UNIT 3 THE LIVING MACHINE:

By the turn of the century, geologists had come to regard the major surface features of the Earth, especially the continents and ocean basins, as fixed and permanent. Today we find that the entire surface of the Earth is mobile, with continents moving about, breaking up and reforming, and ocean floor being created and destroyed. We now know that the ocean floor is far younger than most of the continents. The interactions between moving blocks of the Earth's surface, called plates, accounts for many of the dominant geological processes, such as earthquakes, volcanoes, and mountain building. This new view of the Earth is described by the theory of plate tectonics, which we shall explore in this unit.

In 1915, Alfred Wegener published a book entitled The Origin of Continents and Oceans that shocked the scientific world. In it, he advanced the startling hypothesis that the continents of the world were actually moving and that at one time the Atlantic Ocean did not exist (Figure 3-1).

Wegener's hypothesis brought down a storm of controversy because it challenged established scientific wisdom. The prevailing view of the Earth was based on continuity of geological processes and on the permanence of major surface features such as continents and ocean basins. Wegener's concept of drifting continents was a radical departure in thought for the geological community.

In spite of this, the idea was not simply dismissed as a crackpot notion. Though not a geologist, Wegener was an established scientist, a German meteorologist and polar explorer. Furthermore, he had done his homework. He marshaled evidence in support of his idea from a wide variety of sources, the bulk of which was drawn from publications of respected geologists.

The story of the development of his idea during the next half century is one of the most fascinating chapters in the history of science. Far from ignoring Wegener, geologists of the world hotly debated the notion of continental drift for the next eleven years.

That the debate continued at all shows us that Wegener had many supporters as well as detractors among geologists. Paleontologists (geologists who study the fossilized remains of ancient organisms) found much to admire in Wegener's handiwork. They had long wondered why so many fossils of land or shallow-water organisms were found in identical form in widely separated continents such as South America and Africa. The intervening deep ocean should have formed an insurmountable barrier to migration for such species.

As an example, consider the relatives of the monkey-like lemur which can be found living in trees in Madagascar and India. The lemur cannot swim any great distance, so how could its ancestors have migrated thousands of kilometers from the island of Madagascar to the Asian continent?

Wegener had a ready answer: in the Carboniferous period 300 million years ago, India was attached, along with Madagascar, to the east coast of Africa. They were part and parcel of a huge supercontinent that, he claimed, existed in the Carboniferous and Permian Periods. This supercontinent has since been given the name Pangea, meaning "all lands." Thus, the lemur could be found in widely separated localities because its original homeland was subsequently split. One piece (Madagascar) remained close to its original position near Africa, but the other (India) traveled to the north at an average rate of some 5 cm (2 inches) per year. Fossil associations between Africa and South America were explained similarly.

Other scientists found much to doubt in this scenario. Some geophysicists pointed out that continents are made up of low density rocks like granite, while the ocean floors are composed of a harder, denser rock called basalt. So how could the softer rocks of the continent plow through the hard rocks of the ocean floor without the continent breaking up and becoming deformed in the process?

There continued to be many other arguments, pro and con. In the middle 1920s, two important conferences were held -- one in London, the other in New York. Wegener was invited to defend his ideas and did so with great persuasion.

Even so, at the end of the London conference, two of his critics expressed what seemed to be the majority view:

After the American conference, which went even more poorly for Wegener, the idea of drifting continents passed into an oblivion that was to last for over 25 years.

The story of how the hypothesis of continental drift was revived and reworked into a new theory of plate tectonics is a long one -- too long for us to treat in detail here. However, the story is related in three fine books: Walter Sullivan's Continents in Motion, Ursula Marvin's Continental Drift: The Evolution of a Concept, and William Glen's The Road to Jaramillo. While interesting as history, these books also serve as fascinating guides to the workings of science.

Our approach will take a different route. First, we shall examine the essential concepts of plate tectonic theory as it currently is understood. Then, at the end of this unit, we shall return briefly to the historical perspective and try to answer the question: Did the scientific community blunder in rejecting Wegener's hypothesis?

What is meant by the name of the theory: plate tectonics? The word tectonics shares a common root with the word architecture -- it relates to processes of construction. In geology, the word has a further connotation implying motion of huge blocks of rock. The name, then, simply states that the surface of the Earth is constructed of a series of moving plates.

We may make an analogy to a piece of an eggshell, in that it is rigid and brittle, and its thickness is small compared to its breadth. Because the Earth is spherical, a plate that made up part of its surface would have to share its spherical shape.

Let us take the latter analogy a step further. Consider a hard-boiled egg that has been dropped onto the floor. Its shell has cracked into a number of pieces. The cracks surround each plate of eggshell and define its boundaries.

Each eggshell plate is still as rigid as the original whole, but now the existence of the cracks allows each piece to move slightly in relation to its neighbor. Hence, the eggshell has gained a degree of mobility that it did not have before it cracked. In the egg, each fragment of shell can only move a fraction of a millimeter before it is stopped by its neighbors. If we want to move a piece of eggshell a significant distance, it would be necessary to overlap it with other eggshells in some places and to leave gaps between pieces in other places.

Earth plates move in a similar manner, and as a result they can move very long distances. Notice that so far we have referred only to the motion of plates on the Earth rather than to the motion of continents. This is the critical difference between modern plate tectonics and Wegener's continental drift. The plates may contain continents, or ocean floor, or both.

The most intense geological activity tends to occur around the perimeter of the plates, at the boundaries where one plate interacts with its neighbors. Where the edges of the plates grind together, tear apart, or slip past one another, they produce earthquakes.

On a map of the world the locations of earthquakes during a recent seven-year period outline the plate boundaries for us.

Figure 3-3 diagrams the major plates of the world. Compare the two maps and note how the boundaries are clearly shown by the earthquakes in the oceans, but are often broad or indistinct on the continents.

Now consider the North American Plate. It contains not only the North American continent, but also the western half of the North Atlantic Ocean floor. The western boundary of the plate is the San Andreas fault in California; the eastern boundary is the crest of the Mid-Atlantic Ridge, a huge undersea mountain chain that bisects the Atlantic from north to south.

