07/27/2025
After Juan L. Gonzalez joined UT-RGV in 2009, a colleague took him to a residential neighborhood down the road from the auto shop. There, Gonzalez stared up at a 17-meter hill of the same volcanic ash. To his surprise, virtually no research about the local ash had been published, so he began studying it. Lab testing revealed that the ash was 27.4 million years old and probably came from a single eruption of a volcano in the Sierra Madre Occidental range in modern-day Mexico.
At that time, the area was near the shoreline of the Gulf of Mexico and likely covered with a maritime forest similar to the one preserved at the Sabal Palm Sanctuary in Brownsville. When the ash fell, in a matter of days or months—“an instant, in geologic time,” Gonzalez says—it covered the plants and stymied the normal decay process. Over millions of years, groundwater circulating through the ash replaced the plants’ cellulose with minerals in a process called silicification. What’s left is quartz in the shape of the original plant—such as petrified palm, the state stone.
Ash created the petrified wood that early peoples carved into tools and 20th-century architects used to decorate local homes. It was the source of uranium mined in South Texas from the 1950s to the 1990s, and it was mixed into the cement used to build Falcon Dam 30 miles northwest on the Rio Grande. This unlikely location is one of a handful of spots where visitors can touch it themselves.
This ash stretches across Starr County and has played a key role in the region’s geological and cultural history, but its presence is threatened by encroaching development.
Learn more about the volcano that shaped South Texas and the UT-RGV project, “Ancient Landscapes of South Texas,” here: https://texashighways.com/culture/history/how-a-volcano-shaped-the-south-texas-landscape-millions-of-years-ago/
