Geography Dictionary

Debris – any fragmented rock material i.e. that produced by weathering or erosion.

Decentralization – movement of people, government functions or employment opportunities out from an established central area to a relatively peripheral one. Identified at city, regional and national scale.

Deciduous woodland – woodland or forest made up of trees which have broad, flat leaves and which shed them annually during autumn, or fall. They remain bare through the winter months when insolation is too low for efficient photosynthesis and water may be locked in snow, before growth restarts in spring.

Declination – the latitude where, on any particular day, the sun is 90˚ above the horizon at solar noon i.e. when the sun is aligned between true north and true south.

Decomposer – an organism that converts organic matter into its inorganic chemical components which are then recycled through an ecosystem. Most common ones are bacteria and fungi.

Decomposition – breakdown of organic material in the litter by detritivores. Allows the release of energy and nutrients into the soil for recycling.

Deep-sea trench – see ocean trench.

Deflation – wind removal of small, grain-size rock particles such as sand. 

Deforestation – removal of forest cover due to cutting or burning, or a combination of the two.

Deglaciation – the removal of glacier cover (glacial retreat) when ablation outstrips accumulation.

Deglomeration – the dispersal of businesses from an area due to rising costs, especially those of specialized labour and land rents. The opposite of agglomeration.

Degradation – when high discharge creates a high energy environment in a river channel leading to a lowering of the channel bed.

Deindustrialization – fall in the percentage contribution of secondary industry to an economy in terms such as value of input to GDP and importance as an employment sector.

Delta – a depositional feature found at river mouths under certain conditions. Where the river enters the body of water (sea or lake) there is energy loss and load is deposited. If the rate of deposition exceeds the rate of removal in any current within the body of water then the material will build up. As the upper surface approaches sea-level the river may be forced to split into distributary channels which remain free of further deposition. Plants may colonize the top of the deposit and help it to build up above the sea level to form new land. Three types are identified:

• arcuate – a fan-shape with the ‘point’ at the river mouth.
• cuspate – a triangular shape with the ‘point’ facing the body of water and base at the river mouth.
• bird’s-foot – as it suggests, with the ‘toes’ pointing out to the body of water.

It is now thought that the different shapes are formed according to the salinity of the body of water. High salinity means slow mixing and so the sediments flows further out across the body of water creating a cusp. Low salinity means quick mixing and deposition and so more arcuate shapes.

Demographics – the statistical characteristics of a population’s births, deaths, age/sex structure etc.

Demographic transition model – a theory of population change over time. Seeks to explain population increases and decreases through variations in the birth rate and death rate. Based on observations of historical changes in parts of the EMDW, it is now applied more generally though with variable levels of confidence. A major criticism is its failure to take into account changes due to migration.

Demography – the study of demographics.

Dendritic – description of a stream pattern that is random and creates a tree-like pattern.

Dendrochronology – observation of a core or cross-section of a tree trunk shows a pattern of essentially concentric rings, each representing a year of growth. The number of rings can therefore be used to date the tree. Further, wider rings (higher rate of growth) indicate wetter years, and vice versa. The study can therefore help in the construction of past climate.

Denudation – stripping of surface cover. Can apply to both vegetation and soils.

Dependency ratio – the proportion of working (economically active) people to non-working (non-economically active) people in a country by the formula:

            people aged 0-14 + people aged over 65  x 100

                               people aged 15-65

Thus suggests how many non-workers are supported by every hundred workers. While these age groups do not reflect economic status accurately, inaccuracies tend to balance out.

Dependent variable – one which is directly affected by another e.g. water temperature will vary with depth, but depth is not affected by water temperature.

Deposition – the placing down of material being transported by an agent of erosion due to a loss of energy.

Depression – weather system of the mid-latitudes, where warm, tropical air meets cold, polar air causing the tropical air to rise and thus creating an area of low pressure. Characterized by a circular pattern of isobars, a warm front, a cold front and inward blowing, anti-clockwise winds.

Deprivation – a situation where quality of life is below that of what can be expected for a particular place at a particular time.

Deprivation cycle – much the same as the cycle of poverty but as applied to EMDW inner city areas. Low wages and poverty mean overcrowding and no investment in housing. These cause poor health and stress and low levels of education and training, meaning low skill levels in the population, restricting employment opportunities and maintaining the situation of low wages and unemployment.

Deregulation – the removal of rules, regulations and laws previously imposed on an industry. Often welcomed as the removal of monopoly rights which improve competition and opportunity, they can also lead to the weakening of protection for workers and consumers.

Derelict land – land which has fallen into disuse. Most commonly applied to land that has been built on or heavily impacted by primary activities such as mining and quarrying.

Derelict land grant – in the UK, government funding for the improvement of derelict land.

Desertification – the spread of desert, or desert conditions, from an established desert area into the surrounding area. A function both of physical factors such as reduced rainfall, and human factors such as resource depletion due to increased population.

Desire line – a line on a map relating information on the movement of people by joining their point of origin with their destination. Thickness is proportional to the number of people involved in the movement.

Destructive plate margin – in plate tectonics, a plate boundary where the relative movement of the crustal plates is towards each other and where one is subducted beneath the other thus being destroyed as it returns to the mantle. The collision may be between oceanic plates or between an oceanic plate and a continental plate. The denser of the two will subduct. This will never be continental crust as it is always the less dense.

Destructive wave – a steep, high frequency wave which causes a net loss of material from the beach as the backwash is stronger than the swash.

Detour index – a measure of connectivity in a network which compares actual distance between vertices with the straight line distance.

                        DI = shortest possible actual route

                                    Straight-line distance

Detritivore – an organism which feeds on dead organic material.

Developed – an out of date term for economically more developed countries. The terminology changed to recognize that development can be measured in many different ways, not all of them linked to wealth although it is a highly important factor both directly and indirectly.

