Pilot Knob (Austin, Texas)
Encyclopedia
Pilot Knob is a hill that is part of an extinct volcano located seven miles south of central Austin, Texas
, near Austin-Bergstrom International Airport
and McKinney Falls State Park
.
from Waco
to Austin, San Antonio
, and Del Rio
. All of these volcanoes have been extinct for millions of years.
The Pilot Knob volcanic complex consists of four small, rounded hills (including Pilot Knob proper) forming the volcano's core area in an area two miles in diameter. The hills are composed of trap rock
which is an erosion-resistant, fine-grained mafic
volcanic rock. The complex rises above a circular lowland drained by Cottonmouth Creek and is underlain chiefly by volcanic ash and other pyroclastic debris
. Several smaller bodies of trap rock occur in the volcanic ash. A topographic rim surrounding the Cottonmouth Creek lowland to the north is formed by sedimentary rock, mainly lithified beach sediments composed of shell fragments and reworked volcanic ash that accumulated in the shallow waters around the volcano.
time, Central Texas was part of a vast marine shelf on which carbonate
rocks were deposited with the entire area gradually subsiding as sediments were laid down. The volcano formed when magma
worked its way to the surface and encountered water-laden, unconsolidated sediments with the existing water rapidly vaporizing into steam resulting in an enormous explosion that formed an explosion crater
. Explosive eruptions continued at Pilot Knob as new magma encountered more water in the volcanic ash. Gradually, an ash cone
was built up over the explosion crater. Eruptions of ash
continued until the mound grew above the level of the shallow sea. Ash beds, now altered to clay, occur interbedded with limestone
and marl
of the Austin Group around Pilot Knob; these ash beds provide evidence for subaerial eruption
s at Pilot Knob. The Pilot Knob ash cone eventually built an unstable slope on the sea bottom, resulting in mudflows of ash and carbonate mud which tore up the underlying carbonate mud in places and injected itself into the carbonate mud at other places. The subaireal Pilot Knob ash cone allowed the intrusion of magma into the mound without contact with sea water, resulting in quieter lava eruptions. Such magma cooled and solidified to form the core and satellite areas of the trap rock. Some of the trap rock bodies are the erosional remnants of lava
flows, due to their apparent dip away from the central core area. Cooling joints exposed on a hill about 1,500 feet west of Pilot Knob suggest a dip of that trap rock body towards the center of the core area, possibly indicating that it is the erosional remnant of a cone sheet injected outwards from a central, discordant intrusive body of magma. Exposures at other bodies of trap rock are not generally good enough to determine their exact implacement, but some, at least, are probably plugs of solidified intrusive magma. Magnetic anomalies on the northeast flank of the core area suggest a buried trap rock body within the ash mound, possibly a cone sheet or lava flow.
As volcanic activity diminished, beaches developed around the volcano. One such beach deposit, now lithified and resistant to erosion, extends several miles to the north of the volcano. It appears along Onion Creek, where it is responsible for both Upper and Lower McKinney Falls. The entire shelf continued to subside after volcanic activity ceased, and muds of the Taylor Group gradually covered the entire volcano. During the Tertiary
era, the central Texas area was uplifted, exposing the volcano as younger sedimentary rocks were eroded from the Cretaceous volcanic rocks. Today the terrain at Pilot Knob reflects the relative resistance to erosion of the different rock types that appear around the volcanic complex.
of the Upper Cretaceous sedimentary rocks of the Austin area. The volcanism occurred during the deposition of the upper part of the Dessau and the Burditt formations of the Austin Chalk. These strata are only about one-third as thick near Pilot Knob as they are in other localities around Austin. The faunal zones of the two formations, that is zones within the formations where certain fossils are particularly abundant, are also telescoped in the vicinity of Pilot Knob. Two beds of altered ash occur within the Dessau Formation on Rinard Creek near Pilot Knob. The exact relationship of the beach rock of the northern flank of Pilot Knob with the normal Austin Chalk has been demonstrated to be part of the Exponderosa erraticosta zone, although it may extend down to an age equivalent to the upper part of the Dessau in some areas along Onion Creek. The beach rock is now known as the McKown Formation of the Austin Chalk. Because the volcanism at Pilot Knob occurred during the deposition of the Dessau and Burditt Formations, an age of Lower Campanian (83 to 79 million years BP) can be interpreted for the volcanism according to the ages of the formations in the Austin Chalk, correlated to radiometric dated sections in the Rocky Mountain
Region.
method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive isotope
of potassium
(potassium 40) to argon 40, an isotope of argon
, an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work. The age of Pilot Knob volcanism dated through the potassium-argon method is 79.5 +/- 3 million years, and agrees with the age derived by correlation with fossils to other radiometrically dated deposits.
