Geologic time &
Yosemite's History
37°44'59"N 119°35'44"W
In order to understand Yosemite, we have to understand geologic time!
Think of a geologist as a detective, trying to solve the mystery of Earth's history. In order to determine what has happened in Earth's past, these geology detectives study the present. Then, geologists are able to put events in a timeline to explain what they see in the present using relative and numeric dating.
What is relative and numeric dating?
Relative dating is the process of putting an age to an object relative to the objects surrounding it. For example, I can say that my sister is older than my brother. Therefore, my brother is younger than my sister. Geologists use this technique to often determine the age of rock layers!
Numeric dating is determining the exact age of an object using various scientific methods (i.e. isotopic/zircon dating). For example, I can say that my sister is 22 years old and my brother is 18 years old. Geologists can use these methods to give numerical ages to rocks in order to determine when events occurred.
Using these two dating principles, geologists have successfully been able to date major events in Earth history, including the first appearance of humans, the great dinosaur extinction, the beginning of life, and the formation of the Earth--4.56 billion years ago.
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Wait. Let me say that again.
The Earth is 4.56 billion years old.
The true expanse of that number is incredibly hard to grasp. Watch this fun, short video that scales the major events of Earth's history into the size of a football field.
In that video, the entire history of the Earth was scaled into the span of one football field. In this example, only 1 inch represents 1.3 million years.
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Let's take a look at that football field!
The blue, orange, green, and red lines should look familiar, as they are what you learned about Earth's history already.
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Why did we put Earth's history on a football field? Earth's age can be incomprehensible, as it is difficult to imagine what 4.56 BILLION years really feels like. However, a football field is something that you have encountered in your life and has a distance that you can easily imagine. Hopefully, putting Earth's history on a football field gives you a better idea of how old the Earth truly is, and gives you a better concept of geologic time.
In order to learn about the formation of Yosemite, we have to zoom in to the last four yards on the football field, represented by the photo on the right. Take a second to think about how SMALL that is in comparison to the entire field! Yosemite's formation began about 2 and a half yards ago, which, in geologic time, was 125 million years ago (125 mya). That was a LONG time ago. What was even going on back then?
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This is where we begin "Yosemite National Park: The Magmatic Story".
Once upon a time, hot liquid magma rose up (or intruded) from deep inside the Earth underneath what is currently the Sierra Nevada mountains and began a process called incremental growth. This process of incremental growth occurred for 40 MILLION years ending 85 million years ago (85 mya). What is this process of incremental growth? How did it create the incredible park we see today? To answer these questions, I am going to take you back in time to the formation of Yosemite!
Incremental Growth
Imagine a cake (yum!). In order to see the gooey layers of filling, we must cut the cake and look on the inside. Now, imagine that that cake is Yosemite, the yummy gooey layers are magma, and it's between 125 - 85 mya.
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The figure on the left breaks down the process of incremental growth into steps. The tan background represents Earth (the cake!) and the color red represents hot magma (the filling!). The darker the red gets, the colder the magma is until it fully cools and becomes rock (represented by the color black).
The steps of incremental growth:
Step 1: Magma intrudes into the Earth 10 kilometers deep underground. The rock that the magma intrudes into is called the host rock. Over time, this magma cools to form igneous rock. Since this rock formed underground, we call this rock a pluton.
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Step 2: After the first magma (A) cools, two more magmas intrude into the host rock (B1+B2).
Step 3: Before magmas B1+B2 are able to finish cooling, a new magma (C), intrudes into the existing plutons. This new, hot magma heats up units A, B1, and B2. Rock A becomes liquid again. Now, all four units are interacting and mixing together!
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Step 4: As more time passes, all four units cool to form a pluton.
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Step 5: Now, how do we get these plutons that are at least 10 kilometers deep in the ground to be exposed at the surface? After all, Yosemite isn't 10 kilometers underground. Two processes need to occur for this: uplift and erosion.
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Uplit occurs on a very large scale on Earth, where entire areas of Earth's crust are lifted closer to the surface. Think of a forklift lifting a crate up, that is what is happening to our Earth in certain areas!
Uplift isn't the only process that brought these plutons to the surface, however. Yes, uplift can bring rock at 10 km depth higher, but there is still so much rock on top of them. Where does all of that rock go? We can thank erosion for that.
Erosion is the movement of Earth's materials (rock, for example) from one area of the Earth to a different location. In the case of Yosemite, this happened because of glaciers! Glaciers are essentially huge frozen rivers that have the ability to break and carry rock great distances, and Yosemite used to be covered by them.
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Yosemite is known to be one of the most beautiful locations on Earth. Now you know that we can thank incremental growth, uplift, and glaciers for that! It took millions of years, but it was well worth it for that view.
37°42'54"N 119°40'36"W