MOVING THE CONTINENTS; CREATING THE OCEANS
Ours is the blue planet, just as Mars is the red planet. Spectacular photos taken from space confirm the color of our planet beyond any doubt. It is not entirely blue, of course: there are splotches of brown in arid land areas, greens in areas of dense vegetative growth, the brilliant whiteness of ice in the polar regions, the white swirls of clouds. But mostly it is blue, because of the blue vastness of the oceans.
Oceans cover more than 70% of the Earth's
surface. In fact, as author Brad Matsen and artist Ray Troll have
pointed out (Matsen and Troll, 1994), an appropriate name for
our planet is Planet Ocean. (The term Planet Oceanus has also
been used: Pinet, 1992.) We ourselves, being land dwellers (except
for those who sail the oceans or make our livings by ocean fishing),
tend to forget this fact. It is a fact that makes our planet unique
in the solar system and probably quite rare even among the extrasolar
planets (those outside our own solar system) that we are likely
Earth's liquid water is stored in the low-lying parts ("basins," to the geologist) of the planet's surface, as oceans, seas, and, on land, lakes. Though this is obvious, as water always flows to the lowest point (indeed, in classical times the "element" water was thought to be heavier than the "element" earth because it "sought" -- that is, flowed down to -- the lowest ground, while most other things made of earth, including rocks, did not), the existence of such basins is not something that ought be taken for granted.
After all, as those who study lakes (they're called limnologists) well know, lakes are always filling in with sediment and ceasing to exist. Those who have visited Yosemite Valley in Yosemite National Park may also know this, as within the past several decades, much of that which used to be Mirror Lake, under the towering face of Half Dome, has filled in and become the grassy Muir Meadow. Such filling is the ultimate fate of all lakes.
The same fate should also befall oceans
(and their smaller cousins, seas). Over many tens and hundreds
of millions of years, sediment, washed from the land by rivers,
ought to fill them up. And certainly there has been enough time
for that to have happened. But it hasn't. Why not? Why does Earth,
after all these billions of years, still have ocean basins?
Ocean basins exist because of what earth scientists call plate tectonics. Tecton was the name of a carpenter, mentioned in Book 5 of Homer's Iliad as the father of Phereclus, who built the Trojan fleet. Tecton, in fact, means carpenter in Greek. The term tectonics derives from the work of a carpenter, carpentry, or, more generally, construction. Geologists use the term tectonics to refer to the large-scale forces and movements that construct and shape the crust of the Earth. The general term tectonics includes things like earthquakes and the folding and faulting of Earth's rocky surface. Plate tectonics, however, specifically refers to the movement of the enormous plates which comprise that surface, like large tiles covering a kitchen floor.
The largest of these plates are thousands of kilometers (a kilometer = 0.6 mile) across; the smallest (called "microplates") are mere slivers, perhaps a few hundred kilometers long but only a few tens of kilometers across. (One such sliver is believed to be a microplate stretching from the San Francisco Bay Area north into Humboldt County.)
The theory of plate tectonics has its earliest origins in observations that people made just as soon as enough was known of the lands bordering the Atlantic Ocean to be able to draw maps of the area. The western coast of Africa had become familiar to Europeans as a consequence of a succession of voyages made during the latter part of the 15th century. Once the "New World" was discovered by Columbus, many additional voyages by Europeans provided a rough understanding of the eastern limits of the Americas.
Early maps of the then known world revealed that the Atlantic was a sinuous ocean that separated matching expanses of land on either side. Without the approximately 5000 kilometer (3000 mile) wide ocean separating them, the continents would have provided an almost perfect fit, like pieces of a colossal jigsaw puzzle. But without any known mechanism by which continents could have broken apart and moved such great distances, the matter was relegated to the status of a curiosity.
