In ECONOMIC GEOGRAPHY, agglomeration refers to the clustering of activities and people at nodal points, as for example in towns and cities. This clustering is prompted by CENTRIPETAL FORCES in spatial organization and by the need to achieve AGGLOMERATION ECONOMIES.
Vibration of the Earth’s crust that occurs following an EARTHQUAKE, and results from minor adjustments of rocks along a fault-line after the main rupture. Aftershocks may continue for hours, days or even longer periods of time, and may cause considerable destruction and loss of life where buildings have been weakened by the initial earthquake. There can be hundreds of aftershocks after a major earthquake.
A term applied to the action of wind. Aeolian transport is an important coastal process involving the transportation of sand and the construction of sand dunes. It is also important in arid environments where rocks may become blasted by the sand.
FOG developed in air that is moving in a horizontal direction. The air, which is initially warm and moist, is cooled to DEW-POINT as it passes over a cold land or sea surface. Advection fog forms in mid-latitudes in winter (for example, when tropical maritime air crosses a land-mass previously cooled under anticyclonic conditions), and in spring and early summer (when very warm tropical air is cooled by contact with a relatively cold sea surface, giving sea fog). It is also particularly common at the convergence of warm and cold ocean currents (e.g. off Newfoundland, where warm air from above the Gulf Stream drifts over the Labrador Current, to give up to 100 days of fog each year)
A hill-slope that, in the Northern Hemisphere, faces southwards or southwestwards, and thus receives the maximum amount of sunshine and warmth. It is the opposite side to the UBAC (the shady side). On adret slopes the tree line may be significantly higher, and certain geomorphological processes more active (e.g. FREEZE-THAW WEATHERING in a PERIGLACIAL climate). The cultivation of crops such as vines may be more successful on adret slopes because conditions promote ripening.
One of three principles underlying Christaller’s CENTRAL PLACE THEORY and governing the spatial arrangement of a central place relative to its MARKET AREA. It applies where advanced systems of centralized administration have developed and where six centres of a given order fall entirely within the hexagonal market area of a higher-order central place. This arrangement, with a K-VALUE of 7, ensures that there is no shared allegiance. It thus avoids the unsatisfactory situation of one settlement being located within the administrative area of more than one higher-order central place.
In PERIGLACIAL conditions, where PERMAFROST exists, only the upper layer of ground thaws in summer. This upper layer, which is affected by summer thawing and winter freezing, is the active layer. Its lower limit is the permafrost table, which forms an impermeable surface and causes the active layer to be poorly drained. At its maximum, the active layer may reach a depth of 3-6 m, depending on summer temperatures, the duration of the thaw season, soil composition (GRAVEL favours deeper thawing than peaty soils), SOIL MOISTURE and content, and the density of the plant cover. Within the active layer, processes such as SOLIFLUCTION and FROST HEAVE can be highly effective. Engineering structures can become unstable on active layers and, where possible, piles are driven into the permafrost or structures are raised above the ground surface.
Acid rain describes any form of precipitation with high levels of nitric and sulfuric acids that have been released from industrial chimneys, and in particular from coal-burning power stations. Acid rain been held at least partly responsible for: (i) the ACIDIFICATION of rivers and lakes in upland areas; (ii) the widespread destruction of fish and other wildlife; (iii) the serious degeneration of coniferous forests in many parts of Europe (such as S Scandinavia, which may have been seriously affected by rains contaminated over Britain). Action has been taken to reduce the emission of sulphur dioxide from power stations by the use of filter mechanisms and by political agreements
Some Governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes, and sulfur dioxide is produced by volcanic eruptions. The chemicals in acid rain can cause paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues.
Rotting vegetation and erupting volcanoes release some chemicals that can cause acid rain, but most acid rain falls because of human activities. The biggest culprit is the burning of fossil fuels by coal-burning power plants, factories, and automobiles.
When humans burn fossil fuels, sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere. These chemical gases react with water, oxygen, and other substances to form mild solutions of sulfuric and nitric acid. Winds may spread these acidic solutions across the atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows across the surface in runoff water, enters water systems, and sinks into the soil.
Processes involved in acid deposition (note that only SO2 and NOx play a significant role in acid rain).
Acid rain has many ecological effects, but none is greater than its impact on lakes, streams, wetlands, and other aquatic environments. Acid rain makes waters acidic and causes them to absorb the aluminum that makes its way from soil into lakes and streams. This combination makes waters toxic to crayfish, clams, fish, and other aquatic animals.
Some species can tolerate acidic waters better than others. However, in an interconnected ecosystem, what impacts some species eventually impacts many more throughout the food chain—including non-aquatic species such as birds.
Acid rain also damages forests, especially those at higher elevations. It robs the soil of essential nutrients and releases aluminum in the soil, which makes it hard for trees to take up water. Trees’ leaves and needles are also harmed by acids.
The only way to fight acid rain is by curbing the release of the pollutants that cause it. This means burning fewer fossil fuels. Many governments have tried to curb emissions by cleaning up industry smokestacks and promoting alternative fuel sources. These efforts have met with mixed results. But even if acid rain could be stopped today, it would still take many years for its harmful effects to disappear.
Individuals can also help prevent acid rain by conserving energy. The less electricity people use in their homes, the fewer chemicals power plants will emit. Vehicles are also major fossil fuel users, so drivers can reduce emissions by using public transportation, carpooling, biking, or simply walking wherever possible.
Volcanic LAVA that is rich in silica and flows slowly owing to its high viscosity and quickly solidifies and form a steep cone. Acid lava forms steep-sided, dome-like volcanoes. is sticky or viscous rich in silica. Acid volcanoes have often have violent explosion.
That part of a glacier or ICE SHEET lying above the EQUILIBRIUM LINE, on which the dominant process is the addition of snow and ice. Winter snowfall and avalanches are the main forms of accumulation and are regarded as INPUTS into glacier SYSTEMS. In the accumulation zone, net winter accumulation exceeds net summer ABLATION. The accumulation zone thus comprises a layered structure, with a series of winter accumulation layers separated by summer ablation surfaces.
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