In this part . . .
The natural world is big and complex. People want to make sense of it. That, as it turns out, is one of the principal tasks of geography.
Earth’s surface is a mosaic of landforms covered by a rich variety of natural vegetation that is produced by diverse climate-types. Complementing this is a world of water, most of which is out of sight (and usually out of mind as well), on which life as we know it depends. None of these phenomena “just happen.” Instead, mountains, plains, forests, climates, precipitation, oceans . . . every aspect of the physical world is the result of one or more natural processes that help explain the world we see and live in.
In this part, you will learn the key concepts and concerns of physical geography, which describes and analyzes the distribution of natural phenomena over Earth’s surface. Yes, Earth is a big complex world. And as you will see, physical geography makes sense of it.
In This Chapter
Examining the inside of Earth
Theorizing about plate tectonics
Giving rise to mountains
Shaking and baking with earthquakes and volcanoes
Considering terms: natural hazards or natural processes?
E arth originated about 4.7 billion years ago as a molten fireball and has been slowly cooling ever since. As a result, and after so many years, the outermost portion has hardened into a layer of rock called the lithosphere (from the Greek lithos, meaning stone). Most of this layer is so hot that it would literally fry your feet, along with the rest of you.
Fortunately, however, the outermost portion of the lithosphere is relatively cool. This sub-layer, called the crust, is no more than between 5 and 40 miles deep, so it accounts for a very small portion of planet Earth. But the crust has a degree of importance that is out of proportion to its volume because you live on it. The crust is your home.
The crust is also home to every kind of landform you have ever seen or will see — mountains, valleys, plateaus, plains, and so on. These features and more give character to different parts of Earth and are among the first things that come to many peoples’ minds when they think about geography. And indeed geomorphology, the study of the nature and origins of landforms, is an important sub-field of geography.
Landforms don’t just happen, however. Instead, they are products of a global war of sorts is engulfing your “crusty home.” The combatants are two powerful opposing sets of forces that shape and reshape Earth’s surface. On the one hand, and the subjects of this chapter, tectonic forces (from the Greek tekton, meaning “builder”) build up the Earth’s crust. The pressures involved here are mighty enough to literally make mountains out of molehills, and also cause earthquakes to occur and volcanoes to erupt. Tectonic force has been modifying the crust for as long as crust has existed and will continue to do so for billions of years to come. Thus, I can say with complete confidence that the force will be with you always.
On the other hand, gradational forces wear down the crust. Given enough time, they can transform today’s mountains into tomorrow’s molehills. Gradational forces are the subjects of Chapter 7.
Starting at the Bottom: Inside Earth
The source of power for tectonic force lies deep within the Earth. For that reason, tectonic forces are sometimes called endogenous forces. This comes from the Greek endon, meaning within, and another old word that is the source of “genesis.” So endogenous forces have their genesis, or origin, within the Earth.
It would be great if you could go there and see what’s going on, but that’s impossible. The average distance from Earth’s surface to the center is 3,960 miles, and no human has ever come close. Several books and movies have portrayed such fanciful feats, but the truth is that people have barely penetrated the crust. Miners in South Africa have gone down about two miles, and if that’s not the record, then the real one can’t be much farther. So instead of going on a fantastic journey, you must settle for a diagram (as shown in Figure 6-1). Looking at it may cause you to wonder, “Well, if nobody’s ever been down there, then how do you know what it looks like?” Great question! And the answer is, it’s based on informed speculation. Check out the “How do we know what’s down there?” sidebar for details.
The composition and temperature of Earth’s interior are the reasons nobody has ever gone there and probably never will. Most of that realm is molten or almost molten. Thankfully, not only is it out of sight and out of mind, but also out of touch. Were it not for the insulating crust, life as we know it simply would not exist.
Directly beneath the lithosphere lies the asthenosphere. Measured in the thousands of degrees Fahrenheit, its rock assumes a plastic, almost molten quality. Directly beneath the asthenosphere is a vast volume of somewhat stronger rock and below that liquid iron of the outer core and solid iron of the inner core that is hotter still (as shown in Figure 6-1).
Altogether, that vast volume of incredibly hot stuff is a powerful source of pressure — tectonic force. Indeed, it is mighty enough to create and rearrange continents, and in the process build mountains and cause earthquakes to occur and volcanoes to erupt. This knowledge has been available for only a couple of decades. But the idea of a force powerful enough to move continents has been around for centuries.
Moving Continents: Big Pieces of a Big Puzzle
The continents on either side of the Atlantic Ocean can easily be viewed as giant pieces of a jigsaw puzzle. If that thought has never occurred to you, then take a look at a world map, focusing on the Atlantic coastlines of Africa and Europe on the one hand, and North and South America on the other. Imagine you could place your right hand on the former, your left hand on the latter, and push the pieces together. It really does look as if they would fit together, doesn’t it?
