Austin feels laid-back. The small down-town area has bars and restaurants with live music of varying quality. One person does morning percussion on the sidewalk. There are quite a few self-driving cars on the roads, a few with bored-looking drivers but many devoid of driver or passengers, making one wonder what they are there for. Presumably the different companies are testing them out. Most drive well, perhaps a bit more smoothly than the non-self-driving ones, and taking more care not to block roads near traffic lights. One car shows erratic driving, seemingly randomly changing lanes, but this might be caused by a car from the other direction intruding into the wrong lane. Driver at fault. Outside of the city, but never in it, a few cybertrucks appear, electric but with the appearance of ancient world-war-I tanks. Their curious mix of past and future feels like typical Texas. History means something here, ranging from the many memorials to its brief independence to its curious state flag. The museum of Texas history has quite a lot of history (no surprise there), with a focus on the colonial era and slavery. One floor is dedicated to modern (i.e. 1960’s) Texas. The small exhibition on problems of oil focusses on spillage and carefully avoids any mention of global warming. And Texas is clearly on the frontline of this, with temperatures well above average during our visit and farmers very worried about the on-going drought. The nearby Texas Capitol is fantastic, built with rocks from the state, making it an emblem of state geology. In Texas, geology is there to be used.

Drive south to San Antonio and you pass a series of quarries, all on the west side of the road. In the Natural Bridge Caverns, water has done its own quarrying, creating large underground rooms full of strange limestone formations and full of tourists. Although it lies on the line of the quarries, as an expensive touristic attraction it should be safe from those. Go southeast instead and the landscape becomes more featureless. Go northwest from Austin and hills appear with the ancient granite dome of the Enchanted Rock – an easy climb were it not for the heat. Different directions – different rocks. Austin lies at a geological border.

Austin also has a volcano. It was active a long time ago but the remnant is still visible in the city landscape: Pilot Knob. And there is a faultline which runs through Austin. Although not currently active, the fault zone is recognizable in the cityscape.

Texas and the Gulf

The geology of Texas is basically a simple one. There are several zones, arranged roughly concentric around the Gulf of Mexico, which are all called ‘plains’ (for good reason, although it includes the Hill Country northwest of Austin), and with older ages and higher elevations further from the Gulf. The only outlier is at the centre, where some ancient rock comes to the surface, with some of the most scenic scenery of Texas. Austin lies at the border between the coastal plains and the central plains.

The coastal plains are exactly what the name implies. Once, the Gulf of Mexico spread far inland from current coast, a time when the name ‘Gulf of America’ actually made sense. (Renaming it now seems a bit irrelevant since everyone just refers to it as ‘the Gulf’.)

The Gulf of Mexico is in effect a compact failed ocean. It is an oceanic basin which remains almost entirely surrounded by continental crust. The northern half of the Gulf is shallow and has mainly continental crust. The oceanic part lies further south, wide in the west but narrow in the east. The spreading rate was high in the west, and slow to ultraslow in the east, with an end-point near Miami. This difference in spreading rate caused the crust to the south to rotate away from the north: this became the Yucatan block. In effect, the Yucatan opened the Gulf of Mexico.

The spreading may have been initiated by a hot spot. The evidence for this comes from an apparent hot spot track which has been split in two by the spreading. We can envisage that the hot spot caused spreading and thinning of the continental crust, which dropped below sea level and was invaded by a shallow sea. This happened some 160 million years ago. The sea left salt deposits that are still present in the Gulf. After some 10 million, oceanic crust began to form along a short spreading rift. This split the salt deposits which are now found on either side and not at the centre.

The oceanic spreading ceased after 10 to 20 million years, around 140 million years ago. This places the formation of the Gulf during the late Jurassic, some 30 to 50 million years after the central Atlantic ocean opened, but 20 million years before South America split from Africa.

The borderline between the oceanic and continental crust in the Gulf is not well known. North American rivers have been depositing their sediment in this small ocean every since it formed. This continues to this day, with the Rio Grande and especially the Mississipi rivers. In places the sediment is 20 kilometers thick! There may be oceanic crust buried underneath this sediment but it is hard to tell. But in any case, the basin associated with the Gulf of Mexico is much larger than the Gulf itself. It includes the entire coastal plain, underlain by the thinned continental crust of 160 million years ago with 10-20 km of sediment on top. The Gulf coast is rather more than knee-deep in mud.

