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Old Thursday, May 19, 2011
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Default Guideline Thread for Geography

AoA,

if need any help regarding Geography, plz submit ur quries in this thread
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Earth Materials and Processes

The materials that make up the Earth are mainly rocks (including soil, sand, silt, dust) . Rocks in turn are composed of minerals. Minerals are composed of atoms,

Processes range from those that occur rapidly to those that occur slowly

Examples of slow processes
Formation of rocks
Chemical breakdown of rock to form soil (weathering)
Chemical cementation of sand grains together to form rock (diagenesis)
Recrystallization to rock to form a different rock (metamorphism)
Construction of mountain ranges (tectonism)
Erosion of mountain ranges

Examples of faster processes
Beach erosion during a storm.
Construction of a volcanic cone
Landslides (avalanches)
Dust Storms
mudflows
Processes such as these are constantly acting upon and within the Earth to change it. Many of these processes are cyclical in nature.



Rain comes from clouds - falls on surface, picks up sand, silt and clay, carries particles to river and into ocean. Water then evaporates to become clouds, which move over continents to rain again.


Most surface rocks started out as igneous rocks- rocks produced by crystallization from a liquid. When igneous rocks are exposed at the surface they are subject to weathering (chemical and mechanical processes that reduce rocks to particles). Erosion moves particles into rivers and oceans where they are deposited to become sedimentary rocks. Sedimentary rocks can be buried or pushed to deeper levels in the Earth, where changes in pressure and temperature cause them to become metamorphic rocks. At high temperatures metamorphic rocks may melt to become magmas. Magmas rise to the surface, crystallize to become igneous rocks and the processes starts over.

External Processes

Erosion- rocks are broken down (weathered) into small fragments which are then carried by wind, water, ice and gravity. External because erosion operates at the Earth's surface. The energy source for this process is solar and gravitational.

Internal Processes

Processes that produce magmas, volcanoes, earthquakes and build mountain ranges. Energy comes from the interior of the Earth, Most from radioactive decay - nuclear energy
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The Earth -- What is it?

The Earth has a radius of about 6371 km, although it is about 22 km larger at equator than at poles.


Internal Structure of the Earth:

Density, (mass/volume), Temperature, and Pressure increase with depth in the Earth.

Compositional Layering

Crust - variable thickness and composition
Continental 10 - 50 km thick

Oceanic 8 - 10 km thick

Mantle - 3488 km thick, made up of a rock called peridotite

Core - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni)


Guideline Thread for Geography-earthint.gif

Layers of Differing Physical Properties

Lithosphere - about 100 km thick (deeper beneath continents)


Asthenosphere - about 250 km thick to depth of 350 km - solid rock, but soft and flows easily.


Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.


Outer Core - 2250 km thick, Fe and Ni, liquid


Inner core - 1230 km radius, Fe and Ni, solid

All of the above is known from the way seismic (earthquake waves) pass through the Earth as we will discuss later in the course.

Surface Features of the Earth

Oceans cover 71 % of Earth's surface -- average depth 3.7 km. Land covers remaining surface with average of 0.8 km above sea level


Guideline Thread for Geography-surfacefeatures.gif

Ocean Basins

Continental Shelf, Slope, and rise
Abyssal Plains
Oceanic ridges
Oceanic Trenches
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Minerals

The Earth is composed of rocks. Rocks are aggregates of minerals. Minerals are composed of atoms

Definition of a Mineral:

Naturally formed it forms in nature on its own (some say without the aid of humans]


Solid ( it cannot be a liquid or a gas)


With a definite chemical composition (every time we see the same mineral it has the same chemical composition that can be expressed by a chemical formula).


and a characteristic crystalline structure (atoms are arranged within the mineral in a specific ordered manner).
Examples

Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure. Thus, glass is not a mineral.


Ice - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula H2O, and does have a definite crystalline structure when solid. Thus, ice is a mineral, but liquid water is not (since it is not solid).


Halite (salt) - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure. Thus halite is a mineral.

