Working through this chapter of the study guide will enable you to:

1. Understand the general processes of erosion, including weathering and mass wasting.
2. Name and describe the basic types of erosion processes that wear down the surface of Earth.
3. Understand how running water, glaciers, and wind transport erosion debris over long distances, thus dramatically changing the surface features of our planet.
4. Explain how the hydrologic cycle affects Earth's freshwater supply, and how this water can be retrieved and used by humans.
5. Discuss the general erosion patterns produced by water, wind, and waves.
6. Describe the topography of the submerged portion of Earth's crust known as the seafloor.

Discussion


Section  23.1Weathering

Weathering is the physical disintegration or chemical decomposition of rock at or near Earth's surface. Physical weathering occurs when pressure causes rock to fracture and break into smaller pieces. Internal stresses during the formation of rock often cause cracks or crevices to form. Such joints allow water to penetrate into the rock, after which freezing may occur that can result in the buildup of pressure and the eventual splitting of the rock. In areas where the temperature regularly drops below freezing, ice can often form. As water freezes it expands, and strong pressure is exerted on the surrounding rock. This internal pressure may force the rock to break apart in a process known as frost wedging. This pressure can lead to fracture and fragmentation, reducing the size of rock formations until the pieces can be carried away by wind or running water.

Plants and animals are not a major factor in weathering, but they do play some role. The roots of plants can invade and grow in rock joints, causing further fracturing of the rock. Burrowing animals such as earthworms loosen soil and bring it to the surface where moisture and air are mixed with it, facilitating chemical weathering processes. Human activities such as digging and plowing produce the same effect. All physical weathering processes leave the fragmented rock with the same chemical composition that it had before it decomposed. That is not always true, however, for chemical weathering.

Chemical weathering involves chemical changes in the composition of rock. Such changes are enhanced by heat and moisture, so chemical weathering is most prevalent in hot, moist climates. Acidic solutions such as carbonic acid are the primary agents in most chemical weathering. Carbonic acid is formed when rain or groundwater combines with carbon dioxide from the atmosphere or from plant decay. This acid reacts readily with limestone, converting the rock into calcium bicarbonate, which in turn can be dissolved and carried away by water in solution. The overall result is extensive surface weathering or the formation of large underground caverns. When large-scale underground chemical weathering has occurred, the ceilings of huge underground caverns may sometimes collapse, producing deep sinkholes.


Section  23.2Erosion

Erosion refers to any downslope movement of surface or near-surface material, together with the processes that cause such movement, under the influence of gravity. Running water, glacial ice, and wind are responsible for the erosion of tremendous amounts of material each year. Running water refers primarily to streams and rivers that not only cause erosion but also transport eroded material over long distances.

Streamflow occurs between well-defined banks. The eroded material carried by the flow of water in streams is called stream load. Stream load is classified into three types: dissolved load, suspended load, and bed load. Bed load consists of large rocks and particles that are bounced or rolled downstream by the current. Bed load can be worn finer and finer during transport and may eventually become part of the suspended load. Suspended load is made up of fine particles of clay, soil, and organic material that are not heavy enough to sink to the bottom of the stream bed. Dissolved load consists of water-soluble material that is carried along by the water once it has been dissolved.

The movement of water through a streambed or riverbed also causes erosion. Young rivers have deep V-shaped valleys that show rapid down-cutting erosion. Such erosion can only occur when there is a rapid vertical drop in elevation such as that common in young mountains or rugged highlands. When a river flows over flat lands, such as on lower slopes and wide plains, it begins to twist and turn as it erodes material from one bank and deposits part of its load on the opposite side, producing looplike bends in the river channel called meanders. As they become more prevalent, the twisting may become so pronounced that the river can cut between two loops, forming a new, straighter course and leaving a water-filled meander loop called an oxbow lake. The river eventually tends to broaden out, forming a wide flood plain. The accumulated suspended and bed loads collect at its mouth, where a large, wide land area called a delta is often formed.

