Landforms (World of Earth Science)
Landforms are the mesoscale topographic features that define a regional landscape. Climate and plate tectonics ultimately determine the system of processeslate tectonic motion,
gravity, erosion, and deposition by water, wind, and icehat interact in complex feedback loops to erect and destroy continental landforms. Climate, the regional pattern of seasonal precipitation, temperature, and wind-flow, varies with latitude, altitude, and physiographic location. Motion of Earth's lithospheric plates, called plate tectonics, determines the global geography of mountain chains, volcanoes, continental landmasses, and ocean basins, as well as the distribution of rock types. The morphology of landforms suggests the processes that created them, and landforms are clues to deciphering the present and past geological setting of a region.
Climate often dictates the mix of sedimentary processes acting to create landforms. For example, eolian, or wind-formed, landforms are common in arid environments, whereas glacial landforms like moraines and kettle ponds are found in cold regions with adequate snowfall. The abundance and seasonality of annual rainfall determines the intensity of weathering and erosion by flowing water. In rainy regions, a dendritic pattern of V-shaped stream valleys often dominates the landscape. Low-gradient stream valleys are filled with fluvial landforms like meandering channels, oxbow lakes, natural levees, and point bars, while sediment-choked mountain streams form broad, gravelly plains of intertwined channels and longitudinal bars.
Air currents also shape landscapes by sorting and reworking dry sediments in arid environments. Eolian land-forms include numerous types of sand dunes: cresent-shaped barchans, merged barchans called transverse dunes, parabolic blowout pits, flow-parallel linear dunes, and large composite dunes called draas. Very large dune fields called ergs cover extensive areas of the major deserts. Seasonal temperature fluctuations affect the phase of water. Erosion by frozen surface water creates dramatic glacial features like fjords, hanging valleys, and cirques in high-latitude regions. Evaporation of standing water in hot, arid, desert environments forms a unique array of desert landforms, including ephemeral playa lakes and coarse-grained alluvial fans where mountain streams enter arid basins.
The tectonic plate setting and past geology of a region also affects its topography. Motion along tectonic plate boundaries creates tall mountain belts, volcanic chains, regions of broad uplift, and zones of subsidence. Escarpments, extensional valleys called grabens and pull-apart basins, and linear pressure ridges form along fault planes where rocks bodies move relative to one another. Gravity failure of tectonically over-steepened slopes creates mass wasting deposits like mudflows, slumps, and slides. Streams that follow out-crop patterns of folded sedimentary and metamorphic rocks create ridge and valley provinces. Resistant layers of horizontal strata in tectonically uplifted areas form broad tablelands. Caves, sinkholes, and other so-called karst features form when acidic rainwater falls on carbonate rock layers. Volcanic activity creates numerous distinctive landforms, including craters, lava flows, eroded volcanic necks, and, of course, the conical or shield-shaped volcanoes themselves. Beach and shoreline dynamics along continental margins form an array of coastal landforms, from erosional sea cliffs and stacks to depositional deltas, barrier islands, and spits.
The continents can be divided into physiographic land-form provinces delineated by plate tectonic features and climatic zones. (Antarctica is an exception because ice covers about 90 percent of its land surface.) The Colorado Plateau, in the southwest United States, is an example of one such province, and a tour of its dramatic landscape illustrates the complex web of processes acting to sculpt its landforms. The Four Corners region is arid because the prevailing Pacific westerly winds have dropped most of their moisture in the high Sierras of California and Mexico before reaching it. Streams flowing onto the Plateau from the Rocky Mountains, however, provide ample erosional and depositional energy. The Plateau is a broad zone of tectonic uplift that has exposed a thick sequence of flat-lying sedimentary strata to deep incision of steep-walled canyons by flood-prone streams. Erosional remnants like flat-topped mesas, conical buttes, arches, and spires remain between the incised fluvial channels. Winds sort and transport grains of siliciclastic sand, creating dunes and sandblasting the landscape.
Humans have also affected landscape evolution in the modern southwest desert. Construction of dams along the major rivers, especially along the Colorado River, has led to stream regrading and a resulting change in the pattern of fluvial erosional and depositional landforms. Evaporation from man-made reservoirs, like Lake Powell and Lake Mead, has reduced the amount of water in downstream river segments and increased the humidity around the lakes. Water use by human cities, industries, and especially irrigated agricultural lands has changed the regional hydrologic cycle.
See also Canyon; Desert and desertification; Karst topography; Sedimentation; Stream valleys, channels and floodplains