Pilot Knob
Volcano Type: Cinder cone
Volcano Status: Extinct
Last Known Eruption: late Cretaceous
Summit Elevation: 169 m 554 feet
Latitude: 30°09′43″N
Longitude: 97°42′22″W
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.
In Late Cretaceous time, Central Texas was part of a vast marine shelf on which carbonate rocks were deposited, which the entire area gradually subsided as the sediments were laid down. The volcano formed when magma, working its way to the surface, encountered water-laden, unconsolidated sediments, which it then vaporized the water rapidly 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 eruptions 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.
Pilot Knob
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 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.
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, 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.
Pilot Knob can be reached by driving out 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.
Originally posted 2011-05-14 01:41:34.