, of about the same age, have been studied. The Uvalde rocks are of the same igneous province as the Pilot Knob rocks; both are centers in a chain of volcanic and intrusive complexes that parallel the buried Ouachita structural belt in central and south Texas. The Ouachita
belt may have provided a zone of crustal weakness where magma could rise from great depths through the crust to the surface.
Two types of magma have been determined to have generated in the Earth's upper mantle by partial melting of mantle
rocks. One of these magma types is basaltic in composition, composed of plagioclase
, titanium-rich pyroxene
, and olivine
. The other magma type crystallized to nepheline
, melilite
, pyroxene
, and, in some of the rocks, plagioclase
. Some nepheline-bearing igneous rocks contain rounded inclusions of spinel peridotite
, an ultramafic rock that may represent fragments of the upper mantle incorporated into the magma during its ascent to the surface. Most of the igneous rocks on the surface probably are not representative of the original magmas formed in the mantle; certain processes, such as separation of phenocrysts from the liquid from which they crystallized have altered their original chemical compositions at depths shallower than the mantle.
By inference, the rocks at Pilot Knob crystallized from partial melts of the upper mantle. Only rocks of the second series have been discovered at Pilot Knob; no basaltic rocks have been located. No mantle fragments have been located in Pilot Knob igneous rocks, but their presence is suspected, and some of the olivine phenocrysts in the rocks may be fragments derived from the upper mantle.
past Austin-Bergstrom International Airport. The hill is visible on the right after crossing Onion Creek. A circuit of the volcanic complex can be made by turning right off Highway 183 onto Dee Gabriel Collins Road and heading west to the intersection with McKinney Falls Parkway. Turn left and follow the Parkway to the intersection with Colton Bluff Springs Road, where a left turn leads back to Highway 183. Dee Gabriel Collins closely follows the northern topographic rim of the volcano formed by the escarpment of the Cretaceous beach rock. The lowland is drained by Cottonmouth Creek and underlain by soft, easily eroded volcanic ash and other pyroclastic deposits, and the central core area formed by the resistant trap rock are easily visible to the south of Dee Gabriel Collins. About 2/3 of a mile east of the intersection of Bluff Springs Road with McKinney Falls Parkway, the trap rock of south Pilot Knob crops out along the road. This rock is very similar to that which forms the center of the volcano, and may represent the eroded remnant of a lava flow erupted along the flanks of the volcano or having flowed down the south side from the apex.
Except for the road margins, Pilot Knob volcano lies entirely on private land.
Austin, Texas
Austin is the capital city of the U.S. state of :Texas and the seat of Travis County. Located in Central Texas on the eastern edge of the American Southwest, it is the fourth-largest city in Texas and the 14th most populous city in the United States. It was the third-fastest-growing large city in...
, near Austin-Bergstrom International Airport
Austin-Bergstrom International Airport
Austin-Bergstrom International Airport is a mixed-use commercial airport located southeast of the central business district of Austin, Texas, United States. It covers and has two runways and three helipads.The airport began passenger service on May 23, 1999...
and McKinney Falls State Park
McKinney Falls State Park
McKinney Falls State Park is a state park located at the southeastern edge of Austin, Texas, United States around the confluence of Onion and Williamson Creeks and is administered by the Texas Parks and Wildlife Department. The park opened on April 15, 1976 and is named after Thomas F...
.
Introduction
Pilot Knob is one of around 75 late-Cretaceous Period volcanoes scattered around Central TexasTexas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
from Waco
Waco, Texas
Waco is a city in and the county seat of McLennan County, Texas. Situated along the Brazos River and on the I-35 corridor, halfway between Dallas and Austin, it is the economic, cultural, and academic center of the 'Heart of Texas' region....
to Austin, San Antonio
San Antonio, Texas
San Antonio is the seventh-largest city in the United States of America and the second-largest city within the state of Texas, with a population of 1.33 million. Located in the American Southwest and the south–central part of Texas, the city serves as the seat of Bexar County. In 2011,...
, and Del Rio
Del Rio, Texas
Del Rio is a border city in and the county seat of Val Verde County, Texas, United States.. Del Rio is connected with Ciudad Acuña via the Lake Amistad Dam International Crossing and Del Río-Ciudad Acuña International Bridge...
. All of these volcanoes have been extinct for millions of years.