Large masses of ice that have formed on Earth's surface are called glaciers. They occur in almost all cold, mountainous areas, so the number of glaciers is much greater than you might think. Today there are well over 1000 glaciers in the United States alone. In some regions more snow falls each year than eventually melts. Glaciers form when many years of accumulated snowfall are compressed by the overlying weight of new snow, producing thick layers of ice. Glaciers generally form high up in the mountains in sloping valleys. As the ice accumulates, it begins a slow downhill movement. If a glacier forms in a flat region, its accumulated weight forces it to flow outward from its center.

As large ice sheets move over Earth's surface, they loosen and carry away material, producing U-shaped valleys as they descend from the mountains. Material moved along by a glacier is ground fine by abrasion, but before this can happen, large boulders are sometimes scraped across the bedrock, causing deep grooves that are characteristic of glacial movement. As glaciers spread out and melt, they leave sediment deposits called drift. When such a process is not accompanied by the transport action of meltwater, the deposited material is referred to as till. Till deposits are not sorted or layered, as is the case in meltwater runoff or stream erosion. Large deposits of till found near the end or sides of a glacier form extensive ridges of debris known as moraines.

Wind and waves also contribute to the erosion of Earth's surface. Wind can transport light dust particles over great distances, but this process is restricted to much shorter paths for heavier sand and soil. Dust storms can be very violent and destructive, and the end results of the movement of heavier airborne erosion debris are sand dunes and the sand blasting of local stationary surfaces. Another important erosion process is wave action. Waves work to break down lake and ocean shores in slow but relentless pounding that can erode away even the hardest rock, given enough time.

In mass wasting the driving force is primarily gravity. The loose rock and soil on high elevations is known as overburden, and it is pulled downslope toward lower levels. Landslides can be rapid, as in the processes known as rockslides and mudflows, or slower, as in a persistent process like slumping. The slowest of all mass wasting processes occurs when particle-by-particle movement progresses in a systematic way over years of time in a type of erosion called creep. Creep is generally imperceptible as it moves weathered debris slowly down a steep slope.


Section23.3 Groundwater

The water cycle redistributes Earth's supply of freshwater and saltwater over the planet's surface. About 98% of all water on Earth is considered saltwater. Of the 2% freshwater that is available, most of it is frozen in glaciers. The freshwater that is readily available for use to sustain life is a very precious commodity, especially in some arid and semiarid climates where groundwater is either very deep underground or is not available at all. Any rain that falls in such regions evaporates quickly, leaving almost no surface water to cause erosion and sediment transport.

The hydrologic cycle involves the evaporation of water from large bodies of water into vapor that can form clouds and finally rain. A great amount of rain occurs in the highlands, so this water must flow back to the lakes and oceans, producing erosion in the process. A large portion of this freshwater seeps into porous rock and makes up the groundwater that, except for glaciers, serves as our greatest reservoir of freshwater. The volume of porous rock filled with water is known as the zone of saturation. The upper boundary of this zone is called the water table.

Groundwater supplies much of the usable water for human consumption and is thus a very valuable commodity. In many places man is extracting groundwater from underground aquifers faster than it can be replaced by natural processes like rain or snowfall. It is also quite easy to contaminate our useable groundwater supplies with sewage, industrial waste, fertilizers, etc., and it is very important that care be taken to preserve this vital commodity. Although water is considered a renewable resource, it cannot be treated as inexhaustible and must be a high priority in future pollution control and conservation activities.


Section  23.4The Shoreline and Seafloor Topography

About 70% of Earth's surface is covered by oceans that average nearly 4 km in depth. The water in the oceans is constantly in motion. Three primary motions associated with ocean waters are surface waves, long-shore currents, and tides. The transport of large quantities of seawater over great distances is responsible for keeping the oceans mixed and fairly uniform in mineral content.

The ocean floor is far from flat, as was supposed for many years. Exploration with sonar, deep-ocean submarines, and depth probes has shown a complex system of underwater volcanoes known as seamounts and deep seafloor trenches. There are actually some large flat areas of seafloor, called abyssal plains, on which layers of sediment are continually collecting. This sediment is consolidated into sedimentary rock that later may be forced to the surface by various other geologic processes to produce fold mountains. Other important underwater areas are the edges of the continental plates known as continental shelves. These shallow ocean regions make up about 5% of Earth's entire surface. The transitions between continental shelves and the deep-ocean abyssal plains are referred to as continental slopes.

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