Region 1: Rocks of the Superior Upland

The Superior Upland is the midwestern portion of the Canadian Shield, which extends from Minnesota to Nunavut and Greenland to New York. The shield as a whole represents the first portion of the North American continent to have emerged above sea level.

The Canadian Shield is the exposed portion of a larger craton. Its extremely old, hard rock was resistant to the weathering of repeated glacial advances, compared to the softer sedimentary strata to the south in the Central Lowland region, and this resulted in a higher, more rugged geographic upland.

A craton can be thought of as the heart of a continent—it is typically the oldest, thickest, and most stable part of the bedrock. It is also usually far from the margins of tectonic plates, where new rock is formed and old destroyed. This very mature rock has usually been metamorphosed at some point during its history, making it very hard.

The oldest rock in the region, and indeed the United States, is the Morton Gneiss. It was initially formed 3.6 billion years ago, during the Archean Eon. For hundreds of millions of years, magma welled up from the mantle, creating the foundation of granite below the surface, a rock composed mainly of quartz, mica, and feldspar. These minerals have densities that are lower than both those of oceanic crust and the average density of the mantle, allowing this lighter rock to essentially float on the upper portion of the lithosphere. The lithosphere, in turn, is pushed around on the asthenosphere, which effectively acts as a conveyor belt (Figure 2.2).

Over its exceedingly long history, this rock was metamorphosed, resulting in alternating bands that are distinctive of gneiss. Upwelling continued for hundreds of millions of years, injecting more and more magma into the region, which helped to metamorphose existing granite into gneiss. This magma would then cool to form yet more granite. This cycle continued until at least 2.7 billion years ago, though subsequent erosion could have erased rocks that are younger still. The result is a sequence of metamorphic and igneous rocks spanning nearly a billion years, remnants of which are found in northeastern Minnesota, northern Wisconsin, and parts of Michigan’s Upper Peninsula.

Figure 2.2: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Figure 2.2: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Unless rock layers are overturned, older rocks are found at the bottom and younger rocks are found at the top of a sedimentary sequence. This is known as the Law of Superposition.

One of the chief causes of the metamorphosis of the ancient granites of the Superior Upland was the collision of the Superior Province with the Minnesota River Valley terrane around 2.7 billion years ago. This triggered a period of intense mountain building, called an orogeny, as the two small continents pressed on each other, causing their rock to buckle, which forced some of the rock up as mountains and some down to be subjected to even greater pressure and heat (Figure 2.3).

Figure 2.3: The Superior Province collides with the Minnesota River Valley (MRV) terrane, crumpling the margins of both continents.

Figure 2.3: The Superior Province collides with the Minnesota River Valley (MRV) terrane, crumpling the margins of both continents.

What happens to a rock when it is metamorphosed?

When rocks are subjected to high enough temperatures or pressures, their characteristics begin to change. The weight of overlying rock can cause minerals to realign perpendicularly to the direction of pressure, layering them in a pattern called foliation, as exemplified in gneiss and schist. Recrystallization, as seen in marble and quartzite, results as rock is heated to high temperatures, and individual grains reform as interlocking crystals, making the resulting metamorphic rock much harder than its parent rock.

Contact metamorphism describes a metamorphic rock that has been altered by direct contact with magma. Changes that occur due to contact metamorphism are greatest at the point of contact. The further away the rock is from the point of contact, the less pronounced the change.

Regional or dynamic metamorphism describes a metamorphic rock that has been altered due to deep burial and great pressure. This type of metamorphic rock tends to occur in long belts. Different types of metamorphic rock are created depending on the gradients of heat and pressure applied.

After a 200-million-year gap, the rock record picks up in the Proterozoic with 2.4-billion-year-old quartz sandstones and dolostones, which were deposited in a shallow marine environment. In some cases, these sedimentary rocks preserve ripple marks and stromatolites. Ancient glacial sediments are also found in the Superior Upland, evidence of the Huronian glaciation, which is thought to be the first-ever ice age on Earth. Following a gap of about 100 million years from 2.2 to 2.1 billion years ago, the sandstones reappeared and extended to 1.9 billion years ago.

