Hidden beneath the seemingly ordinary hill lies a remarkable secret. This is Pilot Knob. More than just a hill it’s the eroded core of an extinct volcano. Pilot Knob stands as a testament to a fiery past. Millions of years ago this quiet hill erupted violently.
Pilot Knob is one of approximately 75 late-Cretaceous volcanoes scattered across Central Texas. Imagine this area a vast marine shelf. Magma forced its way to the surface encountering water-saturated sediments. The result was a massive explosion creating an explosion crater. More eruptions followed building an ash cone above the shallow sea. This cone eventually became unstable leading to mudflows.
The volcano’s activity eventually ceased. Beaches formed around it. One such beach deposit created McKinney Falls. The area later rose exposing Pilot Knob. The terrain reflects the erosion resistance of the different rock types.
Geologists used two methods to date Pilot Knob’s volcanic activity. Stratigraphy examined rock layers and fossils. Isotopic dating used the decay of potassium-40 to argon-40. Both methods point to an age of 79 to 83 million years ago. The magma originated from the Earth’s upper mantle. Partial melting created two magma types. Only one type is found at Pilot Knob.
Pilot Knob’s location is easily accessible. Drive south of Austin on Highway 183 past the airport. You’ll see the hill to the right after crossing Onion Creek. A drive along Dee Gabriel Collins Road provides a circuit of the volcano. You can even see some trap rock along the road. These remnants possibly represent an eroded lava flow.
Today Pilot Knob is a peaceful reminder of a dramatic past. This quiet hill witnessed violent explosions millions of years ago. The plaque commemorates this unique geological history. It’s a place where the earth’s fiery past meets the tranquility of present-day Texas. Pilot Knob silently speaks of a time when dinosaurs roamed and volcanoes ruled this landscape. Remember Pilot Knob’s story as you leave this site.