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America's Volcanic Past
Big Bend National Park, Texas

"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

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Map, Location of Texas

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
  • Brief Geologic History
  • Burro Mesa
  • Chisos Mountains
  • Christmas Mountains
  • Fool's Gold
  • Goat Mountain
  • Government Spring Laccolith
  • Mule Ears Peaks
  • Ross Maxwell Scenic Drive
  • Tule Mountain
  • America's Volcanic Past - Texas

Big Bend National Park

Big Bend National Park:
The abundance, diversity and complexity of visible rock outcrops is staggering, especially to first-time observers. From 500 million year old rocks at Persimmon Gap to modern-day windblown sand dunes at Boquillas Canyon, geologic formations in Big Bend demonstrate amazingly diverse depositional styles over a vast interval of time. For most of us, time is measured by the passing of days, years and generations. The concept of geologic time, however, is not so easily understood. Events that occurred 2 million, 26 million or as many as 120 million years ago are, at best, difficult to comprehend. Since astronomers now place the age of the earth at approximately 4.6 billion years we should perhaps consider ourselves fortunate that the oldest rocks found in the Big Bend are only about 500 million years old.


Excerpts from: U. S. National Park Service Website, Big Bend National Park, 2002

   
Brief Geologic History

500 million years ago:1
Accumulation of deep sea sediments along the southern edge of North America. Sandstones, shale, and limestone at Persimmon Gap and near Marathon.

300 million years ago - First major mountain building:1
For a period of at least 200 million years, ending some 300 million years ago in the Paleozoic Era, a deep-ocean trough extended from present-day Arkansas and Oklahoma into the Big Bend region of far West Texas. Sediments from highlands to the north accumulated in that trough to form layers of gravel, sand and clay. With the passing of time, these layers became sandstone and shale beds. About 300 million years ago these strata were "squeezed" upward by collision with a continent to the south to form the ancestral Ouachita mountains. Subsequent erosion over an interval of 160 million years left only the roots of those mountains visible. These remnants may be observed today in the Ouachita Mountains of southeastern Oklahoma, in the immediate vicinity of Marathon, Texas, and in Big Bend National Park near Persimmon Gap.

135 million years ago - Deposition:1
A warm, shallow sea invaded the Big Bend during the Cretaceous Period, some 135 million years ago, providing the setting for deposition of lime mud and the remains of sea-dwelling organisms such as clams and snails. Limestone layers formed from those shallow muds are now visible throughout much of the Big Bend. They comprise the dramatic walls of Santa Elena, Mariscal and Boquillas canyons, the entire range of the Sierra del Caballo Muerto (Dead Horse Mountains) and the magnificent cliffs of the Sierra del Carmen in Coahuila, Mexico, towering above Rio Grande Village.

100 million years ago - Fossils:1
Approximately 100 million years ago the shallow Cretaceous sea began a gradual retreat to its present location, the Gulf of Mexico. Sandstone and clay sediments that formed along the retreating shoreline are found in lowlands surrounding the Chisos Mountains. Shallow water strata of this episode contain the fossil remains of oysters, giant clams, ammonites, and a variety of fishes and marine reptiles. Near-shore deposits in Big Bend have yielded petrified wood, fossil turtles and crocodiles -- one almost 50 feet long! Deposits from further inland contain fossil remains of a variety of dinosaurs.

Second major mountain building:1
Near the end of the Cretaceous Period, a west-to-east compression of the earth's crust marked the beginning of the second major mountain-building period in Big Bend. This compression, which began in Canada, moved gradually southward, uplifting and folding ancient sediments to form the Rocky Mountains. In Big Bend National Park, Mariscal Mountain represents the southernmost extension of the Rockies in the United States. Broad uplift punctuated by upward folding exposed both the erosion-resistant lower Cretaceous limestones and the less resistant overlying sandstones and clays to the onslaught of erosion. Limestone cliffs throughout the region continue to be eroded today; most of the more easily removed sandstone and clay is gone from the mountains.

42 million years ago - Volcanic Activity:1
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 (Cenozoic Era). Upwelling magma lifted the mass now known as the Christmas Mountains, fracturing and weakening overlying 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.

30 million years ago - Volcanoes:1
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. Volcanic activity was not continuous during these eruptive cycles. Periods of hundreds of thousands or perhaps millions of years passed between eruptions. During the quiet interludes the forces of erosion carved new landscapes, many of which were destined to be buried under layers of ash and lava from later eruptions. Life returned to the land only to be displaced by future eruptions.

Intrusive igneous rocks and continuing volcanism:1
Elsewhere in the Big Bend rising magma sometimes failed to reach the surface. Instead, it spread within existing layers of rock, uplifting and fracturing overlying strata. Once the magma cooled and crystallized it formed solid masses of erosion-resistant intrusive igneous rock which have now been exposed by erosion of the overlying material. Maverick Mountain, the Grapevine Hills, Nugent Mountain and Pulliam Ridge are among many examples in Big Bend of such "frozen" magma chambers.

26 million years ago - Basin and Range and igneous intrusions (dikes):1
Beginning some 26 million years ago, stresses generated along the West coast of North America resulted in stretching of the earth's crust as far east as Big Bend. As a result of these tensional forces fracture zones developed which, over time, allowed large bodies of rock to slide downward along active faults. The central mass of Big Bend National Park, including the Chisos Mountains, from the Sierra del Carmen to the east to the Mesa de Anguila to the west comprises such a block of rocks dropped downward by faulting. The western slopes of the Chisos Mountains provide evidence of additional activity within the same fracture zones. Near the old ranch on the Ross Maxwell Scenic Drive stand a number of parallel ridges to the east of the road. These ridges are the eroded remains of tabular intrusions of magma along the Burro Mesa fault. The layers of volcanic ash into which the magma intruded are being actively removed by erosion, leaving the more resistant "dikes" of intrusive rocks standing in bold relief.

2 million years ago - The Rio Grande drainage established and major canyons formed:1
Mountain building by compression, volcanism and tension all served to form the framework for today's landscapes in Big Bend National Park. Erosion of higher lands resulted in the filling of surrounding basins. Eventually basins from El Paso to Big Bend were filled and subsequently linked by the Rio Grande. Achieving through-flow to the Gulf of Mexico only within the last 2 million years, the Rio Grande ranks as the youngest major river system in the United States. Once established, the Rio Grande served, and continues to serve, as the conduit for material removed by erosion.

   

Burro Mesa

Burro Mesa:1
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]

Burro Mesa:2
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.




Chisos Mountains

Chisos Mountains:2
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.

Chisos Mountains:1
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]




Christmas Mountains

Christmas Mountains:1
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.




Fool's Gold

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

Goat Mountain:2
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

Tule Mountain:2
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


Excerpts from:
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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06/14/02, Lyn Topinka