Geomorphology and human activity

Geomorphology is a science that classifies landforms and describes and analyzes the origin and evolution of surface features. It is interdependent with other branches of geology—such as sedimentology and hydrology—that study the processes that act on marine and planetary features. Geomorphology is of immense global importance because of the ever-increasing activity of humans as geomorphic agents. As the human population increases in numbers and complexity, increasingly affects by geomorphic processes.

Landforms

Geomorphology is the scientific study of topographic surface features and naturally occurring Earth processes. The landform is the basic unit of systematic analysis of geomorphology. Landforms can be considered constructional (coral reefs and volcanic eruptions, for example). However, most landforms are erosional because of the intensity of atmospheric weathering and erosion. During landscape analysis, a geomorphologist must not only analyze current surface features but also reconstruct rock units and landforms before the present landscape.

Time is a major factor in geomorphology in that landforms are altered by geologic processes over time. A theoretical analysis of landform evolution uses a time range of millions of years. Practical or applied geomorphology uses a time scale of hours to years because of the need to predict changes on a human scale, such as how long an excavated area will remain stable during hot, humid conditions. Using radioactive isotopes establishes the absolute age of rocks as well as the exposure age of rock surfaces, thus making estimation of absolute rates of land surface change possible. The geomorphic application of isotopic dating to land surface changes can be established when an ice sheet retreats from a region.

Central to the geomorphic study of any region is its structure or specific physical properties. Studies of landform structure reveal the type of rock present, its mineral composition, grain size or crystalline structure, and rock strength. Also, studies of landform structure indicate the depositional sequence of the rock material and any naturally occurring stresses that the rock unit has been subjected to, resulting in faults and fractures.

A full range of processes contributes to produce a landscape. The action of wind, water, and ice erodes rocks and transports the eroded material to deposition sites. Gravity pulls down large materials that rise above ground level. Rates and intensity of geomorphic processes vary. Surface weathering rates may be extremely slow, such as a few centimeters per thousand years, or quite rapid, as measured in an avalanche that may travel at fifty meters per second or more. Intensity of geomorphic processes is governed by climate, vegetation present, and elevation of the surface feature.

Solar radiation provides the energy for geomorphic processes at a greater rate than all other energy sources combined. Other energy sources are the gravitational and inertial forces associated with the mass and motion of the Earth, moon, sun, and other planetary objects. Additional energy comes from the outward flow of heat, caused by radioactivity, from the Earth's interior.

The two major geomorphic processes are constructional processes—such as mountain building and volcanic eruptions—and erosional processes—such as weathering, erosion, and the action of wind, rivers, and glaciers. Other geomorphic processes are sediment movement and deposition, biologic processes as they pertain to vegetative cover, near-shore and ocean systems, and human-driven processes. There are few spheres of human activity that do not create landforms directly or indirectly.

Predicted temperature increases (2 to 6 degrees Celsius per century) and global warming as a result of the buildup of greenhouse gases in the Earth's atmosphere are not new phenomena. Although it became a significant issue in environmental management debates only during the last two decades of the twentieth century, this process has operated, with fluctuations, over the past 4.6 billion years of the Earth's history.

Global warming studies focus on the atmospheric, hydrospheric, and biospheric consequences of climate change. Geomorphological consequences of climate change are significant because of potential changes to landscapes and human occupation of these surfaces. Potential geomorphological changes include climatic alterations affecting vegetation cover and agriculture, which could alter soil erosion, surface water runoff, river siltation, and flooding patterns; changes in frequency, magnitude, and geographical extent of tropical storms, which could affect land erosion, especially in coastal zones; changed patterns of rainfall, with excessive rainfall resulting in increased flooding or low amounts of rainfall affecting human water supplies; elevated temperatures at high altitudes and latitudes, which may alter ice and snow distribution and the extent of permafrost; and raised sea level caused by the reduction of glaciers and ice sheets, impacting coastlines through beach narrowing, cliff erosion, and delta formation.

These geomorphic changes may seem catastrophic, but it is helpful to review them in the light of environmental changes in the geologically recent past. Since the end of glaciation, about 10,000 years before the present, sea level has risen approximately 121 meters, with periods of rapid sea level rise of 14.6 millimeters per year between 13,000 and 7,500 years ago and twenty-four millimeters per year between 12,500 and 11,500 years ago. Although these rapid sea-level changes do not compare with current predicted values, they demonstrate that surface features such as river deltas and coral reefs can continue to exist. Both landforms are capable of quick evolution, river deltas because of sediment supply and reefs by organic structural growth. Both landforms have survived post-Pleistocene rises in sea level and, if not perturbed by further human activity, should continue to do so. The systems that may be most threatened by the projected sea-level rise are land-based human systems such as agriculture.

