I still remember my first geology class as a freshman in college. I was so certain geology was for me that I was ready to declare my major before I even got to campus (very few geology majors start this way). It didn’t matter that I’d never had an Earth Science class or knew the first thing about rocks, but I knew geology was the gateway to movie-worthy jobs like Paleontology and Volcanology. The first time I was given a tray of rocks and they asked me to figure out how they were different. I didn’t have a clue beyond ‘sparkly vs. not sparkly.’ But telling rocks apart isn’t some innate skill, it is the result of observation. Anyone can do it, if you take the time to look at a rock for its parts, not just the whole. With the right push from my lab instructor it didn’t take long to start seeing all the differences that I now take for granted when looking at rocks. The size and shape of crystals, the weight, the way they break etc. I then learned how to look at the rock with more than just my bare eyes, how to slice up the rock and look at the millimeter and micron size differences, and even to use x-rays and look at the atomic differences. Now the big difference between rocks are so obvious that my husband and I will try to identify rock types from the car while driving down the highway (and do a decent, though imprecise job of it). A lot changes when you know what to look for and how to look.
A lot of geology is the interpretation of old rocks and landforms, to figure out what happened in the past. We look for evidence of slow things like erosion by wind or tectonic deformation (100’s to 100000000’s of years). We look for things that happen quickly like meteorite impacts and volcanic eruptions (seconds to years). With enough observations we can look at a landform and be able to tell if it was formed by a fast or slow process. But not all rocks or landforms are always that cooperative. Since geologic processes don’t just stop after something cool happens, most landforms, even the fairly young ones, have changed since they formed. Take for example, Mt St Helens. After 30 plus years the deposits have been altered by wind, rain and plants. It is a fascinating laboratory where we can watch our planet change. But if you want to look at a 400 million year old rock, sometimes that change makes the job of interpreting it harder.
Also, while the processes that shape the Earth are numerous, and the rocks that change are many, sometimes very different processes can form very similar looking landforms. I already wrote a post about holes in the ground. There is a great word for this called 'equifinality' that I just learned the other day from a neat blog post about gullies and channels on Earth and Mars by @PanetGeomorpho. Here, on In the Company of Volcanoes, we compared holes formed by the relatively slow collapse of limestone caves to impact craters made meteorites, and maar craters. At first glance, and even second and third, these landforms have a lot in common. To make matters worse, when we compare landforms we also need to keep in mind that what we are studying likely did not form yesterday so we didn’t see the process, and the landscape has changed since that landform was created. Comparing fresh impact craters to old impact craters is hard enough on its own, but if we want to compare fresh sink holes to ancient volcanic craters where do we begin? The short answer is one observation at a time.
|Large sinkhole formed in 1972 in Central Alabama. 425 ft long, 350 ft wide, and 150 ft deep. Courtesy of USGS.|
On my recent trip to Morocco I was very excited that we were going to see a landform that may be familiar to many readers because it has appeared in movies like the Mummy, Hidalgo and Spectre. Gara Medouar is one of the names for this raised circular ridge. It has been used to represent whatever Hollywood needed at the time, from an impact crater to an ancient Egyptian city. But what is it really? Geologists, and our patient friends and family, enjoy the challenge of trying to identify filming locations and landscapes in movies (See this awesome blog post by @trueanomalies trying to do just that). What is extra exciting is taking that hypothesis and going to the site in question and hanging out with local experts to find out if you were right.
|Gara Medouar as it appears in Spectre (MGM 2015).|
This ring-like ridge has some features that are similar to an old eroded impact crater and to a tephra ring from a phreatomagmatic volcano. My quick look at images before I left said that a tuff ring, a small volcano made by explosions between magma and water, seemed pretty reasonable. Since I have a fondness for volcanoes, that became my favored interpretation that I was hoping to test on my field trip. You can see the similarity to Fort Rock in Oregon that is an eroded tuff ring (below). Even though I knew that part of Morocco isn’t particularly volcanically active, and it is not impossible, but rare, to have one of these volcanoes all by itself, I was still excited to get to see it in person and find out for myself.
|Fort Rock in Oregon is an eroded Tuff Cone, or a mostly phreatomagmatic volcano. It has been severely eroded so only part of the structure is left. Gary Halvorson, Oregon State Archives, via Wikimedia Commons|
|Gara Medouar in Morocco from space. Note the layers of rock visible and they mostly point toward the center of the structure. You can also see all the roads made by the movie industry for filming. Image from Google Earth.|
|Zoomed out image of the area around Gara Medouar. In the lower central part of the image we can see a larger structure also formed by inward dipping ridges, but it is much larger and more elongate.|
The evidence of this collision was visible in all the outcrops we visited on my trip. Rocks that would have formed in nice gentle flat ocean basins were now tipped on end, bent and wrinkled and generally messed up.
|Excellent example of a tight fold in the Anti-Atlas mountains on our way to see the Sahara desert. The cliff is about 8 m high.|
On the way to Gara Medouar the rocks had a gentle undulating sort of deformation. Where the folding took place over kilometers making local highs and lows that were later eroded to make long curved ridges with layers of rock exposed in the crests and sides of these ridges. We frequently look at folds in 2 dimensions (image above) but this all happens in 3 dimensions, so the rock in this area would have resembled an egg carton. These round folds were then exposed by millions of years of erosion. Because of their shape what is left at any given time only shows part of the shape. So our movie star Gara Medour is truly an excellent actor, playing any role that movies require of it, but it remains a humble eroded fold.
|Folded Devonian rocks that were not yet covered by a sandstorm about 2 kilometers from Gara Medouar. The deformation that these rocks show is similar to the small ring-like structure that is famous in movies.|
Unfortunately for my group, when we got to the site we were beset by a mild, but impressive to the out of towners, wind storm. So when I looked to Gara Medouar I saw merely a shadow of a ridge and lots of flying sand. The sand did not deter us from looking at the excellent fossils of cephalopods from the 400 plus million year old ocean, nor a seasoned local from selling some of his pristine fossil finds. It did, however, prevent me from taking my triumphant photo of the landform's tectono-sedimentary past up close. Our hosts had seen studied it numerous time, and could show us ample evidence in the landscape and photographs to support their interpretation, and while I was disappointed to not touch it, I left satisfied being reminded that there are always more options than we initially expect. For more information on Moroccan Geology check out the Ibn Battuta Centre. As our group was full of planetary geologists interested in eolian processes the stop was considered a success, if not for the original reasons. That is one of the many reasons I enjoy being a geologist, most adventures turn out different than expected, but there is always something to be learned.
|Consolation prize, Devonian fossils and a wind storm!|