Rocks of the Midwestern US
Figure 2.1: The rock cycle.
A rock is a naturally occurring solid substance composed of one or more minerals. Broadly speaking, there are three types of rock: sedimentary, igneous, and metamorphic. The rock cycle describes the many processes that produce rocks, while also illustrating the differences between rock types. One type of rock may be transformed into either of the other types, often with the help of other parts of the Earth system, including plate tectonics, the water cycle, and biological processes, to name a few.
Sedimentary rock is formed by the lithification of sediments (e.g., unconsolidated mineral and organic particles created through the weathering of other materials, like rock and organic matter). Typically, sediments are created in an environment where erosion is a dominant force, and they are transported by wind, water, or ice to a depositional environment. For example, a rushing river can wear away the rock it is flowing over, and it also has enough energy to transport the resulting sediment to a lake. The water slows down, losing energy, and deposits the sediment on the bottom of the lake.
Sedimentary Rock Classification
Sedimentary rocks are classified by their sediment size or their mineral content, and each one reveals the story of the depositional environment where its sediments accumulated and were eventually lithified.
Lithification of sediments occurs in several ways. As sediments build up and lower layers are buried more deeply, they may become permeated by water. Minerals dissolved in the water are precipitated, filling the spaces between particles and cementing them together. This cementation helps to form many common sedimentary rocks, such as shale, sandstone, and most conglomerates. The evaporation of water may also form sedimentary rocks by leaving behind evaporites (previously-dissolved minerals) like salt. Deposits of calcium carbonate, usually created through the accumulation of calcium carbonate skeletal material (such as clams and corals), form the sedimentary rocks limestone and dolostone.
Igneous rocks form from the cooling of magma (molten rock underground) or lava (molten rock at the Earth’s surface). When magma cools slowly underground, it has time to produce large crystals that are visible to the naked eye. Rocks that form in this manner, such as granite, are called plutonic. Molten rock that breaks through the crust to the surface (usually through a volcano) cools quickly as its heat escapes to the atmosphere. This produces volcanic or glassy rocks with very tiny crystals or no crystals at all.
Igneous Rock Classification
Igneous rocks differ not only in their cooling rates and subsequent crystal sizes, but also in their chemical compositions. Rocks found in continental crust, such as granite, have high silica content and low iron and magnesium content. They are light in color and are called felsic. Rocks found in oceanic crust, like basalt, are low in silica and high in iron and magnesium. They are dark in color and are called mafic.
Metamorphic rocks form from pre-existing sedimentary, igneous, and metamorphic rocks that are exposed to increased temperature and pressure as a result of plate movements, very deep burial, or contact with molten rock or superheated water. The minerals within the rock recrystallize and realign, forming a much harder rock. This process destroys features in the rock that would have revealed its previous history, transforming it into an entirely new form.
See Chapter 1: Geologic History for a description of the mountain building episodes that created much of the metamorphic rock, and read about the rift valley that created many of the igneous rocks as a result of magma intrusions and volcanic events.
Metamorphic Rock Classification
Metamorphic rocks are classified differently depending on the protolith (parent rock) they are made from. The following chart shows common rocks and the metamorphic rocks that they can become.
As you read through this chapter, keep in mind that once you understand the geologic events that have affected a given region, you should be able to predict the type of rocks found in that area. The Midwest has seen many dramatic changes, which are reflected in the diversity of the rocks found throughout the area. The oldest rocks of the Midwest formed more than three billion years ago from sand deposits, but being buried and “baked” for hundreds of millions of years metamorphosed them. Many of these rocks are largely composed of iron-rich minerals and are therefore, not surprisingly, mined for ore. Metamorphic rocks make up the majority of the bedrock of the Superior Upland. Sedimentary rocks, often fossiliferous, are found in every state. They represent the overwhelming majority of bedrock in the Midwest and include some important natural resources such as coal, salt, gypsum, and limestone, and some of their layers serve as aquifers. Igneous rocks, while not as abundant, are also widespread.
Why do we see different kinds of rocks at the surface?
As you walk across the surface of the Earth, you will observe an amazing variety of rock types. If all rocks were flat-lying layers and there was no erosion, then we would only see one type of rock exposed on the surface. Often, however, rocks have been worn away (eroded), and the underlying layers are now exposed at the surface. Layers of rock may also be tilted, folded, or faulted to reveal the underlying rocks at the surface.
When rocks are flat-lying layers and there is no erosion, folding, or faulting, the person walking across the surface sees only one rock type.
When rocks are worn away (often by streams), the person walking across the surface sees the underlying layers of rock exposed.
When rocks are folded or tilted, the person walking across the surface sees several layers of rock exposed.
