This webpage was developed as part of the 2021 Wisconsin Science Festival, but you can learn from this page and do the activity anytime!
Wisconsin is proud of its geological assets. Among the state symbols, like the state bird and state animal, Wisconsin’s geological state symbols include: the state soil (Antigo Silt Loam), the state mineral (galena), and the state fossil (the trilobite). This activity focuses on one of the most abundant of the state’s symbols — the state rock, or Wisconsin red granite. Red granite was officially made the state rock in 1971 due to its abundance, economic value, and uniqueness to the state of Wisconsin.
Granite occurs in a variety of colors, so what gives Wisconsin’s state rock its trademark red hue? Red granite’s color is due to the high amounts of alkali feldspars (a type of feldspar mineral high in potassium) it contains.
What type of rock is granite?
Granite is a type of igneous rock. This broad category of rocks forms when hot, molten rock cools down — as the rock cools, it crystallizes and solidifies.
To identify an igneous rock that could be granite, look for rocks with crystals. Look for shiny, flat surfaces within the rock — these are the crystal faces.
The rock we’re looking for has interlocking grains, or in other words, all the rock’s grains are touching each other along their crystal faces. These grains should be large enough to be seen by the naked eye with no magnification needed. Geologists call this texture phaneritic. Rocks with crystals too small to be seen without the aid of magnification are called aphanitic.
Is your igneous rock granite?
So you’ve found a rock with interlocking crystals that you can see without magnification! But is this rock granite? The answer depends on what minerals are in your rock. And if your rock has all of the components we describe below, it just might be red granite!
All rocks are made of minerals. The primary minerals in granite are feldspar and quartz, and mica. You’ll learn to identify each of these minerals here. Sometimes, there are other darker minerals that are likely amphibole/hornblende depending on the specific type of granite.
Note: All of the photos used in the examples below are of rocks in the wall at the entrance of Picnic Point on the UW-Madison campus. Not all example photos are of red granite. Photo examples were selected because they best illustrate mineral properties.
These minerals are the distinct pink/red crystals you see in red granite—specifically, they are alkali feldspars. Alkali feldspar crystals come in varying shades of pink and red, from light pink to salmon to brick red. Other types of feldspars in granite are white or clear, resulting in a wide range of granite colors.
Feldspar crystals will have sharp angles and shiny crystal faces. Geologists call these distinctive angles and flat, shiny faces cleavage. Cleavage is related to the crystal structure of a mineral. Minerals with cleavage will break along cleavage planes. Feldspar crystals have two planes of cleavage that meet at 90° angles. See this resource for more explanation on identifying cleavage using feldspars as an example.
Though not as common as quartz, potassium-rich feldspar is an abundant mineral. Learn more about feldspar in Wisconsin.
Quartz has no cleavage; instead, it exhibits conchoidal fracture. Conchoidal fracture means that there are no distinct angles or planes of fracture in the mineral. Unlike feldspar, quartz fractures in more smooth, concave, or curved patterns. Luckily, there are other characteristics we can use to identify quartz. If you have found a quartz crystal in your sample, you’ll see that it is likely milky white in color and a little dull on the surface, or as geologists would say, it has a vitreous luster. Luster (the way a mineral reflects light) is another diagnostic feature geologists use to identify minerals. The two major categories of luster are metallic and non-metallic. There are many subcategories within each broad category. See this website for more information on luster.
Quartz is a component of many rocks besides granite, and it is the most common mineral in existence. Learn more about quartz in Wisconsin.
While the vast majority of minerals in red granite are either quartz or feldspar, a small portion of red granite is made of mica. These crystals will appear to be a shiny gray-brown or clear in color. Mica forms as flaky sheets and is very easily scratched compared to other minerals like quartz and diamond. The hardness of minerals is yet another identifying factor geologists use to determine what mineral they are examining. See this article on Mohs’ hardness scale by UW-River Falls emerit geology professor, Bill Cordua.
There are two common types of micas that occur in rocks, muscovite and biotite. Biotite is the darker of the two and can be found in different parts of Wisconsin. Learn more about biotite in Wisconsin.
Go to a place where you can see several different kinds of rocks. A great example is the rock wall at the entrance to Picnic Point, part of UW–Madison’s Lakeshore Nature Preserve. While this self-guided activity was designed with Picnic Point’s rock wall in mind, you can test your granite-identification skills anywhere that you have a variety of rocks available — even a gravel driveway could work!
- Look for igneous rocks—ones where you can see interlocking crystals. Rocks with interlocking grains in bands or layers are likely metamorphic, don’t be fooled.
- Try to find an igneous rock in which you can identify all of red granite’s major minerals (feldspar, quartz, and mica).
- Not all the granites in the wall are red granite! Look for other examples of granite in the wall. There are other types of granite to be seen in the rock wall; can you find one?
If you want an additional challenge:
Can you find the major minerals we learned to identify (feldspar, quartz, and mica) in any other rocks in the wall? Remember that igneous rocks have interlocking crystals—feldspars and quartz can be abundant in other rock types as well, but they may look slightly different in sedimentary and metamorphic rocks than they do in igneous rocks.
For another challenge:
Another kind of rock that you can find in the wall is rhyolite. This rock is closely related to red granite—the key difference is that rhyolite is an extrusive igneous rock, meaning that it formed above the surface and cooled more quickly, while red granite is an intrusive igneous rock, meaning that it formed underground and cooled more slowly. Rhyolite looks similar to red granite in composition and color, but because it cooled faster when it formed, rhyolite’s crystal grains are much finer (smaller) than the grains in granite. Can you find any rhyolite in the wall? What similarities can you see between that rock and the red granite you identified earlier? What differences can you see?
“Key to the Glacial Erratics in the Rock Wall at Picnic Point Entrance,” by Philip Brown, David Liebl, and David Mickelson for the Friends of the Lakeshore Nature Preserve.
Did you enjoy this activity? Were you successful in identifying our state rock? Did you encounter any difficulties? Let us know!
This activity was developed by Carsyn Ames, WGNHS samples curator, with the assistance of WGNHS geologist Sarah Bremmer. It was inspired by an educational event led by Philip Brown, David Liebl, and David Mickelson for the Friends of the Lakeshore Nature Preserve in 2017.
Some of the text for this page was adapted from the digital version of Laboratory Manual for Introductory Geology, by Bradley Deline, Randa Harris, and Karen Tefend, © 2016 University System of Georgia, and is used under a CC BY-SA 4.0 license.