Driving down McKinney Falls Parkway today, you wouldn’t think to look twice. It’s a small hill, about 100 feet to 150 feet high, that resembles a typical Hill Country view. But it’s the material underneath that makes it unique: volcanic igneous rock.
It’s hard to imagine Austin 80 million years ago, when the volcano, now called Pilot Knob, was active. Back then, the climate was tropical, the area covered by a very shallow sea with an abundant sea life. There were all kinds of dinosaurs roaming around, so tall that they could walk from present-day Fort Worth to San Antonio without getting their heads under water. There were earthquakes from volcanic eruptions. But most imposing of all were the violent explosions.
“There was quite a lot of activity, just here in Central Texas,” says Leon Long, professor emeritus of geology at the University of Texas.
The hill we see today is only part of the original volcano, which once stood several thousand feet high. Long suspects Pilot Knob to be the only exposed submarine volcano in Texas that geologists know of. This means it was partly submerged under water and partly above the surface, with seawater seeping into the throat of the volcano and creating steam explosions as it encountered hot magma. The volcanic material was pulverized, thrust high into the air and eventually settled into the sea floor.
“These (explosions) ended up producing an explosion crater, which was several hundred meters deep,” says Don Parker, professor emeritus of geology at Baylor University. “Around the volcano, sediments indicate that there were beach rocks, composed mostly of shell material, surrounding the explosion crater. “
Seen from above the water’s surface, the volcano probably looked like a shallow water lagoon with beaches around it, Parker says. Waves crashed against its sides. Today, fossils of sea creatures can be found in the surrounding area.
Pilot Knob was formed during a period of volcanic activity during the Upper Cretaceous Age, about 120 million years ago. These volcanic eruptions were associated with what Long calls a fundamental geologic line in Texas that separated the ancient continental crust from more-recently formed land.
The S-shape curved geologic boundary stretched from Dallas, swooped down through Austin and to Del Rio and would later form the Balcones Fault system around 14 million years ago. Its effects are seen in today’s landscape: Below the fault line lies the coast and inland lowlands between Austin and the Gulf of Mexico; above the fault line, the Hill Country and high plains.
“This was a zone of weakness inherited from previous structural events,” Parker says. “In the bigger picture, Pilot Knob is not alone, there are at least another half-dozen or so smaller volcanoes in the Austin area. Most of them are buried by the city, underneath buildings, and not very well-exposed. Pilot Knob is the largest of these and the best exposed of them.”
As the sea retreated and millions of years passed by, erosion took its course on the volcano. While the volcanic material is soft and easily eaten away, limestone is more resistant to erosion. Visitors to McKinney Falls State Park can see the effects of the once-active volcano there: Some of the volcanic ash bed is exposed underneath the limestone overhangs of the upper and lower falls.
But Pilot Knob helped create much more than a swimming spot; it also helped farmers prosper. Its volcanic material made for very rich soil found in the vicinity immediately surrounding the volcano. The Sassman family farmed the area since the late 19th century, and their property included a section of the volcano. John Sassman, who was interviewed for a 1957 English paper archived at the Austin History Center, said he was bothered by the constant stalking of geologists and curious people who wanted to get a closer look at the extinct volcano. There had been rumors of oil, but a drilled well settled the case by turning up nothing.
Another group of ranchers, the Collins and Rucker families, has owned much of the north side of the volcano since the turn of the century. A short story written by Ada Cecilia Collins Anderson, seen in the family scrapbook archived at the Austin History Center, described one of her earliest memories as looking up in wonder at what to her was a “mountain.” The family referred to it as “the Knob.”
There are legends surrounding the mysterious hill: Spaniards were said to have buried gold on its highest peak and indigenous tribes to have buried treasures. Nothing has been found except some volcanic igneous rocks. The name itself is thought to have come from the area’s first settlers, who used the “knob” as a landmark because it could be seen from miles around.
Today, the hill overlooks some ranch houses and a construction site. But it hasn’t caused worry for a very long time; the volcano is indeed extinct, not dormant. That means it’s not active, nor is it likely to ever be again.
“It’s not going to erupt anymore,” Long says. “It hasn’t in millions of years, so why would it now?”