Geomorphology of wet climate areas

Wet climate geomorphology is the study of landforms and land-forming processes in humid regions of the Earth. These landforms correspond roughly to tropical rainforests, savannas, and humid-midlatitude climatic regions. Each is distinct because of differences in temperature and precipitation.

Climatic Geomorphology

Geomorphology is a branch of geology and physical geography concerned with the study of the Earth's landforms and the processes that create them. There are several ways of looking at those processes and, as a result, several branches of geomorphology. One is climatic geomorphology, which is the study of the role of climate in shaping the land. Wet climate geomorphology is a component of climatic geomorphology that exclusively addresses land areas having relatively high annual precipitation. This is a natural grouping since water is instrumental in the wearing down and sculpting of the land.

and Erosion

Before looking specifically at the landforms and processes of the humid regions, it is first necessary to examine the major natural processes operating on the land, no matter what the climate. These are weathering, transport, and deposition of rock and mineral matter. The first two processes are sometimes collectively referred to as erosion, although a more accurate definition of erosion would be the removal of rock and mineral matter from an area. Weathering may be either mechanical, the physical breakdown of rocks into smaller and smaller particles, or chemical, the chemical alteration of rocks and minerals into new, more stable forms that may become part of a soil. The most important type of mechanical weathering is freeze-thaw, in which freezing water expands within rocks and fractures them. Mechanical weathering, as well as transport of weathered debris, is suppressed in hot, wet climates, where there is little or no freezing and a thick vegetation cover protects the soil from being removed and carried away (eroded). In temperate climates such as Europe and the eastern United States, mechanical weathering is somewhat more important and produces huge amounts of loose rock and mineral particles that can be eroded.

Chemical weathering is most active in tropical wet regions, where both temperature and precipitation are high. It occurs only in the presence of abundant percolating subsurface water and breaks down many common rock-forming minerals into three components: clays, quartz, and dissolved particles (ions). The clay and quartz remain behind, unless eroded, and form a residue from which soils develop, while the ions dissolved in the subsurface water may recombine to form new minerals such as iron oxide. They may also be used by plants or be transported downward in percolating water and eventually deposited into streams and rivers. Some common minerals, and, therefore, the rocks that contain them, are resistant to chemical weathering and do not erode easily.

Transport of weathered rock and mineral matter occurs primarily by running water, either as sheet flow (when an unchanneled sheet of water flows down a slope) or as channeled flow (in gullies, streams, and rivers). Where the protective cover of vegetation is thick, sheet flow is ineffective as a transporting agent. Where it is thin, sheet flow can erode the underlying soil as fast or faster than it can form. Where very steep slopes exist, landsliding becomes an important transporting agent. Deposition of weathered and transported debris occurs when the velocity of the running water decreases—at breaks in slope or in the floodplains of rivers, for example.

Morphoclimatic Zones

Wet climate landforms can be subdivided into three types based on temperature and the seasonal amount and distribution of precipitation. These landforms, or “morphoclimatic zones,” are tropical rainforests, where both temperature and precipitation remain high all year long; savannas, or tropical wet-dry regions, where both temperature and precipitation are high but precipitation is restricted to a rainy season; and humid-midlatitude regions, where precipitation is moderate but uniformly distributed throughout the year and temperatures undergo large seasonal fluctuations. Other examples of morphoclimatic zones include kipukas, which are areas of rocks surrounded by lava that are not covered by lava flows. Steptoes are kipukas that protrude over the lava field.

Tropical Rainforests

The tropical rainforests of the world are confined to a belt extending 10 to 15 degrees of latitude north and south of the equator. Examples are the Amazon River Basin in South America, much of central Africa, and the Indonesian Islands. In these areas, high temperatures and rainfall produce a thick vegetation cover even on steep slopes. As a result, sheet flow is minor, and most rainwater infiltrates the ground, where it promotes chemical weathering. In these areas, mechanical weathering is inhibited, but chemical weathering is intense and produces a deep mantle of weathered rock and mineral grains above fresh bedrock. This mantle of unconsolidated debris is called regolith and may extend more than 100 meters below the land surface. The downward percolation of subsurface water is so intense and constant that the dissolved ions that it carries are completely removed, leaving a residue of clay, quartz, and iron oxides. Soils in tropical wet areas, therefore, tend to be infertile below the top meter or so.

In mountainous parts of the tropical rainforest where fast-moving streams have high energy, streams cut rapidly down through the easily eroded regolith and produce deep, narrow valleys with steep, V-shaped slopes separated by narrow ridges. The regolith on these slopes sometimes becomes saturated with water, resulting in landslides. This type of topography is typical of areas such as Papua New Guinea and parts of the Amazon basin.

