There are 4 Primary divisions of Rocks around Folsom Lake.
Ultramafic
Ultramafic (or ultrabasic) rocks are dark-colored igneous and meta-igneous rocks that are rich in minerals containing magnesium and iron ("mafic" minerals) and have a relatively low content of silica. The Earth's mantle is thought to be composed of ultramafic rocks. Most of the exposed ultramafic rocks have been found in orogenic (mountain-forming) belts. The scientific study of ultramafic rocks has been revealing some of the geological processes in the Earth's history. Ultramafic rocks contain very low amount of silica and are common in the mantle.
Metamorphic
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet.
Metamorphic rocks were once igneous or sedimentary rocks, but have been changed (metamorphosed) as a result of intense heat and/or pressure within the Earth's crust. They are crystalline and often have a “squashed” (foliated or banded) texture.
Granodiorite and Granitic
Granite and granodiorite are intrusive igneous rocks that slowly cool deep underground in magma chambers called plutons. This slow cooling process allows easily visible crystals to form. Both rocks are the product of the melting of continental rocks near subduction zones.
These rocks are both classified as granitic, because they both are rich in quartz. Granite contains mostly potassium feldspars and has a low percentage of dark iron and magnesium minerals. In contrast, granodiorite contains more plagioclase (calcium and sodium) feldspar than potassium feldspar and has more dark minerals. Thus it is a darker color than granite.
Volcanic Mud Flows
A lahar (Indonesian) or volcanic mudflow is a mixture of volcanic debris and either hot or cold water which flows down the side of a volcano. Lahars occur when large amounts of water quickly mix with either newly erupted volcanic material or old material resting on slope. The water comes from either melted ice, breached crater lakes, and engulfed river or rainfall. It resembles flowing concrete with particles from clay to boulder size, and gains new material as it erodes its bed.
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Today you will be focusing on the metamorphic rocks of the area. At the coordinates you should find yourself at an outcropping of rocks near the parking area/boat ramp. Here you will be making some observations of the rock you see and, using the information provided here, answering the questions. There are also some other caches in the area if you like to take a little hike around this part of the lake you could perhaps find some of the other divisions of rocks mentions above while also taking in the wonderful and picturesque location.
If you use the nearby parking lot, there is a daily fee to park. If you don’t want to pay for parking, there are hiking and bike trails to and from the area, but the hike is significant.
Before logging this as found, answer the following questions and message the answers you discover.
Questions:
- Your User/Geocaching Name and the name of those with you (only need to send answers once as a group)
- How do metamorphic rocks form and what do they form from?
- Using the Metamorphic rock image above, which do the rocks you see most closely resemble? What grade of metamorphism does this outcropping of rocks appear to be in?
- How do you think increased metamorphism would affect the rocks here?
- Describe the surface of these metamorphic rocks, texture, color, layering, etc. (Struggling with this questions, take a picture of your GPS on the rocks you are trying to describe).
- Look closely at the rock and you will see various crystals/crystalline particles embedded within. Describe the crystals you can see. How big is the largest crystal you see?
- OPTIONAL: Take a picture of yourself at this location.
Sources:
https://opengeology.org/textbook/glossary/
https://www.newworldencyclopedia.org/entry/
https://www.slideshare.net/ctzomes/folsom-lake-state-recreation-area