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Mineralogy
When we were talking about geologic time and Yosemite's history, we were thinking of the BIG picture. We talked about processes that took millions of years to occur and plutons that took up miles of space. When people visit Yosemite, they usually look at the giant cliffs and waterfalls to bask in Yosemite's glory. However, some of the best ways to truly understand Yosemite isn't through a big picture, but through a microscopic lens. We can learn a lot about the formation of Yosemite by simply picking up a rock and studying the minerals within it.
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Therefore, our next task is to study the minerals found in Yosemite including their formation and how we can identify them. Using this information, we will be able to classify the type of rock that Yosemite is known for.
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Look at the picture on the left to see what a typical rock from Yosemite looks like. Do you notice the different colors and shapes within the rock? Take a minute to make some observations about what you can see!
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Those different colors and shapes are minerals.
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Minerals are the BUILDING BLOCKS of rocks and are very, very important! Minerals are everywhere and they are the clues that geologists use to determine how and where a rock formed. Each mineral is unique and needs very specific environments to form. Using this information, geologists are able to name rocks based on the minerals they are composed of.
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But don't worry! Although there are over 4,000 minerals that have been identified, we are only going to focus on the ones we can see in Yosemite!
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Do you remember the process of incremental growth? As discussed in the last section, the rocks in Yosemite formed because of magma cooling, meaning that we already know that the rocks in Yosemite are igneous.
When this magma was cooling to form rock, minerals were crystallizing. The video on your right shows what a mineral crystallizing in magma might look like. Minerals grow from the center to the edges--just like tree rings!
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Due to the specific conditions in Yosemite millions of years ago, specific types of minerals crystallized in this magma: plagioclase, hornblende, biotite, quartz, sphene, and K-feldspar. We are able to distinguish these minerals by observing their various characteristics including color, habit (what does it look like?), luster (is it shiny? dull?), cleavage (what shape does it grow in?), etc.
These various minerals grow in unique ways! Minerals crystallize at various rates, temperatures, and in various shapes! Look at the video below to see what mineral growth looked like in Yosemite millions of years ago.
Let's look at each of these minerals in their purest form and use this information to further classifying what type of rock we are looking at.
Look at the chart above. Take a minute to observe how each of these minerals have unique characteristics! Isn't it beautiful? Each mineral is either labeled as felsic or mafic. Felsic refers to light-colored minerals that are mostly composed of silica and oxygen (as well as other minerals like Na, K, and Ca). Mafic refers to dark-colored minerals that are also composed of silica and oxygen, but contain a fair amount of magnesium and iron.
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One of the first steps in identifying an igneous rock is to classify it as felsic, mafic, or intermediate based on the minerals that is is made out of. A rock that is mostly composed of felsic minerals is typically a light color and is considered felsic. A rock that is mostly composed of mafic minerals in darker in color and is considered mafic. A rock that is equally composed of both felsic and mafic minerals is called intermediate!
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Let's look back at that image of the rock from Yosemite... Would you classify it as a felsic, mafic, or intermediate rock?
TERMS TO REMEMBER
Minerals:
Felsic:
Mafic:
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We are getting very close to determining the type of rock that Yosemite is known for! So far we know that the rock is plutonic (cooled underground--as discussed in Geologic Time), felsic (based on composition), and igneous (formed from the cooling of magma). This means that this rock is called... GRANODIORITE (a variation of granite)!
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Don't get too excited! We have only looked at one rock in Yosemite. Let's go on a hike around the park to see what some other rocks look like!
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Now let's observe this rock that we can find at a different location in Yosemite! Navigate through the slide to compare this image with the previous rock we studied. What differences can you see?
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Notice how the rock in this new image has more mafic minerals than the previous rock that we observed! Due to the greater number of mafic minerals, this rock is classified as intermediate.
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So can we still say that the rocks found in Yosemite are granodiorite?
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Well, since the rocks in Yosemite vary depending on the amount of mafic/felsic minerals present in the rock, we cannot call all of the rocks granodiorite. However, these rocks are still a variations of granite, so we call them granitoids!