Big Bend National Park

Intrusive Igneous Processes

Big Bend National Park exhibits all of the previous classifications of igneous rocks. One 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.

Laccoliths

	Laccoliths are a second category of intrusive structures that are visible in Big Bend National Park. Laccoliths form when magma is injected between layers of rock. The pressure of the magma being injected is high enough that the overlying strata are forced upward; this forms the laccolith's characteristic mushroom shape. A laccolith is a concordant feature.

	Government Spring Laccolith is an example of a laccolith whose overlying strata have been eroded away, exposing the structure's mushroom shape. This particular laccolith is composed of syenite, a mafic intrusive igneous rock. Government Springs Laccolith is located just north of the Chisos Mountains and is visible from several major roads. Other laccoliths in the park include the Grapevine Hills, Paint Gap Hills, and the Rosillos Mountains.

Dikes

Volcanic Necks