|The floor of the Atlantic
This is what the Atlantic Ocean would look like with its water removed. (The use of sonar has allowed us to measure the depth to the bottom in innumerable places.) The mid-Atlantic Ridge is quite prominent, but note two of its characteristics. First, the peak of the ridge is doubled. That is because in the middle of the ridge is the rift, from which molten rock is extruded, creating the ridge itself, and, in the past, the entire ocean floor. Second, the ridge is broken in numerous places by linear features at roughly right angles to the ridge. These are transform faults, which represent places where the ridge is offset. Surrounding the Atlantic are four large tectonic plates: those of North America, South America, Eurasia (including Europe), and Africa. The Caribbean Plate, a smaller plate, is barely visible just north of South America and east of Central America. (Siebold and Berger, 1982)
It was an early twentieth century German meteorologist and glaciologist (one who studies glaciers) who recognized that there was evidence for the movement of continents that went well beyond the matching coastline contours on either side of the Atlantic. That evidence included similar fossils and rock types on continents now far distant from one another. On the basis of such similarities, Albert Wegener (1880-1930) formulated his theory of "continental drift": that continents had somehow drifted from positions they had occupied in the past to different positions in the present day.
Still, there was no good mechanism to accomplish such a colossal moving task (though Wegener did indeed correctly guess that the mechanism was mantle convection, he never suspected that the crust was divided into enormous plates), and, for the most part, Wegener's ideas were dismissed. There were indeed a few geologists who thought his theory had merit, but they were mostly folks from the Southern Hemisphere, where the evidence for an earlier consolidation of continents, and their subsequent "drift," was stronger. But, with the exception of a tiny number of courageous advocates from the Northern Hemisphere -- and, at the time (the 1920s and thereafter), the study of geology was completely dominated by scientists in Europe and the United States -- Wegener's ideas were considered highly unorthodox, if not downright loony.
(My friend and former mentor Eldridge Moores was fond of relating the tale of a Princeton professor who in the 1950s used to give an annual talk on continental drift, apparently just to deride it. When evidence supporting the idea that the continents had indeed moved started accumulating in the late 1950s and early 1960s, these talks became less and less well attended, until they finally ceased altogether.)
During and after World War II, partly prompted by the greater military dependence on submarines, there was increasing interest in the nature of oceans and the geology of the ocean floor. Early on, it was noted that the rock that forms the bottom of the ocean is fundamentally different from that which forms the continents and continental margins. Continental rock is highly varied, changing considerably from one location to another, here sedimentary, there metamorphic, there again igneous: sandstone, limestone, mudstone, slate, shale, marble, granite, andesite and many others. Overall, the minerological composition of continents is close to that of granite. But while ocean basins may have extensive sedimentary deposits as a consequence of river runoff and the normal rain of dead organisms into their depths, the bedrock of the ocean everywhere is basalt.
Basalt is an "extrusive" igneous rock. It is unlike granite, which is an "intrusive" igneous rock, produced by the intrusion of granite-composition magma intruding from below into crustal rocks rather erupting onto the surface. Granite cools slowly underground, producing crystals visible to the naked eye. By contrast, basalt erupts and flows from the earth (hence is "extrusive"), cooling sufficiently rapidly to prevent large crystals from forming. (To see the tiny mineral crystals in basalt, therefore, it is necessary to use a small magnifying lens.)
That the floor of the ocean should be basalt was for many years a puzzle for geologists. One part of the puzzle was that the deeper ocean floor seemed young, relative to the age of the continents. If it was indeed young, at least by geological standards, then there had to be some mechanism which had produced it recently -- at least "recently" in a geologic sense.
Clues to the riddle had to wait until 1957-58, the International Geophysical Year (actually a year and a half), during which the most extensive oceanographic studies ever (until then) undertaken were conducted. One of the most notable revelations of these seafloor studies was that there was a mountain range on the ocean floor. In fact, it was by far the longest mountain range on the planet, meandering some 65,000 kilometers (40,000 miles) through the Atlantic, Pacific, Indian, and Arctic Oceans.
Today we refer to this enormous mountain range as the mid-ocean ridge. It earned the designation mid- because the section which was originally discovered lies in the middle of the Atlantic Ocean, but in other oceans it is not so centrally located. It some places its topography is relatively smooth, in others quite rugged. Generally, however, it drops gently to the floor of the ocean (about 4 to 6 kilometers, or 2 1/2 to 3 1/2 miles, deep) from a typical elevation of about 2 to 3 kilometers (1 1/2 to 2 miles) below sea level.
