Big Bend National Park

Intrusive Igneous Processes

Big Bend National Park exhibits all of the previous classifications of igneous rocks. Some of the most prevalant igneous features in the park are intrusive. Many times between 70 and 20 million years ago, the Big Bend region experienced great intrusions of magma. Many of the resulting intrusive features can be seen today as a result of erosional processes carrying away the overlying sedimentary rocks. Laccoliths, dikes, sills, and volcanic necks are many of the intrusive features that can be seen throughout the park, most of which can be viewed while driving on the park's highways.

Volcanic Necks and Vents

	Volcanoes, of course, are extrusive igneous features -- but part of a volcano cools underground and is considered an intrusive igneous feature. This feature is known as the volcanic neck or plug. The picture to the left is a depiction of the cross-section of the volcano that cooled, leaving the volcanic neck below.

The photograph to the right is a classic example of a volcanic neck. It is located on the Maverick Drive between Panther Junction and Ross Maxwell Drive on the north side of the road. Notice from the photograph that the volcanic body is slowly eroding away, leaving the more resistant igneous neck exposed.

The photograph to the left displays a massive volcanic neck that is surrounded by a layer of tuff. Scattered throughout the tuff is scoria, a blackish red igneous rock with many large vesicles. Vesicles are chambers of gas that get trapped in a rock while it is cooling. This volcanic neck is located a few miles south of Tuff Canyon on the west side of the road. A few volcanic vents peek up from the tuff surrounding the huge neck. These vents were once underground veins of molten rock.

Peeping up from the tuff flow described above, this volcanic vent resembles petrified wood both in texture and in appearance. The "woody" texture shown in the photograph is actually caused by how this volcanic vent cooled. When molten rock cools rapidly, it expands and often this results in cracks on the surface of the rockface. These cracks are commonly called cooling cracks.

Laccoliths

Igneous Processes Page