A Brief Introduction to Igneous Petrology

A Brief Introduction to Igneous Petrology

Mickey MacKie, Harvard University

I study geology because I need to know Earth’s past in order to understand my place in the universe. I love being able to walk around and use clues from rock formations to read their past. The world is an open history book, or so I thought.

As it turns out, my understanding of geology was limited to sedimentary rock. There are also igneous and metamorphic rocks, which are formed by varying conditions of heat and pressure. These unfamiliar rock types have surrounded me since I arrived in Juneau. I was reminded of my ignorance every single day. It drove me nuts. I couldn’t read this landscape. Juneau Icefield, what was your history?

Then came Jen Witter, an igneous petrologist from Alaska Pacific University. Here was my chance at enlightenment. The rain stopped, the clouds cleared, and a few other students and I accompanied Jen on a hike up Taku B, the peak above Camp 10. We scrambled over beautiful, glorious igneous and metamorphic rock that I couldn’t understand.  Jen explained some of the formations and minerals, and I began to grasp the events that had created the terrain. We saw amphibolite, granodiorite, and various intrusions of molten rock.

  Mafic inclusions in a felsic melt (photo: Mickey MacKie)

Mafic inclusions in a felsic melt (photo: Mickey MacKie)

We worked our way to the top. The weather was happy enough to make up for its previous rage. I stood on top of the world and looked down. The Taku Glacier was sprawled far beneath our feet. I could see the gleam of sunshine on metal from camp down below. To our left lay the dirty depression of a drained glacial lake. Jen collected samples. Seth found a bottle of Tums with a pencil and notebook inside. It held the signatures of JIRPers before us. We added our own and lorded over our kingdom for a while before heading down.

  View of the drained lake next to Camp 10 (photo: Mickey MacKie)

View of the drained lake next to Camp 10 (photo: Mickey MacKie)

 Mickey on top of Taku B (photo: Katie Popyack)

Mickey on top of Taku B (photo: Katie Popyack)

  The crew on top of Taku B (photo: Aaron Chesler)

The crew on top of Taku B (photo: Aaron Chesler)

Jen told me that over 100 million years ago, the Pacific plate was pushed, or subducted under another plate. This created melting in the subduction zone and caused a plume of magma to form and rise within Earth’s crust. This plate was simultaneously subducting under the North American plate and caused melt to occur there as well. Eventually, that plate became almost entirely subducted so that the Pacific and North American Plates began to collide. This caused a thickening in the crust and an increase in temperature at depth, generating a melt that mixed with and altered rocks on the surface. These are some of the rocks seen south of Camp 10. Thanks to Jen’s sampling, the rocks on Taku B will soon be analyzed to determine their place in Juneau Icefield’s history.