landscapes of hills and craters.
A recent and notable hydrothermal explosion occurred
in 1989 at Porkchop Geyser in Norris Geyser Basin. The remains of this
explosion are still clearly visible today as an apron of rock debris 15 feet
(5 m) across surrounding Porkchop’s central spring. In the 1880s and early
1890s, a series of powerful hydrothermal explosions and geyser eruptions
occurred at Excelsior Geyser in the Midway Geyser Basin. Some of the
explosions hurled large rocks as far as 50 feet (15 m).
Much larger hydrothermal explosions have occurred at
Yellowstone in the recent geologic past. More than a dozen large
hydrothermal-explosion craters formed between about 14,000 and 3,000 years
ago, triggered by sudden changes in pressure of the hydrothermal system.
Most of these craters are within the Yellowstone Caldera or along a
north-south-trending zone between Norris and Mammoth Hot Springs.
The largest hydrothermal-explosion crater documented
in the world is along the north edge of Yellowstone Lake in an embayment
known as Mary Bay. This 1.5-mile (2.6 km)-diameter crater formed about
13,800 years ago and may have had several separate explosions in a short
time interval. What specifically triggered these very large events is not
firmly established, but earthquakes or a pressure release caused by melting
glaciers or rapid changes in lake level may have been a significant factor.
These very large and violent hydrothermal explosions
are independent of associated volcanism. None of the large hydrothermal
events of the past 16,000 years has been followed by an eruption of magma.
The deeper magma system appears to be unaffected even by spectacular steam
explosions and crater excavations within the overlying hydrothermal system.
Although large hydrothermal explosions are a feature
of Yellowstone’s recent geologic history, most explosions in historical
times have been relatively small and have left craters at most a few yards
across. For example, in early 2003, a long linear fissure appeared on a
hillside above Nymph Lake, north of Norris Geyser Basin, venting steam and
throwing bits of rock onto the surrounding hillside. Although most
hydrothermal explosions in the park are small, their remains can be noticed
by observant visitors and attest to the nearly continuous geologic activity
at Yellowstone.
Go to the
Volcano Main Page
Text from the United States Geological Survey website.
Yellowstone Net is Produced by
Bruce Gourley,
Russ Finley,
&
Tim Gourley.
Copyright 1997-2005 by Bruce Gourley.
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