1.7 Physical Geography Perspectives

As discussed early in this chapter, physical geography is concerned with the processes that shape the Earth’s landforms, climate, and vegetation. In this sense, physical geographers are similar to scientists from other disciplines who focus on these phenomena, though as geographers they often look at problems and depict the results of their analysis spatially. In this book, physical geography provides a backdrop for the many aspects of human geography we discuss. Physical geography is a fascinating, large, and growing field of study worth exploring in greater detail. What follows are just the basics of landforms and climate.

Landforms: The Sculpting of the Earth

The processes that create the world’s varied landforms—mountain ranges, continents, and the deep ocean floor—are some of the most powerful and slow-moving forces on Earth. Originating deep beneath the Earth’s surface, these internal processes can move entire continents, often taking hundreds of millions of years to do their work. However, it is external processes that form many of the Earth’s landscape features, such as a beautiful waterfall or a rolling plain. These more rapid and delicate processes take place on the surface of the Earth. Geomorphologists study the processes that constantly shape and reshape the Earth’s surface.

Plate Tectonics

Two key ideas related to internal processes in physical geography are the Pangaea hypothesis and plate tectonics. The geophysicist Alfred Wegener first suggested the Pangaea hypothesis in 1912. This hypothesis proposes that all the continents were once joined in a single vast continent called Pangaea (meaning “all lands”), which then fragmented over time into the continents we know today (see Figure 1.25). As one piece of evidence for his theory, Wegener pointed to the neat fit between the west coast of Africa and the east coast of South America.

For decades, most scientists rejected Wegener’s hypothesis. We now know, however, that the Earth’s continents have been assembled into supercontinents a number of times, only to break apart again. All of this activity is made possible by plate tectonics, a process of continental motion discovered in the 1960s, long after Wegener’s time.

The study of plate tectonics has shown that the Earth’s surface is composed of large plates that float on top of an underlying layer of molten rock. The plates are of two types. Oceanic plates are dense and relatively thin, and they form the floor beneath the oceans. Continental plates are thicker and less dense. Much of their surface rises above the oceans, forming continents. These massive plates drift slowly, driven by the circulation of the underlying molten rock flowing from hot regions deep inside the Earth to cooler surface regions and back. The creeping movement of tectonic plates fragmented and separated Pangaea into pieces that are the continents we know today (Figure 1.25E).

Plate movements influence the shapes of major landforms, such as continental shorelines and mountain ranges. Huge mountains have piled up on the leading edges of the continents as the plates carrying them collide with other plates, folding and warping in the process. Plate tectonics accounts for the long, linear mountain ranges that extend from Alaska to Chile in the Western Hemisphere and from Southeast Asia to the European Alps in the Eastern Hemisphere. The highest mountain range in the world, the Himalayas of South Asia, was created when what is now India, situated at the northern end of the Indian-Australian Plate, ground into Eurasia. The only continent that lacks these long, linear mountain ranges is Africa. Often called the “plateau continent,” Africa is believed to have been at the center of Pangaea and to have moved relatively little since the breakup. However, as Figure 1.25 shows, parts of eastern Africa—the Somali Subplate and the Arabian Plate—continue to separate from the continent (the African Plate).

Humans encounter tectonic forces most directly as earthquakes and volcanoes. Plates slipping past each other create the catastrophic shaking of the landscape we know as an earthquake. When plates collide and one slips under the other, this is known as subduction. Volcanoes arise at zones of subduction or sometimes in the middle of a plate, where gases and molten rock (called magma) can rise to the Earth’s surface through fissures and holes in the plate. Volcanoes and earthquakes are particularly common around the edges of the Pacific Ocean, an area known as the Ring of Fire (Figure 1.26).

Landscape Processes

The landforms created by plate tectonics have been further shaped by external processes, which are more familiar to us because we can observe them daily. One such process is weathering. Rock, exposed to the onslaught of sun, wind, rain, snow, ice, freezing and thawing, and the effects of life-forms (such as plant roots), fractures and decomposes into tiny pieces. These particles then become subject to another external process, erosion. During erosion, wind and water carry away rock particles and any associated decayed organic matter and deposit them in new locations. The deposition of eroded material can raise and flatten the land around a river, where periodic flooding spreads huge quantities of silt. As small valleys between hills are filled in by silt, a floodplain is created. Where rivers meet the sea, floodplains often fan out roughly in the shape of a triangle, creating a delta. External processes tend to smooth out the dramatic mountains and valleys created by internal processes.

Human activity often contributes to external landscape processes. By altering the vegetative cover, agriculture and forestry expose the Earth’s surface to sunlight, wind, and rain. These agents in turn increase weathering and erosion. Flooding becomes more common because the removal of vegetation limits the ability of the Earth’s surface to absorb rainwater. As erosion increases, rivers may fill with silt, and deltas may extend into the oceans.

Climate

The processes associated with climate are generally more rapid than those that shape landforms. Weather, the short-term and spatially limited expression of climate, can change in a matter of minutes. Climate is the long-term balance of temperature and moisture that keeps weather patterns fairly consistent from year to year. By this definition, the last major global climate change took place about 15,000 years ago, when the glaciers of the last ice age began to melt. As we have seen, a new global climate change is occurring in our own time, produced in part by human activity.