Composition of Minerals

The variety of minerals we see depend on the chemical elements available to form them. In the Earth's crust the most abundant elements are as follows:

O, Oxygen 45.2% by weight

Si, Silicon 27.2%

Al, Aluminum 8.0%

Fe, Iron 5.8%

Ca, Calcium 5.1%

Mg, Magnesium 2.8%

Na, Sodium 2.3%

K, Potassium 1.7%

Ti ,Titanium 0.9%

H, Hydrogen 0.14%

Mn, Manganese 0.1%

P, Phosphorous 0.1%
Note that Carbon (one of the most abundant elements in life) is not among the top 12.

Because of the limited number of elements present in the Earth's crust there are only about 3000 minerals known. Only 20 to 30 of these minerals are common. The most common minerals are those based on Si and O: the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens covalently bonded to one silicon atom

Properties of Minerals

Physical properties of minerals allow us to distinguish between minerals and thus identify them, as you will learn in lab. Among the common properties used are:

Habit - shape

Color

Streak (color of fine powder of the mineral)

Luster -- metallic, vitreous, pearly, resinous (reflection of light)

Cleavage (planes along which the mineral breaks easily)

Density (mass/volume)

Hardness: based on Mohs hardness scale as follows:
Talc

gypsum (fingernail)

calcite (penny)

fluorite

apatite (knife blade)

orthoclase (glass)

quartz

topaz

corundum

Diamond

Formation of Minerals

Minerals are formed in nature by a variety of processes. Among them are:

Crystallization from melt (igneous rocks)


Precipitation from water (chemical sedimentary rocks, hydrothermal ore deposits)


Biological activity (biochemical sedimentary rocks)


Change to more stable state - (the processes of weathering, metamorphism, and diagenesis).


Precipitation from vapor. (not common, but sometimes does occur around volcanic vents)
Since each process leads to different minerals and different mineral polymorphs, we can identify the process by which minerals form in nature. Each process has specific temperature and pressure conditions that can be determined from laboratory experiments. Example: graphite and diamond, as shown previously.

Rocks - Mixtures of Minerals

Mixtures or aggregates of minerals are called rocks. There are three basic kinds of rocks, each type is determined by the process by which the rock forms.

Igneous Rocks - form by solidification and crystallization from liquid rock, called magma.


Sedimentary Rocks - form by sedimentation of mineral and other rock fragments from water, wind, or ice and can also form by chemical precipitation from water.


Metamorphic Rocks - form as a result of increasing the pressure and/or temperature on a previously existing rock to form a new rock.

Each of these rock forming processes results in distinctive mineral assemblages and textures in the resulting rock. Thus, the different mineral assemblages and textures give us clues to how the rock formed.
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What are Rocks?

Rock: any naturally formed, firm and coherent aggregate of minerals that constitutes part of the Earth. Despite considerable diversity, we can classify all rocks into three types, according to how they formed:

Igneous rocks (from Latin "ignis" meaning "pertaining to fire") are formed by cooling and solidification of molten rock material and typically represented by an interlocking aggregate of silicate minerals.

Sedimentary rocks (from Latin "sedimentum" meaning "settle") are formed from particles of pre-existing rocks by cementation or other processes at the Earth's surface.

Metamorphic rocks (from Greek "meta" meaning "change" and "morpho" meaning "form") are formed within the Earth's crust by solid-state transformation of pre-existing rock (igneous, sedimentary or even metamorphic) as a result of high temperature, high pressure or both.
Within each group, rocks share common origins, but do not necessarily look alike. Difference are clue to how and where a rock formed.

What are Igneous Rocks?

Early study of igneous rocks was shrouded in controversy. In an attempt to organize rocks into a simple easily understood system, the 18th century german mineralogist Abraham Werner proposed that all rocks were precipitated in layers from a universal sea. Active volcanoes were explained by burning of subterranean coal beds. Because he was highly respected by his peers, Werner's theory, called Neptunism, gained wide acceptance and was not questioned. Neptunism, however, had a number of problems: e.g., the volume of rock assumed to have been precipitated was much greater than could have been in solution.

Eventually Werner's theory was disproven by studies of volcanic rocks. The geological community came to accept Plutonic theory, the belief that igneous rocks originate as molten rock material deep in the Earth. The new theory got its name from the Greek god of the underworld, Pluto.