The Pilot Knob volcanic complex consists of four small, rounded hills (including Pilot Knob proper) forming the volcano's core area in an area two miles in diameter. The hills are composed of trap rock
Trap rock
Trap rock is a form of igneous rock that tends to form polygonal vertical fractures, most typically hexagonal, but also four to eight sided. The fracture pattern forms when magma of suitable chemical composition intrudes as a sill or extrudes as a thick lava flow, and slowly cools.Because of the...
which is an erosion-resistant, fine-grained mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
volcanic rock. The complex rises above a circular lowland drained by Cottonmouth Creek and is underlain chiefly by volcanic ash and other pyroclastic debris
Pyroclastic rock
Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...
. Several smaller bodies of trap rock occur in the volcanic ash. A topographic rim surrounding the Cottonmouth Creek lowland to the north is formed by sedimentary rock, mainly lithified beach sediments composed of shell fragments and reworked volcanic ash that accumulated in the shallow waters around the volcano.
History of the Volcano
In late-CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
time, Central Texas was part of a vast marine shelf on which carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
rocks were deposited with the entire area gradually subsiding as sediments were laid down. The volcano formed when magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
worked its way to the surface and encountered water-laden, unconsolidated sediments with the existing water rapidly vaporizing into steam resulting in an enormous explosion that formed an explosion crater
Volcanic crater
A volcanic crater is a circular depression in the ground caused by volcanic activity. It is typically a basin, circular in form within which occurs a vent from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of great depth...
. Explosive eruptions continued at Pilot Knob as new magma encountered more water in the volcanic ash. Gradually, an ash cone
Volcanic cone
Volcanic cones are among the simplest volcanic formations. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption...
was built up over the explosion crater. Eruptions of ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
continued until the mound grew above the level of the shallow sea. Ash beds, now altered to clay, occur interbedded with limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
and marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
of the Austin Group around Pilot Knob; these ash beds provide evidence for subaerial eruption
Subaerial eruption
A subaerial eruption is a volcanic eruption that has occurred on the surface. They generally produce pyroclastic flows, lava fountains, and lava flows, which are commonly classified in different subearial eruption types, including Plinian, Peléan, and Hawaiian eruptions. Subaerial eruptions...
s at Pilot Knob. The Pilot Knob ash cone eventually built an unstable slope on the sea bottom, resulting in mudflows of ash and carbonate mud which tore up the underlying carbonate mud in places and injected itself into the carbonate mud at other places. The subaireal Pilot Knob ash cone allowed the intrusion of magma into the mound without contact with sea water, resulting in quieter lava eruptions. Such magma cooled and solidified to form the core and satellite areas of the trap rock. Some of the trap rock bodies are the erosional remnants of lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, due to their apparent dip away from the central core area. Cooling joints exposed on a hill about 1,500 feet west of Pilot Knob suggest a dip of that trap rock body towards the center of the core area, possibly indicating that it is the erosional remnant of a cone sheet injected outwards from a central, discordant intrusive body of magma. Exposures at other bodies of trap rock are not generally good enough to determine their exact implacement, but some, at least, are probably plugs of solidified intrusive magma. Magnetic anomalies on the northeast flank of the core area suggest a buried trap rock body within the ash mound, possibly a cone sheet or lava flow.
As volcanic activity diminished, beaches developed around the volcano. One such beach deposit, now lithified and resistant to erosion, extends several miles to the north of the volcano. It appears along Onion Creek, where it is responsible for both Upper and Lower McKinney Falls. The entire shelf continued to subside after volcanic activity ceased, and muds of the Taylor Group gradually covered the entire volcano. During the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
era, the central Texas area was uplifted, exposing the volcano as younger sedimentary rocks were eroded from the Cretaceous volcanic rocks. Today the terrain at Pilot Knob reflects the relative resistance to erosion of the different rock types that appear around the volcanic complex.
Dating the Volcanic Activity
Two different approaches have been used by geologists to date the volcanism at Pilot Knob. The first approach was through the stratigraphy of the rocks at and around Pilot Knob and the fossils they contain. The second approach was by isotopic age dating of the igneous rocks. The two means are very different, but supplement each other nicely.Stratigraphy
The volcano at Pilot Knob exerted a profound influence on the stratigraphyStratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of the Upper Cretaceous sedimentary rocks of the Austin area. The volcanism occurred during the deposition of the upper part of the Dessau and the Burditt formations of the Austin Chalk. These strata are only about one-third as thick near Pilot Knob as they are in other localities around Austin. The faunal zones of the two formations, that is zones within the formations where certain fossils are particularly abundant, are also telescoped in the vicinity of Pilot Knob. Two beds of altered ash occur within the Dessau Formation on Rinard Creek near Pilot Knob. The exact relationship of the beach rock of the northern flank of Pilot Knob with the normal Austin Chalk has been demonstrated to be part of the Exponderosa erraticosta zone, although it may extend down to an age equivalent to the upper part of the Dessau in some areas along Onion Creek. The beach rock is now known as the McKown Formation of the Austin Chalk. Because the volcanism at Pilot Knob occurred during the deposition of the Dessau and Burditt Formations, an age of Lower Campanian (83 to 79 million years BP) can be interpreted for the volcanism according to the ages of the formations in the Austin Chalk, correlated to radiometric dated sections in the Rocky Mountain
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
Region.