This ridge, it was discovered, actually produces the ocean floor, via a central rift which extrudes basaltic lava. The heat maintains the ridge in its relatively high position; as its lava cools and moves away from the ridge in both directions over geologic time, it becomes more dense and compact, and the ocean basin deepens.
The creation and destruction of ocean floor. Ocean floor is created at mid-ocean ridges (shown in the middle of the diagram) and over millions to tens of millions of years pushes the continents (here, just one continent) aside. In some places (as along most of the coast of the Atlantic Ocean), the ocean and adjacent continent are part of the same plate, and the ocean plate does not drop down ("subduct"). Elsewhere, as shown here, the ocean and continent are on different plates, and the ocean plate, being heavier, subducts. In other areas, ocean plates subduct beneath other ocean plates (as shown on the right side of the diagram). Wherever subduction occurs, it produces volcanic activity 150 to 300 kilometers (about 100 to 200 miles) in the direction of the subduction. (Not all volcanoes owe their origin to subduction, however, and this diagram shows both mid-continent and ocean island volcanoes. The blobs below them are rising magma.) Note the long arrows. These indicate the churning of solid rock (some of it fully solid; some probably about the consistency of the stiffest taffy) in Earth's mantle, the presumed driving force for plate tectonics. (Fowler, 1990)
The actual creation of ocean floor at mid-oceanic ridges explains why the bedrock of the ocean floor is uniformly basalt, because it is basaltic lava which is extruded. In addition, the density of basalt (about 3.3 times the density of water) explains why the ocean floors should be relatively lower than the continents (which average about 2.8 times the density of water). The continuous creation (and destruction) of ocean floor also explains why the ocean floor is relatively young: after being produced at the mid-ocean ridges, it moves slowly away and eventually gets recycled down great trenches back into the interior of the planet. Thus the most ancient ocean crust, found in the northwestern Pacific, is at most only about 190 million years old. Much continental rock, by contrast, is over a billion years old, and some of it is considerably older.
Ocean basins, therefore, exist because they are continuously created, and because their rock is denser and heavier than other rock of the Earth's crust. Just as a more dense piece of wood floats lower in the water than a less dense piece does, so also does a large mass of more dense rock "float" lower on the surface of the Earth than does a large mass of less dense rock. The density difference between ocean basin rock and that of continents consequently produces a striking and instructive contrast between the average elevations of land and ocean floor, revealed in the hypsographic curve (hypso means altitude or elevation). The curve shows that the average height of the continents above sea level is about 3/4 kilometer (about 1/2 mile), while the average depth of the ocean floor is 3 3/4 kilometers (about 2 1/4 miles). Thus as long as plate tectonics continues on Earth, there will be ocean basins -- whether water is there to fill them or not!
Ocean basins are created, and they are
also destroyed. Logically this has to be the case, because the
Earth is not expanding over time, but remains the same size. After
it is extruded at mid-oceanic ridges, the ocean floor basalt is
carried by plate motion away from those ridges at a pace far slower
(only 5 to 10 cm/2 to 4 inches per year) than that of a snail,
cooling, contracting, and becoming even denser. Eventually, it
drops down beneath other oceanic or continental plates, producing
great submarine trenches up to 11 kilometers (6 1/2 miles) deep
as it does. This process is called subduction, and it is the reason
that the ocean crust is so much younger than that of the continents.
Over the course of hundreds of millions of years, ocean crust
Because ocean floor is created at rifts, and is destroyed in trenches, the size of an ocean basin depends on the relative balance of these two processes. In the case of the Atlantic, rifting and the production of ocean floor continues as it has for many tens of millions of years, and there is no significant subduction going on (exceptions being with the Antilles and the Scotia Arcs, the former in the easternmost Caribbean; the latter southeast of South America). Thus the Atlantic keeps growing slowly in size.