Magma and Lava

Magma is the term used to describe naturally occurring molten rock material beneath the Earth's surface. Mobility of this liquid within the Earth is controlled by its physical properties, density and viscosity. Being a liquid, it is less dense than solid rock and thus, tends to rise buoyantly within the Earth as long as it is lighter than the surrounding or country rocks. Lava represents hot streams or sheets of magma that flow over the Earth's surface. Two types of igneous rocks can form from magmas:

Intrusive (Plutonic) igneous rocks are produced by cooling and crystallization of magma beneath the Earth's surface. The resulting igneous body may represent solidification of a magma chamber or reservoir in which magma would have been stored during movement toward the surface.

Extrusive (Volcanic) igneous rocks are produced rapid cooling and crystallization of magma on the Earth's surface. Volcanoes represent the vents from which molten silicate material, solid rock debris, and gases escape from the subsurface. The volcanic products may be coherent (lava) or fragmented (pyroclastic) material. Pyroclasts are produced when the erupting magma is torn apart by violent explosions within the volcanic vent.
Magmas are composed of the major elements (O, Si, Al, Mg, Fe, Ca, Na and K) that form the Earth. The dominant component of most magmas is silicon dioxide (silica), which constitutes 35-79% of the liquid. Magmas are grouped into compositional categories based on silica content: ultramafic (245% silica), mafic (45-52% silica), intermediate (53-65% silica), and felsic or silicic (>65% silica).

Origin of Magma
Geophysical studies demonstrate that except for the outer core, the Earth's interior is solid. Thus, there must generally be insufficient internal heat generation to melt pre-existing rock. Nevertheless, magmas demonstrate that melting must occur, although it is probably incomplete or partial melting rather that complete melting of solid rock material. The necessary melting conditions are present along divergent plate boundaries and above subduction zones. Melting always produces a magma that is more silica- and alkali-rich than its source rock.

Crystallization of Magma

Magma is molten rock material and dissolved gases (e.g., water, carbon dioxide, and various sulfur gases including H2S and SO2). Magma temperatures range from 700û to 1200ûC. Cooling magma begins to solidify through crystallization of minerals and release of gases and hydrothermal fluids. The crystallized minerals may be carried along in the rising magma as suspended solids (crystals).

N.L. Bowen (1922) demonstrated that a mafic parent magma can produce intermediate to felsic rocks as a result of progressive crystallization. Bowen determined the sequence of mineral crystallization from a basaltic magma, and showed that two different types of reaction occur between crystalline solids and the magma as it cools. Continuous reaction series minerals react with the melt to form new crystals with a different composition but a constant atomic structure. Discontinuous reaction series minerals react with the melt to form new crystals with both a new composition and a new atomic structure.

If the crystallizing minerals are continuously removed from contact with the magma, the magma composition will change from basalt through andesite to rhyolite. Generation of rhyolite (granite) from a basaltic liquid would require ninety percent solidification of the parental magma. However, geological observations indicate that there are ten times more granitic than mafic plutons, suggesting the granitic magmas must also be generated in other ways.

Separation of earlier-formed minerals, called fractional crystallization or crystal fractionation, can occur due to (a) crystal (gravitational) settling due to density differences with the magmatic liquid, (b) filter pressing or compaction of the crystal-liquid mush, and (c) differential flow. The separated crystals may settle and accumulate to form cumulate igneous rocks at the bottom of a magma chamber.

Magma may undergo additional compositional changes during ascent from its source region. The rising magma may react with and partially or completely melt the surrounding crustal rocks, incorporating elements that were originally present in the wall rocks. Partially melted inclusions of country rock in many magmas attest to such crustal contamination or assimilation. Different composition magmas within the same magmatic plumbing system may also intermingle or mix during ascent, such that magma mixing produces a hybrid magma of intermediate composition.