People began puzzling over this almost as soon as those shorelines were accurately mapped. Many were convinced the “fit” was too good to be mere coincidence. The implication, of course, was that there was once a super-continent that broke into big pieces that subsequently moved apart. But how could this happen? What gigantic force was responsible? And where did it come from?
How do we know what’s down there?
Our understanding of Earth’s interior rests on a combination of inference, analysis of alien objects, sound waves, and rocks and minerals. By alien objects, I do not mean UFOs, but instead meteorites and such that have fallen to Earth. These uniformly reveal a high percentage of iron. Because these alien objects are the result of the same process of planetary formation that produced Earth, the assumption is that the proportion of iron in these objects is probably about the same for planet Earth. That suggests an incredible amount of iron beneath your feet.
Earthquakes produce sound waves. Over the past several decades, seismologists (people who study earthquakes) have placed within the crust hundreds of “listening devices” that record and analyze sound waves made by earthquakes. Some of these waves, it turns out, have peculiar characteristics: They cannot penetrate liquids, or liquids deflect them, or they travel at different speeds through liquids and through solids with different characteristics. Analysis of the tracks and characteristics of literally hundreds of such waves, plus the previous inference concerning iron, provide much of the input for Figure 6-1. Also, geologists have studied lots of rocks and minerals that have been thrust up through the Earth’s crust. Analysis of these materials reveals a relative scarcity of iron, which suggests this substance must be concentrated deep within the Earth.
Where have you gone, Gondwanaland?
Various explanations were proposed. In the 1850s, Antonio Snider suggested that during Noah’s time, the Earth had several deep volcano-related cracks. Water pressure during The Flood exacerbated these cracks, created the continents, and moved them apart. A few years later, Eduard Suess proposed that a super-continent he dubbed Gondwanaland (after geological area in India) had fragmented and broken apart for reasons that he did not identify or endorse.
Alfred Wegener, mover and shaker
But the greatest theorizer of all was Alfred Wegener, a German geographer and meteorologist. In 1915, he proposed a theory that explained not only the shapes and locations of continents, but also the geography of mountains. According to Wegener, Earth’s surface once consisted of a single super- continent called Panagea (“all the Earth”) and a single world ocean called Panthalassa (“all the seas”). Pangea subsequently broke into two pieces of roughly equal size: a northern component called Laurasia, and a southern component called Gondwanaland(borrowing from Suess). Both of these, in turn, later broke up. Pieces of Laurasia became North America, Central America, Greenland, Europe, and Asia. Pieces of Gondwanaland became South America, Africa, Australia, and Antarctica.
These pieces, Wegener suggested, subsequently “drifted” apart, hence the popular name for his theory, continental drift. These continental “rafts” did not float on water, but instead bulldozed their way over other firmament (ocean bottom, typically). Now, at some time in your life you have probably watched a bulldozer do its thing. It scrapes the surface and produces a pile of debris as it moves forward. As Wegener saw it, that is exactly what the continental rafts were doing: scraping the Earth and producing mountains and mountain ranges along their leading edges. Thus, the “rafts” containing North and South America were drifting westerly, bulldozing as they went, the result being the chain of mountains that today extend along the west coast of the Americas from Alaska to the southern tip of South America.
Wegener’s theory was basically correct, but it remained unproven until after his death because the matter of how Pangaea was broken up was unresolved. Obviously a mighty force was required to break up Pangaea and cause its pieces to move. But what could that force be, and where did it come from? Again, that was the puzzle within the puzzle.
Imagine a mountain range about 10,000 miles long and nobody knew it existed until the latter half of the twentieth century. Sound bizarre? Well, it really happened, and of course, there’s a catch of sorts. The range in question is the Mid-Atlantic Ridge. As the name implies, it pretty much runs down the middle of the Atlantic Ocean. It was virtually unknown until ocean-floor mapping (a very deep subject!) revealed its presence.
Through use of remotely controlled submersibles that carried cameras and other instruments, it was learned that this mountain range is basically a 10,000-mile long active volcano. The Mid-Atlantic Ridge is the product of magma, molten material from beneath the asthenosphere. A series of deep cracks, or fissures, in the lithosphere run the full length of the Ridge. Over the years, magma has
oozed upwards through the fissures in response to tectonic force
piled up on the ocean floor
hardened to form the Mid-Atlantic Ridge
But this oozing is ongoing. Slowly, but inevitably, other magma is rising up through the fissures, to cool and harden, and in doing so, elbowing to either side of the fissure the previously hardened magma. The result is a spreading sea floor. The Atlantic Ocean is getting wider. The New World and the Old World are moving apart.