By contrast, the Pacific plate, which is the largest of several plates that comprise the floor of the Pacific basin, is being created in the eastern and southern Pacific and being subducted in the north and west. It is believed that this plate is being destroyed more rapidly than it is being created, so the Pacific Ocean (particularly the North Pacific) is actually steadily -- but slowly -- getting smaller.
If this process were to continue, or
if the creation of Pacific plate oceanic crust were to slow or
cease, the ocean basin could entirely disappear. That was indeed
the fate of the ocean plate -- thousands of miles wide -- that
once separated India from the rest of Asia. In the course of about
50 million years, the plate completely subducted (except probably
for a tiny on-land sliver of ocean crust that often remains as
evidence), and India collided with Asia, pushing up the Himalayas.
Using specific rocks that originally formed ocean crust (called
ophiolites), geologists can determine where other ocean basins
which no longer exist once were.
The usual way that earth scientists illustrate how the plate tectonics process works is by the boiling soup analogy. Think of a thick soup boiling in a pot. The surface of the soup has places which are seething, and others where a foamy scud has formed and collected. The turnover of liquid in the seething places is extremely rapid,with boiling hot liquid rising to the surface, pushing outward, and dropping quickly back down (a heating process is known to physicists and geologists as convection). The seething liquid often seems to drop down near the places where the floating scud has accumulated. The scud itself exhibits relatively little movement, and may form rather large, frothy islands.
In the soup analogy the source of the heat is an electrical heating element or burning gas. For the Earth, the heat source is the core.
The core of the Earth is enormously hot (about 4300°C/7740°F). It is also under enormous pressure, which keeps the inner portion of the core solid despite the heat, though the outer core is liquid. The heat of the core, which is largely composed of iron, keeps the rocky layer above it -- a thick layer called the mantle -- hot, and keeps it in extremely slow, convective motion.
The scud is very much like the continents, accumulating gradually but changing little over time. The seething liquid, by contrast, is like the ocean basins, in a continual change. The soup analogy is a useful one but obviously has some serious limitations. Unlike the soup, the Earth has a thin but solid crust. But the soup clearly illustrates a basic geologic process called fractionation. The constant churning of the soup separates its lighter parts (that is, the lighter fraction) as scud, while the thick and heavy liquid (the heavier fraction) continues to seethe.
Another limitation of the soup analogy
is that soup time is extremely short compared to geological time.
The surface of the geological "soup" -- the tectonic
plates, that is -- only moves a few inches per year at most. Moreover,
there is very little liquid involved. Yes, lava does ooze out
of rifts in the mid-ocean ridges, and spew or blast out of various
types of volcanoes, but most of the movement of the earth's surface
does not involve molten rock. Instead, most moving rock is either
quite solid, or, down below where the temperatures are hotter,
perhaps the consistency of stiff taffy. In fact, it is this very
stiffness which explains why this sort of geologic movement is
It is possible to understand how ocean basins formed by looking at how they are currently forming. (Surprisingly, perhaps, that process did not just take place in the geologic past, but actually does go on today!) The process starts when an elongated "crack" begins to open in a continental area. This "crack" geologically is called a fault, and usually is not a real physical opening in the earth, no more than California's San Andreas Fault is. There are lots of faults in the rocky crust of the Earth, some big (stretching a thousand or more kilometers -- 600 or more miles) and some quite small and local (just a few meters --yards), but the kind of fault which eventually produces an ocean basin is both rare and unusual. It is produced by forces quite deep in the earth, extrudes lava as it grows, and is beset by earthquakes throughout the millions of years of its existence.
A few million years after it has begun to open, it is a low-lying area usually surrounded by volcanic mountains or highlands. Being low-lying, it will fill with water. A glance at a map of eastern Africa reveals just such a landscape. All along the eastern border of Congo, and continuing along the eastern border of Malawi, there is a string of elongated lakes, smaller in the north (Lake Albert, Lake Edward, Lake Kivu), but considerably larger and longer to the south (Lake Tanganyika, Lake Nyasa). These freshwater lakes help define the long system of faults known as the East African Rift, because it is undergoing the geologic process known as rifting.
