- Dr. Alison Graettinger
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.
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| What is so special about this rock? It has sparkly bits and not sparkly bits! This is a piece of the Fish Canyon Tuff (one of the largest explosive volcanic deposits in the world!) and is made of biotite crystals, pieces of other rock, pieces of pumice, and wee bit of lichen. Knowing all that helps us figure out how it formed. Step 1: what is rock made out of, Step 2: what does it take to pulverize other rocks then stick them back together in this combination (hint: volcanic explosion). |
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.
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.
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| 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.
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| 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 |
For anyone who cannot get to Morocco we can
look at Google Earth. Using these images we can see the shape of the structure
and check out its neighbors and see there are other similar features nearby. The
Gara Medouar itself shows some layering that has been eroded. The layers appear
to mostly be dipping in toward the inside of the structure.
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| 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. |
The first thing we can see when we zoom away from the famous
landform is the roads and infrastructure built up by the movie
industry. We also can see lots of other larger ridges and mountains. To the
south of the landform we see a much longer ridge that has more of an elliptical
shape and is much larger than Gara Medouar, but still shows layers dipping
inward. There aren’t many similar circular structures nearby. If there had been
that might have supported the volcanic hypothesis. While it being alone would
be in line with a meteorite impact structure, the inward dipping layers
suggests another model of formation is needed. This consistency across multiple
large structures suggests there is something about the rock they are formed
from. To learn more about this, we need to get closer again.
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| 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. |
Our group traveled from Marrakech through the Atlas Mountains
to the desert. Along the way we stopped to look at many fascinating outcrops of
rock to learn about fossil life and the tectonic forces that formed the Atlas Mountains
(Africa smashed into North America). As
we got closer to Gara Medouar we stopped to look at the rocks that formed the
nearby mountains. They are all Devonian (350-400 million years old) limestone
full of fossils of awesome marine creatures that lived in an ocean between Euramerica
(what would later become North American and Europe) with Gondwana (northern
Africa now). Around 300 million years ago these two continents smashed together
to form the Atlas Mountains and on the other side of the ocean formed the Appalachians.
The collision of continents is a slow messy process that deforms rocks for 100’s
of kilometers in either direction away from the collision zone. The rocks we
are talking about in Morocco were crumpled by this process creating folds and
faults. Later, about 60 million years ago, the Atlantic Ocean formed from a rift
between these two continents, separating the mountain ranges.
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.
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| 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.
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| 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.
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| Consolation prize, Devonian fossils and a wind storm! |