Texture of Igneous Rocks

Texture refers to variations in the sizes and shapes of mineral grains in a rock, and the type of relationships between the grains. Texture is determined by:

(1) Rate of Cooling - a primary control on texture, determines the relative rate of crystal nucleation and growth.

slow cooling - few large crystals produced by growth rate greater than nucleation rate
rapid cooling - many small crystals produced by nucleation rate greater than growth rate
quench - glass produced where ions have no time to organize into crystals

(2) Magma composition and Temperature - control magma density and magma viscosity (or its internal resistance to flow)

high silica melts are viscous and crystallize at low temperatures (<850ûC). Ions have difficulty migrating through liquid and organizing into crystals
low silica melts are fluid (low viscosity) and have high temperatures (850û-1200ûC). Ions easily migrate through liquid and organize into crystals
higher silica content, the higher the viscosity
(d) higher temperature, the lower the viscosity
(
3) Gas content of magma - High gas content reduces viscosity, leading to larger crystals even at low temperatures.

Igneous Rock Textures

Phaneritic texture is where individual mineral grains (crystals) are visible with the naked eye. The coarse-grained texture indicate slow cooling, and is typical of intrusive rocks.

Aphanitic texture is where individual mineral grains (crystals) can't be seen with unaided vision. The fine-grained texture indicate rapid cooling, and is typical of volcanic rocks.

Vitric or glassy texture indicates rapid cooling or quenching of the magma, best exemplified by obsidian or high-silica (rhyolite) glass.

Vesicular texture describes an aphanitic rock characterized by preservation of cavities (vesicles) originally filled by escaping gases. Highly vesicular basalts (low-silica magma) are called scoria, whereas highly vesicular rhyolite (high-silica magma) is known as pumice.

Porphyritic texture describes a rock, known as a porphyry, in which large crystals (phenocrysts) are surrounded by a fine-grained matrix (groundmass). The texture indicates non-uniform cooling (slow cooling followed by a period of rapid cooling).

Pyroclastic texture denotes a rock made up of broken volcanic particles (pyroclasts) that are fused by heat or cemented together by finer grained material into a rock. The term is derived from "pyro" meaning "fire" and "Klastos" meaning broken.

Pegmatitic texture indicates that the igneous rock is characterized by an extremely coarse-grained texture. Abnormally large (ª1 cm) crystals (locally containing rare metals such as Li, Be, or Ta in Li-mica, beryl or tantalum oxides) are formed from water-rich magmatic solutions (hydrothermal fluids).

Igneous Rock Classification
Igneous rocks are grouped on the basis of texture and mineral assemblage. Compositional categories based on silica content also apply to magma types that cool to form different types of volcanic rocks: komatiite (ultramafic), basalt (mafic), andesite (intermediate) and rhyolite (felsic). If the same magma cools to form intrusive igneous rock, the corresponding rock names are peridotite (komatiite), gabbro (basalt), diorite (andesite) and granite (rhyolite). Differences in magma composition are reflected by the mineral assemblage, or variety and abundance of different minerals, in an igneous rock.

Ultramafic rocks include peridotite (olivine-pyroxene rock), dunite (olivine rock) and pyroxenite (pyroxene rock). These rocks are predominantly intrusive in nature throughout Earth history. Volcanic equivalents, known as komatiites, primarily existed during early Earth history (22.0 Ba), and there are no known modern examples. The Earth's mantle is thought to be composed of peridotite.

Mafic rocks are black, dark gray or dark green in color, and composed primarily of olivine, feldspar (calcium plagioclase) and pyroxene. Basalt, aphanitic but locally porphyritic or vesicular mafic volcanic rock, is the most abundant igneous rock of the Earth's crust, forming the ocean floor and volcanic oceanic islands. Gabbro is the phaneritic intrusive equivalent of basalt, and composes the deeper ocean crust.

Intermediate rocks are medium-gray color, and composed of amphibole and feldspar (intermediate plagioclase) together with some pyroxene and biotite. Andesites, locally porphyritic, are intermediate volcanic rocks found in volcanic chains on continental margins and in island arcs above subduction zones. Diorite is the phaneritic intrusive equivalent of andesite, and comprises many of the batholiths found associated with subduction processes.