Accurate geomorphic predictions and changes to landscapes are troublesome because exact weather patterns and distributions of extreme climate events are difficult to assess. Knowledge of average climatic changes does not automatically assign specific locations of weather systems and events. While it is true that landform changes are altered by climatic conditions, local geology, hydrology, topography, and land use also play major roles in landform evolution. The climatic, oceanic, and biospheric repercussions of global warming provide the basic ingredients for geomorphic change, but human activity will determine the scale, extent, and rate of the resultant changes.

Human apprehensions about adverse changes to the biosphere and human socioeconomic systems have created the modern concerns about global warming. Concern is heightened because the predicted trends of climate change have never been experienced by humans. The predicted climate changes, if they fully occur, are similar to climatic conditions prior to the last glaciation. Finally, humans now recognize that they are the main influence in climate change and are capable of modifying the rate of this predicted change.

Coastal Zone

Major coastal processes are those associated with moving water in near-shore environments. Three forces act upon water to create wavesastronomical, meteorological, and tectonic. Astronomical forces are the driving force for tides, which, in turn, influence the width of the shoreline and growth patterns of the near-shore flora and fauna. Waves produced by meteorological forces operate to modify the coastline. Storm-generated waves and sea swells, along with ocean currents, operate to erode the coastline, transport eroded materials, and deposit debris in coastal areas. Tectonic forces such as earthquakes and volcanic eruptions produce dangerous seismic sea waves known as tsunamis. These three large-scale forces are not affected by human activity, but landforms in the coastal zone have long been altered and used by humans.

Early changes to the coasts by humans were indirect and unintentional because of small populations. Direct and intentional use of the coastal zone began with farming on flat delta land. Lands in these areas were reclaimed or extended by canalization and diking. Reclamation, as a coastal modifier, continues to the present. In the Netherlands, more than 3,800 square kilometers of land has been gained by reclamation in the last eight hundred years. A 1984 study of reclaimed land in Singapore revealed that the country is now 10 percent larger than when it was founded.

Another direct, intentional modification of the coastal zone is using shoreline stabilization structures such as seawalls, breakwaters, and jetties. A seawall or breakwater structure is placed parallel to the shore to protect an eroding coastline. Sand-bearing coastal waves are slowed down by seawalls and drop their load of sand up-current from the structure. Down-current from the barrier, the water picks up more sediment to replace the lost load, and the down-current beach is eroded. Jetties built perpendicular to the shoreline also operate to erode and redistribute sand, creating an unnatural scallop shape along the shoreline. Coastal structures placed by humans severely alter the shape and stability of the coastline. Beach erosion is not halted, nor is the shoreline stabilized.

A review of the state of the marine environment for the United Nations Environment Programme in 1990 reported that the open ocean, because of its large diluting capacity, is still relatively clean, despite measurable levels of artificial radioactive material and synthetic organic compounds. Oil slicks from tanker collisions, explosions, navigational or mechanical errors, and offshore oil installations continue to put a human face on ocean pollution. Likewise, coastal areas near large populations are clearly linked to human activity and exhibit detectable increases in phosphate concentrations from sewage and agricultural discharges. The amount of nitrates (a component of fertilizers) found in coastal waters is increasing, as are areas of eutrophication, unusual plankton blooms, and excessive seaweed growth.

River Systems

Fluvial systems carry water, sediments, and dissolved materials and minerals downstream. Streams, driven by gravity, cut channels and scour the stream bottom and sides with their sediment load. Also, streams deposit sediments in artificial reservoirs and natural freshwater and marine basins. Both stream erosion and deposition create landforms, with stream valleys being one of the most frequent and widespread landforms in North America. Although the portion of water involved in streams constitutes only a small percentage of the total water in the hydrologic cycle, human interference with stream processes perturbs the global water system. The hydrologic cycle is the constant circulation of water from the sea through the atmosphere by evaporation and its return to the land, streams, lakes, glaciers, and the subsurface by precipitation. Water eventually returns to the atmosphere by way of transpiration and evaporation from plants, landforms, and ocean basins.

Human occupation of cities has a profound effect on the hydrological cycle because humans interrupt and rearrange areas where water is naturally stored, such as lakes and groundwater-bearing aquifers. In Urban Hydrology (1984), M. J. Hall discusses links between hydrology and urbanization. First, the replacement of vegetated soil with impermeable surfaces (asphalt) reduces water storage in the soil horizon, the slow percolation of water into aquifers, and the transpiration mechanism in the hydrologic cycle, which allows moisture to return to the atmosphere. Second, with large amounts of precipitation on solid surface areas, the velocity of water flow is increased, moving the water to stormwater systems rather than natural stream channels, where evaporative processes can occur. Third, construction activity clears the land surface, disturbs the soil layer, reshapes natural slopes with potentially unstable slopes, and leaves a limited vegetation cover or builds additional impermeable surfaces. If overloaded with large quantities of construction-derived debris, streams that routinely carry a small amount of sediment may experience various changes in their physical and biological characteristics. These changes include deposition of sandbars or dunes in the channel. Coarser sediments carried in the water may increase the erosion of channel banks. Bottom-dwelling flora and fauna may be blanketed with sediments, reducing the viability of fish species that feed on such stream organisms.

