Pilot Knob (Austin, Texas)
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Pilot Knob is the eroded core of an
extinct volcano A volcano is commonly defined as a vent or fissure in the Crust (geology), crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Earth ...
located in
Austin, Texas Austin ( ) is the List of capitals in the United States, capital city of the U.S. state of Texas. It is the county seat and most populous city of Travis County, Texas, Travis County, with portions extending into Hays County, Texas, Hays and W ...
, United States. It is near
Austin–Bergstrom International Airport Austin–Bergstrom International Airport, or ABIA , is an international airport in Austin, Texas, United States, serving the Greater Austin metropolitan area. Located about southeast of downtown, it covers and has two runways and three heli ...
and
McKinney Falls State Park McKinney Falls State Park is a state park in Austin, Texas, United States at the confluence of Onion Creek and Williamson Creek. It is administered by the Texas Parks and Wildlife Department. The park opened on April 15, 1976 and is named after ...
.


Introduction

Pilot Knob is one of around 75 late-
Cretaceous Period The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 million years, it is the ninth and longest geologi ...
volcanoes scattered around
Central Texas Central Texas is a region in the U.S. state of Texas roughly bordered on the west by San Saba, to the southeast by Bryan- College Station, the south by San Marcos and to the north by Hillsboro. Central Texas overlaps with and includes part ...
from
Waco Waco ( ) is a city in and the county seat of McLennan County, Texas, United States. It is situated along the Brazos River and I-35, halfway between Dallas and Austin. The city had a U.S. census estimated 2024 population of 146,608, making i ...
to Austin,
San Antonio San Antonio ( ; Spanish for " Saint Anthony") is a city in the U.S. state of Texas and the most populous city in Greater San Antonio. San Antonio is the third-largest metropolitan area in Texas and the 24th-largest metropolitan area in the ...
, 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 Trap rock, also known as either trapp or trap, is any dark-colored, fine-grained, non-granitic intrusive or extrusive igneous rock. Types of trap rock include basalt, peridotite, diabase, and gabbro.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A ...
which is an erosion-resistant, fine-grained
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
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 Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...
rock, mainly
lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificati ...
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
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
time, Central Texas was part of a vast marine shelf on which
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
rocks were deposited with the entire area gradually subsiding as sediments were laid down. The volcano formed when
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
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 A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...
. Explosive eruptions continued at Pilot Knob as new magma encountered more water in the
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
. 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 Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
and
marl Marl is an earthy material rich in carbonate minerals, Clay minerals, clays, and silt. When Lithification, hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. M ...
of the Austin Group around Pilot Knob; these ash beds provide evidence for
subaerial eruption In volcanology, a subaerial eruption is any sort of volcanic eruption that occurs on the land surface, or in the open air "under the air". They generally produce pyroclastic flows, lava fountains and lava flows, which are commonly classified in di ...
s at Pilot Knob. The Pilot Knob ash cone eventually built an unstable slope on the sea bottom, resulting in mud flows 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 subaerial Pilot Knob ash cone allowed the intrusion of magma into the mound without contact with sea water, resulting in quieter
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
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 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 emplacement, 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 A cone sheet is a type of high-level igneous intrusion of subvolcanic rock, found in partly eroded central volcanic complexes. Cone sheets are relatively thin inclined sheets, generally just a few metres thick, with the geometry of a downward-poi ...
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 (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
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 Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
rocks. The two means are very different, but supplement each other nicely.


Stratigraphy

The volcano at Pilot Knob exerted a profound influence on the
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
of the Upper Cretaceous sedimentary rocks of the Austin area. The volcanism occurred during the deposition of the upper part of the
Dessau Dessau is a district of the independent city of Dessau-Roßlau in Saxony-Anhalt at the confluence of the rivers Mulde and Elbe, in the ''States of Germany, Bundesland'' (Federal State) of Saxony-Anhalt. Until 1 July 2007, it was an independent ...
and the Burditt formations of the
Austin Chalk The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas. The formation is made up of chalk and marl. Fossils The putative galloansera ...
. 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 The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...
(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 The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of Western Canada, to New Mexico in ...
Region.


Isotopic age

The potassium-argon method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
of
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
(potassium 40) to argon 40, an isotope of
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, 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, Texas Uvalde ( ) is a city in and the county seat of Uvalde County, Texas, United States. The population was 15,217 at the 2020 census, down from 15,751 in 2010. It is the principal city in the Uvalde, Texas Micropolitan Statistical Area. Uvalde is ...
, 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 The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...
by partial melting of mantle rocks. One of these magma types is basaltic in composition, composed of
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
, titanium-rich
pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...
, and
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
. The other magma type crystallized to
nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatit ...
,
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 ( Ca, Na)2( Al, Mg ...
,
pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...
, and, in some of the rocks,
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
. Some nepheline-bearing igneous rocks contain rounded inclusions of spinel
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...
, an
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...
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 south of Austin on Highway 183 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.


In popular culture

The second season of the American
procedural drama A procedural or procedural drama is a cross-genre type of literature, film, or television program which places emphasis on technical detail. A documentary film may also be written in a procedural style to heighten narrative. A popular subgenre is t ...
television series '' 9-1-1: Lone Star'' featured a scenario where Pilot Knob reactivated, and the characters' attempts at saving lives as a result of the eruption.


References

{{reflist 2) http://legacy.lib.utexas.edu/geo/ggtc/ch4.html Cretaceous Texas Volcanoes of Texas Extinct volcanoes of the United States Cretaceous volcanoes