|From rift to ocean. The continent of Africa is slowly being torn apart along the East African Rift. The map on the left shows, in brown, the location of the rift. Note that many of the region's lakes have parallel brown lines. These represent the walls of the grabens in which the lakes lie. A graben, as shown in the diagram on the upper right, is a block of crust that has dropped down relative to the surrounding area. Grabens are produced as the crust is pulled apart and broken. Here the pulling is the result of the movement of East Africa plate toward the east, that is, in the direction of the Indian Ocean. (The current situation is similar to that shown in b of the previous diagram.) As rifting continues, this area will eventually resemble that of the Red Sea, shown in the diagram on the lower right. (The arrows indicate the direction of rifting.) Newly created ocean floor (basalt) is indicated by the stippling. (This situation is like that shown in c of the previous diagram.) (East Africa map from Duff, 1993, Figure 29.9, p. 672; Graben diagram is from Press and Siever, 1986, Figure 4-27, p. 93; Red Sea map is from Duff, 1993, Figure 29.7, p. 670.)
As the rifting process continues, the rift eventually meets the sea, and the freshwater is replaced by the saltwater of the ocean. Look further north on that map of Africa, and you will notice the Red Sea. Here we see the rifting process at a later, more developed stage. The sea is connected to the ocean; it contains salt water; it is longer and wider than the lakes of the East African Rift. On the other side of the globe, in North America, Mexico's Gulf of California displays rifting at a similar stage of development.
Such were the origins of the Atlantic Ocean. About 180 million years ago, what we now refer to as Africa and Europe were attached to and part of the Americas. Then rifting began, first in the north central area, and then, tens of millions of years later, in the south. The great S curve of the Mid-Atlantic Ridge had begun to create the ocean floor that would push Africa and Europe roughly 2500 kilometers (1500 miles) to one side, and the Americas roughly 2500 kilometers to the other. The Atlantic, currently about 5000 kilometers (3000 miles) across, continues to grow, adding a few inches to its width each year.
The plate tectonics process produces the major features of the ocean floor.
Adjacent to the continents are the continental margins; these are underwater extensions of the continents, composed of continental-type rocks rather than oceanic basalt, and largely covered with sediment washed into the ocean by the rivers of the land. Further from the continents lies the great deep of the ocean, called the abyssal plain (4 to 6 kilometers deep, or 2.5 to 3.5 miles), which, because it is so distant from the continents, gets rather little continental sediment, but often receives a slow, steady rain of organic debris from the overlying ocean.
Immediately off the edges of the continents lie the continental shelves, situated in relatively shallow water. In some places, such as the Atlantic coast of the Americas, the shelf is relatively wide, extending as much as about 100 kilometers (60 or so miles) out. In others, as along the Americas' Pacific coast, the continental shelf is relatively narrow, dropping off only 10-15 kilometers (6-9 miles) offshore. Some shallow seas (called epicontinental or epeiric seas), like Hudson Bay in North America, and the Ross and Weddell Seas on the edge of Antarctica, also are underwater portions of continents.
Geologically, the continental shelf is defined as that part of the continental margin that is between the shoreline and the continental slope (or, when there is no distinct continental slope, a depth of 200 meters). It is characterized by a gentle downward gradient of 0.1°. Shelves are not only adjacent to continents and under relatively shallow water, but they also share the general geology of the continents: rock compositions are similar, granitic in overall composition, and of the same general age.
Further from the continent is the continental slope, the more distant portion of the continental margin. The slope tends to be fairly narrow compared to the continental shelf, but it lies deeper and has a gradient of about 3 to 6%. Though steeper than the adjacent continental shelf, and even occasionally as steep as 15%, these gradients are certainly not precipitous. The continental slope drops from the continental shelf (at a depth of about 200 meters, or 600 feet) to depths of several kilometers. (The slope and the "passive basins" of the same depth are the parts of the ocean where, as we will see, methane hydrates are found, because temperature and pressure conditions are right. With hundreds to thousands of meters [600 to about 6500 feet] of water overlying them, and with the frigid temperatures at such depths, conditions allow the formation and preservation of these exotic, icy substances.)