Subsequent deep-ocean mapping revealed other spreading sea floors in other oceans. Here was the explanation of how continents had split apart and moved! Here was the solution to the great puzzle that had been driving people nuts for years
Getting Down to Theory: Earth Benedict?!
In a manner of speaking, Earth’s lithosphere is like an eggshell — a thin, hard, brittle outer covering that encases a big mass of goo. Due to tectonic force, that earthly eggshell has broken into many pieces. Now, I want to be careful with this eggshell analogy. Just about everybody has dropped an egg and dealt with the mess that resulted. That won’t happen to Earth. You have no cause for a nightmare in which your home planet becomes scrambled Earth, or Earth Benedict. Just remember that, like an eggshell, Earth’s lithosphere is a brittle veneer that bears no resemblance to what is inside, and is easily cracked.
The Theory of Plate Tectonics explains what has been happening over the years. Here are a few points from that theory:
Tectonic force has broken up the lithosphere into 13 large pieces, or plates.
Tectonic force causes the plates either to move apart, collide, or slide by one another.
Mountains, volcanoes, and earthquakes result when plates collide.
Earthquakes may also result when plates slide by each other.
Taken together, these statements constitute much of the Theory of Plate Tectonics.
How fast are the continents drifting?
Not very fast at all. Indeed, the proverbial “snail’s pace” is much, much faster. Just as a hoot, I measured the distances between a couple of pairs of points on opposite sides of the Atlantic Ocean where the Old and New Worlds once were joined. The average distance is about 4,000 miles. Geologists say “the big breakup” occurred about 200,000,000 years ago. That means the Atlantic Ocean has widened from zero to 4,000 miles over a period of 200,000,000 years. I’ll spare you my math, but it shows that the Atlantic Ocean has widened at a rate of about 1.25 inches per year. That, of course, involves two plates that are moving in opposite directions, so the speed of a single plate is about half the speed of 1.25 inches per year. (In case you are wondering, if Columbus repeated today his voyage of discovery, he would have to sail about 210 feet farther than he did in 1492.) In other words, things move slowly, but consistently. We have good evidence that the speed of plate movement varies somewhat, but at any rate, the slowest snail would leave the fastest tectonic plate far behind.
Tectonic force has broken the lithosphere into 13 plates that vary in size. The Pacific Plate, for example, covers millions of square miles. In contrast, the Juan de Fuca Plate, which borders the Pacific Northwestern States of the U.S., is barely visible on the map (as seen in Figure 6-2). Every plate is either on the move or is being affected by the movement of a neighboring plate. Arrows on the map indicate the direction in which different plates are moving. As geographers have seen, the Mid-Atlantic Ridge marks a boundary at which neighboring plates are moving apart. Other spreading sea floors also are evident on the map. But, if plates are moving apart along some of their boundaries, that means there must be other locales where they are meeting head on, or sliding by each other. Three important results are mountains, earthquakes, and volcanoes.
Making Mountains Out of Molehills
Mountains and mountainous terrain are pretty hard to miss, and everybody knows them when they see them. Likewise, most people recognize the aesthetic appeal of mountains, their value as recreational resources, and the problems they pose for surface transportation systems, land settlement, and agriculture. In addition, however, mountains are of particular interest to geography for three reasons:
They are climate makers. Mountains may cause some areas to have abundant moisture and others to be bone-dry. Thus, they are major factors in the geography of climate, as you will see in Chapter 9.
They are culture makers. Mountainous terrain has historically tended to isolate people and impede their ability to share ideas and material things. Thus, as you will see in Chapter 13, they have tended to encourage development of separate cultures, act as barriers between cultures, and in general, serve as major factors in the geography of culture.
They are country makers. Because they are such visible landscape features, mountains and mountainous features — such as ridges — have often been used to designate frontiers between countries and states. Thus, as you will see in Chapter 14, they are major factors in political geography.
For now, however, the focus is on the causes and consequences of mountain building per se rather than the climatic, cultural, or political effects. As regards causes, mountains owe their immediate origins and locations to three processes: folding, faulting, and subduction. Each is discussed in the following sections.
Folding the crust
When plates collide head-on, a couple of outcomes are possible. One is folding, in which the crust buckles in response to the compression, and may eventually assume a rather wave-like appearance, as you can see in Figure 6-3. You can crudely simulate that sequence by doing the following: Put a piece of paper on the surface of a desk or table and place your right and left fingertips on opposite edges. Then very slowly move your hands together. Hopefully, the paper will assume a wave-like form as your fingers approach each other. Remember, however, that the Earth area represented by that paper contains hundreds of square miles, and that the convergence of your finger tips mimic plate movements that span millions of years.