Developing – a term for a country or region that is enjoying ever increasing levels of economic development. May be an ELDC, or have moved to an intermediate stage known as newly industrialized country.

Development – use of resources, natural and human, to achieve higher standards of living. Once focused solely on economic factors, now encompasses social measures and ideas such as freedom. Can be used in general terms e.g the ‘level of development’ or in specific terms e.g. a development project such as a power station being built.

Development area – region earmarked for economic support.

Development gap – the disparity in development between the EMDW and ELDW.

Development model – a theory of development – why and how it occurs. Two popular ones are Clarke-Fisher and Rostow.

Devonian - in geologic time, a period lasting from 408m to 360m years ago.

Dew – condensation of water directly onto ground surfaces such as leaves or car windscreens. Rapid heat loss at night causes air closest to the surface to reach its dew point.

Dew point – temperature at which air becomes saturated i.e. relative humidity is 100%.

Diastrophism – movements producing positional changes in the crust. Orogenic changes are large scale folding and faulting processes which lead to landforms such as fold mountains. Epeirogenic changes are more gentle processes of uplift.

Diffluence – the breakaway from a large glacier of a smaller, secondary flow of ice which then crosses a drainage divide. May involve uphill movement. Has important implications for post-glacial drainage patterns.

Diffusion – spread through space over time. In geography, usually applied to the uptake of an innovatory piece of technology or idea.

Dip slope – the gentle slope formed by the upper plane of the dipping, harder bed of rock in a cuesta.

Discharge – the volume of water in a channel passing a particular point in a particular time, usually cubic metres per second or cumecs. Calculated by multiplying cross-sectional area of the river by its velocity.

Discontinuous permafrost – between 50˚N and the Arctic Circle where mean annual temperature is -1˚ to -5˚ there will be patches of permanent permafrost perhaps 50 metres deep, separated by areas of little or no permafrost which are kept warmer by local conditions such as rivers.

Discordant coast – a coast of headlands and bays where the different rock types are perpendicular to the coastline.

Discrete variable – a variable which can only be measured in whole, individual units if it is to have any real meaning e.g numbers of people.

Diseconomies of scale – at large-scale levels of output inefficienies may creep in and cause unit costs to rise.

Dispersal – the movement of people or organisms from their area of birth.

Disposable income – income left after taxes and necessities have been paid. This can be spent on goods or services wanted, or saved.

Dissolved load – those minerals that have been taken into solution and are carried along by a river.

Distance decay – the negative correlation of distance to interaction between two points i.e. the greater the distance, the lower the amount of interaction.

Distance ratio – gradient of a slope measured by dividing vertical change by horizontal change. Expressed as a percentage or a ratio.

Distributary – a stream which splits away from the main channel and never rejoins it. Common across deltas as they are so close to the sea that the channel has no space to find its way back. When a channel does rejoin the main stream, it is braided.

Distribution – two uses:

• movement of goods from producers to places where they are sold.
• spatial pattern of where an observed feature appears within a particular area.

Distribution channel – the route a product takes from producer to consumer. Becomingly increasingly direct as producers offer products direct through the internet.

Diurnal – in geography, daily, or of each day, where a day means the full 24 hour period.

Divergent plate margin – see constructive plate margin.

Diversification – a strategy for spreading business risk whereby a business branches into new industries or markets to protect against potential, unpredictable problems in their core business.

Doldrums – see intertropical convergence zone.

Dormitory settlement – a rural settlement that is increased in size due to the influx of new residents from a nearby urban area. Incomers may be retiring or ‘escaping’ the negative aspects of city life while retaining jobs in the urban area to which they commute. Often leads to a decline in services as the incomers have private transport and spend most of their income in the urban area.

Doubling time – number of years taken for a population to double in size (number).

Downward spiral – decline occurring in a vicious circle or negative cumulative causation.

Draa – vast desert sand dune large enough to carry smaller dunes across its surface.

Drainage basin – the area from which a river channel receives water.

Dredging – removal of sediment from the bottom of a sea or river. Carried out to improve navigation or to obtain material for construction uses and beach nourishment.

Drift – material deposited by glacial and fluvioglacial processes.

Drought – a lack of precipitation into an area for a long period of time. Modern droughts are thought to be intensified by population pressure which may help to remove water available for local evaporation.

Drumlin – a mound of glacial debris formed into a smooth, elongated feature whose apex is skewed towards the rear (in terms of the direction of movement). The steeper slope at the back of the feature is called the stoss end. Origin still being debated. Thought to be the result of deposition followed by erosion/shaping.

Dry adiabatic lapse rate (DALR) – the rate of fall in air temperature by adiabatic change as unsaturated air gains altitude. Approximately 1°C per 100m.

Dry valley – valley, usually found in upland areas of limestone or chalk, demonstrating the characteristics of a typical upper course river, but containing no river.

Dual economy – a country having one or two core areas which far outstrip the development of the surrounding peripheral area.

Dumping – large-scale selling of a good in another country at below-cost price to earn foreign currency, get rid of excess production or attack that country’s domestic producers.

Dune – ridge or mound of sand formed by wind conditions in arid and coastal areas.

Dust bowl – in the USA, the area of degradation of the agricultural land of the High Plains region in the 1930s when overexploitation of the land exposed the area to a higher impact when drought arrived. Drove many farmers from the land. Modern ideas, methods and technology have since led to recovery.

Dyke – three meanings:

• a wall or embankment constructed parallel to the coast to protect against flooding.
• a ditch in a fenland area
• a wall-shaped intrusion of magma which cuts across bedding planes.

Dynamic equilibrium – lack of change in a system as inputs and outputs remain in balance. If changes do occur, then feedbacks will allow for correction.

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