The shift in emphasis from continents to plates is an important one. It resolves one of the principal difficulties of Wegener's hypothesis. In the plate tectonics model, the continents do not move through a stationary ocean floor: the ocean floors are moving also.

The way these plates move, and how they interact with one another at their boundaries, will provide us with a model for understanding many of the geological processes that occur on the Earth's surface. This is a theme that will run throughout much of the rest of this course.

Plate tectonics has proved to be a unifying principle for the geological sciences: a kind of megatheory that philosophers call a paradigm. With its introduction, we have gained a conceptual tool of immense power and a whole new way of looking at Planet Earth.

How is it possible for the plates to move at all? They are, after all, made of solid rock. Let us examine in some detail the uppermost parts of the Earth.

To begin with, earthquakes make waves -- vibrations similar to sound waves that can travel through the deep parts of the Earth. The speed with which these waves travel through different rock layers can tell us much about the density and rigidity of each layer. The study of these waves is called seismology.

Seismologists have noted that in the depth range of 100 to about 300 km (60 to 190 mi) beneath the surface, earthquake waves tend to travel more slowly than at either shallower or greater depths and they also lose their strength more quickly in that zone. For this reason it is called the low velocity zone. The physical properties of the rock in this zone indicate that it is solid but soft -- rather like asphalt on a hot day.

Many materials have this property of becoming soft at elevated temperatures, even though technically they remain classified as solids. The effect is particularly noticeable at temperatures that approach, but do not actually reach, the material's melting temperature. At cold temperatures, the same material can be quite rigid and brittle. A stick of butter that has been in the freezer is very hard, while one that has been sitting at room temperature for several hours can be spread easily with a knife. Rocks behave in a somewhat similar manner. It is believed that rocks in the low velocity zone are very near their melting temperature and that this accounts for their soft or weak character. The soft zone probably extends somewhat deeper than the low velocity zone, and this expanded region is referred to by the name asthenosphere, (derived from the Greek for "weak zone").

The upper 100 km of the Earth behaves in a much more rigid manner because there the temperature is far below the melting point. This zone is called the lithosphere. The name is also derived from the Greek and refers to the more familiar rock-like behavior of this region. It is the lithosphere that makes up our rigid plates, and we shall refer to them as lithospheric plates.

Lithospheric plates can move as coherent pieces because they can slide about on the mushy underlying asthenosphere. Taking the egg analogy another step further, we can liken the asthenosphere to the slippery layer below the shell that enables us to peel it from the hard-boiled white.

The proportions of the eggshell are similar to those of the lithosphere. The radius of the Earth is 6371 km (3959 mi), and so the lithosphere extends less than one-sixtieth of the way to the center of the Earth. Lithosphere and asthenosphere together constitute only the top ten percent of the Earth.

The division of the upper parts of the Earth into lithosphere and asthenosphere is based on the physical property of rigidity. The same kind of rock could occur in both regions and yet, because of differences in temperature, be more rigid in one than in the other.

The lithospheric plates are themselves layered; this time based not on temperature differences but on different types of rock materials. On this basis, we can divide the lithosphere into crust and a portion of what is known as the mantle (see Figure 3-4).

The crust is made up of all the surface rocks familiar to geologists: granite, basalt, limestone, and so on. The mantle is composed of ultramafic rocky materials that are denser than the crust. It is separated from the crust by a sharp boundary called the Mohorovicic discontinuity, or Moho, for short. Look at Figure 3-4 and try to recognize that the division into crust and mantle and into lithosphere and asthenosphere are two very different ways of describing the uppermost layers of the Earth. The mantle differs from the crust in having a higher density (about 3.3 grams per cubic centimeter -- g/cc). The average density of the crust is about 2.7 g/cc. Compare this to the density of water -- 1.0 g/cc.

The lithosphere includes both crust and the uppermost mantle (in the region where it is cool and quite rigid), while the asthenosphere includes that portion of the mantle in the depth range of roughly 100 km (60 mi) to 640 km (400 mi) where the mantle is near its melting point and rather soft or mushy. The boundary between lithosphere and asthenosphere is not at all sharp. Indeed, the asthenosphere is hard to detect, and may not even exist in some regions. It often is identified most clearly beneath the oceans.

Figure 3-4 also shows that the crust varies greatly in thickness. Beneath the oceans, the crust is only about 5 km (3 mi) thick, and composed mostly of basalt. Beneath the continents, the crustal thickness averages about 35 km (22 mi), increasing to 70 or 80 km (44 or 50 mi) beneath the largest mountain ranges, such as the Himalayas. The shape of the Moho is an inverted and exaggerated version of the surface topography.

These large changes in the depth of the Moho are readily understood once we recognize that the crust is less dense than the mantle. As a result, the crust has buoyancy, and literally "floats" on the denser mantle.

HM 3-2 (A-B): Images of bouyancy and isostasy.

A) Isostasy (meaning literally "to stand above") relates the difference in density between the ice and surrounding water and the amount of water displaced by ice beneath the surface of the water to a bouyancy force which supports a certain height of ice above the surface of the water. Because of the relatively small difference in density between frozen and liquid water, the iceberg shown below extends much further below the surface of the water than above. (Iceberg in Wohlstenholme Fjord, Dike Mountain in background. Northeast of Thule Air Base. Nunatarssuaq region, Greenland, images from USGS)

B) Isostasy explains why the continents have a different average elevation than the surrounding oceanic basins. Isostasy also predicts that regions of elevated topography (such as Mt. Cook in New Zealand shown below) must be supported by a deep crustal "root" extending into the mantle. Seismic studies have confirmed such continental root zones beneath Planet Earth's major mountain ranges.

Figure 3-5 shows how this works using as an example blocks of ice floating in water. An ice cube floats in tap water such that 10% of its mass protrudes above the water surface and 90% of its mass is submerged. As a result, the bottom of a thick block will float at a greater depth than that of a thin block. Also, the top of a thick block will stand at a higher elevation than the top of a thin block.

