Travel Reference
In-Depth Information
Y
ELLOWSTONE
C
ALDERA
E
VENT.
The third and latest explosion was 639,000 years ago and the
best understood. The earlier ones were similar but have been partly obscured by the later
events. This one formed the present Yellowstone Caldera. Rocks and tephra blown out this
time are called the Lava Creek tuff You can see this tuff on the west side of the road just north
of Gibbon Falls
[GEO.9],
at Tuff Cliff near Madison Junction, and at the top of the Lewis
River Canyon cliffs on the South Entrance Road
[GEO.10].
This latest eruption, with tephra
accumulations occasionally exceeding 1000 feet (300 m) in thickness, produced a volume of
240 cubic miles (1000 cu km) of tephra, which makes it intermediate in scale between the two
previous caldera events.
In comparison with the impressive eruption at Mount St. Helens, Washington, in May
1980, the Yellowstone Caldera (or Lava Creek) eruption blew out about one thousand times
more rock. The Mount St. Helens output of volcanic debris was about 0.25 cubic miles (1 cu
km).
The elliptical caldera resulting from this last eruption is approximately 30 by 45 miles (48
by 72 km). A good place to see the northern edge of the caldera is along the east-west stretch
of road that leads from Madison Junction to Gibbon Falls.
You may have noted that the intervals between the three caldera-forming events were
800,000 and 661,000 years, and the last caldera explosion was 639,000 years ago. We'll discuss
below whether we are due for another soon.
P
OST-
C
ALDERA
L
AVAS.
After each of the cataclysmic caldera-forming eruptions, magma spilled
out onto the caldera floor at intervals for several hundred thousand years as rhyolite flows
(see “Rhyolite and the Plateau of Fire,”
page 49
), filling much of the caldera. This explains why
there is no deep hole or crater now visible in or near the Yellowstone Caldera.
It is difficult to get a sense of the Yellowstone Caldera when you're in the park. In fact, the
existence of a caldera of this size was not proposed until the 1960s. After many years of geolo-
gic mapping, geoscientists realized the following: first, vast areas outside the oval-shaped area
consisted of just one volcanic unit, the Lava Creek tuff, and it formed at just one time; second,
most of the lavas within the oval area were younger than the Lava Creek tuff; and third, tephra
seen in other states (as far away as Iowa) came from the Yellowstone area and has proven to
be the same as the Lava Creek tuff.
A good place to get a feel for the enormous caldera event is along the Grand Loop Road in
the vicinity of Gibbon Falls, south of Norris Junction. The 5 miles of highway between Madis-
on Junction and Gibbon Falls parallel the caldera edge
[GEO.11].
On the north side of the
road, you can see a cliff whose upper part is Lava Creek tuff This tuff was left when the cal-
dera, which was on the south side here, slid down along a
fault
during its collapse (see Figure
