4 1.4 How Do Geologists Study the Earth?

The answer to to this question is… it depends! The specific tools, techniques, and instruments geologists use to answer questions depends of the subfield of geology and the precise question being asked. The general process of geology is the same for any other science: geologists form hypotheses and test them.

For some questions this is straightforward. For example, geological engineers can test how a rock will respond to the stress of constructing a building on top of it by taking a sample to a lab and compressing it using pistons. For other types of historical questions, however, such as answering whether an area was ever covered by a glacier in the past, there is no way of directly conducting an experiment. Instead, the geologist has to ask what evidence that glacier would have left behind.

In this way, a lot of geologists operate like forensic scientists at a crime scene: their job is to reconstruct the past from the clues left behind. However, just like on a forensic TV crime show like CSI: Crime Scene Investigation, the process of testing hypothesis often involves a lot of going to a lab to test whether a ‘clue’ could have been created in the way you think it was. So, even the historical parts of geology involve experimentation for confirmation.

The rocks in image from near Tangier, Nova Scotia, show evidence that they were once covered by a glacier: the scratches were formed when glaciers slid over them. We know this from observing the impacts modern glaciers on rocks. The large loose boulders and smooth curves of outcrop are yet more evidence that a glacier was once here. View source.

Over the coming weeks, as we examine different areas of geology, we look in depth at both the facts geologists have uncovered, as well as a the tools and methods they’ve used to do so.

An Example of the Scientific Method at Work in Geology

Consider a field trip to the stream shown in Figure 1.5. Notice that the rocks in and along the stream are rounded off rather than having sharp edges. We might hypothesize that the rocks were rounded because as the stream carried them, they crashed into each other and pieces broke off.

Figure 1.5 Hypothesizing about the origin of round rocks in a stream. Source: Steven Earle (2015) CC BY 4.0 view source

If the hypothesis is correct, then the further we go downstream, the rounder and smaller the rocks should be. Going upstream we should find that the rocks are more angular and larger. If we were patient we could also test the hypothesis experimentally, by marking specific rocks and then checking back to see if those rocks have become smaller and more rounded as they moved downstream.

If the predictions turn out to be correct, we must still be careful about how much certainty to attach to our hypothesis.  Although our hypothesis might seem to us to be the only reasonable explanation, someone could argue that we have the mechanism wrong, and the rocks weren’t rounded by bumping into each other. If our experiment didn’t specifically check for the mechanism (e.g., by looking to see if chips fall off the rocks and the rocks are made smoother) then we would have to acknowledge the possibility.  We needn’t abandon the hypothesis as a useful tool for making predictions, but it is necessary to be open to the possibility that other things might be going on. If someone demonstrates conclusively that our hypothesis is wrong, then we have to discard the hypothesis and come up with a better one.