If this still seems obscure, you can try a simple experiment by floating different size cubes or chunks of ice in water. The tallest chunks will stick out of the water the most, and they will also reach to the greatest depth. For the case of a mountain, the buoyant "root" beneath it is approximately 4-1/2 times the height of the mountain itself, as shown in Figure 3-4.

The concept of a solid block floating in a liquid is a familiar one; less familiar is the situation where a light solid "floats" in a denser solid. Such is the case for the crust and mantle. In order for this to work, the mantle must be able to flow somewhat in order to accommodate vertical motions of crustal blocks. Because of the high temperatures and pressures found in the mantle, it is reasonable to expect this kind of behavior provided that it takes place slowly, over a long period of time.

Density of the blocks is also a factor. The denser the block, the lower it will float in the mantle, just as ice, having a higher density than cork, will float lower in water than an equal size block of cork.

Isostasy is the principle that surfaces on the earth are generally at an equilibrium height. For example, presently most of the continental land mass is between zero and one kilometer above sea level. Isostasy comes from is- plus the Greek word stasia (standing), together meaning "to stand above". The word isostasy means this general equilibrium of the earth's crust.

An iceberg floating in the ocean is a familiar example of the principles of isostasy. An iceberg has a density of about 0.92 g/cc. Water in the arctic regions usually has a density of about 1.02 g/cc. The iceberg is less dense than the surrounding water and floats. Its height above the water can be calculated using Archimedes' principle of buoyancy. This principle of buoyancy states that an object immersed in a fluid is buoyed up by a force equal to the mass of fluid that it displaces. The overall Earth situation is shown in the second part of Figure 3-5. The oceanic crust is rich in the elements Fe and Mg. The continental crust is rich in the elements Al, Fe, Ca, Na, and K. This lateral variation in density suggests that the continents are somewhat like icebergs floating on the underlying mantle. Because they are less dense than the oceanic crust, and this crust consists mostly of basalt. The continents "float" at a height above that of the basalt. The waters in the Earth's oceans merely collect in the lowest regions; these are regions floored by the more dense basalt.

The concept of low density crust floating on high density mantle material, isostasy, at one stroke explains both the changing depth of the Moho and the major changes in elevation of the Earth's surface: the Earth's topography.

We can now make some progress toward answering a very important question: Why is the surface of the Earth divided into continents and ocean basins? We can see from Figure 3-4 that continents exist where the crust is thick, and that ocean basins exist where the crust is very thin. Actually, we have only rephrased the question: Why is the crust thick in some places and thin in others? Nonetheless, we have taken an important step toward understanding the origin of continents and oceans. Whenever we produce a thick crust composed of a light rock like granite, then we will have produced a continent.

According to this definition, the edge of a continent is where the crust changes from thick to thin. This is not necessarily the location of the seashore. In many cases, the water filling the ocean basins covers a part of the edge of the continent, called the continental shelf. The seashore is then substantially inland from the true edge of the continent.

This rather long preamble has been necessary to define the essential structure of the Earth's crust and lithospheric plates. We shall make frequent use of these concepts later on.

The plates are all interlocking, so when a plate moves, something has to give. The simple case shown in Figure 3-6 can help us discover how this works.

In this illustration, we consider only two flat plates. Plate A is a square cut from surrounding plate B. If we move it to the left in a rigid manner, that is, without distorting its shape, then we produce three distinctly different types of boundaries. We produce a gap on its right margin, an overlap on its left margin, and a side-by-side sliding motion on its top and bottom margins, where there is neither gap nor overlap.

If the concept is not clear, trace the top half of Figure 3-6 on a piece of paper, cut out plate A so it is loose from plate B, and slide it to the position shown in the bottom half of the figure. Watch what happens on all four boundaries.

Nature tends to behave in the same way. Plate margins may be classified according to three main types corresponding to the kinds of motions demonstrated by our simple model. They are spreading ridges or rifts, subduction zones, and transform faults. Spreading ridges occupy boundaries where two plates are pulling away or diverging from one another; subduction zones are found where two plates are converging with one plate diving beneath the other; and transform faults are characterized by a mostly horizontal side-by-side sliding motion where neither gap nor overlap are produced. In addition, transform faults connect to other plate boundaries at either end where the sliding motion abruptly changes to divergence at a spreading ridge or convergence at a subduction zone. It is the transformation of spreading motion to side-by-side motion at either end of a transform fault that gives this feature its name.

Our flat paper model is obviously an over-simplification of how plates move. Figure 3-7 is a cross section or profile view of the lithosphere and shows how spreading ridges and subduction zones work in the real world. To fully understand it, let us look more closely at each kind of plate boundary.

Spreading ridges or rifts occur where two plates are moving apart. The resultant gap is constantly filled with newly created crustal material. As it happens, the gap is always filled with oceanic crust, never with continental crust. We shall see why a little later on.

Figure 3-8 diagrams what happens at an oceanic spreading ridge. As the two plates move apart, cracks form in the rift valley at the summit of the ridge, and the magma from the hot, molten region below fills and seals the cracks. This is how new ocean crust is added to each plate. This process, called sea-floor spreading, is repeated indefinitely, with new cracks forming and more lava intruding and solidifying, ever widening the ocean floor.

Spreading oceanic ridges are found in all the major ocean basins of the world. These ridges are huge structures whose summits often stand two to three kilometers (7,000 to 10,000 feet) above the surrounding ocean floor. They are nearly all interconnected into one vast ridge system that stretches for some 65,000 km (40,000 mi) (see the spreading ridges in the world's oceans in Figure 3-3). Far broader than most mountain ranges, individual spreading ridges are typically 1,000 to 4,000 km (600 to 2,500 mi) wide at their base. Indeed the suboceanic ridge system constitutes the greatest mountain range on Earth.

The rift valley seen at the summit of the ridge in Figure 3-8 is often the site of volcanic activity. In recent decades, undersea technology has provided researchers the means to study this phenomenon. During the early 1970s, a joint French-American project investigated a section of the Mid-Atlantic Ridge south of the Azores. Dubbed FAMOUS, for French-American Mid-Ocean Undersea Study, the project sent manned research submersibles 3,000 meters (10,000 ft) below the sea's surface to visit the rift valley.

