Dikes are vertical wall-like structures that form as a result of magma being
injected into the fractures of rocks. Dikes are discordant features.
This means that they cut through existing layers of rocks. Being intrusive,
dikes crystallize below the ground. When the surrounding rock is eroded,
dikes are exposed and often appear as dark walls of rock. Pictured to the
left is a rhyolite dike that cuts through a bed of shale.
||On the road that leads west from Panther Junction, many dikes can be
seen before you reach Tuff Canyon. Pictured to the right are two dikes
that run parallel to the road. One is in the foreground of the picture,
and the second runs atop the small hill on the right hand side. When
many dikes appear within close proximity, they termed a dike
||Before you reach Tuff Canyon, the road cuts through a rhyolite dike
that runs perpendicular to the road. This dike extends about a mile
to the east. An important feature to note about this dike is the cross
section of the dike created by the road cut. The size of the
phenocrysts in the dike increase in size as they extend toward the
center of the dike. This implies that the magma forming this dike
cooled faster on the edges of the dike and slower toward the center.
||Mule Ears Peaks is a prominent landmark in Big Bend National Park. The
peaks are found to the east of Tuff Canyon. These peaks are actually two
dikes that run parallel to each other. Noticethere is
still some country rock between them. At one time, that rock spanned
the two peaks.
The picture to the right shows a contact between a rhyolite
dike and the shale country rock which it cut through. There
is a layer of rock between the rhyolite and the shale that has
been metamorphosed. Typically, shale when metamorphosed turns
into slate or phyllite. However, in this case, the degree of
metamorphosis that occurred was not as great and the resultant
rock is known as baked earth.