Rocks of the Northwest Central US
The amazing diversity of rocks in the Northwest Central records several billion years of history—from 3.8-billion-year-old Precambrian granites to sedimentary deposits from the most recent ice age. Colliding plates, rifting, inland seas, deposition, erosion, igneous and metamorphic activity, and recent glacial processes are all part of this story. The Northwest Central’s different rock types influence its topography and tell us where to look for certain fossils or natural resources. Each type of rock forms in a particular environment under particular conditions (Figure 2.1).
Figure 2.1: The rock cycle shows the relationships among the three basic types of rock.
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 differences between the 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, such as 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, such as 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) such as 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. When magma comes to the surface (as lava), it cools quickly so that individual crystals are not visible, resulting in a volcanic rock such as basalt. In some circumstances, lava may cool so quickly that crystals do not form at all, creating a glassy rock such as obsidian. Smaller fragmental rocks that cool quickly at the surface form during explosive eruptions; these are called pyroclastic rocks, and they are composed of a variety of different volcanic ejecta.
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.
Every rock is capable of being melted, weathered, or changed by heat and pressure. Any rock that has been subjected to intense heat and pressure can recrystallize into a metamorphic rock. This process destroys features in the rock that would have revealed its previous history, transforming it into an entirely new form as the minerals within realign. The pressure to transform a rock may come from burial by sediment or from compression due to plate movements, while the heat may come from very deep burial or from contact with magma.
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. For example, when plates collide, compression and friction melt the crust. The rising magma forms igneous intrusions that crystallize below the surface, producing large-grained igneous rocks such as granite. The rising magma may break through the surface in the form of volcanoes, creating volcanic rocks such as basalt. Tectonic collision also leads to increased heat and pressure, creating metamorphic rocks. Basins adjacent to mountains fill with transported sediment, producing thick sequences of sedimentary rock.
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 farther 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.
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.