Among the bizarre features observed by the team of scientists at the sea floor were pillow basalts, which are bud-like rounded lava rocks about the size and shape of a pillow, caused by the extrusion of molten lava directly into cold ocean water.

A rift valley might seem like an unusual feature for a ridge top -- most continental mountains don't have them. But the oceanic ridges are not ordinary mountains. The rift valleys develop because the ridges are spreading and creating new oceanic crust.

A simple model demonstrates how the process works (Figure 3-9). A number of books are placed in the usual way on a shelf, but without bookends. The end books will tend to flop over, and if we try to insert an additional book into the center of the group, the configuration shown in the diagram will develop. Our model would be more realistic if the books were added from below rather than from above, but it does demonstrate a number of features shared by actual spreading ridges. Consider what happens when books tip to one side (Figure 3-10). As they tip over, they also slide against one another as shown by the pair of arrows.

If we replace our books with blocks of rock, then the sliding action is called faulting and the plane of contact between the two blocks is called a fault plane. It occurs in a spreading ridge as shown in Figure 3-10. The faults cut the ridge crest into a series of blocks arranged somewhat like our books, except that the amount of tipping is much smaller than is the case for the books. Because the oceanic crust continues for great distances on either side of the ridge, the crustal blocks never have room to flop over onto their sides.

Instead of inserting whole blocks, however, the crust is extended by the intrusion of magma into the faults from below. The magma cools in the widening faults and forms sheet-like layers of basalt called dikes. Because there are many faults and cracks over the source of magma , half of the newly-formed dikes spread out to the left and half to the right, adding equally to each plate.

Slippage along the faults usually is not smooth and steady, but tends to proceed in a series of jumps with intervening periods of quiet. This is due to the friction between the blocks of rock in the fault plane. Energy stored during a quiet period is released suddenly, creating an earthquake.

The map in Figure 3-2 shows that the earthquakes associated with the oceanic ridges form a very narrow band about the ridge crest. The faults are actively slipping only while they are very close to the rift valley, and they become dormant once they have traveled some distance from it. In Figure 3-2, the seismically active oceanic ridges in the middle of the Atlantic and Indian Oceans are especially evident.

The continuous pattern of shallow earthquakes along the oceanic ridges indicates that all of these ridges are actively spreading at the present time, constantly renewing the floor of the world's oceans.

The structure of the oceanic crust has been studied through seismology, actual drilling of the oceanic crust by the Deep-Sea Drilling Program, and the study of suites of rocks which are believed to have formed as oceanic crust and later have been pushed onto the edge of a continent at a subduction zone. From this evidence, the top of the oceanic crust is believed to consist of a layer (about 1 km thick) of pillow basalts atop a thicker layer of closely spaced sheets of solidified magma called dikes oriented similarly to the faults in Figure 3-10. In fact, the crustal blocks themselves are made up of the sheeted dikes, so new faults and dikes intrude older dikes that had formed earlier by the same process.

In addition to the rift valleys, the ocean ridges show another striking difference from normal mountains. Continental mountain ranges have isostatic roots -- the crust becomes thicker beneath them, providing the buoyancy necessary to support their weight (see Figure 3-4). Beneath the oceanic ridges, however, the crust becomes thinner (Figure 3-8). The height of the ridge is supported by the buoyancy, not of the crust but of the hot mantle material below it. Hot rock expands and becomes more buoyant and molten rock is lighter yet. This also explains why the oceanic plate created at the ridge subsides toward the deep ocean floor as it moves away from the ridge: the mantle beneath it is cooler and more dense, and the aging oceanic crust also cools, becoming more dense and sinking farther into the mantle.

Subduction zones are the second type of plate boundary. They occur where two plates are coming together and overlapping. Figure 3-7 shows the structure of a subduction zone where two lithospheric plates are converging. In this case, the plate on the left is overlapping the plate on the right, which is being pulled or forced down (subducted) into the asthenosphere.

Let us consider the case in which the subducted plate is oceanic. This process is very different from that at the spreading ridges. Here we are destroying, not creating, plate. Also, the spreading process is a very symmetric one, with new material being added equally to each plate. When oceanic plate is being subducted, the process is asymmetric, with only the oceanic plate being destroyed. Subduction zones are characterized by earthquakes within the subducting slab, down to depths as great as 600 km (370 mi). This contrasts with the case of spreading ridges, which are characterized by shallow earthquakes confined to depths of less than 70 km (44 mi).

Two important geological features result from subduction of ocean floor and are shown in Figure 3-7: trenches and andesitic volcanoes.

Oceanic trenches are significant features of the ocean floor. The deepest is the Mariana Trench in the southwestern Pacific, which reaches to 11,000 m (36,100 ft) below sea level. The trench floor lies some 5,500 m (18,000 ft) below the surrounding ocean floor. The trenches result from the tendency of the descending plate to drag the edge of the overriding plate downward. Look at Figure 3-7 and make sure you understand how this occurs.

HM 3-7 (A-E): Earthquakes and Seismicity.

A) Pacific plate subduction near New Zealand and the Mariana Trench

B) Mariana Trench (three dimensional view below begins just north of New Zealand) north is at the top of the view

C) Structure of the Pacific plate subduction zone and Japanese Island arc. A corner of the Eurasian continent is seen along the left-hand portion of this figure.

D) Earthquake epicenters 1960-1990. Plate boundaries shown in yellow.

E) Seismic epicenters-National Earthquake Information Center. (Click below to enlarge image for easier viewing).

A line of andesitic volcanoes is found parallel to the subsea trench, directly over the downgoing lithospheric slab. These volcanoes tend to erupt andesitic lava. If the topmost plate is also oceanic, then a volcanic island arc -- an arcuate chain of islands paralleling the trench -- is formed in the ocean. The Aleutian Islands stringing out to the southwest from Alaska is a good example of a volcanic island arc (see the map in Figure II-5 and note how they parallel the Aleutian trench).