The origin of the Gulf of Mexico is sometimes described as part of the westward growth of the Tethys Ocean. But the Central Atlantic Ocean had already opened at this time, and the Tethys was located on the east side. The Gulf formed on the west, too late to be part of the Tethys. It was an independent event which may have been related to the rifting in the central Atlantic. The Gulf did eventually open to the Pacific Ocean, thus helping to split North and South America.

At the termination of the oceanic rifting in the Gulf, there may have been volcanism around the Gulf Coast. The evidence is deeply buried and the indications come only from seismic reflections. It would be similar to the harads in Arabia north of the Red Sea, caused by sideways convection of heat from the spreading centre.

The formation of the Rocky Mountains (or at least its precursors) led to a new event. On the east side of the mountains, the ground sagged and a seaway formed. This is called the Western Interior Seaway and it stretched from north to south across America. Around 100 million years ago, the Gulf connected to it and the seaway now covered much of Texas. A long period followed during which the region remained under a shallow, tropical sea. As in many places around the world, at this time of high sea levels and high temperatures, a thick layer of chalk was deposited at depths of a few hundred meters. It is now known as the Austin chalk, but is also found around Dallas and extends as far as Kansas. Volcanic ash layers show the presence of marine volcanoes, dated to 80 million years ago – we will come back to those. Where plankton was less abundant, limestone (in effect, non-biological chalk) was deposited. It is found (and quarried) across much of Texas. The Edwards plateau consists of limestone.

After 70 million years ago, the seaway began to close and retreat towards the Gulf, and the limestone and chalk deposition was replaced with sedimentation. This was part of the so-called Laramide orogeny, a period between 80 and 50 million years ago with uplift of the southern Rocky Mountains, over time extending further east. (The same orogeny formed mountains from Mexico to Alaska.) During this phase, the Chicxulub impact left its waste all over the northern Gulf.

In later eras, sea levels went up and down, while the thick sediment depressed the crust further. Sometimes the sea flooded the coast and sometimes the coast pushed back. This coastline see-saw continues to the present day. Sedimentation from the major rivers has built large deltas which in general have extended the coast line outward. But at the moment, sea level rise, in places amplified by extraction of ground water, is reversing this trend. This would be a good time to sell that beachfront property and move inland as the see-saw reverses – perhaps to Austin. History provides a warning. It may not repeat, but it can rhyme.

The faults of Austin

Starting some 25 million years ago, the Edwards plateau began to uplift (or uplift further), thereby forming the Hill country. This uplift re-activated an ancient fault zone on its southern and eastern edge. It is called the Balcones fault zone, and it forms an escarpment which delineates the edge of the Hill country and separates it from the coastal plain.

The zone is 600 kilometers long and 40 kilometers wide; it runs across Texas northeast to southwest where it ends in the Gulf. The zone is a mess, containing many series of short faults. The faults are ‘normal’, meaning the movement is mainly up/down. Individual faults may either have their ‘up-side’ on the northwest or on the southeast, but combined the northwest side is ‘up’. The downside is covered by sediments from Gulf incursions: this hides some of the total amount of uplift of 350-500 meters.

The age of the Blacones fault zone has been uncertain, with both young (25 million years mentioned above) and old dates (80 million years) being suggested. Dating of veins in some of the faults now indicate that both were roughly right. Several faults are dated to similar ages of around 61-57 million years. This was when the Laramide uplift reached the region, making it the likely cause of the break. The Edwards Plateau likely already obtained some uplift at this time. It is also the time that limestone formation in the region ceased and was replaced by river sediment: the Gulf was withdrawing and the rivers were eroding the new Rockies and dumping the waste into the sea.

