Travel Reference
In-Depth Information
ing North American Plate. In this theory, the “plume” is not formed in a stationary hot area
deep in the mantle but is formed at a shallow depth by the movement of the plates them-
selves. This idea comes from interpretation and modeling of new seismic and other geophys-
ical data recently gathered by the USArray, a transportable array of thousands of geophysical
instruments comprising a dense grid covering the continental United States. Geoscientists are
actively developing new interpretations with lots of variations. The new interpretations can
explain the Yellowstone hot spot, the Newberry volcanic chain, and other features of the in-
termountain west such as the still-active basalt flows on the Snake River Plain more than 60
miles (100 km) southwest of Yellowstone. This theory does not explain how magma is gen-
erated at the front and along the sides of the fragments nor why the magma contains certain
elements.
Both the mantle plume and the fragmenting plate theories have ardent adherents. This is
an exciting time for geoscientists as they acquire new data, build new models, and confer with
each other in attempts to explain the Yellowstone hot spot.
II. Are we due for another caldera eruption?
We saw above that the last caldera-forming eruption was 639,000 years ago, while the pre-
ceding two major eruptions were at intervals of 800,000 and 661,000 years. Time intervals
between such major eruptions are not regular, but we are approaching the time when another
massive explosion could occur. How likely is that—and should we be concerned? What evid-
ence is there of the likelihood of a new caldera catastrophe?
The earth at Yellowstone is restless. Usually a thousand and frequently several thousand
small earthquakes occur every year. Most of these are far too small to be felt but are recorded
by the Yellowstone Volcano Observatory's (YVO) network of 26 seismic stations within or
near the park. Earthquake swarms, several hundred earthquakes that occur within several
days or weeks, are common. There have been at least 73 such swarms between 1983 and 2012.
Most of these have occurred outside the caldera on the northwestern part of the park, some
on the inside of the caldera on its northwest side, and a few inside the caldera on the southeast
side under Yellowstone Lake.
Areas within the park also rise and fall, and the YVO has special recording stations for
slow ground movements (changes in location, elevation, and tilt), geyser temperature, gas
discharge, and water levels. In addition, historical data tells us that the Mallard Lake
dome
[GEO.25]
just east of Old Faithful and the Sour Creek dome
[GEO.26]
east of Hayden Valley
(see map on
page 302
)
both rise (inflate) and then fall (deflate) in irregular cycles. In fact, the
Sour Creek dome area rose a total of 36 inches (91 cm) between 1923 and 1984. It then fell
somewhat for a decade but recently has been rising again; the Mallard Lake dome has moved
similarly.
