America's Volcanic Past
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"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:
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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
formed:1
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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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