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

1. Indicate how plate tectonics has been able to explain continental drift and seafloor spreading, volcanic activity, earthquakes, mountain building, and many other geologic processes that have shaped the surface of our planet.
2. Explain the causes of earthquakes, how they are detected and recorded, and the scales used to indicate their strengths.
3. Describe the overall internal structure of Earth.
4. Define isostasy and tell how this concept can explain the variations in elevation between mountains and the seafloor.
5. Describe how existing rock strata can be deformed, and what tell types of formations are created by the application of compressive forces within Earth's lithospheric plate system.
6. Explain how the various types of mountains are built and give examples of each type.

Discussion

Although the processes are often very subtle, the surface of Earth is constantly changing. No comprehensive explanation of these changes had been found until early in the 20th century when the ideas of continental drift and oceanic seafloor spreading were formulated. These processes were the keys to the development of an overall theory of plate tectonics that has been able to account for many widespread and diverse geological phenomena.

One result of crustal movement is the occurrence of earthquakes. These tremors of our planet's crust happen quite frequently, but fortunately most of these disturbances are too weak to even be noticed. Occasionally, however, large-scale earthquakes occur that produce enormous amounts of damage and even loss of life. These destructive earthquakes are naturally of great interest and concern to the population of our planet. Most earthquakes are caused by the interaction of huge lithospheric plates as they move across the surface of Earth.

The motion of plates in the lithosphere produces a dynamic system in which these massive slabs of rock must interact with each other at plate boundaries. In some areas, these plates converge, while in others they diverge or simply slide past one another. Especially at convergent regions, the plate boundaries can buckle and fracture under the tremendous pressures created as the plates are forced together. This action can lead to massive uplifting and folding that is the basis for mountain ranges and other geologic surface features that characterize Earth's crust. Several types of faulting can occur in these processes. Fault-block mountains and fold mountains are the direct result of such plate boundary interactions. The formation of the third type, volcanic mountains, has already been explained in Chapter 21.


Section  22.1Continental Drift and Seafloor Spreading

Geologic studies have suggested for many years that the continents tend to move across Earth's surface. The first indication of this was the surprising jigsaw-puzzle-like fit of the Atlantic coasts of Africa and North and South America. Additional evidence was found by a German meteorologist named Alfred Wegener in several areas, for example, similarities in biological species and fossils found on the various continents suggested that they were once physically connected. Also, the continuity of geologic structures such as mountain ranges, and glaciation in the Southern Hemisphere further suggested that the continents were once connected as a single unit in the past. To account for his observations Wegener proposed that a giant supercontinent once existed that he called Pangaea. According to his theory, this supercontinent slowly broke apart and formed the continents that moved across Earth's surface into their present positions.

Wegener's theory of continental drift was not generally accepted until 1960, when an American geologist named Harry Hess proposed a mechanism called seafloor spreading that could account for the movement of the continents. The basic idea behind seafloor spreading is that convection cells bring new molten magma to the surface of the ocean floor, where it forms into mid-oceanic ridges and forces the continents slowly apart. Deep-sea drilling for core samples of the crust across these ocean ridges eventually provided conclusive proof of this theory when the samples showed younger, thinner sediment layers near these mid-oceanic ridges and older, thicker layers at ever increasing distances on either side. Symmetrical strips of magnetic anomalies on either side of these ridges were also discovered about this time, which helped strengthen the argument. These strips show remanent magnetism that was locked into the new seafloor as it cooled. This new seafloor was then forced slowly apart by new up-welling magma, producing the symmetrical magnetic patterns.


Section  22.2Plate Tectonics

The theory of plate tectonics is an extension of the ideas of seafloor spreading and continental drift. About 20 surface plates have been identified as making up the lithospheric surface of Earth. These plates are driven about by convection cells in the asthenosphere. Some are pulling apart, forming divergent boundaries, whereas other are pushed together at convergent boundaries. A third type of relative motion is also possible where plates slide past one another, creating transform boundaries. Plates are considered to be passive parts of the lithosphere that are driven across Earth's surface by deep thermal convection processes in the dynamic asthenosphere.

If objects of various densities are placed in an even denser liquid, the densest objects will float lowest in the liquid. This means that more of their volume will be submerged and less will protrude above the liquid's surface. Low-density objects will float higher, like a cork on the surface of water. The mantle of Earth is not actually liquid, but the asthenosphere is relatively plastic and so can make structural adjustments. The pliable nature of the asthenosphere allows segments of the lithosphere to "float" on this plastic sublayer.

The ability of Earth's crust (technically, the lithosphere) to float on the asthenosphere is the basis for an important geologic concept known as isostasy. Isostasy explains variations in surface heights by assuming that since the rocks that make up continents and ocean basins have different densities, these areas of Earth's crust will float at different elevations. The average density of continental rock is less than that of the ocean basins and both float on the same plastic sublayer, so the continents must float higher because of their lower average density.