The Appalachian Mountains of the eastern United States are a prime example of a folded mountain range, as illustrated by Figure 6-4. They do not, however, coincide today with a plate boundary. This tells geographers that plates and plate boundaries come and go over the broad expanse of geologic time. In some cases, therefore, mountains mark an active plate boundary where mountain making is in progress. In other cases, mountain ranges mark ancient (extinct) plate boundaries, and are themselves mere eroded remnants of what used to be. Today, the highest peaks in the Appalachians are in the 6,000 feet range. But orientation of various rock layers suggests to geologists that in their ancient heyday, the Appalachians towered 30,000 feet and more above sea level. That’s higher that Mt. Everest, the tallest mountain on Earth today.
Making resources accessible
The geography of mining often coincides with the geography of mountains because the tectonic processes that make the mountains also serve to reveal the presence of valuable ores and minerals and facilitate their accessibility to humans. The coal reserves of the Appalachians are a case in point.
In the diagram of folded mountains (see Figure 6-3), assume that coal occupies the second layer, or strata, of rock from the surface. In the flat terrain on the left of the diagram, one can see no indication of a valuable resource underfoot. On the right, however, folding may reveal the coal. If not, then subsequent erosion — say, a river that “cuts through” the landscape — may reveal the presence of valuable strata. This, in turn, may give rise to economic activity that — such as in the case of coal mining in the Appalachians — is largely synonymous with the region.
Whose “fault” is it?
In addition to folding, head-on collisions of plates may also produce mountains by faulting. In this case, a series of deep fractures develop through the crust. Over time, the immense pressure attendant to the slow-motion collisions between plates may cause large-scale rock units to be raised (producing horsts) or lowered (producing grabens) along the fault lines that mark the intersections of the fractures and Earth’s surface (as you can see in Figure 6-5). Mountains of significant size may result. And, actually, they have — the highest mountains on Earth, the Himalayan Range, are products (fault-block mountains) of uplift along fault lines produced by collisions between the Indo-Australian Plate and the Eurasian Plate. In this range are found Mt. Everest, the highest mountain on Earth (29,028 feet), plus K-2, Kanchenjunga, Makalu, and every one of the twenty tallest summits on Earth.
As you read this, the collision between the Indo-Australian and Eurasian Plates continues. And as a result, the elevations of many Himalayan peaks are getting higher. Mt. Everest, for example, is growing by about one-half inch per year.
Plate tectonics: A four-letter word!
That word is slow. Mt. Everest is growing by about a half inch per year. Slow. And since the break-up of Pangea, it has taken the continents hundreds of millions of years to travel a few thousand miles. Slow. If you didn’t read it, then please check out the earlier sidebar on the speed of continental drift. (Slow!) Tectonic force has massive power, but the continents and crust have massive weight. That means massive friction of resistance needs to be overcome if anything is going to be moved. Looked at over long periods of time, therefore, alterations to the crust due to tectonic forces tend to happen real slowly. Or so things normally occur. As befits a four-letter word, things can get nasty. A dark side to the force is out there.
Experiencing Earthquakes: Shake, Rattle and Roll!
The study of plate movement rather naturally leads one into the field of seismology, the study of earthquakes. These are inevitable and dangerous consequences of plate tectonics. Why “inevitable?” Well, an earthquake is a sudden movement of the Earth’s crust. Given plate tectonics, earthquakes are inevitable. That is, sooner or later they’re bound to happen.
During an earthquake, gazillions of tons of crust are moved. The amount of pressure required to do that is incredible; and it does not accumulate in a day. Therefore, at a given location, on an active plate boundary, earthquakes are not everyday ongoing events. Instead, it takes long periods of time — years and decades — for tectonic force to build up enough pressure to move a mass of crust.
The mechanics are crudely similar to inflating a balloon until it bursts. As the volume of air increases within, so does the pressure and tension along the surface of the balloon. Eventually, the pressure exceeds the balloon’s capacity to contain it, and the balloon gives way . . . pop! Obviously, crust does not inflate and pop, but pressure does build-up slowly, especially along plate boundaries. Tension increases and keeps increasing over the years. And finally, perhaps after decades of pressure building, the crust just can’t take it any more. And so it suddenly gives. That is, it suddenly moves, releasing the built-up pressure.
Because tectonic force exists everywhere, an earthquake can happen anywhere. But given what you have read about plate tectonics, it should come as no surprise to see that the geography of earthquakes (see Figure 6-6) largely coincides with the geography of plate boundaries (see Figure 6-2). Zones of spreading sea floors are prime candidate locales. So, too, are areas where plates collide. And so, too, is another possibility that has yet to be mentioned.
