A GEOLOGICAL HISTORY OF RIB MOUNTAIN, WISCONSIN

Keith Montgomery Ph.D.
Department of Geography-Geology
University of Wisconsin - Marathon County

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HERE are some quick FAQs in response to questions I have received.

THIS IS ABOUT MORE THAN JUST RIB MOUNTAIN ......

I don't just want to tell a geological story here -- you know, "A long time ago on a primaeval continent ....".  If all you want is a short chronological account then proceed straight to the summary and exit. Of course I want to get to that, but I also want to involve you in thinking like a geologist.  So, I will draw your attention to the rocks we find in Marathon County and we will use them as evidence to piece together the history of the mountain. All this will not be done in historical order, but in the end it will all come together. Like all the best scientists, we will proceed by first asking good questions! However, to stop this from getting too long there will be some facts (such as dates) that I will supply and that we will just have to accept. If you want further information on local geology you can check out my sources, or you can contact me for further information.

Our only hope of figuring out what went on in Marathon County in the geological past is to study the rocks we find in north central Wisconsin in light of what we know about similar rocks being formed today in the world. Taking the knowledge we have of rocks in today's world back through time is called "uniformitarianism." Only by applying this knowledge can we come up with a rational, scientific account of what happened in the geologic past in Marathon County (or anywhere else, for that matter) -- anything else is pure speculation and liable to mythologizing. Doing this and accepting the conclusions we come to requires intellectual honesty and courage.
 
History of the glacial theory  Development of the glacial theory

THE "MOUNTAIN" AND SOME ADJACENT HILLS .....

Rib Mountain does not stand alone. Adjacent to it are Mosinee Hill (Upper and Lower) and Hardwood Hill. Together, these hills form a sweeping crescent that encircles Ninemile Swamp.

Rib Mountain (formerly Rib Hill) consists of a impressive ridge almost 4 miles long that peaks at 1924 ft above sea level. Rib Mountain is not the highest point above sea level in the state: that honor is due Timm's Hill, near Ogema in Price County (1952 ft above sea level). However Rib Mountain does stand higher above the surrounding terrain (670 ft) than any other hill in the state -- this is the "local relief" (a local guidebook once claimed that Rib Mountain "towers 2000 ft above Wausau" -- a statement that clearly confuses "elevation", or height above sea level, with "relief"!).

As the map shows, Mosinee Hill is located 1.5 miles south-southeast of Rib Mountain and consists of two peaks (the Upper and Lower) joined by a "saddle". The Upper hill (to the north) is 1610 ft above sea level while the Lower one is 1472 ft asl. The local relief of the Upper hill is 465 ft. Hardwood Hill, 3.5 miles to the southwest of Rib Mountain, is a single conical peak, 1610 ft asl with a local relief of 300 ft.

All three hills are composed of rock of the same composition and have a similar geology (details below). Therefore, whatever can be inferred about Rib Mountain will also apply to Mosinee and Hardwood Hills.
 
 

SOME MYTHS

Some people think Rib Mountain is an extinct volcano.  Seen "end-on", as one approaches from the west or east along Highway 29, the cross section of the "rib" does indeed make the mountain appear to have the conical shape of a volcano (this is also true of various angles on Mosinee Upper Hill and Hardwood Hill). However, in the case of Rib Mountain at least, this apparent shape is just an illusion, as we can see from the map (above).

Then again, some people believe Rib Mountain to be volcanic because of this plan (map) form. They see the semi-circle of the three hills and believe that altogether they represent the fragmented remains of the base of a large volcanic mountain. Perhaps they see it as an ancient, fragmented version of the Crater Lake (Mt. Mazama) caldera. In fact, by coincidence, the dimensions are just about correct for this interpretation.

However, there is no geological evidence to support a volcanic interpretation.

So, sorry, these hills are not volcanic in any way, shape, or form!
 
 

THE REALITY (more complex, of course, but much more interesting) .....

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.

So, first, we must explain how the quartzite came to be embedded in the syenite. Let's begin with the syenite itself.
 
 

THE SYENITE AND WHAT IT REPRESENTS .....

Syenite is an intrusive igneous rock (as is the red granite for which Marathon County is justly famous). 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:

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.
 
 

XENOLITHS .....

How do such huge volumes of magma actually get into the landscape? How do they find the space?  And how did the quartzite get into the syenite?

Well, to find space for the magma, it seems that the pre-existing or "country" rocks may be pried apart by the force of the intrusion and that large blocks of the country rock sink into the magma to be melted down or "assimilated." This entire process is called "magmatic stoping." In central Wisconsin, as elsewhere in the continent at this time, there may also have been some tension in the continental crust that also helped to pull it apart and create some space for the intrusion.

