Physical geography

Physical geography is a branch of science that studies the natural processes and features of Earth's surface. It is a separate discipline than cultural geography, which concentrates on human-defined boundaries and people's impact on the environment. The focus of physical geography is on the four subsystems, or "spheres," of Earth—the atmosphere, biosphere, hydrosphere, and lithosphere. Individual fields of study within these subsystems include climate, weather patterns, erosion, vegetation, rivers and oceans, glaciers, and biological ecosystems.

Background

Ancient humans believed the environment and natural processes of Earth were created and shaped by a host of deities and other supernatural forces. With the development of civilizations, humans began to look for more rational explanations for the world around them. The Chinese, Babylonians, Egyptians, and Phoenicians were among the first peoples to explore regions outside their homelands, chronicling their journeys in historical records and maps. The ancient Greeks were among the first civilizations to go beyond mapmaking to note the physical characteristics and natural elements of the lands they lived in and visited.

In the fifth century BCE, Greek historian Herodotus wrote one of the first geographical works in history when he described the environment and geography of the Persian Empire. The word geography—Greek for "to write about or describe Earth"—was first used by third century BCE scientist Eratosthenes. By using the measurement of shadows cast at two different points on Earth, Eratosthenes was able to measure the circumference of the planet with a fairly reliable degree of accuracy for the time. During the early days of the Roman Empire, Greek historian Strabo compiled a seventeen-volume record of his travels, describing geographical regions from Britain to India.

Human understanding of geography progressed little during the medieval period after the fall of Rome. That began to change with the intellectual rebirth of the Renaissance and the Age of Exploration in the fifteenth century. As nations sent sailing vessels to the far regions of the globe, the need to understand the rapidly expanding world created a new interest in geography. In the seventeenth century, German geographer Bernhardus Varenius categorized the discipline into three distinct branches. The first examined the dimensions and composition of Earth; the second focused on natural processes such as climate and tides; and the third studied the cultural regions of Earth. The highly influential work became a standard of geography for nearly a century.

Overview

Human scientific knowledge increased dramatically from the eighteenth to the twentieth centuries, with significant advancements in fields such as biology, botany, geology, and zoology. By the mid-twentieth century, geographers began incorporating these disciplines into the study of geography. In 1964, geographer William Pattison divided the field into four traditions—the spatial tradition, area studies tradition, human-land tradition, and Earth sciences tradition. Physical geography is an element of the Earth sciences tradition.

The atmosphere—one of the four subsystems of physical geography—is an envelope of gases that surrounds Earth. It is composed of about 78 percent nitrogen, 21 percent oxygen, and trace amounts of other gases and water vapor. The atmosphere rises to a height of about 6,214 miles (10,000 kilometers) and is divided into five main layers—the exosphere, thermosphere, mesosphere, stratosphere, and troposphere.

The exosphere is the topmost layer and marks the boundary of outer space. The thin thermosphere can reach temperatures as high as 273 degrees F (1500 degrees C). The mesosphere is the coldest section of the atmosphere with temperatures dropping to about –130 degrees F (–90 degrees C). The stratosphere contains the ozone layer, which protects Earth's surface from the Sun's ultraviolet rays. The troposphere is the closest layer to the surface and contains about 80 percent of the atmosphere's total mass. The majority of the planet's weather occurs in the troposphere and is caused by variations in moisture, pressure, temperature, and the movement of air masses.

The biosphere is the region on Earth where living organisms exist. Because life exists from the depths of the oceans into the atmosphere, the biosphere encompasses parts of the other three spheres of the planet. The total range of the biosphere is about 12 miles (19.3 kilometers), although the majority of Earth's life exists in a zone about 10 feet (3 meters) below ground or 1,640 feet (500 meters) below the ocean's surface to about 3.7 miles (6 kilometers) above sea level.

The biosphere includes a diverse group of biomes, an ecological region of similar animal and plant life. Biomes are predominantly affected by regional climates, landscapes, and temperature variations. Examples of biomes include deserts, tundra, tropical rain forests, coral reefs, and grasslands.

The hydrosphere refers to Earth's waters. It includes not only oceans, lakes, and rivers but also groundwater in rocks and soil, water vapor in the atmosphere, and frozen water at the poles or in glaciers. Scientists estimate the total volume of water on Earth to be about 333 million cubic miles (1.4 billion cubic kilometers). The oceans contain more than 97 percent of the planet's water. Water moves through the hydrosphere in a cycle, falling from clouds as rain or snow, seeping into rocks and flowing into rivers, lakes, and the oceans, and eventually evaporating back into the atmosphere.

The lithosphere includes Earth's mantle and crust—the two outermost solid layers of the planet. The lithosphere extends from Earth's surface to the asthenosphere, the lower layer of the mantle where intense pressure and heat have begun to liquefy rock. The oceanic lithosphere, or crust below the oceans, is a denser layer of rock than the continental lithosphere, or crust found on land. The continental lithosphere is thicker, reaching depths from 25 miles (40 kilometers) to 174 miles (200 kilometers) below the surface.

The lithosphere is segmented into fifteen slowly moving tectonic plates that fit together like a large jigsaw puzzle. These plates are the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca. The movement of the plates causes intense pressure and heat and is responsible for earthquakes, volcanoes, and the formation of mountains.

Bibliography

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