also decentralized, and businesses that relocated toward the suburbs tended to
cluster together in a single, concentrated space, the shopping mall, effectively dis-
placing some of the benefits offered by the CBD. In the densely populated eastern
seaboard of the United States, urban decentralization has resulted in an almost con-
tinuous band of urban development, themegalopolis. Some of the negative effects
of urban decentralization include the economic decline of the inner city in many
instances in the 1950s and 1960s; the appearance of so-called urban sprawl, charac-
terized by vast tracts of land containing nearly identical homes; and greater social
isolation of families and communities. Slowed somewhat bygentrificationand
other counter trends, urban decentralization continues to occur in many urban areas.Urbanization
The process whereby an increasingly larger percentage of a givenpopulationlives
in spatial clusters considered to be “towns” and “cities.” The criteria that qualify a
settlement as “urban” frequently vary from country to country, meaning that what
is considered an “urban place” in one location may not be defined as such in another
nation-state. In the United States, the U.S. Bureau of the Census considers
any place having a concentrated population greater than 50,000 inhabitants to be
an “urban area,” and any population center of at least 2,500 residents outside of an
urban area and having a community identity and name to be “urban.” Urbanization
is typically associated with a number of related processes: industrialization, eco-
nomic development, declining average family size, rising levels of literacy and edu-
cation, and others. In some cases a single urban place will come to dominate a
country’s urban geography in terms of both population size and economic func-
tion—this city serves as theprimate city. In 2008 the United Nations estimated that
for the first time in history, more than half of humanity lived in urban areas, and that
the fastest rates of urbanization are occurring in the developing world. Urban places
are not located on thelandscaperandomly. Many factors may affect the rate of
growth of an urban area, including transportation linkages,natural resources
located nearby, economic opportunities,and even the local physical geography
can play a role in urbanization. Geographers have developed a number of
approaches to explain the patterns of urbanization and to determine the characteris-
tics that may influence urban growth and development.Central Place Theory
holds that urban places are spatially arranged in a hierarchical framework, based
on the sophistication of economic functions offered by each city or town.
Urbanization on a significantscalefirst took place in ancient Sumeria, along the
course of the Tigris-Euphrates river system. The rich soils of these river valleys350 Urbanization