 Whatever the exact means of intrusion, it is not unusual to find chunks of the pre-existing rock still intact within a pluton. These chunks are called "xenoliths." Most xenoliths are the size of small boulders but, almost incredibly, it seems that the quartzite of Rib Mountain and the hills to the south are gigantic xenoliths buoyed up and rotated in the streaming, viscous magma! This is how they came to be embedded within the syenite. The geology map of the area shows that the quartzite hills are arranged in a rough semi-circle within the crescent of the syenite pluton. The shape of the pluton has determined the arrangement of the hills.
 
 

.....AND MONADNOCKS

However this really does not entirely explain the relatively high relief we find here. Let us now consider this aspect of the landscape. Again, let's start with the syenite.

Recall we said that syenite is intrusive. So how do we come to find it now at the surface of the landscape?  

The answer is, in the 1.5 billion years since its intrusion, erosion in the landscape by rivers and other agents, such as glacial ice, has worn away the  rocks that the syenite intruded. Indeed, erosion has also worn away a good deal of the syenite that encapsulated the quartzite! But the quartzite, unlike the syenite and those other rocks, has survived the elements on account of its famous hardness. Rib Mountain and it neighbors are thus examples of "monadnocks". This is how the relief has come to be formed in this part of the state. But let us note that Rib Mountain and its neighbors are not unique in this respect: The Baraboo Hills in south central Wisconsin provide another example of the resistance of quartzite to erosion.

For the sake of complete accuracy, however, it should be pointed out that the local hills have survived not one, but two periods of erosion since the intrusion of the syenite. 

So we can say that a new Rib Mountain (the one on which we can stand today) fully emerged sometime in the past 300 million years!
 
 

WHERE DID THE QUARTZITE COME FROM?

We could end here, if we wish. But all of this begs one final question: If the quartzite pre-existed the syenite, then when and how was the quartzite formed? To answer this question we must venture even further back in time, back almost to the first continents of Earth.

Quartzite is sandstone that has been altered ("metamorphosed") by heat to the extent that the original quartz grains of the sandstone have been fused with the silica that served to cement them together in its original sedimentary state. So now we must explain the original sandstone!

Like sand we find today along rivers and on beaches, the sand grains of the sandstone must have come from the erosion of the land. This land must have been created even further back in time. It is thought that this land was part of an "Archaean Supercontinent" (think of this as an Archaean version of the Phanerozoic's "Pangaea"). The erosion that produced the sand was happening on this ancient continent about 2.5 - 2.2 billion years ago. Much like today, this sand accumulated on beaches or in river valleys and was later buried and cemented into the solid rock of sandstone. Although this happened a long time ago, as you walk about Rib Mountain you might notice in the quartzite some of the original ripples of the sand.

But when was the sandstone metamorphosed? By the heat of the syenite magma, or at some other, earlier time? Well, detailed investigations of the local geology seem to suggest that it might have happened during the subduction that formed the Penokean Mountains, but certainly prior to the time of syenite intrusion. We know this because the quartzite xenolith contains some other minerals, such as sillimanite, that require greater pressures and heat for their formation than would be present even within the magma of the syenite intrusion.
 
 

CHRONOLOGICAL SUMMARY

1. Archaean Supercontinent splits 2.5 billion years ago, much like Europe and North America have split and formed the Atlantic in more recent geologic time. Erosion of this ancient continent produces sand (among other things) that accumulates and soon forms sandstone.

2. Subduction starts to happen about 2.0 billion years ago, sweeping oceans and continents back together. The Penokean Mountains are formed. Perhaps at this time the sandstone is metamorphosed into quartzite. Then, at a slightly later time, an intrusion of syenite incorporates various large blocks of this quartzite.

3. The Penokean Mountains and some of the syenite in their roots are eroded away between 1.5 and 0.6 billion years ago, revealing the quartzite xenolith of today's Rib Mountain.

4. The landscape is submerged and possibly buried under accumulations of various sedimentary rocks roughly between 600 and 300 million years ago.

5. Erosion in the past 300 million years removes the sedimentary rocks from this part of the state. As before, Rib Mountain survives this erosion and stands tall on account of the legendary toughness of its quartzite!
 
 

TAKE A QUIZ -- not easy!

 

COMPILATION OF NOTES

Here is a copy of all the notes, in order of appearance.


 

ANOTHER SITE ON WISCONSIN GEOLOGY YOU MIGHT LIKE ........

A geologic tour of the Baraboo Ranges and Devil's Lake.
 
History of the glacial theory  Development of the glacial theory