This is an Earth Cache! No physical hide is here, but rather a lesson on geology instead! Using the rocks at this location and the following lesson please answer the questions below!
In elementary and middle school science most of us learned about the three major types of rocks - igneous, sedimentary and metamorphic. This earthcache will examine the difference between granite, an igneous rock and gneiss, a metamorphic rock.
The bedrock around Granite Falls is dominated by ancient metamorphic rocks called gneiss (pronounced "nice"). These are not “granite” in the strict sense — they’re older rocks that have been transformed by heat and pressure deep in the Earth’s crust.
The gneiss in this part of the Minnesota Riveer Valley formed during the Archaen eon, roughly 3.6 to 3.8 million years ago, making them among the oldest rocks in the continental United States. They were later intensely deformed and metamorphised around 2.6 to 2.7 years ago.
Originally formed deep underground, these gneisses are now exposed along the Minnesota Riveer Valley because of:
- Uplift and erosion over billions of years
- Massive carving by glacial meltwater during the end of the last Ice Age (e.g., by the catastrophic drainage of Lake Agassiz), which stripped off younger rocks and uncovered this ancient basement.
Granite exhibits a coarse-grained structure resulting from the gradual cooling of magma beneath the Earth's surface. This slow cooling process facilitates the formation of interlocking mineral crystals, predominantly composed of quartz, feldspar, and mica. The overall appearance of granite varies from speckled patterns to more homogenous colorations, rendering it visually distinct.
Gneiss, a metamorphic rock, demonostrates a foliated texture attributed to the impact of high pressure and temperature on pre-existing rocks, such as granite, shale, or schist. The recrystallization of minerals during metamorphism leads to the development of distinct layers, manifesting as alternating bands of light and dark minerals. This foliation contributes to the visual differentiation of gneiss from granite.
Granite and gneiss are both common rocks, but they differ in how they form, how they look, and their structure.
1. How they form
- Granite is an igneous rock. It forms when molten magma cools slowly beneath the earth's surface.
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- Gneiss is a metamorphic rock. It usually forms when granite (or a similar rock) is changed by intense heat and pressure deep within the Earth.
2. Appearance
- Granite has a speckled, uniform look. The mineral minerals (quartz, feldspar, mica) are mixed evenly with no layers. Looks like "salt and pepper" or spotted arrangement
- Gneiss has a banded or striped appearance, with light and dark mineral layers clearly visible.
3. Structure
- Granite has a chunky random crystal arangement.Crystals are chunky and randomly oriented.
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- Granite has a foliated structure,meaning its minerals are aligned in layers due to pressure.
Malachite jewelry
In order to log a find on this EarthCache send me the answers to the following questions, and post a photo of you or a geocaching item(required): .
1. Near the posted coordinates is a large column or pillar of stone (lying on the ground). This column was on a building prior to being placed here at the museum. Examine the column. What do you observe? What can you observe in the column that tells you that it is gneiss or granite.
2. Approximate the height of the large granite outcropping in front of you to the East..
3. What can you observe in the outcropping that shows you this is gneiss or granite?
4. Take a picture of you or a personal item showing the outcropping?