by Steve Winters
Often the most fundamental questions are the simplest. To a geologist, The Question is this: What’s a rock? The definition is quite simple: rocks are aggregates of minerals. (The definition of a mineral, it turns out, is a little vexing but can be simply defined as any naturally occurring inorganic crystalline solid, and let’s leave it at that for now.)
So, then, why don’t geologists study minerals instead? What’s so special about rocks? Actually, geologists must know their minerals quite well, but generally only in the service of learning more about their rocks. Why? Because rocks contain information about the Earth that minerals never can.
One of the best examples of information minerals are never privy to is the seemingly simple question a geologist must ask every time when facing a road cut or other outcrop: Which way is up? That is, what was the original direction of up when the sediments of sand or silt or lava were laid down to (later) make the rock we see? Minerals are mute on which was is up (oddly, some minerals can tell you which way is north, but that’s another story).
Geologists ask this fundamental question all the time because it sets the chronological stage as we examine an outcrop. Assuming the sediments or lava were originally deposited in horizontal sheets, the older bed or stratum must be on the bottom and the younger strata must be on top. Since strata are almost always at least a little disturbed (tilted, folded, or, heaven forbid, turned upside down), geologists need a simple and reliable method of determining the up direction. Rocks, especially sedimentary ones, to the rescue! Geologists look for something called sedimentary features and top (or bottom) indicators such as ripple marks, mud cracks, and the tracks and trails of animals (including dinosaurs) as they walk or crawl on or through the sediment.
Once geologists have figured out which way is up, they’re in a good position to reason out the sequence of geologic events. And that’s the intriguing work at the heart of geologic inquiry.