Other faults are found to be much younger, with one dating to 30 million years ago and the other to 12 million year. It indicates re-activation, perhaps (as previously suggested) a second phase of uplift caused by the tectonic mishaps in the western US. (The quiet interregnum may have been related to the change of plate movements in the Pacific Ocean 45 million years ago, seen in the bend in the chain of the Hawaiian hot spot, but this is speculation.) Another suggestion is that the re-activation was caused by the sediment filling up the Gulf and pressing it down. In fact, this is a stronger possible explanation for the older fault formation date: in that case the fault zone would have developed from subsidence of the Gulf crust rather than uplift of the Edwards Plateau: it was gravity driven. It is difficult to separate the two effects, as uplift causes increased sedimentation: the two are linked.

In Austin, the Balcones faults mainly (but not exclusively) run west of the city, on the far side of highway 1. Austin lies on the Colorado river (no, not that one, but another one of that name) which comes off the Hill Country, flowing through the escarpment. The cliff on the river where it enters the coastal plains is called Mount Bonnell, and it is a popular place for a short climb to a good viewing point. The city has grown around it, bringing the Balcones Fault Zone into the city suburbs. If this sounds risky, remember that there has been little or no activity for many millions of years. There are many faults associated with the zone, and one of these runs on the other side of the Austin downtown. However, on the Texas risk register this does not warrant a mention.

Go south to San Antonio, and the I-35 runs parallel to the Balcones Fault Zone, on the side of the coastal plain. (Within the city limits, the road runs briefly on the edge of the escarpment.) Those limestone quarries that can be seen from the road are located on the fault zone. That is what uplift does: it brings geological riches to where they can be profitably mined. There is money to be made from Texan faults.

Volcanoes of Austin

The same risk consideration applies to Austin’s volcanism. It was a long time ago. And it was only one.

Pilot Knob volcano lies about 2 kilometers south of Austin’s international airport, 0.5 km from the township of that name. It is on the coastal plain but only a few kilometers from the edge of the escarpment. It doesn’t look much. (There are locations with the same name in Iowa, Kentucky and several other states – don’t be confused by some of the images returned by google.) There is a small hill, with three more outcroppings to the south and east, reaching 60 meter above the surrounding plain. The total diameter is 2.5 kilometers. The nearby McKinney Falls shows evidence of pyroclastic falls.

But this is only the eroded remnant of what once was a larger volcano. The area covered by volcanics (lava and ash) is 60 km² which may conceivably put the airport runways partly within it. In laid-back Austin, your plane may land on debris from an ancient volcano.

And it really is ancient. The volcanics have been dated to around 80 million years ago. Even though this volcano is associated with the Balcones Fault Zone, it is considerably older than the fault. The eruptions happened well before the up (or down)lift. This makes Pilot Knob a submarine volcano, as the Western Interior Sea still covered this region sea at this time. Chalk formed, and in fact this layer is called the Austin chalk. The volcano grew to above sea level. This allowed reefs to grow around it, also leaving some remains. It was Surtsey-like but located in a tropical sea, and it formed a small atoll. The depth of the surrounding sea can only be estimated, but the pyroclastic layers appear to have been deposited in water below wave depth. A depth of order 100 meters seems plausible and this fits with the depth of the Western Interior Seaway. It all points at a Surtsey-size volcano. Pilot Knob is often described as Texas’ largest volcano, but this was no St Helens. Not everything in Texas is Texas-sized; it’s (reportedly) largest volcano was no cybertruck but more like a mini-cooper.

Pilot Knob was not an isolated event. There are some 400 identified volcanic plugs spread out over a distance of 400 km, all roughly following the Balcones Fault Zone. The volcanic rocks are all located within the Austin chalk, and therefore are of similar age. It is called the Balcones Igneous (or magmatic, or volcanic) Province. One of these sites is also in Austin, at Williamson Creek Volcanic Mound. This site is about 5-km north of Pilot Knob, much closer to Austin downtown and located in the densely populated suburb of East Congress. The tuff layer is up to 10 meters thick, with a 1-meter ash layer, and contains large chalk blocks thrown out by the explosive (phreato-magmatic) eruption. It is considered part of the Pilot Knob volcanics, but may have come from a separate vent.

