Glacier National Park
05.13.2021 - By PlanetGeo: The Geology Podcast
Today we are talking about the geology of Glacier National Park! We cover three major things: 1- The Colors: The rocks in Glacier have stunning colors and you can see them almost anywhere in the park. The main colors are various shades of red, green, brown, and gray. First off, focus on the reds: These rocks have a mineral called hematite (Fe2O3). Hematite forms when iron bearing minerals react with O2 to produce the iron oxide. This happens either on land or in shallow enough water so the minerals have access to free O2 from the atmosphere. In Glacier, we know this definitely happened in shallow water. How do we know this? These red rocks contain sedimentary structures such as ripple marks that only form in shallow water….. If rocks have as little as 3% hematite, it can stain the rocks bright red.
Green: These rocks formed in deeper water. In chemistry, we call this a reducing environment (as opposed to an oxidizing environment for the red rocks). Because there is less O2 present in the deeper water, the iron doesn’t combine with oxygen, and instead binds with silica to produce dark minerals like hornblende, biotite, and others. But, these minerals aren’t as stable as hematite and turn into a green mineral called chlorite with burial. So why do the red and green rocks alternate as they are stacked up on top of each other? It’s a testament to fluctuating sea level or “shifting shorelines”. Basically red = shallow and green = deeper.
2- The Mountains: Most of the rocks we see in the park were sedimentary. A shallow sea was depositing sedimentary rocks for a very long time (over 18,000 feet). The sea was actually a result of early continental rifting and higher sea level.
Long after the deposition of the rocks we see, tectonics began to influence the area. Sedimentary rocks are laid down flat. The rocks in Glacier are not flat - they are folded and tilted. This tectonic event is the same event that formed the Rockies extending from Canada to Northern Mexico. An oceanic plate was subducting beneath the North American plate. At this point, the mountains were tall, but rather rounded. They didn’t look like they do now. They were not carved, jagged, and sharp. That would have to wait until the Ice Age beginning about 2 million years ago.
3- Glaciers: Beginning 2 my ago and ending 20,000 yrs ago, glaciers grew and filled the valleys with immense systems of moving ice. These powerful, slow, relentless glaciers carved Glacier NP into the rugged beauty we see today. Although there are active glaciers still today Glacier NP is named for the features that formed during the Ice Age rather than being named after the remnants barely hanging on today. Glacier NP is a textbook for features like cirques, aretes, U-shaped valleys, Lakes fill in these U-shaped valleys. On the map view they look like fingers! Very similar to Finger Lakes area in western NY State and formed similarly, glacially scoured valleys that now have lakes in them. Another interesting difference between Glacier and other parts of the Rockies is that there are no foothills to the East (think Denver, Calgary, etc). You go from Prairies to Mountains very quickly. This is because glaciers did a TON of erosion and all the sediments were deposited in the valleys to the East. So, the foothills are there, just buried by pieces of the mountains
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