Felsic rocks are light-colored, locally glassy (obsidian), and composed of quartz and potassium feldspar with minor sodium plagioclase, biotite and amphibole. The volcanic rock, rhyolite, characterizes continental volcanoes and is typically associated with extremely explosive volcanic activity. The explosive nature of rhyolite volcanism reflects the magma's high viscosity and gas content relative to mafic or even intermediate magmas. Granite is the phaneritic intrusive equivalent of rhyolite, and comprises many of the batholiths found within continental crust. Pegmatite is an extremely coarse-grained granite, that forms from residual, water-rich magmatic fluids.

In addition to composition, pyroclastic rocks are further subdivided according to fragment size and type:

tuff is a pyroclastic volcanic rock consisting of broken crystals and pieces of volcanic glass less than 2 mm in diameter. Welded tuffs occur where particles were hot enough to fuse together after coming to rest.
volcanic breccia is a pyroclastic volcanic rock composed of consolidated, angular volcanic particles greater than 2 mm in diameter.

Intrusive Igneous Rock Bodies
Magmas crystallized beneath the Earth's surface form intrusive bodies of igneous rock known as plutons. The term pluton (after the Greek god Pluto) refers to any igneous intrusion regardless of size, shape or composition of the magma. Classification of plutons is based on:

Geometry of intrusion:
size
shape
Relationship to surrounding rocks:
concordant or boundaries parallel to layering in surrounding rocks
discordant or boundaries cut across layering in surrounding rocks

Tabular Plutons
A sill is a concordant body, few cm to >1 km thick, produced when magma is injected between layers of older sedimentary or volcanic rock, and are generally composed of intermediate to basic composition magma.
A dike is a discordant body, few cm to >100 m thick, produced when magma is injected along fractures in surrounding rock layers. Dikes ftypically form from magmas of basic to granitic composition. Ring and Radial dikes are discordant bodies having either a concentric (circular) or radial distribution; develop above a large subsurface intrusive body (batholith or stock) or adjacent to volcanic pipes or necks .
A lopolith is a spoon-like shaped concordant body similar to a sill except the floor and roof sag downward. The intrusions are generally magma of intermediate to basic composition.
Massive Plutons
A laccolith represents magma that pushes overlying rock layers upward to form a condordant, mushroom-shaped, sill-like body, typically comprising magma of intermediate to granitic composition
A batholith is a discordant magma body with exposed surface area of more than 100 square kilometers; typically consists of multiple intrusions. Batholith are usually magma of granitic composition with minor intermediate varieties

A stock is a discordant magma body with exposed surface area of less than 100 square kilometers; may represent exposed portion of a much larger intrusion. It is usually magma of granitic composition with minor intermediate varieties.

Volcanic pipes and necks are discordant bodies that represent the upper part of the conduit that connects the volcanic vent (crater) with an underlying magma source (magma chamber or reservoir). Volcanic necks are erosional remnants of magma that solidified in the pipe or conduit.
Mechanisms of Magma Emplacement
Batholith are found in the roots of mountain systems, and have been suggested to form by:

Granitization - an exxtreme type of metamorphism by which country rock is altered to granitic composition by ion-rich solutions. Contact with country rock is gradational

Forceful Injection - Country rock is deformed and forced aside as viscous magma slowy rises buoyantly. Magma may also move upward by stoping or detaching and engulfing pieces of the country rock. Xenoliths are unmelted remnants of the surrounding rock found in the upper parts of intrusive bodies. Contact with country rock is sharp. Most batholiths show characteristics consistent with an injection emplacement.
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Old Friday, May 20, 2011
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Default Urban Geography

Urban geography is a branch of human geography concerned with various aspects of cities. An urban geographer's main role is to emphasize location and space and study the spatial processes that create patterns observed in urban areas. To do this, they study the site, evolution and growth, and classification of villages, towns and cities as well as their location and importance in relation to different regions and cities. Economic, political and social aspects within cities are also important in urban geography.

In order to fully understand each of these aspects of a city, urban geography represents a combination of many other fields within geography. Physical geography for example is important in understanding why a city is located in a specific area as site and environmental conditions play a large role in whether or not a city develops. Cultural geography can aid in understanding various conditions related to an area's people, while economic geography aids in understanding the types of economic activities and jobs available in an area. Fields outside of geography such as resource management, anthropology and urban sociology are also important.