Isotopic Age
The potassium-argonPotassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(potassium 40) to argon 40, an isotope of argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work. The age of Pilot Knob volcanism dated through the potassium-argon method is 79.5 +/- 3 million years, and agrees with the age derived by correlation with fossils to other radiometrically dated deposits.
Source of the Magma
The igneous rocks of the Pilot Knob volcano have not, as yet, undergone rigorous petrological study, although such studies are in progress. Similar rocks near Uvalde, TexasUvalde, Texas
Uvalde is a city in and the county seat of Uvalde County, Texas, United States. The population was 14,929 at the 2000 census.Uvalde was founded by Reading Wood Black in 1853 as the town of Encina. In 1856, when the county was organized, the town was renamed Uvalde for Spanish governor Juan de...
, of about the same age, have been studied. The Uvalde rocks are of the same igneous province as the Pilot Knob rocks; both are centers in a chain of volcanic and intrusive complexes that parallel the buried Ouachita structural belt in central and south Texas. The Ouachita
Ouachita
Ouachita may refer to:In geography:* Ouachita Mountains* Ouachita orogeny the geologic event that raised the Ouachita Mountains* Ouachita River* Ouachita Parish, Louisiana* Ouachita County, Arkansas* Ouachita National Forest...
belt may have provided a zone of crustal weakness where magma could rise from great depths through the crust to the surface.
Two types of magma have been determined to have generated in the Earth's upper mantle by partial melting of mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
rocks. One of these magma types is basaltic in composition, composed of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, titanium-rich pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, and olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. The other magma type crystallized to nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
, melilite
Melilite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is 2[SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color...
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, and, in some of the rocks, plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
. Some nepheline-bearing igneous rocks contain rounded inclusions of spinel peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
, an ultramafic rock that may represent fragments of the upper mantle incorporated into the magma during its ascent to the surface. Most of the igneous rocks on the surface probably are not representative of the original magmas formed in the mantle; certain processes, such as separation of phenocrysts from the liquid from which they crystallized have altered their original chemical compositions at depths shallower than the mantle.
By inference, the rocks at Pilot Knob crystallized from partial melts of the upper mantle. Only rocks of the second series have been discovered at Pilot Knob; no basaltic rocks have been located. No mantle fragments have been located in Pilot Knob igneous rocks, but their presence is suspected, and some of the olivine phenocrysts in the rocks may be fragments derived from the upper mantle.
Location
Pilot Knob can be reached by driving out Highway 183U.S. Route 183
U.S. Route 183 is a north–south United States highway. U.S. 183 was the last US route to be completely paved. The 20 mile segment in Loup County, Nebraska, north of Taylor, was unpaved until 1967. The highway's northern terminus is in Presho, South Dakota, at an intersection with...
past Austin-Bergstrom International Airport. The hill is visible on the right after crossing Onion Creek. A circuit of the volcanic complex can be made by turning right off Highway 183 onto Dee Gabriel Collins Road and heading west to the intersection with McKinney Falls Parkway. Turn left and follow the Parkway to the intersection with Colton Bluff Springs Road, where a left turn leads back to Highway 183. Dee Gabriel Collins closely follows the northern topographic rim of the volcano formed by the escarpment of the Cretaceous beach rock. The lowland is drained by Cottonmouth Creek and underlain by soft, easily eroded volcanic ash and other pyroclastic deposits, and the central core area formed by the resistant trap rock are easily visible to the south of Dee Gabriel Collins. About 2/3 of a mile east of the intersection of Bluff Springs Road with McKinney Falls Parkway, the trap rock of south Pilot Knob crops out along the road. This rock is very similar to that which forms the center of the volcano, and may represent the eroded remnant of a lava flow erupted along the flanks of the volcano or having flowed down the south side from the apex.
Except for the road margins, Pilot Knob volcano lies entirely on private land.