These coordinates take you to an intersection on a country road. Just park your car, get out, and look around. Everything you need to examine to log this Earthcache is within the public right-of-way. DO NOT go beyond the gate, DO NOT trespass on nearby private property.
What you see around you is very familiar to anyone who travels the back roads of Marathon County . People call it “rotten granite,” “crushed granite,” “disintegrated granite,” or “unconsolidated granite,” but to a geologist it is GRUS.
GRUS is a special type of granite. Granite is an igneous rock formed under great heat and pressure below earth's surface. The different minerals crystallize separately as the molten rock cools, giving it a speckled appearance. The rate at which it cools determines the size of the crystals.
You are standing on the Nine Mile Pluton (a pluton is a large body of igneous rock formed when molten magma cools underground) which was formed over 1.4 billion years ago! Other bodies of rock in Marathon County include the Wausau Pluton, the Stettin Pluton, and the Wolf River Batholith Some quarries in Marathon County yield hard granite that can be cut and polished, yet a few miles away front end loaders scoop crumbly grus out of the ground practically ready to be spread on roadways. Grus contains much the same minerals as “regular” granite. What made grus different? Geologists propose several theories--here are some major points:
a) The simplest explanation is that over an extremely long time, before plants and animals existed, the granite was subjected to intense weathering and erosion, which turned the edges of the feldspar crystals into clay, and so the granite began to crumble. These processes progress most quickly in tropical climates, and when the rock is constantly wet.
b) Grus develops most often in mafic granites – that is, granites that contain iron and other heavy metals which oxidize, forcing the grains of feldspar and iron-rich biotite apart.
c) Grain size seems to be a factor, as fine-grained granites do not seem to be transformed into grus.
d) Grus development seems to occur at the margins of a pluton. As the molten rock intrudes into the older, hardened rock far below the earth’s surface, a shear zone develops. A shear zone is similar to a fault, except instead of a single crack, there is a larger zone of deformation containing microcracks.
e) Interesting facts: Wisconsin was once part of an ancient continent and was near the equator; and the type of granite (syenite) found in the area is formed in areas of subduction where one tectonic plate slides under another, where a continent meets the ocean.
To receive credit for this earthcache, answer the following questions and send them to us
1. On the west side of the road is a row of boulders resting on natural grus. One of these boulders is different from the others. Describe how it is different – compare grain size, color, proportions of the different minerals, and smoothness.
2. Look at the grus next to the boulders. Then cross the road to the graveled intersection. Look closely at the individual bits of gravel. How are these two areas of gravel different?
3. Although the last ice age (85,000-12,000 years ago) did not cover this area, earlier ice ages did. Can you think of two possible reasons the crumbly grus of the Nine Mile Pluton did not erode away? Hint: look north
4. Look up the dictionary definition of "rot." Now that you know theories of how and when the rocks of the Nine Mile Pluton formed, why or why not is "rotten granite" a good description of grus?
Bonus: in your own words, describe the sequence of events that might have led to the formation of the grus you see today.