Definition of a City
An essential component within urban geography is defining what a city or urban area actually is. Although a difficult task, urban geographers generally define the city as a concentration of people with a similar way of life based on job type, cultural preferences, political views and lifestyle. Specialized land uses, a variety different institutions and use of resources also help in distinguishing one city from another.
In addition, urban geographers also work to differentiate areas of different sizes. Because it is hard to find sharp distinctions between areas of different sizes, urban geographers often use the rural-urban continuum to guide their understanding and help classify areas. It takes into account hamlets and villages which are generally considered rural and consist of small, dispersed populations, as well as cities and metropolitan areas considered urban with concentrated, dense populations.

History of Urban Geography

The earliest studies of urban geography in the United States focused on site and situation. This developed out of the man-land tradition of geography which focused on the impact of nature on humans and vice versa. In the 1920s, Carl Sauer became influential in urban geography as he motivated geographers to study a city's population and economic aspects with regard to its physical location. In addition, central place theory and regional studies focused on the hinterland (the rural outlying are supporting a city with agricultural products and raw materials) and trade areas were also important to early urban geography.

Throughout the 1950s and 1970s, geography itself became focused on spatial analysis, quantitative measurements and the use of the scientific method. At the same time, urban geographers began quantitative information like census data to compare different urban areas. Using this data allowed them to do comparative studies of different cities and develop computer based analysis out of those studies. By the 1970s, urban studies were the leading form geographic research.

Shortly thereafter, behavioral studies began to grow within geography and in urban geography. Proponents of behavioral studies believed that location and spatial characteristics could not be held solely responsible for changes in a city. Instead, changes in a city arise from decisions made by individuals and organizations within the city.

By the 1980s, urban geographers became largely concerned with structural aspects of the city related to underlying social, political and economic structures. For example, urban geographers at this time studied how capital investment could foster urban change in various cities.

Throughout the late 1980s until today, urban geographers have begun to differentiate themselves from one another, therefore allowing the field to be filled with a number of different viewpoints and focuses. For example, a city's site and situation is still regarded as important to its growth, as is its history and relationship with its physical environment and natural resources. People's interactions with each other and political and economic factors are still studied as agents of urban change as well.

Themes of Urban Geography

Although urban geography has several different focuses and viewpoints, there are two major themes that dominate its study today. The first of these is the study of problems relating to the spatial distribution of cities and the patterns of movement and links that connect them across space. This approach focuses on the city system. The second theme in urban geography today is the study of patterns of distribution and interaction of people and businesses within cities. This theme mainly looks at a city's inner structure and therefore focuses on the city as a system.

In order to follow these themes and study cities, urban geographers often break down their research into different levels of analysis. In focusing on the city system, urban geographers must look at the city on the neighborhood and citywide level, as well as how it relates to other cities on a regional, national and global level. To study the city as a system and its inner structure as in the second approach, urban geographers are mainly concerned with the neighborhood and city level.
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Default Central Place Theory

Central place theory is a spatial theory in urban geography that attempts to explain the reasons behind the distribution patterns, size, and number of cities and towns around the world. It also attempts to provide a framework by which those areas can be studied both for historic reasons and for the locational patterns of areas today.

The theory was first developed by the German geographer Walter Christaller in 1933 after he began to recognize the economic relationships between cities and their hinterlands (areas farther away). He mainly tested the theory in Southern Germany and came to the conclusion that people gather together in cities to share goods and ideas and that they exist for purely economic reasons.

Before testing his theory however, Christaller had to first define the central place. In keeping with his economic focus, he came to the conclusion that the central place exists primarily to provide goods and services to its surrounding population. The city is in essence, a distribution center.