Splitsville in California
Sometimes neighboring plates do not diverge or collide, but rather slide by each other. The linear break in the rocks that marks the occurrence of this kind of movement is called a transform fault (see Figure 6-5). California’s San Andreas Fault, no doubt the most famous fault line in the United States, is an example of a transform fault and is shown in Figure 6-7. The land on the western side of the fault is part of the Pacific Plate and is slowly moving to the northwest. Meanwhile, the land on the eastern side, which is part of the North American Plate, is slowly moving towards the southeast.
Actually, and as has been seen, “sudden fits and starts” is more accurate than “slowly moving.” Pressure slowly builds on both sides of the fault line. Every so many years, enough pressure accumulates to overcome the friction of resistance offered by gazillions of tons of crust. At that point, parts of California slide by each other — not as continuous slow movement, but rather in short and sudden spurts as earthquakes occur. As a result, California is slowly being torn apart.
People at risk
Ultimately, geography investigates natural phenomena to gain information that is relevant to humans. Earthquakes are particularly significant because of their destructive potential and because so many earthquake-prone areas are densely populated. More important, therefore, than long-term scenarios such as the splitting of California are short-term consequences for Los Angeles, San Francisco, and several other major cities that are on or near a fault line. And California isn’t the only place affected. Examination of Figure 6-4 reveals several other locations where plate boundaries coincide with major metropolitan areas. That includes about a dozen or so major cities along the West Coast of the Americas from Mexico City to Santiago, Chile. The same applies to virtually all of Japan and many parts of Southeast Asia. Then consider the long interface between the Eurasian Plate and its southern “neighbors” that extends from the Himalayas westward through Italy. All told, more than two billion people live close enough to an active fault zone to be in harm’s way.
How earthquakes kill and maim
Collapsing buildings are the cause of most earthquake-related casualties. That is, most people who end up as statistics are either inside or next to a building that experiences structural failure. The walls give out. The roof caves in. The floors of a multistory building become something akin to a pile of pancakes. And people get crushed.
As a result, an adage of sorts goes, “Earthquakes don’t kill people: Buildings kill people.” Of course, those buildings didn’t just up and collapse. It was the earthquake that caused the collapse, and the greater the magnitude of the quake, the greater the likelihood of casualties resulting from collapsing buildings. But rarely does a quake per se prove fatal. Certainly, the attendant terror has been known to induce heart attacks. But shaking earth per se is not a major killer. Usually a side effect does people in, the principal one being structural failure.
Do earthquakes “gobble up” people?
I’ve seen at least three movies in which the earth suddenly splits open during an earthquake and “gobbles up” people who just happen to be standing there. Yes, it has happened, but the likelihood and frequency of such occurrences is roughly once in every couple of thousand blue moons. It takes an extraordinarily powerful earthquake to make the earth “open wide” and do such a thing. Eyewitnesses to the New Madrid earthquakes, discussed in an earlier sidebar, reported seeing the earth split open. Thankfully, seismic events of such magnitude are extremely rare. Reality, however, is not always conducive to box-office success, so Hollywood has a knack for rendering the extraordinary commonplace. Powerful quakes on film that transform people into munchies are examples.
A matter of wealth and culture
Wealth and culture also play major roles in determining earthquake damage and casualties. Countries characterized by low average income and a traditional cultural environment tend to fare far worse than their wealthy, modern counterparts. Say, for example, that equally strong earthquakes strike a major American West Coast city and a city in a developing country. Chances are the toll in human lives and injuries would be far worse in the developing country.
The reason relates to differences in building construction. Buildings with walls of concrete, cinder block, brick, or adobe-like materials have a rather brittle quality, so they tend to “snap” and give way rather readily. It helps if they have a supporting skeleton of steel rods or wooden poles, but even these may prove grossly insufficient in the event of a really strong quake. Unfortunately, literally millions of buildings such as these are located in seismically suspect regions in developing countries. Each is a potential tragedy waiting to happen.
In contrast, superior construction is much more predominant in wealthier settings. Skeletal steel is much more common, as is implementation of the latest thinking regarding earthquake-resistant buildings. In that regard, an ideal model is the way a tree bends in a strong wind, absorbing the punch. The implication is to build structures that bend with the punch — or rather, sway with the earthquake. This is done by attaching girders in a way that produces a skeleton that is rather like your own — it bends.
Building codes in earthquake-prone areas of the United States, and in other parts of the world, now mandate this kind of construction. And these laws clearly are having their desired effect. But modern construction is costly — much more so than the traditional masonry that continues to dominate much of the earthquake-prone world as a matter of tradition and inability to afford the state-of-the-art alternative.
