Big Bend National Park

Extrusive Igneous Processes

Extrusive igneous rock is formed when molten rock reaches and spills over on the surface of the earth. These rocks are quite unique and some display interesting characteristics that give us a clue as to the nature of the rocks' origins.

Most extrusive rocks exhibit an aphanitic texture, meaning that their crystals are not visible to the unaided eye. Some extrusive rocks are riddled with vesicles, small holes in the rock left by gas bubbles during cooling. While others have no vesicles and appear to have a glassy texture. Regardless of their appearance and chemical make-up, all extrusive rocks share one characteristic, they cooled on the surface of the earth.

Pictured to the right is Tule Mountain. Tule mountain is composed of porphoritic andesite and Mule Ear Spring tuff. Both of these rocks belong to the Chisos Formation which formed due to massive volcanic activities during the late Eocene epoch. The rock which Tule Mountain caps is part of the Javelina formation, a layer of bentonite clays occasionally interbedded with sandstones.

	The picture to the left shows the composition of Burro Mesa. This formation can be found at the Burro Mesa Pouroff trail. The rock capping the mesa is an extrusive form of granite called rhyolite. The layer below the rhyolite is the Wasp Springs Flow Breccia Formation, a flow breccia with interbedded tuff. Breccia is an extrusive rock that contains sharp fragments of native rocks. The layer below the flow breccia is the same layer of porphoritic andesite that forms the top of Tule Mountain.

Goat Mountain, located along Ross Maxwell Drive in the southern portion of the park, is an excellent example of a lava flow. If you look closely at the photo on the right, you will notice the V-shaped cross section. This indicates that an ancient river once cut through the lowest layer of volcanic rock. Then a massive lava flow of porphoritic andesite filled the valley. This flow was also present in Tule Mountain, Burro Mesa, and the top of the Chisos Mountains.

	The rock pictured to the left is vesicular basalt. Notice the cavities left behind due to gas bubbles escaping while the rock cooled. This basalt can be found below the ash flows that surround the volcanic vents discussed in the Intrusive Igneous Features page.

The Chisos Mountains is the most prominent extrusive igneous feature in Big Bend National park. Between 30 and 60 million years ago, several large volcanoes erupted. The necks, lava flows, and collapsed calderas of these volcanoes remain today as the Chisos Mountains.