Christaller's Assumptions

To focus on the economic aspects of his theory, Christaller had to create a set of assumptions. He decided for example that the countryside in the areas he was studying would be flat, so no barriers would exist to impede people's movement across it. In addition, two assumptions were made about human behavior: 1) Christaller stated that humans will always purchase goods from the closest place that offers the good, and 2) whenever demand for a certain good is high, it will be offered in close proximity to the population. When demand drops, so too does the availability of the good.
In addition, the threshold is an important concept in Christaller's study. This is the minimum number of people needed for a central place business or activity to remain active and prosperous.

This then brings in the idea of low-order and high-order goods. Low-order goods are things that are replenished frequently such as food and other routine household items. Because these items are purchased regularly, small businesses in small towns can survive because people will buy frequently at the closer locations instead of going into the city.

High-order goods though are specialized items such as automobiles, furniture, fine jewelry, and household appliances that are bought less often. Because they require a large threshold and people do not purchase them regularly, many businesses selling these items cannot survive in areas where the population is small. Therefore, they often locate in large cities that can serve a large population in the surrounding hinterland.


Central Place Size and Spacing

Within the central place system, there are five sizes of communities. A hamlet is the smallest and is a rural community which is too small to be considered a village. Cape Dorset (population 1200), located in Canada's Nunavut Territory is an example of a hamlet. The rank order of central places is:

•Hamlet
•Village
•Town
•City
•Regional Capital
Examples of regional capitals would include Paris, France or Los Angeles, California. These cities provide the highest order goods possible and have a huge hinterland.

Central Place Theory Geometry and Ordering

If visually imagined, the central place is located at the vertexes (points) of equilateral triangles. They then serve the evenly distributed consumers who are closest to the central place. As the vertexes connect, they form a series of hexagons- the traditional shape in many central place models.
This shape is ideal because it allows the triangles formed by the central place vertexes to connect and it represents the assumption that consumers will visit the closest place offering the good.

In addition, the central place theory has three orders or principles. The first is the marketing principle and it is shown as K=3 (K is a constant). In this system, market areas at a certain level of the central place hierarchy are three times bigger than the next lowest one. The different levels then follow a progression of threes, meaning that as one moves through the order of places, the number of the next level goes up three times. For example, when there are two cities, there would be six towns, 18 villages, and 54 hamlets.

There is also the transportation principle (K=4) where areas in the central place hierarchy are four times bigger than the area in the next lowest order. Finally, the administrative principle (K=7) is the last system and here, the variation between the lowest orders and highest orders increase by a factor of seven. Here, the highest order trade area completely covers that of the lowest order, meaning that market serves a larger area.


Losch’s Central Place Theory

In 1954, German economist August Losch modified Christaller's central place theory because he believed it was too rigid. He thought that Christaller's model led to patterns where the distribution of goods and the accumulation of profits were based entirely on location. He instead focused on maximizing consumer welfare and creating an ideal consumer landscape where the need to travel for any good was minimized and profits were held level, not maximized to accrue extra.

Central Place Theory Today

Though Losch's central place theory looks at the ideal environment for the consumer, both his and Christaller's ideas are essential to studying the location of retail in urban areas today. Often, small hamlets in rural areas do act as the central place for various small settlements because they are where people travel to buy their everyday goods. However, when they need to buy higher value goods such as cars and computers, they have to travel into the larger town or city -- which serves not only their small settlement but those around them as well. This model is shown all over the world, from rural areas of England to the United States' Midwest or Alaska with the many small communities that are served by larger towns, cities, and regional capitals.
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Default The Von Thunen Model of Land Use

The Von Thunen model of agricultural land use was created by farmer and amateur economist J.H. Von Thunen (1783-1850) in 1826 (but it wasn't translated into English until 1966). Von Thunen's model was created before industrialization and is based on the following limiting assumptions:

•The city is located centrally within an "Isolated State" which is self sufficient and has no external influences.
•The Isolated State is surrounded by an unoccupied wilderness.
•The land of the State is completely flat and has no rivers or mountains to interrupt the terrain.
•The soil quality and climate are consistent throughout the State.
•Farmers in the Isolated State transport their own goods to market via oxcart, across land, directly to the central city. Therefore, there are no roads.
•Farmers act to maximize profits.