When earthquakes occur under the ocean, the sea floor may rise or fall by a few feet over an area hundreds of miles on a side. This happens in a matter of seconds. With the sea surface suddenly a few feet too high or too low over a huge area, enormous volumes of water are set in motion to bring the sea surface back to level. This produces a long, low wave that moves out across the ocean in every direction at speeds that may exceed 400 miles per hour. In the open ocean the wave is less than 3 feet high, but may be 300 miles across. Although the wave is moving over 400 miles per hour, because the wave is so broad, the 3-foot rise and fall takes 10 or 20 minutes and if it passed under your ship at sea it would not even ripple the surface of your martini. You wouldn’t even notice it. But as the wave encounters shallow coastal waters, it slows down and grows enormously in height, manifesting itself as a tsunami, or large “tidal wave.” The size of the wave is directly related to the magnitude of the earthquake. Strong quakes may produce waves 30 to 50 feet in height, and 100-feet monster waves are not unknown.
Naturally, the destructive potential of these waves in regard to coastal settlements is substantial. Given the seismically and volcanically active plate boundaries that border the Pacific Ocean, more tsunamis affect that ocean’s shores than any other (see Figure 6-4). Japan has been one of the most affected regions, which explains why the word “tsunami” is of Japanese origin.
When a major earthquake occurs on land, devastation is limited to a few tens or perhaps a hundred miles of the point where the quake occurred. The tsunami, however, may travel for hundreds or even thousands of miles with little loss of energy. By means of tsunamis, therefore, earthquakes can wreak havoc far away in places where the earthquake itself is never felt.
Fortunately, global earthquake monitoring now makes it possible to warn tsunami-prone cities of an approaching tidal wave, hopefully in time to evacuate people from low-lying areas. This know-how has been unevenly applied, however. Generally, affluent countries have been able to put in place good civil defense systems while poor countries have not. As is the case with earthquakes, therefore, the geography of wealth and poverty has much to do with the resulting human toll.
A matter of magnitude
The amount of destruction that results from an earthquake depends on a couple of factors, one of which is its strength. Earthquakes vary remarkably in their power. Some can barely be felt. Others can knock you off your feet or buildings off their foundations. Two methods are used to measure the power of earthquakes. Because both make use of a series of numbers, they are referred to as scales.
The Richter Scale
Probably the more famous of the two scales is the Richter Scale, which was formulated in 1935 by a seismologist (one who studies earthquakes) named Charles F. Richter. This scale indicates ground motion in an earthquake. It ranges from 0 to 9, but theoretically can go higher. The numbers are logarithmic. That means each whole number is 10 times greater than the preceding whole number. Thus, an earthquake that measures 7.0 on the Richter Scale releases 10 times more energy than one that measures 6.0, and 100 times as much as one that measures 5.0. Of course, the ground motion of earthquakes is not limited to multiples of the number 10. Thus when earthquakes are reported in Richter terms, you may see numbers such as 6.3 or 4.8.
The Mercalli Scale
Less famous is the Mercalli Scale. This was devised in 1902 by a gentleman whose first name was Giuseppe and whose last name you can guess. This measures the intensity or violence of an earthquake, particularly in terms of damage caused to human-built structures. Expressed by a series of Roman numerals, I to XII, the higher numbers reflect increasing damage. Table 6-1 tells you what the numbers of the Mercalli scale mean, as well as (in parentheses) their approximate Richter Scale equivalents (Note: because the Mercalli is a 12-point scale and the Richter is a 9-point scale, not every Mercalli numeral will have a corresponding Richter equivalent).
The New Madrid Earthquake(s)
On December 16, 1811, perhaps the most powerful earthquake in the recorded history of the United States occurred near New Madrid, Missouri, located on the Mississippi River in the extreme southeastern part of that state. I say “perhaps” for two reasons. Scientific instruments that accurately measure earthquakes were not then available. And in the days and weeks that followed, two other major quakes (and literally thousands of much lesser ones) rocked the region. Either or both of those may have been stronger than the first.
How powerful were they? Apparently, each may have registered an 8.0 or higher on the Richter Scale. The Mississippi River changed course. An island in the river disappeared. New land rose. Forests were knocked down for miles around. The ground rolled in visible waves, wiping away houses, gardens, and fields. Fortunately, remarkably few people perished, mainly because the region was then only lightly populated.
The quakes were not flukes, but instead the product of a fault zone that is minor in terms of its length, but major in respect to seismic potential. Geographically, what is most interesting is that New Madrid is more than a thousand miles from the nearest plate boundary. Thus, while the vast majority of earthquakes occur on the fringe of tectonic plates, the New Madrid episodes demonstrate that earthquakes (even very serious ones) can potentially happen anywhere.
