WHAT DOES EVERY U.S STATE FLAG MEAN?

UNDERSTANDING WESTERN SAHARA

THE DEADLIEST ICE AGE EVER

THE SOLAR SYSTEM

THE GULF STREAM EXPLAINED

ALONSO MODEL

A model developed by Alonso in the 1960s to explain the paradox observed in many cities that poor people tend to live close to the city centre on high-value land, while the rich occupy cheaper land close to the city margins (see BID-RENT CURVE, URBAN DENSITY GRADIENT). The explanation is based on the assumption that the income of a household is consumed by three basic costs: (i) subsistence (food, etc.); (ii) housing and (iii) COMMUTING (assuming that, for most people, their place of work is located in or near the city centre). Poor households can make a saving on these costs by: opting for an innercity residential location (so as to be close to their work); limiting the amount of highcost space occupied (i.e. by living at high densities); occupying older and often substandard housing, and by accepting MULTIFAMILY OCCUPATION of dwellings. Conversely, rich families are assumed to have large space requirements. Because they can afford higher commuting costs, they are also able to purchase large amounts of the lower-value land to be found at the edge of the city.

THE LAST STAR IN THE UNIVERSE

ALLUVIUM

This describes the SEDIMENTS laid down by streams. Alluvium is unconsolidated material forming features such as ALLUVIAL FANS, FLOOD PLAINS, RIVER TERRACES and DELTAS. The most common constituents are CLAY and SILT (from the SUSPENDED SEDIMENT LOAD of the stream) and SAND and GRAVEL (from the BED LOAD of the stream). Alluvium is generally regarded as being very fertile and it has the potential to form rich farmland.

ALLUVIAL FAN

A fan-shaped mass of ALLUVIUM (SAND, GRAVEL, COBBLES and sometimes BOULDERS) formed where a rapidly flowing stream leaves a steep and narrow valley and enters a lowland or broad valley. At such points a reduction in gradient, and thus stream velocity, occurs, causing overloading and DEPOSITION. Alluvial fans form in many different locations – for example, where a small tributary valley joins a major glacial trough, or where a desert stream passes through the MOUNTAIN FRONT to the PIEDMONT Zone.

Alluvial fan in southern Iran.

THE EUROPEAN REFUGEE CRISIS AND SYRIA EXPLAINED