A caldera is a large basin-shaped depression that forms when a magma chamber is emptied by lava flows or eruptions and the surrounding area collapses into the emptied chamber. For about a million years, starting around 35 million years ago, the Paisano Volcano spread ash and debris over a wide area. Lava oozed from vents and spread laterally across the surface or moved underground along fractures that eventually formed dikes – like the one you can see from the picnic area.
The next time you drive US Highway 90 between Alpine and Marfa, take a second and pull over at the picnic area thatâ€™s located about five or six miles west of downtown Alpine and take a look to the south, across the highway. You will see a strange looking, narrow line of flat, vertical rocks running along the top of a low ridge in a line parallel to the highway. You might think it looks like a partially destroyed wall or the dorsal fin of a dinosaur. Geologists call it a dike.
Dikes are formed when magma is forced under pressure into cracks, called joints, of underground rock formations. Eventually, this magma cools and hardens, forming a flat, vertical line of very hard igneous rock that cuts across the bedding of the surrounding rock. If the surrounding rock with the dike running through it is exposed to the surface by uplifting, it will begin to erode. Often, the rock that makes up the dike is harder than the surrounding rock and will erode more slowly. Over millions of year, the dike may be left exposed, sticking up out of the ground.
Between Paisano Peak on the east and the bridge spanning the South Orient Railroad on the west, Highway 90 runs through the collapsed caldera of the Paisano Volcano.
There are other places you can see dikes in West Texas. The next time youâ€™re in Big Bend National Park, drive south on Ross Maxwell Scenic Drive, the road that takes you to Cottonwood Campground and Santa Elena Canyon. About 8 miles from the beginning of Ross Maxwell Drive, youâ€™ll see the Fins of Fire off to the left. These are spectacular reddish-brown dikes that run across the terrain in front of the Chisos Mountains. Further on, the road cuts through one of the Fins.
Mule Ear Peaks, also in Big Bend National Park, are the remnants of an extremely large eroded dike.
If you look closely the next time you drive between Fort Davis and Alpine, you can also see a dike that formed from the same upwelling of magma that created Mitre Peak.
Nature Notes is sponsored by the Dixon Water Foundation and is produced by KRTS Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Burr Williams, Executive director of the Sibley Nature Center..
Nature Notes is produced by the Chihuahuan Desert Research Institute and Marfa Public Radio and is sponsored by the Meadows Foundation and the Dixon Water Foundation. Tune in to Nature Notes on KRTS-93.5 FM on Tuesdays and Thursdays at 8:35 a.m. and 4:45 p.m. and again on Thursdays at 7:06 p.m. Visit us online atnaturenotesradio.org.