Stromatolites

Stromatolites are regularly banded accumulations of sediment created by the trapping and cementation of sediment grains in bacterial mats (especially photosynthetic cyanobacteria). Cyanobacteria emit a sticky substance that binds settling clay grains and creates a chemical environment leading to the precipitation of calcium carbonate. The calcium carbonate then hardens the underlying layers of bacterial mats, while the living bacteria move upward so that they are not buried. Over time, this cycle of growth combined with sediment capture creates a rounded structure filled with banded layers.

Stromatolites peaked in abundance around 1.25 billion years ago, and likely declined due to predation by grazing organisms. Today, stromatolites exist in only a few locations worldwide, such as Shark Bay, Australia. Modern stromatolites form thick layers only in stressful environments, such as very salty water, that exclude animal grazers. Even though there are still modern stromatolites, the term is often used to refer specifically to fossils. For more information, see Chapter 3: Fossils.

Beginning about 2 billion years ago, thick and increasingly widespread banded iron formations also appear in the Superior Upland. As with much of the Canadian Shield, most of this rock has been metamorphosed to some degree: sandstones have become quartzite, dolomites have become marble, and banded iron formations have been deformed and folded. The Penokean Orogeny, from 1.9 to 1.8 billion years ago, transformed the area into a primarily erosional environment, so few rocks are preserved from this time. The tectonic activity did, however, force some magma up near the surface where it cooled into the granite found in northeastern Wisconsin and the Upper Peninsula.

See Chapter 1: Geologic History for more information on the Penokean Orogeny and Midcontinental Rift.

About 1.1 billion years ago, a major event in the history of the Superior Upland took place when the North American continent began to split apart along the Midcontinental Rift. It is thought that a hot spot in the mantle caused a great upwelling of magma, centered near where Lake Superior is today. Over the course of 20 million years, the pressure from below caused the crust to spread in all directions, eventually thinning the Canadian Shield considerably.

In some areas, water and carbon dioxide that had been trapped in the magma formed bubbles. After the lava cooled, these bubbles were slowly filled by mineral-rich water flowing through them, depositing layers of fine quartz crystals and enough iron to color the resulting rocks red. Lake Superior agate, the Minnesota state gemstone, was formed in this manner.

The rift eventually failed, but it left its mark on the region. Huge volumes of lava burst through to the surface, forming deposits of basalt. Magma of the same composition cooled underground to form gabbro. The tectonic activity also broke up many of these new rocks, producing large amounts of sediment that were preserved as sandstones and conglomerates. An age of 1.1 billion years is old even for a rock, yet this is currently the youngest exposed bedrock in the Superior Upland. On the southern and western shores of Lake Superior, one can see volcanic sequences, sandstones, and the Copper Harbor Conglomerate, evi- dence of the processes occurring during rifting (Figure 2.4). While the rocks produced by the Midcontinental Rift are only found at the surface around Lake Superior, now-buried legs of the failed rift extend north into Canada, southeast nearly to Lake Erie, and southwest all the way to Kansas (Figure 2.5).

Figure 2.4: The geology around Lake Superior is perhaps the most complex in the Midwest. A variety of sedimentary, igneous, and metamorphic rocks representing more than a billion years of history are found within a few kilometers of each other.

Figure 2.4: The geology around Lake Superior is perhaps the most complex in the Midwest. A variety of sedimentary, igneous, and metamorphic rocks representing more than a billion years of history are found within a few kilometers of each other.

Figure 2.5: The scars of the Midcontinental Rift span nearly the entire Midwest, but are buried deep below the surface in most of the region. The inset box represents the outcrops of Midcontinental Rift rocks found at the surface.

Figure 2.5: The scars of the Midcontinental Rift span nearly the entire Midwest, but are buried deep below the surface in most of the region. The inset box represents the outcrops of Midcontinental Rift rocks found at the surface.