Solar energy is the engine of climate. The Earth’s atmosphere, oceans, and land surfaces absorb huge amounts of solar energy, and the differences in the amounts they absorb account for part of the variations in climate we observe. The most intense direct solar energy strikes the Earth more or less head-on in a broad band stretching about 30° north and south of the equator. The fact that Earth’s axis sits at a 23° angle as it orbits the sun—an angle that does not change—means that where the band of greatest solar intensity strikes the Earth varies in regular sequence over the course of a year, creating seasons. Just how this yearly seasonal pattern works is illustrated by Figure 1.27. The highest average temperatures on the Earth’s surface occur within this band of greatest solar intensity. Moving away from the equator, solar energy strikes the Earth’s surface less directly—at more of an obtuse (wide) angle. This wide angle reduces its heating effect of the sun’s rays and results in lower average annual temperatures.

Temperature and Air Pressure

The wind and weather patterns we experience daily are largely a result of variations in solar energy absorption that create complex patterns of air temperature and air pressure. To understand air pressure, think of air as existing in a particular unit of space—for example, a column of air above a square foot of the Earth’s surface. Air pressure is the amount of force (due to the pull of gravity) exerted by that column on that square foot of surface. Air pressure and temperature are related: The gas molecules in warm air are relatively far apart and are associated with low air pressure. In cool air, the gas molecules are relatively close together (dense) and are associated with high air pressure.

As the sun warms a unit of cool air, the molecules move farther apart. The air becomes less dense and exerts less pressure. Air tends to move from areas of higher pressure to areas of lower pressure, creating wind. If you have been to the beach on a hot day, you may have noticed a cool breeze blowing in off the water. This happens because land heats up (and cools down) faster than water, so on a hot day, the air over the land warms, rises, and becomes less dense than the air over the water. This causes the cooler, denser air to flow inland. At night the breeze often reverses direction, blowing from the now cooling land onto the now relatively warmer water.

These air movements have a continuous and important influence on global weather patterns and are closely associated with land and water masses. Because continents heat up and cool off much more rapidly than the oceans that surround them, the wind tends to blow from the ocean to the land during summer and from the land to the ocean during winter. It is almost as if the continents were breathing once a year, inhaling in summer and exhaling in winter.

Precipitation

Perhaps the most tangible way we experience changes in air temperature and pressure is through rain or snow. Precipitation (dew, rain, sleet, and snow) occurs primarily because warm air holds more moisture than cool air. When this warmer, moist air rises to a higher altitude, its temperature drops, which reduces its ability to hold moisture. The moisture condenses into drops to form clouds and may eventually fall as rain or snow.

Several conditions that encourage moisture-laden air to rise influence the patterns of precipitation around the globe. When moisture-bearing air is forced to rise as it passes over mountain ranges, the air cools, and the moisture condenses to produce rainfall (Figure 1.28). This process, known as orographic rainfall, is most common in coastal areas where wind blows moist air from above the ocean onto the land and up the side of a coastal mountain range. Most of the moisture falls as rain as the cooling air rises along the coastal side of the range. On the inland side, the descending air warms and ceases to drop its moisture. The drier side of a mountain range is said to be in the rain shadow. Rain shadows may extend for hundreds of miles across the interiors of continents, as they do on the Mexican Plateau, or east of California’s Pacific coastal ranges, or north of the Himalayas of Eurasia.

A central aspect of Earth’s climate is the “rain belt” that exists in equatorial areas. Near the equator, moisture-laden tropical air is heated by the strong sunlight and rises to the point where it releases its moisture as rain. Neighboring nonequatorial areas also receive some of this moisture when seasonally shifting winds move the rain belt north and south of the equator. The huge downpours of the Asian summer monsoon are an example.

In the summer monsoon season, the Eurasian continental landmass heats up, causing the overlying air to expand, become less dense, and rise. The somewhat cooler, yet moist, air of the Indian Ocean is drawn inland. The effect is so powerful that the equatorial rain belt is sucked onto the land (see Figure 8.4, Winter and Summer Monsoons). The result is tremendous, sometimes catastrophic, summer rains throughout virtually all of South and Southeast Asia and much of coastal and interior East Asia. The reverse happens in the winter as similar forces pull the equatorial rain belt south during the Southern Hemisphere’s summer.

Much of the moisture that falls on North America and Eurasia is frontal precipitation caused by the interaction of large air masses of different temperatures and densities. These masses develop when air stays over a particular area long enough to take on the temperature of the land or sea beneath it. Often when we listen to a weather forecast, we hear about warm fronts or cold fronts. A front is the zone where warm and cold air masses come into contact, and it is always named after the air mass whose leading edge is moving into an area. At a front, the warm air tends to rise over the cold air, carrying warm clouds to a higher, cooler altitude. Rain or snow may follow. Much of the rain that falls along the outer edges of a hurricane is the result of frontal precipitation.

Climate Regions

Geographers have several systems for classifying the world’s climates. The systems are based on the patterns of temperature and precipitation just described. This book uses a modification of the widely known Köppen classification system, which divides the world into several types of climate regions, labeled A through E on the climate map in Figure 1.29. As you look at the regions on this map, examine the photos, and read the accompanying climate descriptions, the importance of climate to vegetation becomes evident. Each regional chapter includes a climate map; when reading these maps in any chapter, refer to the written descriptions in Figure 1.29 as necessary. Keep in mind that the sharp boundaries shown on climate maps are in reality much more gradual transitions.