Geology and Minerals of Rib Mountain near Wausau Wisconsin

Geology of Rib Mountain
   from: A GEOLOGICAL HISTORY OF RIB MOUNTAIN, WISCONSIN
              Keith Montgomery Ph.D.
              Department of Geography-Geology
               University of Wisconsin – Marathon County

Syenite is an intrusive igneous rock (as is the red granite for which Marathon County is justly famous) which contains very little to no quartz.  An intrusive igneous rock is formed when a mass of molten rock material or “magma” is injected into the landscape from below and solidifies beneath ground. Another term for intrusive rocks is “plutonic” (named after the Classical god of the underworld, Pluto). Large volumes of magma cool to form a type of “pluton” we call a “batholith” (see details in notes on intrusive igneous rock). The slow cooling that occurs beneath ground allows good-sized crystals to form and so intrusive igneous rocks are fairly coarse-grained. (This is also true of granite — you can see its individual crystals as in the note on red granite, above — but syenite, in contrast to granite, has very little quartz in it.)

The intrusion of the syenite’s magma happened about 1.5 billion years ago, or roughly in the middle part of the Proterozoic Eon. About this time in this area several other intrusions occurred. These other intrusions include:

    * First, the huge “Wolf River” Granite pluton that starts just east of Highway J in Marathon County and extends across into Marinette County;
    * Second, there are two other syenite plutons immediately to the north of Rib Mountain. One of these (the “Wausau Pluton”) forms the hill the Westwood Conference Center sits atop, while the other (the “Stettin Pluton”) forms a hill north of Highway 29, about 3 miles to the west of Wausau. You can just about make out both of these hills from Rib Mountain if you know where to look. The syenite of the Wausau Pluton is exposed along the north wall of the “29 Super” parking lot on 17th. St. and along Highway 51 to the west of that. Interestingly, the Wausau Pluton also contains some small quartzite xenoliths, near the summit of the hill. There is also a small syenite mass that forms a small hill south of Mosinee Hill, along Highway 51 near American Concrete.
    * And third, there is the “Ninemile” granite pluton, located between Highways N and 153 south of Rib Mountain, and exposed nearby along Highway N about 2-3 miles west of Highway KK. This granite is “rotten” in several places and is quarried for the gravel this provides (this is the pink stone often found in gravel lots and on the shoulders of many roads around the county). This last pluton has no appreciable relief and underlies much of Ninemile Swamp.

Despite the size of these intrusions, we are not certain of what was happening in the subsurface to melt material there and form the magmas. Today in the world, magma that leads to the formation of granite and syenite is found in zones where ocean floor is being recycled into Earth’s Mantle as a result of “sea-floor” spreading (see previous note on intrusive igneous rock). These zones are termed “subduction” zones, the largest and most continuous of which today forms the so-called “Ring of Fire” of the Pacific Rim. These are also zones of explosive volcanic activity and mountain building (think of the Andes and the Cascades, for example). Such intense geologic activity was also true of the area of today’s Marathon County between 2.0-1.5 billion years ago. Although they are not exactly contemporaneous, perhaps the intrusion of the plutons is in some way related to this activity.

Rib Mountain is composed of a single large block of the rock quartzite, as are each of its two neighbors. These blocks are embedded in a much larger mass of a rock called “syenite.” This syenite can be found at the base of the mountain, down at the level of Highways N and NN. Some of the syenite is exposed to view on the south side of Highway N just west of the junction with Highway KK. You might think of Rib Mountain as an iceberg, with more of the quartzite being below the level of the surrounding syenite plain than above it. The same is true of Mosinee and Hardwood Hills. However, don’t take this “iceberg” analogy literally: The quartzite isn’t really floating in the syenite, but the general image that this conjures up of the quartzite being “embedded” in the syenite is a good one.

From  Google Books – American journal of science, Volume 173 page 287

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