What is geochemistry?

Over at Green Gabbro, Yami says that she gets all technical and jargony whenever she doesn’t want to talk about work to normal people. I don’t do that. I can turn people off just by being very basic.
“I determine the chemical composition of rocks.”
My, what interesting shoelaces we all have.

As Chris recently posted, though, it isn’t about the rocks. Don’t get me wrong, geologists appreciate a pretty rock when we see it. But that isn’t why we study them. We study rocks because they tell us stories. And the stories are very cool.

Take geochemistry, for example. Actually knowing the composition of a rock is in fact pretty dull, if it’s just a list of elements and concentrations. The reason we study them, then, is to discover the process that lead to the composition that we measure in lab.

Different processes change chemical composition in different ways, so by measuring various elemental ratios, we can determine what a rock has been through. To start at the beginning, though, we need to acknowledge the sub-field of cosmochemistry.

Cosmochemistry is generally not the chemistry of the cosmos, it is actually the chemistry of the solar system. We have very few mineralogical materials that predate the formation of the solar system. The vast majority of non-terrestrial rocks that we have access to are from various solar system bodies.

The chief goal of cosmochemistry is to determine how the planets formed and what their composition is. Once the bulk chemistry of the Earth is determined, various processes that act on Earth can then be studied using geochemistry. The processes include, but are not limited to, the formation of continents, the evolution of the atmosphere and ocean, ancient and modern climate, the creation of economically significant ore bodies, and the pollution that results from the exploitation of such deposits. Some of these processes are pretty cool, which is why I like my job.