Travel Reference
In-Depth Information
N
ATURAL AND
H
UMAN
H
ISTORY
Geological History
THE STORIES IN YELLOWSTONE'S ROCKS
The greater Yellowstone area is a showcase for some of the world's best examples of spectacular
geological features. Geology is more than just identifying rocks; it's about the evolution and
structure of the earth beneath us. In this chapter we explain the 3-billion-year development of
the region and speculate about future events. We also discuss some geological features you may
find in the park. Numerals like
[GEO.1]
help you locate such places on the map.
Early geological history
The North American continent has existed for approximately 4.25 billion years, although it was
not always so large. The oldest rocks in the vicinity of Yellowstone Park are about 3.5 billion
years old and contain minerals that are about 4 billion years old. These rocks are accessible at
the roadside on the Northeast Entrance Road in Lamar Canyon
[GEO.1]
and where U.S. High-
way 212 crosses the Beartooth Pass outside the Northeast Entrance.
These very old rocks are
granites
and
gneisses.
They are coarse-grained rocks often contain-
ing mineral crystals one-half inch (1 cm) in diameter. They formed at depths in the earth of 5
to 10 miles (8-16 km) and at very high temperatures. Granites form by crystallizing, that is,
solidifying, from molten rock. They appear as a uniform mixture of light and dark minerals.
Gneisses form when existing rock is pressed and pulled by movements within the earth and is
heated up but not melted. The rock is changed, or
metamorphosed,
and a new rock is formed.
It contains different minerals and acquires an appearance of contorted light and dark bands of
minerals. Since their formation deep below a high mountain range, the granites and gneisses
have been uplifted to the surface and then eroded down. A new mountain range was formed
about 2.5 billion years ago, then also uplifted to the surface and eroded down.
Not much is known about the Yellowstone region from the time of those ancient mountains
until about 500 million years ago, a time gap of 2 billion years.
Along the Gallatin River Canyon north of the park and at a few places within the park, we
can see sedimentary rock layers lying flat in a few cliff faces. Most
sedimentary rocks
are formed
by transport of sediments, that is, rock or mineral particles, to low areas or shallow seas, where
they are deposited. They are transported by wind, streams, ocean currents, or glaciers; then