Dams and reservoirs began to play an important role in water use in the late nineteenth century. The immediate impact of reservoirs is the intentional alteration of flow downstream, usually to increase low flows for year-round water use or to curb floods. Other impacts include the loss of land caused by the filling of the reservoir, increased evaporation from the reservoir surface, groundwater seepage into reservoir walls, channel and bank scouring, and sediment deposition. Inland reservoirs have resulted in major changes to coastlines because of sediment and nutrient starvation. Damming of major streams aggravates beach and delta erosion. Before the Aswan High Dam was constructed in Egypt, the Nile River transported 140 million tons of silt per year to the Mediterranean Sea. Increased erosion of the delta began once Aswan was filled, and the loss of nutrients caused a reduction in fish catches in the Mediterranean Sea. Because the earth's landscapes are dynamic and ever evolving, changes similar to those observed in the Nile River over millions of years are observable in the twenty-first century on a smaller scale. For example, India's Kosi River's flow changed course in 2008, moving over sixty kilometers towards the east and creating a new channel.

Soil

Soil formation is the direct result of physical, biological, and chemical weathering processes acting on rock units and rock-cored landforms. Soil is the active surface layer that mantles most rock masses and supports the growth of rooted plants. It is not just a simple, loose layer with a stock of plants and plant nutrients on the land surface, but rather a specific stratum that regulates biological and geological interactions. Because soil exists between the geological, biological, and atmospheric realms, it is subject to a complicated set of direct and indirect links with surface processes, both natural and anthropogenic. Human activities that can degrade soils include improper cultivation, deliberate deforestation, over-compaction by heavy machinery or trampling of grazing domesticated animal herds, and submergence of land because of drainage basin changes.

Globally, only 11 percent of the land surface (1.5 billion hectares) is naturally appropriate for growing crops. An additional 1.7 billion hectares is available for agriculture if proper land management practices are instituted and followed. The remaining vegetated areas are too cold, hot, dry, or low in nutrients or soil cover to produce crops. Soils can be physically removed by wind and water erosion when land is improperly cultivated. The removal of vegetation by clearing the fields for planting increases erosion from water, and fertile soils are washed from the land. With the protective vegetation and topsoils removed, wind erosion can increase soil loss. Tilling land surfaces parallel to the prevailing wind direction exposes soil to wind erosion. Tilling land across contours and valley slopes also degrades soil layers. It is estimated that the amount of soil and rock moved in agricultural and construction activities each year in the early twenty-first century could fill the Grand Canyon in fifty years.

Chemical pollution of groundwater can occur as a result of the treatment of land surfaces used for agriculture. Both fertilizers, used to replace soil nutrients, and pesticides, used for raising crop yields, can enter subsurface groundwater zones after irrigation waters percolate through the soil horizons. Groundwater is especially susceptible to this type of degradation because of the slow movement of subsurface waters and the prolonged retention times within the aquifer. Salinization of soils can occur when poor irrigation techniques are used. Over-irrigation, poor drainage of irrigated soils, and irrigating with water of high salt content are major causes of this problem. Other issues—such as dam placement and excessive evaporation from rivers in hot, dry climates—complicate soil salinization.

Trees act as a buffer to the erosive processes caused by rain and storms. When trees are removed, much of the rainfall runs directly off the land, taking with it large amounts of soil and sediment that choke stream channels or decrease water quality. Rainforests that are cut and developed for agriculture are particularly susceptible to soil degradation because of the clay-rich layers within tropical soil horizons. Once rainforest trees are removed, massive amounts of rainfall wash the soil areas, removing the small amounts of nutrients and the fine-grained clay materials. Once this stripping of the soil and nutrients occurs, the area is unsuitable for agriculture or reforestation programs. Another form of deforestation consists of stripping the land for the gathering of fuel wood. Of all the degraded land in the world, about 7 percent is the result of firewood gathering and burning, which opens the land to soil erosion by running water and wind erosion.

Landslides are also common geomorphology activities involving soil. Though they are known to occur suddenly and intensely modify the landscape, some landslides occur slowly over time. For example, America's Slumgullion Landslide in Colorado moves at less than .02 meters daily.

Principal Terms

erosion: wearing away of soil and rock by weathering, landslides, and the action of streams, glaciers, waves, wind, and underground water

landform: one of the many features that, taken together, make up the surface of the Earth

landscape: broad term for the land setting of an area

topography: general configuration of a land surface, including its relief and the position of its human-made features

weathering: destructive processes by which rocks are changed though exposure to the atmosphere at or near the Earth's surface, with little or no transport of the loosened or altered material

Bibliography

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