Tuesday, July 24, 2012

Looking At Mars through Arizona

Ryan Jackson

            The southwest is a beautiful place, not due to the plant life which can be quite menacing but instead it has a stark beauty because of the exposed geologic features which abound through the region. This lack of annoying and obscuring vegetation makes it an ideal place to study geology, and in addition the southwestern U.S. bares the evidence of numerous large scale geologic events. This has caused a variety of features from volcanic fields to ridgebacks, and as a result of sheer luck and the arid environment (which doesn’t weather features as quickly as would be seen on the East Coast) the area also plays host to two of the world’s most impressive geologic features—the Grand Canyon and Meteor Crater. These two features, in addition to the volcanic fields, make northern Arizona an ideal place to study Martian analogs.
            So first off, Martian analogs (or planetary analogs) are geologic features on Earth that can be used to gain a better understanding of a feature elsewhere in the Solar System that it corresponds closely to. This works because of the idea of Uniformitarianism which is at the very heart of geology, and can be basically summed up as “the present is the key to the past” and the laws of nature are consistent throughout the Universe. This means that we can look at the processes we see now (like a volcano erupting) and extrapolate this back to a similar lava flow which is hundreds of millions of years old; in addition, because we understand the laws of physics to be uniform throughout space we can extrapolate this event to another planet (allowing for changes in composition, gravity, and other considerations) and understand lava flows there as well.
            This was the basic idea for the field trip, the basaltic Lava Flows of the San Francisco Peaks correlate to basaltic flows all over the terrestrial bodies of the solar system, but in particular the process is representative of hot spot volcanism which also created Olympus Mons, the Solar System’s tallest volcano. Meteor Crater is quite obviously representative of other impact craters which are ubiquitous across the Solar System, but it is rare on Earth as it is so well preserved; and the sedimentary layers exposed within the Grand Canyon could be analogous to the stratigraphy it is hoped the Mars Science Laboratory will find on Mount Sharp in Gale Crater.
            After a day of traveling, our first day was spent in the San Francisco Volcanic Field which is comprised of cinder cones and basaltic lava flows. Cinder cones are small volcanoes made quickly in eruptions which shoot out ash (cinders) which fall back down into a pile, kind of like if you took a handful of sand and poured it out in a stream back onto the ground, the small mound it would form as it piled up is very similar to the formation of the cinder cones. The basalt here has a Hawaiian name “Aa” (pronounced ah-ah) which is named for the sound made when a bare-footed person steps on it—I’m not joking. It is characterized by the blocky texture and is differentiated from pahoehoe which has a ropy texture. This area is important as the volcanism was the result of a hot spot, which aren’t greatly understood, but basically can be summarized by a large plume of magma coming up from the Earth’s mantle and spilling out onto the surface and because Mars doesn’t have plate tectonics, like the Earth does, this is pretty much the only volcanism present there.
            Hotspots are also responsible for some island chains like Hawaii where they form shield volcanoes (the largest type of volcano but they are broad and very gently sloping) which are analogous to Olympus Mons and other giant Martian volcanoes. Hotspots are more or less stationary; however, on Earth the tectonic plates above them move so the volcanism moves with the plate. On Mars, that doesn’t happen so the hot spot continually pumps out lava making the volcano grow larger and larger until the volcanism dies away.
            I have been in large lava flows before, but the scene around our first stop at S.P. Crater (bit of a misnomer as it is a textbook cinder cone) still awed me. The flows stretched out all around us; basalt is my favorite rock and this was a good as Disney Land for me; needless to say my pack was weighed down by samples by the end of the day. The next stop was the apply named Colton Crater, the geology here was very important as this was a volcanic crater which can be at times be very hard to distinguish from impact craters. Colton Crater was once a cinder cone like S.P. but at some point the magma underneath it had come into contact with ground water and turned into a maar. I always imagine maars as giant steam bombs because they kind of are; this giant explosion blew off the top of Colton and left a giant crater there. The coolest part though was the tiny cinder cone within the crater.
            The next day we went to Meteor Crater, the best preserved impact crater on the Earth, to compare the features here to what we saw at Colton. But besides the conspicuously missing slopes we saw at Colton the two sites pretty much looked the same. While we wouldn’t get the chance to observe enough of the crater to see this ourselves, we soon learned that impact craters can be identified by the inverted stratigraphy around the crater. Stratigraphy refers to the different horizontal layers of rock and common sense should tell you the oldest is at the bottom and the layers become progressively younger towards the surface; however, the layers which have been penetrated by the impact are tossed out and flip over past the rim, so that suddenly there are older rocks on top of younger. This set up is a dead giveaway for impact cratering and once it was discovered at Meteor Crater, inverted stratigraphy was used to identify numerous other such impacts all over the world. This concept is highly important for planetary science because on places, like Mars, which has had volcanoes in the past it cannot just be assumed that every crater is from impacts because then it would be easy to miss an important part of the picture. In addition, if maars would be found, it could tell geologists about the ground water present on the planet.
            The last large sight we saw was the Grand Canyon, which was an amazingly beautiful sight, it actually took my breath away when I first saw it. When geologists are feeling poetically they will often compare the rock layers to pages within a book, and talk about “reading” them, if that is the case, then the rock layers would have to be War and Peace. A massive amount of Earth History has been laid bare by the Colorado River, and it is beyond words to describe the phenomenal sight. We hiked down a trail into the Grand Canyon, but as we were limited on time my group only made it about 2.5 miles down the trail akin to just opening the book and skimming the first few pages, but it was still amazing.
            The Grand Canyon’s stratigraphic layers do serve as an analog for Mount Sharp on Mars, in a rough way. The stratigraphic layers in the Grand Canyon were deposited in a variety of different environments, while the depositional environment for Mount Sharp was probably mostly uniform throughout its history. However, the Grand Canyon does provide great practice for reading stratigraphic layers, which will be extremely valuable when the Mars Science Laboratory lands in Gale Crater and begins exploring Mount Sharp. The amazing idea about Mount Sharp is that it is larger than the crater it is in, which means it is impossible for it to have formed during the impact, and must have been built up over time by other process. And like geologists can read Earth history from the stratigraphic layers within the Grand Canyon (or a road cut or just wherever they are exposed) Martian history should be exposed by studying the rock layers exposed within Gale Crater.
            The trip was absolutely amazing, and the opportunity to learn about planetary analogs from numerous scientists who study them was basically a dream come true. The geology we saw are some of the most amazing sites I have ever seen, and being able to connect them to geology across the solar system reminds me why I decided to pursue planetary geology.

No comments:

Post a Comment