Differences in density account for the elevation variations between continental masses and ocean basins, but they do not explain why there are mountains and lowlands on the continents themselves. The key to understanding this difference is in the thickness of the continental crust. In regions where the continental crust is thin, it floats low on the asthenosphere. Plains or lowlands are found in such regions. If the crust is thicker, more of the continental mass will float below the average level of the molten sublayer, but because of the relative thickness more will also protrude above this level. Uplands and even towering mountains can thus exist in areas where the crust is exceptionally thick. Again the concept of isostasy comes into play, and the total surface structure of Earth is in equilibrium. In this case, we can say that the entire crustal system of Earth is in isostatic balance.

Plate boundaries are instrumental in producing changes in Earth's crust. Where plates pull apart, new crust is continually forming, as in the mid-oceanic ridges discussed earlier. If plates are pushed together, three types of activity can occur.

1. If two oceanic plates are driven together, one plate is forced under the other, causing a subduction zone. In this region the lower plate is forced downward into the hot mantle where it melts, releasing gases and low-density magma that work their way to the surface causing volcanoes. This process explains most of the volcanic activity around the Pacific basin Ring of Fire.
2. Where an oceanic plate is pushed into a continental plate, the denser oceanic plate is forced under the lighter continental plate, forming trenches along the coasts of the continents. Large igneous intrusions and volcanic activity are also produced that can build huge mountain ranges such as the Andes in South America.
3. The collision of two continental plates leads to crumpling and buckling of the leading edges of both plates, which produces fold mountains. An example is the formation of the Alps.

Another interaction that can take place between plates is the transform type. As two plates slide past each other along fault lines, large amounts of stored potential energy can build up along their boundary. Such energy eventually must be released, usually in the form of earthquakes, some of which can be very violent. The San Andreas Fault in southern California is an example of a transform boundary. The sudden movement of the plates along this fault results in the initial release of large amounts of stored energy, which can cause major earthquakes. Continued adjustment of the plate boundaries can then lead to additional aftershocks that can often continue for some time after the initial earthquake event.


Section  22.3Earthquakes and Earth's Interior

It is possible to study Earth's crust and even its interior using the wave energy produced by earthquakes. This study is called seismology. Most earthquakes are related to movements of Earth's crust, although some occur as the result of violent volcanic eruptions. Crustal movements happen along large fracture lines known as faults. Rocks along fault lines can be stressed because of crustal shifting, and potential energy can be stored in the elastic rock itself. When this stored energy exceeds the fracture point of the surrounding rock, the crust can snap suddenly back into an unstressed state, producing an earthquake.

The center of an earthquake, no matter where it occurs inside Earth, is called the focus. The point on the surface of Earth directly above the focus is known as the epicenter. The locations of both the focus and epicenter of an earthquake can be determined precisely by analyzing seismic waves that travel across Earth's surface (surface waves) and those that travel through the interior of Earth (body waves). Using the amplitude of seismic waves, it is possible to estimate the amount of energy released by an earthquake. The severity of an earthquake can be measured on either the Richter scale or the Mercalli scale.

Study of seismic waves has led to a better understanding of Earth's internal construction. Four identifiable concentric shells exist: the solid, metallic inner core; the liquid, metallic outer core; the dense, rocky mantle; and the thin, rocky, outer layer called the crust. However, if the interior of Earth is studied in terms of its behavior rather than its composition, its structure is quite different. In this classification the first and outermost layer is called the lithosphere, which has a thickness of about 70 km and includes all of the crust plus the upper part of the mantle. The lithosphere is rigid and brittle, and so is relatively resistant to deformation. Faults and earthquakes occur in this layer. The second layer, the asthenosphere, extends downward from the bottom of the lithosphere to a depth of about 700 km. The asthenosphere plays an essential role in continental drift and seafloor spreading because it moves heat from the inner core of Earth toward the surface by thermal convection. The asthenosphere is plastic and mobile enough to allow slow but powerful convection cells to form, and these cells provide the driving forces necessary to produce continental drift and seafloor spreading.


Section  22.4Crustal Deformation and Mountain Building

Processes by which the major features of Earth's crust are formed and changed because of the relative movement of lithospheric plates results in the deformation of rocks. These processes may take place suddenly, producing earthquakes and other easily observed changes, or they may be extremely slow and subtle. Even though not readily noticed, slow changes often have a greater long-term impact on the structural features of Earth than do rapid small-scale ones.

Two slow processes that produce tremendous changes in Earth's crustal features are folding and faulting. When extreme horizontal pressure is applied, rocks eventually reach their elastic limit and begin to buckle and fold. This process produces wavy-looking geologic formations that consist of alternating "arches" and "troughs" known as anticlines and synclines, respectively. Faults are fractures in the crust caused by the shifting of surface plates. The cause of such motion is the slow convection of plastic rock within the asthenosphere. Such action can be vertical, which produces upthrusts, or horizontal, which leads to compression or stretching of the crust itself. Faults can be classified as normal, reverse, or strike-slip faults. Strike-slip faults are basically the same as the transform faults that were discussed earlier in this chapter.

Mountains differ greatly in size as well as basic structure. Single, isolated mountains can be formed by volcanic action; mountains can also be produced in mountain ranges. Mountains can generally be classified by their characteristic features as volcanic mountains, fault-block mountains, or fold mountains. The basic theory behind mountain building involves the process of plate tectonics, which gives us a good explanation for the hot spots, the upthrusts, and the folds that produce major mountain formations.

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