Travel Reference
In-Depth Information
Firehole Falls
There are cascades in the river both above and below the falls. All are caused by the varying
resistance that different rocks offer to river erosion.
Rhyolite and the Plateau of Fire
Lava is molten rock that reaches the earth's surface, and rhyolite is one type of lava. Rhy-
olite lava, which formed the rocks exposed along Firehole Canyon Drive, is very stiff and
does not flow readily (that is, it's highly viscous), behaving much like asphalt or molasses.
It's so stiff that a lot of force is required to push it out from within the earth. As it's being
pushed out, it rips fragments from the rocks through which it's flowing and carries them
to the surface.
If you stop at Firehole Falls, look closely at the rock face around the parking area. Por-
tions of the rock consist of thin layers—a pencil thickness or less—that extend in the same
direction as the original flow motion. This rock cooled very quickly. It didn't have time to
form crystals throughout, but ended up consisting mostly of what geologists term
glass.
The very dark layers you can see are glass, and the light layers are mostly very tiny crystals
of the minerals quartz and feldspar. Embedded in the rhyolite are rock fragments that have
sharp, jagged edges. These were the bits of rock that the lava ripped out and carried along
as it came toward the surface.
