Sorry if it sounds like I have a crush on him.
Anyway, since I enjoyed the class so much, I figured I would regurgitate some of what I learned for what I hope will be an enjoyable post. I've gone back and forth about what exactly would be best to talk about, but I've decided that the best topic to cover in this format would be Yosemite National Park. It is an ideal fit for this post because it is a bit more contained and easy to understand than some of the other topics in the class, but is still complex enough to convey an element of wonder and mystery. Additionally, it is the park I've had the most personal experience with, which will allow me to convey more understanding than I might be able to otherwise. So let's get started!
I'll begin by just showing some of the park's features in case anyone is unfamiliar with it. When most people think of Yosemite, they think of the valley which comprises the most popular destination within the park. In truth, it is actually more extensive than that, but for the purposes of this post I'll stick mostly to Yosemite Valley because it is sufficient to convey the information I wish to present, and is probably what most readers will experience for themselves at some point in the future.
The valley is known worldwide for its epic granite cliffs, huge waterfalls, and unique rock formations. Here is a sampling:
"No temple made with hands can compare with Yosemite. Every rock in its walls seems to glow with life...Awful in stern, immovable majesty..."
Now at this point, I expect that readers will start dropping like flies. Easily accessible natural beauty now takes a back seat to scientific beauty, which requires a little more work to enjoy. With that in mind, I have erred on the side of length rather than brevity with this post, so I will not be offended if you decide to jump ship. For the sufficiently curious, however, I expect it to be worth it.
You may know that geology is the study of the Earth's physical structure and substance. It also looks at how the Earth's history can inform us about what is happening now, and what will happen in the future.
So how did Yosemite come to exist as it does today? How were these huge chunks of granite created? How did they get to where they are now?
These questions open the door to explain one of the fundamental tools of geology: actualism. Actualism is the use of modern observation to understand ancient phenomena. For example, if you want to know how a particular kind of rock was formed, look for where that rock is forming today. A common example is limestone, which is characterized by the mineral calcite. Calcite is a mineral found in the skeletons of marine organisms, notably coral and shellfish. So to find where limestone is forming today, head to the Bahamas or another shallow tropical sea and observe how calcite is being deposited on the sea floor and over time becoming pressurized into limestone. Similarly, this same idea can help us understand what various regions were like during the formation of particular rocks. For instance, since we observe large slabs of limestone in the Colorado Plateau, we can deduce that the area was once covered in a shallow sea. This might seem like a stretch at first glance, but once you look at all the details, there's really not much guesswork to it.
Again, this is the idea of actualism: using modern observation to understand ancient phenomena. In Yosemite, we see that granite dominates the landscape. So we need to understand how granite forms in order to understand how the rocks of Yosemite came to be. Now it makes sense to tell you at this time that we know that granite forms from slowly cooling molten rock. But then where does the molten rock come from? To get a complete picture of how the rocks of Yosemite formed, we'll head to a place you probably wouldn't expect: the Andes Mountains of South America.
In order to understand the Andes (and in turn, Yosemite), you need a basic understanding of plate tectonics, because at the root of what makes the Andes tick is the collision of two tectonic plates.
I won't assume that everyone is familiar with plate tectonics, so here is a brief explanation. The Earth is not just a big hunk of solid rock. Only the very outer layer, called the crust, is solid rock. Beneath the crust, about 3-6 miles underground, the mantle begins. The mantle is composed of rock that is extremely hot, and sometimes liquid (you can think of this as magma, or underground lava). It is fluid enough that it can flow very slowly over time. The closer to the center of the Earth you get, the hotter this molten rock becomes until you reach the Earth's core, which is composed of liquid metals, and at the very center solid metals (at least this is the notion most commonly accepted, since we've obviously never been there).
Dang it Landon, if you throw me another piece of jargon I'm gonna to smash my computer! Would you get to the point already?!?!
Stay with me.
Like I said, since the Nazca plate is heavier, it subducts. This means it's moving under the South American plate. It's a lot like this:
Here's the important thing. The Nazca plate continues to move downward at an angle. As it goes down, the pressure from the weight of all the rock above increases. At a certain depth, the pressure gets so great that it causes the temperature to rise. Once the temperature increases sufficiently (at a depth a few kilometers beneath the surface), the rock actually begins to melt at the point where the two plates continue to scrape together. Once the rock is melted into liquid form (magma), it begins to rise because it is lighter than the surrounding rock. This causes an upward rising stream of magma to slowly ascend from the point of melting.
You can see how this process works back in that video above. Look off to the right side underground.
During the Mesozoic era, about 150 million years ago, the west coast of North America was in a very similar situation to the modern day coast of South America. A tectonic plate called the Farallon plate was subducting under the North American plate, just like the Nazca plate is currently subducting under the South American plate. This Farallon plate's subduction is also believed to have been responsible for the uplift of the Rocky Mountains later on, and possibly the extension of the west side of North America after that (the distance from Reno to Salt Lake City is thought to have doubled over the last 16 million years). We can still see the remnants of the Farallon plate in the small Juan de Fuca plate off the Pacific Northwest and the Cocos plate off the coast of Central America. Other than that, it has almost completely subducted under the North American plate.
But the important thing is that during the Mesozoic, there was magma production under the area of the Sierra Nevada range. We can assume that the ancient Sierra Nevada was very much like the modern Andes in size and scope, but that doesn't have a whole lot to do with the modern day Sierra Nevada.
Think back to those plutons I mentioned earlier (the big underground bubbles of magma). There were lots of those under the ancient Sierra Nevada. So many, in fact, that many joined together to where there was an almost continuous string of plutons under the west coast of North America. Eventually, the Farallon plate's subduction changed in nature such that it was no longer producing enough magma to replenish the plutons, and the plutons began to slowly solidify. By the extinction of the dinosaurs 65 million years ago, the plutons had pretty much solidified into huge underground hunks of granite. This ran up and down the west coast, and comprised a huge formation of underground rock called the Sierran Batholith. Some of you will probably see where this is going by now.
This process of uplift was greatly accelerated about 16 million years ago. Around that time was when the continent began to expand from the inside (which I mentioned earlier). This was due to other tectonic forces most likely having to do with the Farallon plate, but that's really all you need to know. What is important is that increased tectonic force was applied to the Sierra Nevada region, and faults began to form on the east side of the granite batholiths that began to slowly push the huge granite batholith upward, one earthquake at a time. By the time the most recent ice ages hit about 2 million years ago, the Sierra Nevada had once again become a substantial mountain range due to the uplift of the Sierran Batholith due to faulting on its east side. By this time, Yosemite was largely on its way to looking how it does today.
So by this time, the stage was set for Yosemite, but it was still sort of a “blank canvas”. Over the past few million years, there have been a few of important geological phenomena that have brought it to its present condition.
These kinds of rockfalls were probably what gave the valley it's initial shape, which was later altered by other erosive forces, like glaciers. Here's a great video produced by the park on Rockfalls if you're interested:
The other important erosive force in Yosemite's history is glaciation—the movement of glaciers through the park during the most recent ice ages. Until about 15,000 years ago, glaciers ran through Yosemite and are responsible for a number of characteristics of the valley. First, the glacier bulldozed away any loose rocks on the valley floor, deepening the valley and clearing ground. Second, the glaciers carved the walls of the valley, smoothing the faces of the cliffs and rounding out the shape of the valley. These are really the two main things that glaciation did for the park.
And that's just Yosemite.