Table 6-1 The Modified Mercalli Intensity Scale
Extent of Impact (approximate Richter Scale equivalent)
Not felt. Usually detected only by instruments. (2 or less)
Felt by a few people at rest, especially on upper floors
Hanging objects swing. Felt quite noticeably outdoors. (3)
Felt indoors by many, outdoors by few. Sensation is like a heavy
truck striking a building. (4)
Felt by nearly everyone; sleepers awakened; trees and tele-
phone poles shaken, some dishes and windows broken.
Felt by all; books fall from shelves; glassware broken; building
damage slight. (5)
Difficult to stand; damage to some masonry; damage to build-
ings slight to moderate.
Partial collapse of masonry; chimneys fall; frame house moved
on foundations. (6)
Partial collapse of substantial buildings; underground pipes
broken; conspicuous cracks in ground. (7)
Most structures destroyed; ground badly cracked; large landslides
Few, if any, structures remain standing; bridges destroyed;
broad cracks in ground; railroad rails greatly bent. (8+)
Damage total; waves seen on ground surface; objects thrown
Applied geography: Coping with tsunamis
Urban planners are using geographical knowledge and ideas to help coastal towns in tsunami-prone areas mitigate the impact of future tidal waves. Cities and towns consist of different kinds of land use — parks, apartment buildings, schools, office buildings, and so forth. A key goal of urban planning is to allocate different kinds of land use to different parts of town in ways that benefit local residents. With respect to tsunamis, land adjacent to the coastline is dangerous, but the threat lessens as one goes progressively farther inland. The typical planning response is to allocate land use such that schools and hospitals are away from the danger zone and that population density, in general, decreases with distance to the shoreline. That means, for example, placing residences, apartment complexes, and office buildings away from the shore, while allocating warehouses, open space, and other low-population density land use to the immediate coastal setting. What may sound like plain common sense is, in fact, a geography lesson that several coastal towns have learned by fatal trial and error.
Subducting Plates: Volcano Makers
Sometimes when two plates meet head-on, one overrides the other in a process called subduction (see Figure 6-8). The plate that gets overridden is said to be subducting. Only the oceanic lithosphere created at mid-ocean ridges is dense enough to subduct (sink) very far into the mantle below. When it does it heats up, its upper surface, or the asthenosphere, overlying it partially melts. The result is a local surplus of sorts of molten material that seeks to rise through the lithosphere, and will do so if a convenient fissure or area of weakness provides a path to the surface. When that is accomplished, the result is a volcanic eruption. Thus, the geography of subduction largely determines the geography of volcanoes.
“The Ring of Fire”
Zones of subduction occur in many parts of the world, but are especially prevalent around the shores of the Pacific Ocean. For that reason, the Pacific Rim has an extraordinarily high concentration of active volcanoes and is known as “The Ring of Fire.” Locations include the western coast of South America, the western fringe of Central America, the U.S. Pacific Northwest, much of coastal Alaska from the Anchorage area westward through the Aleutian Islands, Russia’s Kamchatka Peninsula, Japan, The Philippines, and Indonesia. As is the case with earthquakes, billions of people worldwide are directly or indirectly at risk, especially so in “The Ring of Fire.”
Subduction: Another four-letter word?
Subduction is another example of that four-letter word: slow. As a result, the build-up of pressure and the basic “stuff” of an eruption are slow. Therefore, volcanic eruptions happen infrequently, as do earthquakes. And in a sense, that’s the really nasty thing about volcanoes: They erupt so infrequently.
The Hawaiian “hot spot”
Hawaii is clearly a “hot spot” as regards to tourism, but that moniker applies in another way, too. Hawaii is the most volcanically active place on Earth. In most cases, subduction is responsible for volcanoes. But in a relative handful of locales, including Hawaii, the cause is a “hot spot.” In these instances, hot mantle rock is rising to the base of the lithosphere and then rolling back down, like water boiling in a pot. Some of the rock melts as it rises and the magma rises close to or onto the surface. The latter is the case with the Hawaiian Islands, and is the very source of their existence.
For the last several millions of years, the Pacific Plate has been moving to the northwest, passing over a hot spot. Over the eons, magma has come up through a fissure (or vent), issued onto the ocean floor and hardened, becoming a seamount. As magma continues to seep through the vent, the seamount grows, breaks the ocean surface, and becomes an island, which continues to grow as long as the connection with the magma-giving vent remains. Eventually, however, the moving plate severs that connection and the island stops growing. A new seamount — the forerunner of a future island — then begins to grow on the ocean floor. This sequence explains the southeast-to-northwest orientation of the Hawaiian Islands, as well as why all of the islands except for the Island of Hawaii, which is now over the hot spot, are volcanically extinct. It also explains why to the southeast of the Island of Hawaii a seamount exists, which, several thousands (if not millions) of years from now, will become the next island in the Hawaiian chain.