If the topmost plate is continental, then a linear belt of volcanic mountains is formed at the continent's edge, as in the Andes of South America. Take another look at Figure 2-5 and note the relation between the Andes mountain chain, which runs along the west coast of South America, and the Peru-Chile Trench, paralleling it just offshore. In that figure, subduction zones are shown as curved lines drawn along the trenches, with black triangular "teeth." The teeth point in the direction of the downgoing lithospheric slab. In both the Aleutian and Andean subduction zones, note that the teeth point from the trench toward the andesitic volcanoes. Now go back to Figure 3-7 and make sure you understand the relation between the trench, the downgoing lithospheric slab, and the andesitic volcanoes.

This volcanism is different from that associated with the spreading ridges. As a consequence of greater viscosity and a higher content of dissolved gases in andesitic magma, subduction zone volcanoes tend to erupt much more explosively.

The violent eruptions of Krakatoa in Indonesia in 1883, of Mount St. Helens in the state of Washington in 1980, and of Mt. Pinatubo in the Philippine Islands in 1991 were examples of subduction zone volcanism. So were the eruptions of Mt. Pelée on the Caribbean island of Martinique in 1902, which totally destroyed the town of St. Pierre along with all 30,000 of its inhabitants, and of Mt. Vesuvius in Sicily, which wiped out Pompeii and Herculaneum in 79 A.D. and entombed those cities as a kind of time capsule, providing us today with a glimpse of a sample of Roman civilization, frozen at the moment that a subduction zone volcano chose to unleash its fiery death.

Contrast this with the 1973 eruption of Kirkjufell on the outskirts of the Icelandic fishing town of Vestmannaeyjar. Not only were the inhabitants spared, many remained in the town and valiantly defended their homes by sweeping heavy layers of volcanic ash from their roofs. They even tried to stem the flow of lava toward the heart of the village by spraying water onto advancing flows with fire hoses. In the end, much of the village escaped destruction.

Iceland sits astride the Atlantic spreading ridge, and its volcanism, like that of all oceanic ridges, is basaltic in nature. Basaltic magma is relatively fluid and low in volatiles and results in effusive lava fountains and wide-ranging lava flows, but seldom in the kind of cataclysm in which the whole top of a volcano might be blown away.

The structure of a subduction zone shown in Figure 3-7 indicates other features of interest. At the trench, the plate that is being subducted makes a quite abrupt bend and plunges downward into the asthenosphere. The downgoing oceanic crust rubs against the adjoining plate as shown by the coupled arrows in the diagram.

Slippage along the subduction zone and breakage of the subducting slab can give rise to violent earthquakes. In addition, other earthquakes may be generated where the brittle lithospheric slab is being bent near the trench and where it is breaking up as it descends into the asthenosphere. Because rock is a very poor conductor of heat, it takes a long time for the descending lithosphere to heat up, lose its relative rigidity, and be absorbed into the mantle. Once it becomes hot and soft, earthquakes can no longer occur within it. At high temperatures, instead of rupturing, it simply deforms.

Figure 3-11 diagrams the occurrence of earthquakes (shown as dots) beneath the Alaskan island arc. The earthquakes clearly indicate the downgoing slab, while the asthenosphere, shown as a shaded zone, is seismically quiet. Earthquakes occur only in rigid and brittle rock and cannot occur in the asthenosphere because of its soft and malleable nature. Earthquakes at a depth of more than 100 km are found only in subduction zones.

Further proof that the subducting slab is still fairly rigid at depth is shown by the effects of an earthquake occurring at a depth of 400 km (250 mi) within the slab beneath Japan. Earthquake waves travel via a path directly from the earthquake hypocenter, within the subducting oceanic slab, through the overlying asthenosphere and finally through the Japanese arc lithosphere to reach the west coast of Japan, while other earthquake waves travel via a longer path upwards through the subducting oceanic slab and then to the east coast of Japan. Even though path through the direct path through the asthenosphere to the west coast of Japan is much shorter than the path through the subducting slab, the earthquake was felt much more strongly on the east coast of Japan. The reason is that this path lies entirely within the rigid subducting slab and lithosphere, which conducts the waves with little loss of strength, while the path from the hypocenter directly upwards through the asthenosphere and lithosphere must traverse the asthenosphere above the slab, in which the seismic waves tend to lose strength rapidly due to the mushy nature of the asthenosphere.

A) Earthquake epicenters in the Cook Inlet region of southern Alaska

B) Structure of subduction zone with depth. North is to the left, depths are given in kilometers below sea level

C) Perspective figure of Pacific plate subduction beneath the Cook Inlet region of southern Alaska.

D) Earthquake epicenters 1960-1990 northern Pacific basin. The plate boundary between the North America and Eurasia plates is delineated by earthquake activity in Russia and shown by the yellow line.

E) Seismicity of Alaska-National Earthquake Information Center

F) Earthquakes in Alaska during June 1997(one month!) from the University of Alaska Geophysical Institute and the United States Geological Survey.

G) Northern Pacific subduction. Earthquake epicenters 0-100 km, yellow dots, Earthquake epicenters >100 km, red dots, Volcanic centers, purple dots.

One of the largest earthquakes to occur during this century was the Magnitude 8.6 (estimated by some to have been as large as 9.2) "Good Friday" earthquake which occurred March 28, 1964 near Anchorage, Alaska. This earthquake caused destruction over a region of more than 50,000 square miles and was felt over an area of half a million square miles. The largest city in Alaska, Anchorage, suffered terrible destruction. Much of the destruction in the Anchorage region was related to the liquefaction, in which a water soaked sediment when shaken losses strength. Additionally numerous landslides and settling within a 30 block region of the downtown region destroyed large portions of the city. A tsunami generated by the earthquake destroyed several coastal villages around the Gulf of Alaska, killing 107 people. About four hours after the earthquake, at Cresent City, in northern California tsunami waves smashed into the town's waterfront with 6.5 meter waves, killing 11 people and causing $7.5 million dollars in damages. Extensive movement during the earthquake caused resulted in permanent topographic changes of extensive portions of the southeastern Alaska coastline.

