Sunday, August 02, 2020

Geology of the National Parks Through Pictures - Rocky Mountain National Park

My next post about the Geology of the National Parks Through Pictures is a park we hit up after the wife did a race outside of Denver.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website

Colorado State Geological symbols can also be found HERE.


Even though there are a lot of scenic views within Rocky Mountain National Park, the basis for Rocky Mountain National Park is geology. The reason the mountains are here is geology and the features seen on the mountains are geological. So even though this isn't a strictly geological park, geology in imbibed everywhere. 

The obligatory entrance sign

The mountains within Rocky Mount National Park are a result of orogenic events (mountain building) that took place over three periods within the planets history. The first orogeny (mountain building event) took place 1.7 billion years ago during the Precambrian. This event laid down the foundation of faults that will later be activated during the most recent mountain building. The next orogeny, known as the Ancestral Rockies Orogeny, was 285 million years ago during the Pennsylvanian. Afterwards, the Ancestral Rockies were entirely eroded away, but again the faults were left behind. The final mountain building event was the one which produced the mountains we see today. This occurred 70 to 40 million years ago and is known as the Laramide Orogeny.

The Earth is made up of giant plates which are all moving around. When two plates are moving towards each other they push each other up, forming mountains. This is what is currently happening in the Himalayas where the Indian Plate is moving northward into the Eurasian Plate. As the two plates collide, the edges get "wrinkled". It is this wrinkle that are the mountains.  Here is a view of the Rocky Mountains from the visitors center. 

 The Laramide Orogeny occurred due to the presence of a plate off the western coast of the United States known as the Farallon Plate. The Farallon Plate was an oceanic plate that was pushing up against the west coast of the United States and Mexico. Since the plate was made up of oceanic crust, it was denser than the North American continental crust and therefore ended up going beneath the North American Plate in a process known as subduction. As the plate went under North America, it still ended up compressing North America, forming those mountain wrinkles. The mountain building occurred along those long ago created faults that were reactivated during this renewed time of mountain building. Eventually, most of the Farallon Plate was completely subducted, leaving behind only a small piece known as the Juan de Fuca plate off the coast of Washington and Oregon.

 Within the park there are also significant amount of glacial features, this divot out of the side of the mountain being one of them. When a glacier is forming on the side of a mountain, the snow continually accumulates near the peak. Eventually the snow builds up enough that it compresses down into ice, and then eventually the entire ice block starts to flow downhill. As the ice flows downhill it picks up rocks and starts to erode into the mountain that it is sitting on, creating a bowl shaped depression. That bowl shaped depression is what eventually becomes known as a cirque, once the glacier melts entirely away. 

 Here's the view of another cirque from a distance.

 Here are a whole bunch of glacier features, as well as some glaciers still present (where some of the snow and ice doesn't melt completely in the summer time). Within this photo we also have some more cirques. Since cirques erode down into the mountains, creating a bowl shaped depression, these often eroded down into bedrock. The bedrock creates a nice seal against water escaping and eventually can create a lake. This specific type of lake is known as a "tarn". When two cirques are on the the same mountain but going in different directions (i.e. back to back), they can form a ridge between themselves. This steep, knife-like ridge of rock is a glacial feature known as an "arête", which you can also see within the central portion of the photo.

 Here is a view looking down into a cirque. Although there is snow here, it is not a glacier until the snow starts to build up over time and compact into ice. Right now these are just seasonal snow fields.

After the glaciers move downward from the mountains, they start to fill up the valleys between the mountains. This ends up eroding the valleys in a specific pattern. When rivers erode out valleys, the valley cross section forms a "V" shape, where the stream erodes straight downward at the bottom of the V. However, when a glacier enters a valley, it is much larger and erodes over a much larger area. The result is a valley with more of a "U" shape, created as the glacier erodes on all of the edges of the valley. Here is a look towards the mountains centered on a glacial U-shaped valley.   

Another erosional glacial feature is when the glaciers make it further down slope and erode out and widen existing valleys. The valleys were initially eroded by streams and rivers, but glacial erosion widened and flattened these stream valleys far beyond what was initially dug out. After these valleys are eroded out by glaciers, eventually the glacier melts and water returns to the former streams. The large flat areas are then filled in as a series of interconnected lakes called paternoster lakes.

One of my favorite features of western National Parks is the Continental Divide. This is essentially a high point of the country where water at the divide goes one of two ways. Within this picture (which I am facing south when I took), water on the left, or eastern side all flows to the Atlantic Ocean, while all of the water on the right, or western side, all flow to the Pacific Ocean. The signs are not often perfect, but they are pretty close. The Continental Divide is actually a line of mountain ridge tops that runs up the entire length of the continent, dividing the drainage between the two oceans. There are other divides within the country as well, such as the one that divides water going north into Canada's Hudson Bay, or water going into the Great Salt Lake, which is an end-basin where water goes to die.

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