Still further out there may be a continental rise, with a generally smooth topography and a gently inclined gradient of from 0.05% to 2.5%. The continental shelf and slope may be incised by submarine canyons, particularly in areas where large rivers now or in the geologically recent past have cut into submarine sediments as they deliver their waters to the ocean. The continental rise represents the area of the ocean floor where the remaining muddy sediment derived from rivers and continents (that which has not been left behind on shelf or slope) is deposited. Rises are common to passive margins (as along the Atlantic: see below), but not to active ones (as along the Pacific Northwest).
The deepest parts of the ocean basins, however, constitute a striking contrast to the parts of the oceans which are shallower and closer to land. They are extremely deep, dark, and intensely cold. Because these ocean basins are generally flat, and what topography there is is at most gently inclined, they are referred to as abyssal plains. On average, they are several (four to six) kilometers deep. Their extreme depth provides no natural light. Their water temperatures are about or only slightly above 0°C (32°F). In fact, their water may actually be below the freezing point of water -- pure water, that is. Because salt lowers the freezing point of water, highly saline seawater can remain liquid at temperatures down to about two degrees below zero Celsius (that is, to almost 28.4°F).
These, then, are the major features of the ocean floor: the rifts/ridges, which actually produce the bedrock of the ocean basins, the deep floor itself, the zones where subduction is taking place, and the continental margins. Continental margins, however, are not everywhere the same. Where continents are overriding the subducting oceanic plates, there is a good deal of earthquake and volcanic activity. These margins are called active margins, and where they exist, the continental margins are narrower. This is why continental shelves on the Pacific coast of the Americas are less wide than those of the Atlantic coast. Offshore they have (or in the geologically recent past, did have) trenches where ocean crust is (or recently was) subducting.
The continental margin of Northwest Coast of the US is an example of such an active margin. Much of the California coast is prone to earthquakes, both from the San Andreas Fault and many others. To the north is a major subduction zone, just off the northern California coast and along those of Oregon, Washington, and British Columbia. This coast can have rare, but quite powerful quakes. Inland from the coast is the Cascade chain of volcanoes, stretching from Mounts Lassen and Shasta in the south to Mount Baker near the Canadian border. These features of the West Coast are the consequence of the continuous movement of the North American continent as it overruns the Juan deFuca plate; the movement churns up sediments along the coast but keeps the margin narrow.
|Andean-type continental margin, also referred to as an active margin. Named after the Andes Mountains of western South America, another example is that along the coasts of northern California, Oregon, Washington and British Columbia. There, an ocean plate (the Juan deFuca plate) drops down ("subducts") beneath the North American continent, producing volcanoes (the Cascade range). (Moores and Twiss, 1995, p. 46)
By contrast, the continental shelves along both sides of the Atlantic (including the Americas, Europe, and Africa) are relatively wide. Once rifting and the creation of the Atlantic Ocean basin had pushed the continents some distance to each side, the continental margins became quiet, or passive. Although there is some occasional earthquake activity along the North American continental margin (there was a major quake in the Boston area in the early 18th century, for example, and another in Charleston, South Carolina in the late 19th), such activity is rare. And, obviously, there are no volcanoes.
|Atlantic-type continental margin, also called a passive margin. This cross-section through land and ocean reveals that no subduction or subduction-related volcanism is occurring. Continental curst and ocean crust are part of the same plate. The "intrusions and extrusions" are remnants of the rifting activity which produced the ocean basin. (Moores and Twiss, 1995, p. 45)
Plate tectonics is the central integrating concept of geology, just as evolution is the central integrating idea of biology. I employ the phrase "central intergrating concept" rather than the term "theory" because that term is both misunderstood and abused. The term "theory" is misunderstood because it leads some people to think that something described as a "theory" is a mere air-headed idea without foundation in reality or facts to back it up. Nothing could be further from the truth. Moreover, the use of the term "theory" allows opponents -- who almost universally have no or little knowledge of the myriad facts that support the concept -- to disparage theories as without basis. Both plate tectonics and evolution have enormous quantities of evidence to support them; moreover, they explain things which would simply be without explanation otherwise.
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