HM 3-8: Vertical offset (almost 10 feet) measured after Good Friday earthquake, 1964. Permanent topographic changes along extensive portions of the Alaskan coast resulted from this massive earthquake. Close- up view, looking northeast, of apportion of the Hanning Bay fault scarp. The face of the fault scarp is composed of bedrock except near the forest in the background, where the fault offset the pre-earthquake beach. (All images and annotated captions from USGS)

HM 3-9 (A-B): A) Downtown Anchorage, Alaska. More than 30 square blocks were destroyed. Collapse of Fourth Avenue near C Street, Anchorage, due to earthquake caused landslide. Before the earthquake, the sidewalk at left which is in the graben was at street level on the right. The graben subsided 11 feet in response to 14 feet of horizontal movement. This view, prior to clean up operations, is about 180 degrees from that shown in Alaska Earthquake 45ct. Anchorage district, Cook Inlet region, Alaska. 1964.

B) Shear related failure of a multilevel control tower at Anchorage International Airport. A back and forth shear motion will easily destroy otherwise strongly build structures, if strong, through-going structural elements are not present. Additionally local conditions can considerably vary the a) size of earthquake waves, b) the acceleration and deceleration of earthquake shaking, c) the length of time during which shaking occurs, and d) the strength of rock units upon which the structure is built. Within a relative small region, earthquake related effects can vary by a factor of more than 100. In Mexico City during the 8.1 earthquake, local conditions in some small regions results in 40 seconds of strong shaking, rather than the 5 to 10 seconds experience elsewhere.

HM 3-10 (A-C): Examples of slope failure triggered by the Good Friday Alaska earthquake.

A)Close-up of Government Hill elementary school, which was destroyed by the Government Hill earthquake generated landslide.

B) A close-up view of the compressional buckle, the ruptured fuel tank and the revetment at the foot of the landslide near the Alaska Native Hospital. Anchorage, Alaska.

C)The Turnagain Heights landslide in Anchorage, occurred along a steep bluff fronting Knik Arm of Cook Inlet. Its length, which is parallel to the bluff, was about 1.5 miles; its width was about .25 to .50 mile. This landslide reduced to rubble many of the finer homes of the city. Failure here, and in the "L" Street, Fourth Avenue, and Government Hill landslides in Anchorage occurred on horizontal or near horizontal slip surfaces in the Bootlegger Cove Clay, a marine silt of Pleistocene age. Anchorage Alaska.

HM 3-11 (A-C): A) The zone of fresh earth and landslide at the foot of this hillside on Montague Island marks the southwest trending Patton Bay fault, which was reactivated during the Good Friday Alaska earthquake. The northwest side of this vertical fault (on the left hand side of the photo was displaced upward as much as 8 feet with respect to the southeast side. There was, in addition, 9 feet of associated upwarping of the up-thrown (northwest) block, so that total vertical displacement across the entire fault zone was 17 feet. The view is to the northeast.

B) Tsunami damage, Kodiak, Kodiak Island, Alaska

C) Collapse of bridge caused by 1964 earthquake

In January 1995 a Magnitude 7.2 earthquake struck near Kobe, Japan. This city was located along a "seismic gap" or region within which earthquakes were rare (the only large previous earthquake occurring almost 50 years previously) and seismologists had suspected a significant seismic risk in this area. Buildings, freeways and port facilities were built to meet the most exacting standards in effect at the time and designed following special building codes enforced for this region. Despite expert earthquake preparedness the resulting quake destroyed some parts of the city. As had been observed following analysis of damage related to the Mexico City earthquake of September 1985, surprisingly earthquake shaking, acceleration, and length of shaking was found to vary significantly over relatively small regions of the city.

Seismologists have now confirmed this behavior of earthquake waves after analysis of damage following the Mexico City, San Francisco, Los Angeles and Kobe earthquakes. In each case the geological structure of a sedimentary basin and sedimentary rocks on which these cities have been build acts like a kind of lense for earthquake waves. The earthquake waves can become focused and defocused as well as trapped and "echoing" within certain regions of the basins. This results in significant changes in the size of earthquake waves, their peak acceleration or deacceleration, and the length of earthquake shaking. In addition this increased shaking, acceleration, and length of shaking can result in dangerous liquefaction, landslides and ground rupture, each deadly to nearby human structures.

Transform faults are long and relatively continuous where they cut across continents, but tend to appear as short discontinuous segments offsetting sections of spreading ridges on the ocean floor (see Figure 3-3). While subduction zones are often arcuate in shape, the combination of oceanic ridges and transform fault segments tends to form a rectilinear zigzag. It is still not clear just how this zigzag pattern is formed initially.

The transform fault is in many respects an astonishing feature of the Earth. It is simply a fault connecting two other kinds of active plate boundaries, but that is a deceptively simple definition. In the simple model of Figure 3-6, the transform faults connect the spreading ridge to the subduction zone. In the real world, however, the most common type offsets two spreading ridges. Let us examine this type in some detail.

Figure 3-12 is a working model of a transform fault that you can construct and study. Take the time now to cut and fold it and try some simple experiments with it. Notice that your model is very similar to small portions of the Mid-Atlantic Ridge between South America and Africa (Figure 3-3).

Begin with the model in its closed position. The two points marked A and B should be juxtaposed, and the gaps at the two spreading ridges should be closed tightly. Now open the model. Note how new plate is "created" at each spreading ridge and an equal area is added to each of plate A (the shaded plate) and plate B (unshaded). Also, notice that after spreading has occurred, the two ridges are no farther apart than before spreading. Verify this by measuring distance DD' with the model closed and distance EE' with the model open.

The model illustrates the fact that the shape of an oceanic ridge does not change with time. In Figure 3-3, the shape of the Mid-Atlantic Ridge between South America and Africa today remains an exact replica of the rift that initially tore the two continents asunder. The ridge has not altered its shape in over 150 million years of sea-floor spreading.

Now turn your attention to the fault itself. With the model closed, imagine yourself standing on point A, looking across the fault to the juxtaposed point A'. Slowly open the model, watching how point A' moves as seen from point A. If you are standing at point A, you will see point A' move to your right.