In contrast to the rugged topography of mountainous regions, lowlands in the tropical rainforest are quite flat. They contain large rivers such as the Amazon in Brazil and the Congo in central Africa. These rivers meander on wide floodplains tens of kilometers wide and thousands of kilometers long. Many rivers have a peculiar stepped appearance, with occasional rapids or even waterfalls interrupting long stretches of sluggish flow. Tropical rainforests contain some of the world's most spectacular waterfalls, such as Angel Falls in Venezuela (900 meters high) and Victoria Falls in Zambia, Zimbabwe. The waterfalls are produced by resistant rock layers, such as quartzite, that do not chemically weather and that the rivers cannot wear down given the lack of coarse abrasive particles in transport. Another unique landform feature of the tropical rainforest is the occasional presence of large, isolated, haystack-shaped hills devoid of vegetation. These are called monolithic domes, and they present a curious contrast to the surrounding heavily vegetated lowlands. Many of these domes are found around Rio de Janeiro, Brazil. Geomorphologists believe that monolithic domes exist because they are more resistant to weathering than the surrounding rocks. Soils cannot form on them because of steep slopes, landsliding, and lack of the water-retention capacity necessary to promote chemical weathering.

Savanna Regions

Savanna regions are much larger in area than the tropical rainforests and extend north and south as a wide fringe bordering the tropical rainforests. Savanna regions include parts of southern and northern Brazil, southern Central America, central and southern Africa, and parts of India and Southeast Asia—areas containing more than half the world's population. Savannas have a dry season lasting from three to six months. During the dry season, vegetation dies or becomes dormant and chemical weathering decreases significantly as a result of the lack of percolating water. With a decreased vegetative cover, mechanical weathering and erosion of weathered debris by sheetwash increase significantly, resulting in removal of the regolith faster than it can form.

Savanna landscapes, therefore, tend to have a stepped appearance, with vast flat or gently sloping plateaus (pediplains) with thin regolith or none, interrupted by abrupt slopes that lead up to the next higher plateau. This style of erosion is called backwasting and is typical of savanna and semiarid regions. Former high plateaus may be reduced by erosion to isolated remnants called inselbergs. The stepped appearance is augmented by the formation of hard, impermeable layers in the soil called crusts or duricrusts. They form progressively during dry seasons, when percolating subsurface water evaporates and chemically precipitates oxides of iron or, less commonly, aluminum. Once formed, these crusts persist and may grow to several meters in thickness. They protect the underlying, less resistant regolith from erosion unless they are breached, in which case erosion proceeds rapidly by undercutting them, sometimes all the way down to bedrock. In this way, the crusts help preserve flat plateau surfaces at various levels, although the surfaces are progressively reduced in area through time.

When stream flow resumes in the wet season, streams are unable to cut down through the crusts and, therefore, have very shallow, hard-to-recognize channels. Much rainwater runs over the plateau surfaces as sheetwash, carrying with it fine particles of regolith. When flow ceases, areas of shallow ponded water serve as gathering places for animals such as elephants, lions, and wildebeests. The well-known Serengeti Plain in Tanzania, Africa, is an excellent example of this kind of landscape.

Humid-Midlatitude Regions

Landforms in humid-midlatitude regions such as Europe and the eastern United States differ in many respects from the tropical rainforest and savanna regions. Here, the cool winters promote mechanical weathering via freeze-thaw, chemical weathering is moderate because of cooler temperatures, and precipitation is moderate but evenly distributed throughout the year. Streams, therefore, run all year long and carry higher loads of coarse sediment, such as sand and gravel, that chemical weathering has not reduced to smaller sizes. These mechanically weathered sediments act as abrasive tools and wear down even the harder rocks. Stream courses, therefore, do not have the stepped appearance seen in the tropical rainforest. They cut valleys whose slopes recede via landslides and, more important, soil creep, the very slow downslope movement of the moist soil.

Soil creep helps to produce the rounded hills and gentle, curved slopes typical of humid-midlatitude areas such as the Appalachian Highlands of the eastern United States or the gently rolling landscapes of parts of Western Europe. Slopes grade uniformly into lowland floodplains, where coarse sediment is stored. Isolated remnants such as monolithic domes or inselbergs are quite rare in humid-midlatitude regions. Since chemical weathering is only moderately active, soils tend to be thin but rich in plant nutrients because the downward percolation of water is not intense enough to cause complete removal of dissolved ions necessary for plant growth. Humid-midlatitude regions are, therefore, ideal areas for agriculture.

Topographic Maps

The primary interest of the geomorphologist is to determine the nature of the important causative processes acting to form the landscape and to construct predictive models of landscape change based on a knowledge of those processes and their rates. Various methods are used for this purpose, and specific techniques and analytical tools are usually dictated by the problem to be solved. Most geomorphological analyses involve measuring some landscape features, such as slope steepness, and observing the natural processes acting to form it, such as the amount of precipitation, underlying rock type, or type of vegetation cover. The effect of each process must be measured, and its relative importance must be established. Almost all such analyses involve statistical treatment.

Topographic maps, aerial photographs, and geologic maps are essential for any geomorphological study. Topographic maps are two-dimensional representations of the topography of an area. They allow geomorphologists to study landscape features without actually being at a site, and they lend themselves to statistical treatment. For example, the relation between slope steepness and the frequency of landslides can be determined quantitatively because both features have a topographic expression and can be recognized on topographic maps. Such a study may conclude that slopes greater than 40 degrees have 20 percent more chance of landslides than slopes less than 40 degrees. This example, though rather simple, nevertheless has great practical importance to engineers working in wet climate areas.