The basaltic volcanic outcrops have dates between 81 and 85 million years ago, with a second set of more evolved (phonolite) rocks which are a few million years younger. All outcrops are small: this was a large volcanic field but it was not highly active, and all mounts were marine. The mounds typically have multiple vents, as is common in low-intensity volcanics where every magma recharge has to find its own way out.

What caused this volcanic field, and why is it in the same region as the later fault zone? The line of the Balcones Igneous Province follows roughly that of the Ouachita structural belt. This is a mountain belt stretching across the southern US. If you now wonder why you have never seen these mountains, they formed 300 million years ago are now long gone and buried. The suggestion is that the Balcones volcanics made use of this belt as a weakness in the crust. That may be true, but the volcanics only affected this area in Texas and not elsewhere, although the mountain rage extended much further east and southwest. Also, the magma erupted in the volcanoes derived from the mantle and not from thickened mountain crust. This can’t be the full answer.

The second suggestion is that the volcanic activity was caused by sediment build-up in the Gulf of Mexico. This was the beginning of the Laramide orogeny, with the mountains building in the west and the formation of the Western Interior Seaway. Rivers from the new mountains dumped their sediment into the young new ocean – more than 10 kilometers of it, which depressed both the oceanic crust and the surrounding continental crust. This had formed the Gulf basin, shown in one of the images above. The Balcones Igneous Province follows the edge of the basin quite well. The idea is that the depressed crust caused the mantle material to ‘flow’ (an optimistic word for ductile rock) sideways to make space. This mantle material experienced some decompression melt when reaching the edge of the basin where the pressure from above was less. The model explains the location of the volcanoes, the fact that they derived from 100-km deep mantle material, and the small size of the volcanoes which is because the melt fraction would have been low. It was shortly after the thin Austin chalk had been deposited during a worldwide sea level high-stand. At 80 million years, the Gulf was deepening. Thus, the ‘ayes’ have it and this model seems to be more likely. Whether the ancient Ouachita mountains are also involved is up in the air: perhaps their deep crust was what stopped the magma flow and deflected it upward.

A hot-spot origin for this volcanic has also been suggested. It is hard to rue out but there is no direct evidence for its presence and the volcanic seems to be too weak and too spread out for this model.

The volcanics lasted for only a brief time. 20 million years later, the Laramide uplift reached Texas, the Edwards Plateau went up and the Balcones Fault Zone formed at the edge of the depressed basin. The volcanics and the faults have the same origin – different children of the same parents.

Secret Santa Science

There are quite a few scientific papers on the Balcones volcanics, but almost invariably they are behind solid paywalls which are clearly meant to hide rather than reveal their content. Economic reasons can be invoked. The volcanic plugs often act as oil traps. There are more than 100 known oil fields related to these plugs. In Texas, oil is big business. The Texas geological society is not there to disseminate data to the public. It serves different customers.

There is a bit more scientific openness about the Balcones Fault Zone. It also has a commercial side, but related to a much less profitable but perhaps more important resource: water. San Antonio, and to a lesser degree Austin, depend for their water on the Edwards aquifer. The aquifer feeds the major springs in the region and these springs in turn feed the cities. Rain water sinks quickly through the chalk and limestone and becomes stored in the aquifer. The Edwards aquifer follows the escarpment of the Balcones Fault Zone. The individual faults provide pathways for the rain water. The water quality and salinity can change dramatically across a fault. Most recent papers on the fault zone study the interaction of the aquifer with the faults: e.g. see USGS

The fault, the cavern and the aquifer

Remember the Natural Bridge Caverns? The chambers have all been carved out by water. At times there has been more water than the caves could cope with: after a hurricane, the chambers were flooded to the top. But why are there so many large chambers in this cave?

It turns out that the caves follow one of the faults of the Balcones Fault Zone. When a stream crosses a fault, as it does here, the water can percolate down into the fault. Limestone dissolves easily. Once a cave has formed, water will flow along it. Hence this set of large chamber of variable depth which follows the line of the fault. At the bottom, the water enters the aquifer that nearby San Antonio depends on.

Texas lives of its history. Whether oil, cities or tourism, the resources come from seas, faults and volcanics of a deep past. In Texas, history matters.

Albert, December 2025