Guideline Thread for Geography-vt.gif
In an Isolated State with the foregoing statements being true, Von Thunen hypothesized that a pattern of rings around the city would develop.
There are four rings of agricultural activity surrounding the city. Dairying and intensive farming occur in the ring closest to the city. Since vegetables, fruit, milk and other dairy products must get to market quickly, they would be produced close to the city (remember, we didn't have refrigerated oxcarts!)

Timber and firewood would be produced for fuel and building materials in the second zone. Before industrialization (and coal power), wood was a very important fuel for heating and cooking. Wood is very heavy and difficult to transport so it is located as close to the city as possible.

The third zone consists of extensive fields crops such as grains for bread. Since grains last longer than dairy products and are much lighter than fuel, reducing transport costs, they can be located further from the city.

Ranching is located in the final ring surrounding the central city. Animals can be raised far from the city because they are self-transporting. Animals can walk to the central city for sale or for butchering.

Beyond the fourth ring lies the unoccupied wilderness, which is too great a distance from the central city for any type of agricultural product.

Even though the Von Thunen model was created in a time before factories, highways, and even railroads, it is still an important model in geography. The Von Thunen model is an excellent illustration of the balance between land cost and transportation costs. As one gets closer to a city, the price of land increases. The farmers of the Isolated State balance the cost of transportation, land, and profit and produce the most cost-effective product for market. Of course, in the real world, things don't happen as they would in a model
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Default

Rank-Size Rule

In 1949, George Zipf devised his theory of rank-size rule to explain the size cities in a country. He explained that the second and subsequently smaller cities should represent a proportion of the largest city. For example, if the largest city in a country contained one million citizens, Zipf stated that the second city would contain one-half as many as the first, or 500,000. The third would contain one-third or 333,333, the fourth would be home to one-quarter or 250,000, and so on, with the rank of the city representing the denominator in the fraction.

While some countries' urban hierarchy somewhat fits into Zipf's scheme, later geographers argued that his model should be seen as a probability model and that deviations are to be expected.
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Default Primate Cities

A country's leading city is always disproportionately large and exceptionally expressive of national capacity and feeling. The primate city is commonly at least twice as large as the next largest city and more than twice as significant. - Mark Jefferson, 1939

Geographer Mark Jefferson developed the law of the primate city to explain the phenomenon of huge cities that capture such a large proportion of a country's population as well as its economic activity. These primate cities are often, but not always, the capital cities of a country. An excellent example of a primate city is Paris, which truly represents and serves as the focus of France.

They dominate the country in influence and are the national focal-point. Their sheer size and activity becomes a strong pull factor, bringing additional residents to the city and causing the primate city to become even larger and more disproportional to smaller cities in the country. However, not every country has a primate city, as you'll see from the list below.

Some scholars define a primate city as one that is larger than the combined populations of the second and third ranked cities in a country. This definition does not represent true primacy, however, as the size of the first ranked city is not disproportionate to the second.

The law can be applied to smaller regions as well. For example, California's primate city is Los Angeles, with a metropolitan area population of 16 million, which is more than double the San Francisco metropolitan area of 7 million. Even counties can be examined with regard to the Law of the Primate City.


Examples of Countries With Primate Cities

•Paris (9.6 million) is definitely the focus of France while Marseilles has a population of 1.3 million.

•Similarly, the United Kingdom has London as its primate city (7 million) while the second largest city, Birmingham, is home to a mere one million people.

•Mexico City, Mexico (8.6 million) outshines Guadalajara (1.6 million).

•A huge dichotomy exists between Bangkok (7.5 million) and Thailand's second city, Nanthaburi (481,000).

Examples of Countries that Lack Primate Cities

•India's most populous city is Mumbai (formerly Bombay) with 16 million; second is Kolkata (formerly Calcutta) with more than 13 million; and third is less than 13 million.

•China, Canada, Australia, and Brazil are additional examples of non-primate-city countries.

•Utilizing the metropolitan area population of urban areas in the United States, we find that the U.S. lacks a true primate city. With the New York City metropolitan area population at approximately 21 million, second ranked Los Angeles at 16 million, and even third ranked Chicago at 9 million, America lacks a primate city.
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