Geology and Me: Earth Science as I see it

Published 2007-09-02 by Chris

I’m not the kind of person who enjoys writing about myself. It always seems narcissistic to the point of ridicule to concern too many words about yourself to an audience not concerned about anything of the sort. An exception has to be made for this article, as the topic for the first Accretionary Wedge geology blog carnival is a personal self-introduction to the community and a few words on why you chose Earth/planetary science as a field of interest and what makes that area of science so interesting.

Dwelling on the intricate series of events that lead to me dedicating a fifth of my life to a single field of science would likely bore the most patient of you. Rather than sending you to sleep, I’ll recall a short version of how I got into studying earth science and then quickly move into the interesting bits; why it’s worthwhile, why geology is such a great science and what I’m doing insofar as my MSc research is concerned.

Who am I?
My name is Chris. I’m in the final year of my MSc in Earth Science at Victoria University in Wellington New Zealand. I’m studying the geochemistry of chondritic meteorites and enjoy doing research that combines two of my major interests; geology and astronomy.

How did I get here?
In short, on a whim. It’s true for an outrageous majority of the people I took my undergraduate degree with and even most of the lecturers, that geology wasn’t their first flavour choice for the brain freeze inducing ice cream we call academia. The same rang true for me. I’d tried my patience with doing a BSc in computer science right out of high school. I quickly quit out of abject boredom only to start a career as an IT guy. Fixing computers, maintaining servers, writing and designing web pages. A go-nowhere-interesting, do nothing out-of-the-ordinary career choice I’d fallen into like a turbidity flow onto the abyssal plain. I eventually realised I needed to get back to university and do something I was genuinely interested in. So as my early twenties were drawing to a close, I headed back to school.

Taking a suggestion from a geologist friend and colleague, I signed up to all of the first level geology courses that were offered at Victoria, which happens to be my local university. By the end of the year I’d subjectively decreed that computers, dealing with millionths of a second, were intellectually pale in comparison to every major natural science expressed through the theme of millions of years. I changed my major from computer science to geology and here I am almost 5 years later.

What so good about geology?
So far as undergraduate study is concerned, geology is an incredibly social science. Its participants live together for weeks at a time on various field trips, which in turn encourages social mirth and mayhem to ensue both in and out of the field. The oft expressed stereotype of the bearded geologist, sidestepping lava flows en route to the next boutique pub isn’t as far from the truth as many would prefer.

Putting the social side of undergraduate study aside, I can’t express in simple terms just how fantastically broad a subject geology is. And just how complicated it can get. To quote Peter Barrett, one of my lecturers through my undergraduate degree;

Geology is like reading a history book. In a foreign language. With 90% of the pages ripped out.

I’m not sure if he made that up or got it from another truly inspired person, but it’s the perfect way to describe the science as a whole. Geology is where the natural sciences meet time. Physics, chemistry and biology, wrapped up in a enormous temporal package spanning billions of years. Though much has been written about the awe inspiring sites to behold should one gaze into the cosmos, there’s an equal array of wonder below your feet; something most people, unfortunately, can’t fully appreciate.

I’m an igneous geochemist if you go by my research, and I don’t have much time for things such as sedimentology. However, even in the most boring of sedimentary rocks, there’s often a riveting story to be told. Take Wellington, New Zealand, for example. The rock around here is dead boring from a petrographic standpoint. Quartz grey wacke and loess blown into place during the last glaciation. But how did that grey wacke form? Well, millions of years ago as Gondwana was in its death throws and Australia was tectonically rifting from east Antartica, large turbidity flows of sediments, hundred of millions of tons in mass, crashed through the ocean depths, depositing themselves on the ocean floor, specifically the submerged continental mass that would eventually make up part of New Zealand. The image I paint in my mind’s eye of these events is just breath taking. The shear scale of the deposition would be truly magnificent to behold. And this is an area of geology I don’t care much for.

My Research and Specific Geological Interests
On the other hand, the kind of geology I DO care for can be summed up in this description of my MSc research. My MSc concentrates on the origins and chemical/isotopic make up of refractory inclusions in CV carbonaceous chondrites. Or in less-specialised words, I’m studying how the oldest minerals in the solar system formed and what the environment in space was like when they did.

