Travel Reference
In-Depth Information
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Madison Junction. To the northeast about 13 miles (21 km) away are Norris Junction
and Norris Geyser Basin. Upper Geyser Basin and Old Faithful Village are 17 miles (27 km)
to the south.
At the Edge of the Yellowstone Caldera
The road approaching Madison Junction from the west enters one of the major geological
features of Yellowstone Park, the Yellowstone Caldera or huge crater that formed 639,000
years ago. (Figure 6 in the Geological History chapter shows how
calderas
form.) The cal-
dera no longer looks like a vast crater but rather is part of the large Yellowstone Plateau.
The location of the crater's rim is determined by the difference in the rocks on either side
of it rather than by the elevation of those rocks. At the time of the formation of the Yel-
lowstone Caldera, a vast volcanic explosion occurred, which blew out at least 1,000 times
as much rock as the Mount St. Helens eruption in Washington state did in 1980. The mol-
ten rock from a huge underground chamber was blown high into the air and then settled
as tephra (now called the
Lava Creek tuff).
). The chamber extended from here east beyond
the present-day Canyon Village and Fishing Bridge Junction areas and south almost to the
edge of the park. Its boundary is shown by the dashed line on the map given to park visit-
ors and on the map on
page 302
.
The tephra settled out partly within the caldera itself but mostly over the nearby land
and was also carried to most of the southern Rockies, eastward to the Great Plains as far as
Iowa, and southeast to the Gulf of Mexico. So much rock was expelled, along with
super-
heated
water and other gases at high pressure, that the roof of this partly emptied chamber
collapsed, leaving a large hole, the caldera.
The volcanic activity did not cease after the ground collapsed inward. More molten
rock rose into the chamber, forcing large volumes of thick, tacky
lava
out onto the caldera
floor. The lava flows continued intermittently over the course of a few hundred thousand
years, partly filling the caldera and building up a large, high flat area. This explains why
today the caldera no longer appears like a hole or crater.