Turn the model upside down and repeat the process. If you are standing at point A', you will see point A moving to your right. Because of this independence of where you happen to stand, this particular transform fault is said to display right-lateral motion. That is, the motion is side-to-side and the other side of the fault always moves to your right.

Transform faults may also be left-lateral, if the ridges are offset in the opposite sense. To see this, turn the model page over to the other side and trace the positions of points A and A' on the reverse side of the paper. Holding the closed model so that you are looking at its reverse side, open it and note the motions of points A and A'. If you are standing at point A, you will see point A' move to your left.

The combination of ridge segments and transform faults forms a rectilinear zigzag pattern for oceanic plate boundaries that may be seen clearly in Figure 3-3. It is still not clear just how this zigzag pattern is formed initially, but because the pattern does not generally change shape with time, it must have come into existence at about the same time as the ridges themselves. The process by which this happens is still not fully understood.

Where transform faults cut across continents, however, they tend to be long and relatively continuous, with few, if any, spreading segments. The best known and most studied example of a continental transform fault is California's San Andreas.

In Figure 3-13, land and ocean floor to the west of the San Andreas Fault is part of the Pacific Plate and is moving to the northwest, parallel to the fault. To the east of the fault is the North American Plate. (You may also want to look at Figure 3-3.) Where the San Andreas Fault crosses the North American continent it is long and unbroken, but where it goes out to sea, it is cut into shorter segments separated by spreading ridges. Some of the ridge segments themselves are quite short, as in the Gulf of California.

Note that the ridge and fault geometry is similar to that of your paper model, for which right-lateral motion is expected along the fault. This is in fact what is observed along the length of the San Andreas.

Along the fault, the rocky edges of the plates grind against one another. In a few places, the slippage occurs smoothly. Here, any structure such as a fence or road that crosses the fault is offset at a rate of up to six centimeters (2-1/2 inches) per year. But in other places, the fault is jammed and does not move steadily.

As the plates continue their inexorable motion, the forces exerted on the pinned fault build up with each passing year. Finally the rock can stand no more and it breaks, unleashing the pent-up energy as strong vibrations of the ground: an earthquake.

Those vibrations are capable of shaking buildings to the ground, as San Franciscans discovered at 5:12 AM on April 18, 1906. The great earthquake, measuring 8.3 on the Richter scale, lasted less than two minutes. At the end of those two minutes, much of the city was rubble and in the next hour much of the rest was in flames. Seven hundred lives were lost.

The fault had ruptured along 430 km (270 mi) of its length, extending from San Juan Bautista in the south to Point Arena in the north. At places along the rupture, it was found that the ground west of the fault had lurched 4.3 meters (14 feet) northwestward in relation to the ground east of the fault.

Earthquakes associated with transform faults generally occur at very shallow depths -- less than 20 km (13 mi) deep. Because they occur so close to the surface, they often cause more damage then a deep earthquake of similar magnitude. The Anatolian Fault in Turkey is a transform fault that has claimed many thousands of lives during recorded history.

Perhaps the greatest impact of plate tectonic theory for most people has been upon our view of the occurrence of earthquakes. Long regarded as random acts of God that might not recur, many earthquakes are now seen to be recurrent events along plate boundaries. Indeed, a long respite from earthquake activity at a plate boundary is not a sign of safety -- it is a sign of danger. The longer the period of inactivity, the larger the forces that are building up, and the greater the earthquake that inevitably must come.

HM 3-16 (A-B): Earthquakes and Seismicity.

A) Loma Prieta (San Francisco) 1989 earthquake and after shock epicenters. The San Andreas fault is shown by the yellow line running through the region of earthquake activity. Other faults are also shown in yellow. The southern most portion of San Francisco bay is shown at the top of the figure

B) Earthquake activity associated with the Loma Prieta events projected onto the San Andreas fault. The vertical axis is depth in kilometers. The cross section runs along the San Andreas fault with northwest at the right, and southeast at the left of the figure below.

HM 3-17 (MPEG-Video): MPEG video of the three dimensional location of the main shock and aftershocks of the Northridge, California earthquake sequence January 18, 1994 (Magnitude 7.0). California Institute of Technology, Department of Geology and Planetary Science. Double click on the image below, your computer will then display this animation sequence.

HM 3-18: Location of recent earthquakes and aftershocks, Los Angeles region. (California Institute of Technology, Department of Geology and Planetary Science).

HM 3-19 (A-E): Damage related to the Loma Prieta Earthquake.
A) Aerial view of collapsed section of the Cypress viaduct of Interstate 880. Loma Prieta Earthquake. Oakland, California. 1989. (Images and captions from USGS).	B) Pancaked upper deck, Cypress viaduct. Guard rail at right is on lower deck. Loma Prieta Earthquake. Oakland, California. 1989.

C) Support columns of Highway 1 bridge across Struve Slough protrude through road bed. This resulted from collapse of the road bed after the effects of lateral shaking. Loma Prieta Earthquake. Watsonville Area, California.	D) Close-up of damaged reinforcement bars from a viaduct support column. Loma Prieta Earthquake. Oakland, California. 1989. Despite careful design, the steel bars which attached the vertical support columns were destroyed by earthquake shaking. Once these elements were broken, multilayered structures simply collapsed.

E) Collapse of a wall element of an older structure. Collapsed wall of unreinforced masonry Medico Dental Building in the Pacific Garden Mall. Loma Prieta Earthquake. Santa Cruz Area, California. 1989.

There is another part of plate tectonic theory that is perhaps the most curious of all: the hot spots. They may be found on the plate boundaries or in the interiors of the plates, and consist of a line of extinct volcanic features that ends abruptly in a single spot of active volcanism.

The Hawaiian island chain presents the clearest example of a hot spot trace (see Figure 3-14). The eight main Hawaiian Islands are but the most southeastern members of a linear chain of islands that stretch for 3500 km (2200 mi) northwest to Midway Island. Most of the islets are tiny and uninhabited, but in fact they sit on large undersea volcanic mountains whose tops have been planed off by the erosive action of the sea. Coral reefs have built up on these submerged platforms, keeping the last visible trace of the ancient volcanoes from disappearing altogether.

