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:
[This list is just a sample of
various Texas volcanic features or events and is by no means inclusive.]
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Basin and Range:5
The Interior Plains:5 The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability. The Atlantic Plain:5 The Atlantic Plain is the flattest of the provinces. It stretches over 2,200 miles in length from Cape Cod to the Mexican border and southward another 1000 miles to the Yucatan Peninsula. The Atlantic plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf. This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era.
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| Basin and Range |
Texas's Basin and Range:2
Igneous, metamorphic, and sediments.
North-South mountains and basins.
Some complex folding and faulting.
Maximum Elevation: 8,750 feet;
Minimum Elevation: 1,700 feet.
| Big Bend National Park |
Big Bend National Park:4
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.
| Central Texas Uplift |
Central Texas Uplift:2
Granites, metamorphics, and sediments.
Knobby plain surrounded by questas (ridges),
centripetal dips, strongly faulted.
Maximum Elevation: 2,000 feet.
Minimum Elevation: 800 feet.
| Chisos Mountains |
Chisos Mountains:4
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.
Ross Maxwell Scenic Drive:4
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.
| Christmas Mountains |
Christmas Mountains:4
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.
| Davis Mountains State Park |
Davis Mountains:3
The Davis Mountains, the most extensive mountain range in Texas,
were formed by volcanic activity during the Tertiary
geologic period, which began around 65 million years ago.
These mountains were named after Jefferson Davis, U.S. Secretary of
War and later President of the Confederacy, who ordered the
construction of the Fort Davis army post. Most Indian bands passed
through the Davis Mountains, although the Mescalero Apaches
made seasonal camps. As west Texas settlements increased,
raiding in Mexico and along the San Antonio-El Paso Trail
became a way of life for Apaches, Kiowas, and Comanches. Few
Americans had seen the Davis Mountains prior to 1846.
After the war with Mexico, a wave of gold seekers, settlers, and traders
came through the area and needed the protection of a military post -
Fort Davis. Fort Davis was active from 1854 until 1891, except
for certain periods during the Civil War. In 1961,
the historic fort ruins were declared a National Historic Site, and a vast
restoration/preservation program was initiated by the National Park Service.
| Enchanted Rock State Natural Area |
Enchanted Rock:3
Enchanted Rock State Natural Area consists of 1643.5 acres on Big Sandy Creek,
north of Fredericksburg, on the border
between Gillespie and Llano Counties.
It was acquired by warranty deed in 1978 by the
Nature Conservancy of Texas, Inc., from
the Moss family. The state acquired it in 1984,
added facilities, and reopened the park in March 1984,
but humans have visited here
for over 11,000 years. Enchanted Rock was designated a
National Natural Landmark in 1970 and was placed on the National
Register of Historic Places in 1984.
The Rock is a huge, pink granite exfoliation dome,
that rises 425 feet above ground, 1,825 feet
above sea level, and covers 640 acres.
It is one of the largest batholiths
(underground rock formation uncovered by erosion) in the
United States.
Enchanted Rock - National Natural Landmark:8
Located mainly in Gillespie County and extends into Llano
County - 12 miles southwest of Oxford. A classic illustration of a
batholith and of the exfoliation process exposed on its
dome-shaped surface, composed of coarse-grained pink
granite uniform in composition and texture throughout, and
unique in the Llano Uplift area. Owner: State.
DESIGNATION DATE:
October 1971
| Inks Lake State Park |
Inks Lake State Park:3
Inks Lake State Park is 1201 acres of recreational facilities adjacent to Inks Lake on the Colorado River in Burnet County. The
park was acquired by deeds from the Lower Colorado River Authority and private owners in 1940 and was opened to the public in
1950. Inks Lake is located in the Highland Lakes chain (7 lakes) surrounded by granite hills. The water level of Inks Lake is usually
unaffected by drought and is maintained at a normal level most of the time. During flooding situations, the lake level can rise as the
flood waters are passed through Inks Lake to other lakes downstream.
| Volcanic Ash Deposits |
Ash Deposits from Yellowstone Caldera:7
Ash deposits
from these powerful eruptions (Yellowstone Caldera)
have been mapped as far away as
Iowa, Missouri, Texas, and even
northern Mexico.
Excerpts from:
1)
2) Tectonic Map of Texas, 1997, Texas Bureau of Economic Geology.
3) Texas State Parks and Historic Sites Website, 2002
4) U.S. National Park Service Website,
Big Bend National Park, 2002
5) USGS/NPS Geology in the Parks Website, 2001
6) USGS A Tapestry of Time and Terrain Website, 2002
7) Kious and Tilling,
This Dynamic Earth: The Story of Plate Tectonics:
USGS Publication, online web version 1.08
8) U.S. National Park Service, National Natural Landmarks Website, 2003
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