February 17, 2011

Accretionary Wedge #31: "What the heck?!"

This will be my first contribution to the Accretionary Wedge blog carnival, and as a senior undergrad the topic suites me perfectly: What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)?

basic concept of gravity
when applied to rock density
Thanks to Jim Lehane of the Geology P.A.G.E. for hosting AW#31

For my first two years at uni, previously unknown concepts were a given on a weekly basis. But as my studies get more in-depth, the concepts are becoming more esoteric and/or specific. One concept from those early years comes strongly to mind, as it took me some time to grasp before the metaphoric light-bulb turned on: Gravity Anomalies

My physics teachers would drill it into us that acceleration due to gravity is a constant (@ 9.807 m/s2). GRAVITY, G,  IS A CONSTANT! ad nauseum. Then I was eventually presented with an alternate view of the consistency of the constant by my geophysics teacher. I, in my infinite lack of wisdom, and stubbornly sticking by what was told to me by my physics teachers, shirked off his silly idea of minute differences in gravity based on crustal thickness and rock types. I didn't really understand the mechanics of it the way he explained it, and it was never really tested on us students.

The true revelation came during a summer volunteer expedition with a local CGS glaciologist. As one of three heading up to the Matier Glacier within Joffre Lakes park, I got a taste of what experts do, and what instruments they use to analyze receding glaciers and the mountains they rest on. I found out that one such device we lugged up to the top, a microgravimeter, measures the gravitational field at a point. So the glaciologist operated it, got the reading in milligals, and I stood there dumbfounded. He was gracious enough to explain to me the concept of gravity anomalies...how it relates to crustal properties (such as thick, light mountain roots & thin, dense old oceanic trenches), and how we get the Bouguer anomaly, derived from measurements/corrections.
"Rocks have different densities, eg. felsic - ultramafic, and compression of the crust via plate tectonics can thicken the crust, developing structural mountains with or without igneous intrusions. These mountains have thick roots, sometimes 30+km from peak - basement. When such a thick mass is composed primarily of lighter granites, its lighter density is less of an attraction than when compared to a thinner, denser mass of heavier basalts. Denser basalts are found near subduction zones, especially above deep trenches. The phenomenon is comparable on continents as well: The Deccan Traps of Maharashtra measure a noticeable difference in gravity compared to the Himalaya ranges in Uttarakhand."
Upper Joffre lake. Behind would be the tongues of the Matier Glacier.
This is where I learned about gravity anomalies, luckily not by tumbling down to the tarn
Thus I was enlightened to a concept that had previously been muddled in my brain. I've even had a couple opportunities to apply it to my personal and work projects (ie. Cornwall geology post). It's become an increasingly seen concept in higher level textbooks I've perused, and more and more diagrams & cross sections that have caught my eye combine topographical profiles and Bouguer anomaly milligal values (example below).
Simple profile showing Bouguer anomaly values with general topography across the US
In retrospect, I wish I had a time machine, so I could go back and tell my junior undergrad self about how not to take anything for granted in the scientific studies. Geology always seems to smash preconceptions built up by the other science disciplines, and that's something I love about it. For anyone interested, Britain & Ireland geological survey's have done some extensive gravity surveying, and documented some interesting positive & negative zones. Check out the additional links for it.

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Jim L. said...

Welcome to the AW club! Great entry, I hope to have them all organized sometime soon.

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