Below is a picture of my very first mounted meteorite sample (not as dirty as it sounds);

The white bit in this picture is 4.5672 billion years old. It’s one of the oldest solids in the Solar System and dates back to a time when the Sun was just kicking off its fire-juggling party. The minute concentrations of iron in this rock and the iron in your blood are from the same star-derived reservoir. But I digress.

This is a sample of a Calcium-Aluminium rich Inclusion (or CAI) from a carbonaceous chondrite (stoney-iron meteorites). These things formed in a very hot environment, and the minerals within have gone through between one and three stages of melting. The heat inherent in the environment was not due to the sun, but radioactive decay of unstable isotopes such as 26 Aluminium (Half life of ~703 Ka).What I’m doing with these tiny inclusions (which are all less than 10mm in diameter), is determining the major mineral constituents, of each of those I’m looking at the minor or trace element concentrations and finally dating them by determining comparative ²⁶Mg deficits (if any). So what’s involved in each step?

Step 1: Mineralogy: Using an Electron Micro Probe, I am able to determine the major elemental weight percentages of each mineral “phase” of the targeted CAI;

Each shade of grey in the above image is a different mineral. In this case, the lightest phase (Phase 1, points of sampling are orange) is melilite, the second lightest (Phase 2, coloured blue) is pyroxene, phase 3 (in green) is anorthite, and the nearly black phase 4 (in red) is spinel. The above image is an electron back-scatter image of a Type B1 CAI from the carboneceous chondrite NWA 2364.

Step 2: Trace Elements: Using Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS), I am determining the concentrations of 34 chemical elements that are present to an accuracy of a few parts per million (ppm) or less. The elements I’m looking for range through Uranium, Thorium, Lead, Titanium, the Rare Earth Elements (REE) and more. By comparing the concentrations of these elements in different CAIs, whole meteorites and planets, you can get an idea of what was around when each formed and how the areas of formation differed in their respective elemental composition. Did the Earth and these CAIs form in the same place in the solar nebula? The answer at the moment is no, so why did the solar nebula cloud have differing concentrations of elements from one point to another? That’s the big questions relating to how these various bodies formed.

I like to think of this step as a real life game of Asteroids, with a scientific slant.

Step 3: Magnesium Deficits and Dating: Using another kind of ICPMS, namely a Multi-Collector, I will be determining the concentrations of the isotopes of magnesium (Mg) in each of my collected samples. As I mentioned previously, these samples were heated by the decay of ²⁶Al. This particular isotope decays to ²⁶Mg. So the more ²⁶Mg in a sample, the older it is. Any deficit in ²⁶Mg compared to that of the maximum found in CAIs can be correlated to the time between CAI formation and the formation of whatever you’re looking at. So by getting the ²⁶Mg/²⁴Mg ratio from these samples, I can determine their relative ages from oldest to youngest. This is of interest because knowing over what time span CAIs were forming can help you determine whether it all happened at once in a very short time span (and was thus stopped by some process of the sun’s formation), whether there were several exclusive periods of CAI formation (perhaps by injection of ²⁶Al from nearby supernovae), or whether it happened slowly and steadily over 6 half lives of ²⁶Al (most likely).

Why does this count as geology? Well it’s more analytical chemistry. But knowing what was around when the Earth formed and thus what it is made of (i.e, how the chemical composition of the solar nebula changed over time) and how old it is in comparison to other bodies in the solar system, you can build more accurate models of the chemical composition of the materials that make up the crust, mantle and the core of the Earth. This helps in the understanding of how and why things are the way they are. It also makes up the underpinning of mantle geochemistry, volcanic petrology and chemistry, and environmental and atmospheric evolution (which links to the formation of life (abiogenesis) and the like.

In Summation
Geology is broad, and the other posts present in the Accretionary Wedge will surely show this. From dinosaurs to meteorites, sediments to earthquakes, there’s something for everyone in geology, so long as they’re interested in science. If I had to choose a science above all others, geology would be it, because geology is all science.

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