Topography of the Northwest Central US
Does your region have rolling hills? Mountainous areas? Flat land where you never have to bike up a hill? The answers to these questions can help others understand the basic topography of your region. The term topography is used to describe the changes in elevation over a particular area and is, generally speaking, the result of two processes: deposition and erosion. These processes can occur over an enormous range of timescales. For example, a flash flood can erode away tons of rock in a matter of hours, yet which rock is broken down and which remains can depend on how it was formed hundreds of millions of years ago. In the Northwest Central, topography is intimately tied to weathering and erosion as well as to the type and structure of the underlying bedrock, but it is also a story of plate tectonics, volcanoes, folding, faulting, uplift, and mountain building.
Weathering includes both the mechanical and chemical processes that break down a rock. There are two types of weathering: physical and chemical. Physical weathering describes the physical or mechanical breakdown of a rock during which the rock is broken into smaller pieces, but no chemical changes occur. Wind, water, temperature, and pressure are the main media by which physical weathering and erosion occur. Streams are constantly eroding their way down through bedrock to sea level, creating valleys in the process. Given sufficient time, streams can cut deeply and develop wide flat floodplains on valley floors. Streams, oceans, and ice also deposit the material they erode, creating new topographical features elsewhere. The pounding action of ocean waves on a coastline contributes to the erosion of coastal rocks and sediments, while the emptying of a river can lead to the building of a delta.
Pressure release can cause rocks to crack. Growing plant roots can exert many pounds per square inch of pressure on rocks—think of tree roots uplifting and cracking a sidewalk. Additionally, since rocks buried miles beneath the surface are under considerable pressure, if those rocks become exposed at the Earth’s surface (where the rock is under less pressure), the rock may expand and crack in a process called exfoliation (Figure 4.1). Ice can also change the landscape due to frequent episodes of freezing and thawing, causing both temperature and pressure differentials within a rock. On a small scale, as water trapped in fractures within the rock freezes and thaws, the fractures continue to widen (Figure 4.2). This alone can induce significant breakdown of large rock bodies.
Working in conjunction with physical (mechanical) weathering, chemical weathering also helps to break down rocks through changes in the chemical composition of their constituent minerals. Some minerals contained in igneous and metamorphic rocks that are formed at high temperatures and pressures (far below the surface of the Earth) become unstable when they are exposed at the surface or placed in contact with water, where the temperature and pressure are considerably lower. Unstable minerals transition into more stable minerals, resulting in the breakup of rock. Weak acids, such as the carbonic acid found in rainwater, also promote the disintegration of certain types of rocks. Limestone and marble may be chemically broken down as carbonic acid reacts with the carbonate mineral composition of these rocks, forming cavities and caverns. Other sedimentary rocks held together by carbonate cement are also particularly susceptible to chemical weathering.
Glaciers have also contributed to the Northwest Central’s topography. Ice sheets from the last glaciation covered part of this area, and mountain glaciers in the Rocky Mountains were considerably more extensive than they are at present. In mountainous areas, erosion by valley glaciers leaves behind jagged peaks, bowl-like depressions called cirques, and long U-shaped valleys with tributary hanging valleys. Glaciers can both erode and deposit material. As the ice melts, piles of sediment are left behind, forming structures such as moraines, eskers, and drumlins. Glacial lakes are common, as water from the melting ice readily fills depressions. The deposition of fine silt that has been ground from rock by glaciers can lead to the formation of wind-blown deposits called loess.
See Chapter 6: Glaciers to learn more about the ways in which glacial erosion alters the landscape.
Volcanic activity has shaped the land throughout the Northwest Central. Although there are no active volcanoes there today, evidence of past activity—such as volcanic cones and craters, lava flows, dikes, and sills—can be seen in a variety of locations, including tuff beds in western and central North Dakota, and the Shoshone lava field in Idaho. Evidence of hot rock and cooling magma, including igneous rocks (e.g., basalt, andesite, and rhyolite), hot springs, and geysers, can be found in the Yellowstone area, which contains the ancient caldera of a supervolcano. There has been no major volcanic activity in the Yellowstone area within recorded history; the most recent Yellowstone eruption occurred around 630,000 years ago. Nevertheless, there is concern that Yellowstone may erupt again in the future.
See Chapter 10: Earth Hazards to learn about supervolcanoes.
The specific rock type found at the surface has an important influence on a region’s topography. Certain rocks are able to resist weathering and erosion more easily than are others; resistant rocks that overlie weaker layers act as caps and form ridges, while surrounding layers of less resistant rock erode away. The great Western Interior Seaway of the Cretaceous collected and preserved sediments that became sedimentary rocks throughout the Great Plains and Central Lowland of Nebraska and the Dakotas. Sedimentary rocks weather and erode differently than do crystalline (and generally harder) igneous and metamorphic rocks, such as those found in the Rocky Mountains and the Black Hills. Silica-rich igneous rocks have a crystalline nature and mineral composition that resists weathering far better than do the cemented grains of a sedimentary rock. The metamorphic equivalents of sedimentary and igneous rocks are often even more resistant due to recrystallization. There are exceptions, however, such as schist, which is much weaker than its pre-metamorphic limestone or sandstone state.
See Chapter 1: Geologic History to learn about the processes of subduction and uplift.
The underlying structure of rock layers also plays an important role in surface topography. Sedimentary rocks are originally deposited in flat-lying layers that rest on top of one another. The movement of tectonic plates creates stress and tension within the crust, especially at plate boundaries. Intrusions beneath the surface may also cause deformation of the crust. All these different sources of geological stress can deform the flat sediment layers through folding, faulting, or overturning. These terms are collectively used to describe rock structure, and they can also be used to determine which forces have affected rocks in the past. The folding of horizontal rock beds followed by erosion and uplift brings layers of rock to the surface. Tilted rocks expose underlying layers. Faulting likewise exposes layers at the surface to erosion, due to the movement and tilting of blocks of crust along the fault plane. For example, the Basin and Range formed as a result of normal faulting (Figure 4.3A), which occurs due to extensional stresses that create uplifted ranges and downdropped basins. The Rocky Mountains provide another regional example of folding and faulting: this range formed as a result of uplift associated with subduction along the western edge of the North American plate. The shallow angle of the subducting plate generated thrust (reverse) faults (Figure 4.3B) and the onset of the Laramide Orogeny.
Just as we are able to make sense of the type of rocks in an area by knowing the geologic history of the Northwest Central US, we are able to make sense of its topography (Figure 4.4) based on rocks and structures resulting from past geologic events. Topography is a central element of the broader concepts of geomorphology or physiography, which also include consideration of the shape (not just the height) of land forms, as well as the bedrock, soil, water, vegetation, and climate of an area, and how they interacted in the past to form the landscape we see today. A physiographic province is an area in which these features are similar, in which these features are significantly different from those found in adjacent regions, and/or is an area that is separated from adjacent regions by major geological features. The “regions” of the Northwest Central that we use in this book are examples of major physiographic provinces.