I recently completed a partial cross-country drive from Columbus, OH to Los Angeles, CA with a quick stay-over in Tucson, AZ. This drive took me through the middle part of the US that is often known as “fly-over country” since people choose to skip it as they fly from coast to coast. During this monotonous, often mind-numbing drive, I realized that there are some geological wonders along the way that go unnoticed. Here are 4 that I found the most interesting.
1. New Madrid Seismic Zone- Missouri, Tennessee, Arkansas border.
The rumble and tumble of earthquakes is typically associated with the West Coast, but hidden right in the middle of the US is the New Madrid Seismic Zone. This zone represents a weak point in Earth’s crust from a messy, incomplete continental break-up about 750 million years ago. This weak point means that small compressions and contractions east to west in the US can be especially dangerous here.
Despite the long distance to the nearest tectonic plate boundary, this old fault can still cause extensive damage. From 1811 to 1812, four massive earthquakes rattled the surrounding areas. Although there were no seismographs (devices to measure earthquake strength) at that time, researchers used journal entries, newspaper articles, and reports to estimate the power of these shakes to be 10 times as strong as the 1906 San Francisco earthquake. The area surrounding the New Madrid Seismic Zone is still a concern especially for damage in large cities like St. Louis, MO and Memphis, TN.
2. Texas Panhandle.
I’m sorry Texans, but your panhandle is so flat and boring. The old joke really applies: “It is so flat you can watch your dog run away for three days”. Luckily, alternative wind power has utilized this level area to put up hundred of giant windmills: Don Quixote’s worst nightmare.
Literary references aside, I wanted to know why this area was so dang flat and what it could tell me about the geologic past of the area. I hoped the answer would be easy to find, but it took some digging and I’m still not sure I have the whole picture. A 1907 USGS report by Charles Gould told me this: “the surface of the High Plains in generally flat, with nothing to break the severe monotony.” See? Even geologists think it is flat and boring. Anyways, my interpretation of his report is that rivers originating in the Rocky Mountains occupied the High Plains in the panhandle of Texas. Over time, the streams became overloaded with sediment (rocks, dirt, etc.), deposited this sediment in the riverbed, and eventually filled the rivers pushing them into new directions. This process of deposition and river re-working continued until a deposit “several hundred feet thick” covered the area to the east of the Rocky Mountains including Texas.
I had a feeling that the flatness had something to do with water, most outstanding geological features do. However, I was surprised that instead of rivers cutting into the landscape, they actually created a flat surface. Next time I drive through Texas, I might give it a bit more respect.
(Update January 21: I reached out to the USGS for more information about this area. They pointed me to another, more complete resource on the geology of the High Plains. The information was essentially the same, but in more detail. It appears that this part of the Texas was covered and uncovered by water at many times in the geologic past. Each time it was underwater, more sediment was deposited creating such a flat surface. This short investigation has taught me an important lesson: it is hard to pinpoint an exact cause to a certain geologic feature.)
3. Texas Canyon Rest Area / Balancing Rocks, near Dragoon, AZ.
Entering into New Mexico and Arizona the terrain became much more interesting. There were mountains, mesas, and sometimes bizarre rocks. Our last stop before entering Tucson was the Texas Canyon Rest Area, a surprisingly beautiful and well-known pit stop. We stopped here specifically to look at the perched boulders along the road. My parents have made this drive before and knew that it was a great spot to stop. Once again, being the inquisitive geology girl, I wanted to understand why it looked the way it did. The rounded boulders made me feel like I was within the circle of a giant’s game of marbles. Rounded and perched rock formations are not uncommon and even have an official name: precariously balanced rocks (or PBR for short). PBRs can be found in hipster hiking hotspots around the world from Africa to Arizona.
To become so round, these rocks were subject to physical and chemical weathering. Physical weathering is when rocks are broken-down by forces like wind and water, but remain the same type of rock. Chemical weathering changes the rock composition through chemical reactions. I won’t go into the exact make up of the rocks (but see this post), however, eventually the weathering causes the rocks to breakdown in such a way that they become rounded and appear to balance atop other rocks.
Although they are called precarious, the rocks are typically very steady on the shoulders of the other rocks. However, earthquakes can jar these rocks down and can thus be used to monitor earth’s movements. Pretty cool, but I hope no one is at the rest stop when the rocks start to wiggle!
4. San Andreas Fault, California.
You don’t need to be a geology nerd to know about the San Andreas Fault. It is infamous for the fact that one day it will be the center of a break between Southwestern California and the rest of North America (see this classic flash-animated video warning: some NSFW language). This fault is one of the main reasons that Los Angeles and San Francisco are earthquake prone. What I found so surprising on our drive in Los Angeles is that even though I knew the fault line was there, we just drove right over it. I don’t know what I expected. A big sign pointing it out? Some large chasm in the earth? Not sure. Nevertheless, we drove right over this divide without even knowing it.
The San Andreas fault is actually a tectonic plate boundary between the Pacific and North American plates. The plates rub against each other in opposite directions. Think about it this way: place your hands together palms touching. Move your right hand down and your left hand up. That friction and tension between your hands is the same as between the plates. If you press your hands really hard you can understand how an earthquake may occur. The uneven surface of your hands cause them to stick together better in some parts and suddenly slip in others.
Once in Los Angeles, poking my toes into the Pacific Ocean and soaking in the rays, I realized that I hadn’t driven through swaths of nothingness to get here. Instead, I’d peeked into Earth’s history along the way. Next road trip, I’ll still be staring out the window watching the landscape pass, but I will be looking with a closer eye at what I can learn along the way.
New Madrid Seismic Zone:
Gutentag et al. (1984). Geohydrology of the High Plains aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. Regional Aquifer System Analysis Paper 1400-B. Available here: http://pubs.usgs.gov/pp/1400b/report.pdf
Gould, C (1907). The geology and water resources of the western portion of the panhandle of Texas. Water Supply and Irrigation Paper. No. 191. Available here: http://pubs.usgs.gov/wsp/0191/report.pdf
San Andreas Fault