Week Two Reflections: how do layered igneous rocks form?

The second week of Igneous Petrology was a whirlwind! We spent most of the week discussing all of the processes that can cause different rocks to form out of the same body of magma. We looked at several major igneous rock formations that are made up of distinct layers with different mineral compositions, even though they formed from a single body of magma. So how does the same magma body produce different rocks? One of the earliest hypotheses is the idea of gravity settling. Gravity settling is the idea that in a magma chamber heavier minerals that contain elements like iron would sink to the bottom of the chamber as they crystallized, while lighter (and less dense) minerals would float towards the top. Gravity settling has been a popular theory since geologists first began to study layered igneous rocks, but many scientists have started to question it in the recent years. Many of our labs this week focused on studying the distribution of minerals in a layered rock and deciding whether or not they formed through gravity settling or a more complex process. We started off studying a body of rock called the Muskox Intrusion, then looked at more complex formations: The Skaergard formation and the Palisades Sill (located in the eastern US). So how can we tell how the minerals settled in a rock? We looked at the mineralogy, textures, and geochemical data of samples from each layer of the different formations we studied in order to build a hypothesis about how they formed. Initially, many of us thought that the layers were the result of gravity settling, but as we progressed in our research we realized that far more complex processes were involved. Magma chambers are influenced by elaborate convection patterns in addition to gravity. The density and temperature of different crystals can affect how they circulate just as much (if not more) than gravity can. Beyond that, magma chambers don’t necessarily have constant temperatures – the edges are likely cooler than the middle. Occam’s razor holds true in many cases, but it certainly didn’t apply to our labs this week. One of the things I love about the geology major is how it forces me to challenge the initial assumptions I make. The more data I encounter, the more I have to change my hypothesis.

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