Now you may say that is a really stupid statement. Why would anybody want eruptions to happen more often? My point is this: If eruptions happened with greater frequency, then people would get the message. They would more fully realize the dangers attendant to volcanoes and be more likely to avoid the danger zones.
But that’s not how it works. Eruptions happen infrequently. Subduction is slow, and there’s nothing anyone can do to speed things up. So the people who occupy volcanic environs either are unaware of the dangers or like their chances. Maybe they even employ that popular, cuddly term “sleeping volcano,” and pray it doesn’t wake up in their lifetimes. And indeed, most days it doesn’t. But then one day it does.
The big blast
A volcanic eruption is perhaps nature’s most spectacular show. In some cases it consists of lava “fountains” and flows. More commonly, however, the event is a big blast accompanied by massive emissions of steam and hot rock particles of all sizes, rather than rivers of lava.
The power of eruptions is sometimes equated to many atomic bombs. Obviously, nobody wants to be around a volcano when it goes BOOM! But few people tend to live in immediate blast areas, so BOOM! per se is not the big thing you may think, at least in terms of immediate human casualties. In that regard, two other side effects are of greater importance: ash and lahar.
Making an ash of itself
When explosive volcanic eruptions occur, gazillions of tons of ash (tiny rock particles) are thrown into the air as a humongous, dense, and potentially suffocating cloud. This is shown in Figure 6-9. Having weight, these particles eventually fall to earth over a wide area, coating crops, covering roads and houses, and potentially causing severe (and sometimes fatal) breathing problems for people and animals. The eruption of Mt. St. Helens provides an excellent case study of the possibilities (see the “A mountain blows its top” sidebar later in the chapter).
A lahar, a word of Indonesian origin, is a dangerous, fast-moving mudflow. During an eruption, vast quantities of emitted steam may cool rapidly, fall as rain, mix with ash, and form a mudflow. In the cases of very high volcanoes, such flows may be complemented by large amounts of water from rapidly melted snow and glaciers. The resulting lahar may race down flanking valleys for miles and miles from the volcano, burying everything in its path.
In November 1985, the destructive potential played out to its fullest following the eruption of the Nevada del Ruiz volcano in Colombia. Lahars as deep as 150 feet raced down the side valleys. Within four hours, locations as far as 65 miles away — seemingly well beyond the volcano’s reach — were under mud. Hardest hit was the town of Armero, where some 23,000 people were killed and another 5,000 injured.
In a way, a lahar is to a volcano as a tsunami is to an earthquake — a mechanism by which the power of a major tectonic event may be fatally felt far away from the actual event itself. But being far away, the possibility exists for early warning systems that can significantly lessen the number of people who end up as statistics.
A mountain blows its top
I have two atlases that disagree mightily concerning the height of Mt. St. Helens. One says the summit is 9,677 feet above sea level. The other gives an elevation of 8,363 feet above sea level. That’s a difference of 1,314 feet, which is close to the height of the Empire State Building. The reason for the disagreement is one atlas was published before the volcano erupted (May 18, 1980) and the other was published afterwards.
But “erupted” is a bit of a misnomer. The mountain literally blew its top. The U.S. Geological Survey estimates 3.7 billion cubic yards of mountain got blown away. Another 1.4 billion cubic yards of ash got ejected, much of it in a cloud that reached 80,000 feet within 15 minutes. Accumulations of the inevitable ash fall averaged 10 inches 10 miles downwind, 1 inch 60 miles downwind, and 1/2 inch 300 miles downwind. Fifty-seven people perished (some from the blast, others from the suffocating ash-fall), along with an estimated 7,000 big game animals and 12-million Chinook and Coho salmon fingerlings. As you can see, all kinds of numerical facts have been calculated and committed to print. What’s really amazing to me is that the volume of forest that got blown down (4 billion board feet of timber) was enough to build 300,000 2-bedroom homes.
Categorizing Tectonic Processes
Ironically, beautiful mountains are products of powerful processes that can kill or maim. Due to the latter, earthquakes and volcanoes are rather commonly referred to as natural hazards — environmental events that are potentially harmful to humans and their handiwork, such as tornadoes, hurricanes, landslides, and floods. While there is no denying the destructive potential of earthquakes and volcanoes, some students of tectonics are rather put off by the “natural hazard” name, which in their view unfairly demonizes nature and conveniently absolves humans of any responsibility for fatal effects. Better, they say, to think of tectonic events as “natural processes” that only become “natural hazards” when people get in the way, as by building houses and cities in areas at risk.
You may agree with that, or disagree with that, or have no opinion. But the statement underscores the interest of geography in this subject matter. Certainly, tectonic movements are natural processes. But when you consider the geography of these events in relation to the geography of humans and their handiwork, then tectonic forces may assume a degree of importance that goes well beyond their own immense power.