Aerial Photographs

A series of aerial photographs taken at a certain elevation—for example, at 3,000 meters—can be partially overlapped and, when viewed with stereoscopic glasses, can give an exaggerated three-dimensional view of a land area. Such photographs present a visual image of the topography with more detail than can be provided by topographic maps.

Photographs taken at different times can be compared to determine rapid landscape changes and their rates of change. This method is often used in studies of river-channel migrations across a floodplain. Engineers and land planners can then use the information to determine future urban expansion areas.

Geologic Maps and Other Study Tools

Geologic maps are two-dimensional representations of the types and orientations of rocks or surficial materials in an area. They allow geomorphologists to study the relationship between the underlying rock or soil and the topography as expressed on topographic maps or aerial photographs. For example, in the humid-midlatitude Valley and Ridge area of Pennsylvania, elongate ridges stand up to 1,000 meters above intervening valleys. The ridges are composed of resistant, hard sandstone, whereas the valleys are underlain by shale, a weaker and more easily eroded rock.

Other investigative tools have a more specific purpose. These include sieves to measure grain sizes of debris, petrographic microscopes to determine mineral types, and X-ray diffractometers to determine types of clay minerals. In addition, geomorphologists use various techniques to determine the age of landscape-forming events. The techniques used are quite sophisticated and may be relative or absolute. They include carbon-14 dating, fission track dating, amino acid racemization, dendrochronology (using tree rings to determine the dates and climate at which they formed), optically stimulated luminescence (OSL), paleomagnetic dating, and radiometric dating methods. Finally, computers are essential tools in geomorphological studies. They extensively aid in statistical analyses, modeling geomorphic processes, and predicting landscape changes.

Applications of Geomorphological Research

The geomorphological study of wet climate landforms has tremendous practical importance through its ability to predict the effects of human-induced changes in the landscape. For example, pressures for development in nations occupying the tropical rainforest have resulted in massive forest-clearing operations that threaten the very existence of these forests. One effect has been a tremendous increase in erosion of the soil and underlying regolith as a result of the high precipitation and stripping of the protective vegetation cover by sheetwash. Parts of the Amazon basin in Brazil now have a badlands type of topography. Crops will not grow in the remaining bare, infertile regolith, because seeds and fertilizer simply wash away with the regolith. The eroded debris rapidly clogs formerly clear-flowing streams and endangers numerous aquatic plant and animal species. Tropical geomorphologists are now involved in projects to reclaim much of this land and to prevent future landscape degradation.

Many applications of geomorphological research in wet climate areas have to do with the dynamics of river channels and river erosion. Engineering works such as reservoir construction and river channelization have had some unanticipated side effects, notably the severe erosion of land downstream from impoundments or increased flood heights downstream from channelized rivers. Geomorphologists have been able to model these river responses to change and have enabled engineers to construct safer and more effective flood control and water supply works.

A new challenge for geomorphologists is the reclamation of disturbed lands such as old landfills and open pit mines. Proper reclamation is based on an understanding of the landforming processes and how they operate in different climatic regions. With this understanding, geomorphologists have had much success in converting potentially hazardous areas to environmentally safe and economically valuable uses such as golf courses, parks, and playgrounds.

Scientific Value of Climatic Geomorphology

In addition to its practical value, climatic geomorphology has great scientific value. Because climate exerts strong controls on the landforming processes of weathering, transport, and deposition, climatic geomorphologists can recognize large areas of the Earth where past climates were different from the present one. For example, the currently humid-midlatitude region of central Germany retains remnants of flat plateau surfaces formed under a savanna climate. Many, if not most, land areas of the Earth show such polygenetic landscapes (landscapes having more than one origin). This information is used to establish a history of climate change through the last seventy million years.

A rapidly developing area of geomorphology is the study of planetary landforms. Pictures of the Martian surface clearly show valleys similar to those cut by rivers on Earth, yet there is no evidence of modern-day surface water on Mars. Does this mean Mars has somehow lost its water, or could some other agent of erosion, such as wind, have produced the valleys? Twenty-first-century geomorphological research attempts to answer this question and others regarding the formation of planetary landforms.

Principal Terms

chemical weathering: the chemical alteration of rocks and minerals into new forms that are chemically stable at the Earth's surface

erosion: the removal of weathered rock and mineral fragments and grains from an area by the action of wind, ice, gravity, or running water

humid-midlatitude: land area with average temperature of the coldest month less than 18 degrees Celsius but at least eight months with average monthly temperatures greater than 10 degrees Celsius; this area has no dry season

landform: a grouping of genetically related landscapes based on similarity of appearance

mechanical weathering: the physical breakdown of rocks into progressively smaller particles

regolith: the layer of mechanically and chemically weathered rock and mineral matter above fresh, unweathered bedrock

savanna (tropical wet-dry): land area with monthly temperatures averaging greater than 18 degrees Celsius and with one to six months of average monthly precipitation of less than six centimeters

tropical rainforest: land area with monthly temperatures averaging greater than 18 degrees Celsius and monthly precipitation averaging more than six centimeters

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