In many ways, being a geologist is like being a detective. You have to look at the clues left behind and figure out what happened in the past.
“The present is the key to the past” is a principle proposed by James Hutton (“father of geology”) and popularized by Charles Lyell. It suggests that the physical processes we observe today also occurred in the past and were responsible for formations of geologic features.
For example, when you go to the beach and you see ripples in the sand caused by waves, you can safely assume that similar ripples preserved in a rock sample were also caused by waves.
While it is common for someone in New England to see ripples and waves at the beach, it is not possible to see glaciers here, but they left their mark. It took observations of glaciers and glacial landscapes in the European Alps to start understanding what happened in North America. The theory of ice ages is relatively new and what causes ice ages is one of the most exciting mysteries in the earth sciences.
I study past climate change (paleoclimatology). Some of the BIG questions I think about are:
- What causes ice ages?
- What reactions and feedbacks are there in response to a climate change?
- How does a climatic event that occurs in the North Atlantic affect the South Pacific, for example?
Various scientists theorized and wrote about ice ages from 1740s into 1830s (Louis Agassiz), but ice age theory was not widely accepted until the 1870s when a plausible cause was provided. The hypotheses of what causes ice ages continue to be tested and revised to account for what scientists are finding at locations around the world. That is to say, one hypothesis that might explain ice ages in North America (Milankovitch cycles) might not work as well for southern South America. The timing and magnitude of glacier maxima (when glaciers were much larger than today) helps scientists determine when and by how much glaciers fluctuated.
One method used to determine the age is to date moraines (ridges of rocks and boulders) deposited by a glacier marking its previous extents. We use 10Be cosmogenic exposure age dating techniques, and this explanation will sound a bit sci-fi, but here we go. Once the moraine is deposited by the glacier, boulders on the surface of the ridge start to be exposed to cosmogenic radiation (rays that come from beyond our solar system and penetrate through the earth’s atmosphere). The rays cause a spallation reaction (splitting apart) within the oxygen in quartz (chemical composition SiO2). When oxygen is split, one product is the isotope 10Be, which accumulates over time, so the longer the boulder is exposed to these rays, the more 10Be will be in the top few centimeters of the boulder surface. We can then relate the amount of 10Be to a time duration and thus an age of when the moraine was deposited. Crazy huh?
Another method I used to determine past climate from these moraine ages is to use a computer model to simulate growing glaciers out to their moraines. This model can be thought of as a Goldilocks video game, if I make the temperature too cold, the model glacier grows too large and grows past the moraine, if it is not cold enough, the glacier does not reach the moraine. The modelled temperature needs to be “Just Right”. The amount of snow also matters to a glacier’s health, so I change that too (drier means I am basically starving the glacier and wetter means I am overfeeding the glacier), but in New Zealand, temperature has a greater effect on glacier size.
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