America's Volcanic Past
|"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994|
Volcanic Highlights and Features:
|[NOTE: This list is just a sample of various Big Bend features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Big Bend.]|
Big Bend National Park:
500 million years ago:1
300 million years ago - First major mountain building:1
135 million years ago - Deposition:1
100 million years ago - Fossils:1
Second major mountain building:1
42 million years ago - Volcanic Activity:1
30 million years ago - Volcanoes:1
Intrusive igneous rocks and continuing volcanism:1
26 million years ago - Basin and Range and igneous intrusions (dikes):1
2 million years ago -
The Rio Grande drainage established and major canyons
Between roughly 38 and 32 million years ago Big Bend itself hosted a series of volcanic eruptions. Initial activity in this cycle centered in the Sierra Quemada, below the present South Rim of the Chisos Mountains. Subsequent volcanic activity at Pine Canyon, Burro Mesa, near Castolon and elsewhere in the park is responsible for the brightly colored volcanic ash and lava layers of the lower elevations and for most of the mass of the Chisos Mountains. On the west side of the Ross Maxwell Scenic Drive you'll see the vivid colors of rock that make up Burro Mesa. These rocks were deposited by volcanoes active in Big Bend between 32-38 million years ago. [See Ross Maxwell Scenic Drive below]
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.
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.
Between roughly 38 and 32 million years ago Big Bend itself hosted a series of volcanic eruptions. Initial activity in this cycle centered in the Sierra Quemada, below the present South Rim of the Chisos Mountains. Subsequent volcanic activity at Pine Canyon, Burro Mesa, near Castolon and elsewhere in the park is responsible for the brightly colored volcanic ash and lava layers of the lower elevations and for most of the mass of the Chisos Mountains. The Chisos Mountains stand east of the road (Ross Maxwell Scenic Drive). Much of the rock that makes up these mountains are intrusive igneous rocks. [See Ross Maxwell Scenic Drive below]
Near the present northwest boundary of Big Bend National Park, the first of a long series of volcanic eruptions occurred approximately 42 million years ago. Upwelling magma lifted the mass now known as the Christmas Mountains, fracturing and weakening over-lying strata, allowing massive outpourings of lava to spread across the land. The oldest volcanic rocks in Big Bend owe their origins to this eruptive cycle.
Colorful igneous rocks:1
Rainwater contains free oxygen which reacts with sulfur-bearing minerals in igneous rocks. Virtually all igneous rocks in Big Bend contain minor amounts of pyrite, or Fool's Gold, which is iron sulfide. Oxygen-bearing water attacks individual pyrite grains, replacing the sulfur with oxygen to form iron oxide, better known as rust, which provides the warm red and brown colors of igneous rocks in the Big Bend.
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 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.
Government Spring Laccolith:2
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. 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.
|Mule Ears Peaks|
Mule Ears Peaks:2
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. Notice there is still some country rock between them. At one time, that rock spanned the two peaks. Dikes are vertical wall-like structures that form as a result of magma being injected into the fractures of rocks.
|Ross Maxwell Scenic Drive|
East side of the road:1
The Ross Maxwell Scenic Drive travels through an ancient volcanic landscape that has undergone thousands of years of erosion. The Chisos Mountains stand east of the road. Much of the rock that makes up these mountains are intrusive igneous rocks. When the molten rock (igneous) was rising to the surface it did not break the earth's surface (intrusive). It crystallized and became rock deep underground. Erosion has made it visible today. Another type of intrusive rock, called a dike, can be seen radiating away from the base of the mountains. A dike looks like a wall of dark rock cutting across the landscape.
West side of the road:1
On the west side of the road you'll see the vivid colors of rock that make up Burro Mesa. These rocks were deposited by volcanoes active in Big Bend between 32-38 million years ago. Stop at Burro Mesa Pouroff to get a closer look at the large, dark, pumpkin-sized rocks that flew from the volcano. These "bombs" are kept in place by the ash for us to see and touch. From the Burro Mesa Pouroff parking lot, you will see a yellow ash layer sandwiched between a dark red harder ash (rhyolite) and a dark brown rock on the bottom. The bottom brown rock is a mixture of cobbles and gravel of different sizes cemented together (a conglomerate). The dark brown conglomerate was deposited first. Erosion later scoured this conglomerate, causing an irregular surface. Following this erosional period a volcano erupted, leaving yellow ash. Finally, rhyolite ash was deposited. Burro Mesa Pouroff showcases just one of many intriguing stories that lie in the Big Bend landscape.
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.
|America's Volcanic Past - Texas|
1) U.S. National Park Service Website, Big Bend National Park, 2002
2) Texas A&M University, Department of Geology, Big Bend National Park Virtual Field Trip, Texas A&M Website, 2002, created by Cain and Leslie Neal for a Geology 485 Earth Science Education independent study project, 1996
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