As we travel southeast, we find that the islands become larger and younger. We do not encounter recent volcanism until the last two islands of the chain: Maui and the big island of Hawaii. Historic activity is largely confined to the two most southeastern volcanoes on the island of Hawaii. These are the Mauna Loa and Kilauea volcanoes. As this is being written, both are in active eruption, spewing basaltic lava fountains into the air and sending flows down their slopes toward the sea.

Fifteen kilometers southeast of the island of Hawaii, an undersea volcano has been detected. Its name is Loihi, and so far it has built up about 5 km (3 mi) above the ocean floor and now reaches to within 960 m (3,150 ft) of the ocean surface. If we continue southeast from Loihi, we find only relatively featureless ocean floor.

HM 3-4 (A-D): African Rift system and Arabian region.
A) African Rift System, Defense Mapping Agency ETOPO-5 Digital Elevation Model, north is at the top of the image.	B) Earthquake Seismicity 1985-1995. Blue dots-Hypocenter 0-50 km White dots-51-100 km Purple crosses-Volcanic center

C) Seismicity of Africa-National Earthquake Information Center. Note the broad zone of seismicity which marks the collision zone between the Africa/Eurasian plates (along the upper left and central portion of the image) and the Arabian/Eurasian plates (along the upper right portion of the image).	D) Seismicity of Arabia-National Earthquake Information Center. Seismicity results from the collision of the Arabian/Eurasian plates (upper right and central portions of the figure) and the Indian/Eurasian plates (upper right portion of the figure).

HM 3-12 (A-H): Earthquakes and Seismicity. A) Location of main earthquake epicenter (January 17, 1995) and regional seismicity, near Kobe, Japan. National Earthquake Information Center.

B and C): Destruction of strongly built and carefully designed structures was a response to heterogeneous and locally amplified resonance.

D and E): Note that the destruction and intensity of earthquake motion varied from block to block, as also observed when analyzing the destruction associated with the September, 1985 earthquake near Mexico City. In Mexico City 99% of the overall damage was associated with normally strong clay and sedimentary strata being shaken hard and long enough to reduce them to a liquified consistency, resulting in foundation collapse. Similar failure occurred in Kobe.

F and G): Destruction resulting from the Kobe earthquake of 1995

H): Seismic records of the January 1995 earthquake. The notation used is discussed in Unit 4.

HM 3-15 (A-H): Damage caused by earthquakes.
A) 1906 San Francisco earthquake-- Agassiz statue, Stanford University.	B) Damage to Stanford University, 1906 San Francisco earthquake

C) Train thrown down by earthquake of April 18, 1906. The train was standing on a siding. Beyond are the buildings of the Point Reyes Hotel; and at the extreme right the ruin of a stone store which was shaken down.	D) East side of Howard Street near Seventeenth Street, San Francisco. All houses shifted toward the left. The tall house dropped from its south foundation wall and leaned against its neighbor. San Francisco County, California 1906.

E) Transform fault movement, 8.5 feet, location one half mile northwest of Woodville, looking northeast. Marin County, California	F) Northward on Howard Street at Seventeenth Street. The soil at right foreground settled and moved forward. The buckle of railway tracks resulted from this movement. San Francisco. San Francisco County, California. 1906.

HM 3-21 (A-H)
A) Active volcanic center. Daytime oblique aerial view of Halemaumau Crater. Hawaii Volcanoes National Park. Hawaii County, Hawaii. December 4, 1967. All photos and captions from USGS.	B) The very low viscosity of the magma as it is erupted from the volcanic centers assures that this type of shield volcano will in general have very gentle slopes. Snow capped Mauna Loa volcano is seen in this picture.

C) Oblique aerial view of Halemaumau Crater, Kilauea summit. HVO (Hawaiian Volcano Observatory) in foreground. Fumes rising from crater. Hawaii Volcanoes National Park. Hawaii County, Hawaii. May 26, 1972.	D)Makaopuhi lava lake, aerial view. Lava glows through cracks on surface of lake. Kilauea east rift. Hawaii Volcanoes National Park. Hawaii County, Hawaii.

E) Oblique aerial view of the perched lava pond on the north flank of Mauna Ulu satellite shield, east rift zone of Kilauea Volcano; view is to the south. The diameter of the pond is 130 m. Hawaii Volcanoes National Park. Hawaii County, Hawaii.	F) Slump fissure at edge of Alae crater has split Chain of Craters Road. Kilauea east rift. Hawaii Volcanoes National Park. Hawaii County, Hawaii.

G) Ski in Hawaii? This hotspot generated volcano is the largest topographic feature on Planet Earth rising 5,800 meters (19,000 feet) from the seafloor to sea level and then a further 4,169 meters (13,677 feet)--total elevation 9,969 meters! (32,700 feet). However, the Hawaiian Island is not the largest volcano in the solar system, see Unit 9 Tales from other worlds: The solar family (a 21 km high shield volcano on Mars is the tallest known).	H) A'a lava flow at school. Kilauea east rift: Kapoho. Hawaii Volcanoes National Park. Hawaii County, Hawaii. January 1960. Cooling has significantly increased the viscosity of the magma and it is moving very slowly, acting like a kind of tumbling, burning, and grinding bulldozer.

HM 3-5 (A-B) (Stereo Image; please use Red/Blue glasses to view). Afar Region, East African Rift.
A and B) Red/Blue stereo images: Afar Triangle, Ethiopia, East African Rift: This region is the site of active plate tectonics. Eastern Ethiopia and Somalia are breaking away from the rest of Africa, and this part of the Earth's crust is being stretched and pulled apart. Deep valleys and linear fractures (or faults) are abundant in this area. Volcanos sometime straddle these faults where molten rock has oozed up to the surface along these fractures. Dark basaltic lavas can be seen partially filling some of these valleys. Images and caption from LPI/NASA. (Click to enlarge images for easier viewing).