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Seasons of the Sea

Tectonic plates, sometimes called lithospheric plates, are large, irregularly shaped slabs of oceanic and/or continental crust. Much like a flagstone sidewalk, plates come in many sizes. The largest plates, the Pacific and Antarctic Plate, measure thousands of miles across. Smaller plates, such as the Juan de Fuca Plate off the coast of Washington state, may be only a few hundred miles across.


The location of the plate boundaries were first identified by plotting all the major earthquakes around the world between 1961-1967. This provides a surface map of the outlines of the major plates even today (see http://www.iris.edu/seismon/). Earthquakes occur along all three types of boundaries, not just where plates collide. This should be obvious if you think about California, where the Pacific Plate and the North American Plate grind against each other.

All of the continents of the world were once assembled into one giant supercontinent named Pangaea. (Quick, what was the name of the ocean that surrounded Pangaea?) As with all theories, scientists are always looking for different ways to apply them. Plate tectonics is no exception. Recent studies of the Earth's oldest rocks have now confirmed that the breakup of the continents didn't start with Pangaea. In fact, Pangaea appears to represent one stage in an endless succession of breakups and consolidations (much like a long-lasting love affair). Whereas a few years ago we were able to start with the continents as they were 200 million years ago (MYA), now we must push the clock back further and tell you what we know about the continents as much as three billion years ago. (Such is the ever-expanding story of science!) Approximately 1.1 billion years ago, we find a group of continents assembled into a supercontinent known as Rodinia. And if we look back three billion years, we find what appears to be a lone supercontinent, called Ur.

In the pre-Pangaean world, approximately 555 MYA, six continents are recognized: Gondwana (composed of Africa, South America, India, Australia, and Antarctica), Baltica (Scandinavia), Laurasia (North America), Siberia, China, and Kazakhstan. It seems pretty incredible that five of our present day continents used to be one big landmass without distinction, and that Scandinavia was an island, for all intents and purposes. From the looks of it, what we know as the Soviet Union and China (on the same continent today) were pretty chopped up back then. According to the interpretation of the data that comprise this model of the Paleozoic continents, all these land masses rested near the equator and there were no land masses at the North or South Pole. At 490.- 475 MYA, Baltica and Gondwana began to slide towards the east. Gondwana traveled through the South Pole and turned northward around 400 MYA, causing the north-pointing tips of South America and Africa to completely reverse and point south.

At about 310 MYA, the continents began to draw together, much like ice cubes in the whirlpool of a drain. Shortly thereafter, the continents were assembled as "Pangaea" at about 255 MYA. Scientists conjecture that this process of separation and conglomeration represents a 500-year cycle driven by heat in the interior of the Earth.

The mountains caused by this collision of the continents are still visible today, the most notable being the mountain range that contains Mount Everest, the highest mountain in the world. However, the real bits of evidence that Pangaea was not the first supercontinent came from ancient crustal rocks found in North America. Large chunks of crust, called cratons, are found imbedded in newer continental crust. These cratons may be as much as 3 billion years old. Another ancient crustal formation that makes up parts of Alaska is know as a terrane. Terranes are elongate structures occur along faults and are distinct from cratons.

At 225 MYA to present, the story becomes more familiar (or at least it has been told longer). Pangaea breaks into two continents, called Laurasia and Gondwanaland, and subsequently split up into the six continents we know today: North America, South America, Eurasia, Africa, Australia, and Antarctica. This latter breakup of the continents is the one that has been the most studied, and the one that has lent all of our evidence regarding the theory of plate tectonics. However, as scientific research advances and more evidence is gathered, further refinements to this "dance of the continents" is sure to come.

In the theory of plate tectonics, plate boundaries are where the action is. From sea-floor spreading to subduction zones, the boundaries of plates give us clues that divulge the nature of the inner earth. By understanding the mechanisms that drive the movement of the plates, we can better understand phenomena such as the formation of oceanic islands, the nature of volcanoes, and the source of earthquakes.

An area where two plates meet is known as a plate boundary. As you can easily see in the figure of the major lithospheric plates, any plate may be bounded by several other plates. Plate boundaries are where the action is. This is where the driving forces of plate tectonics takes place, and these boundaries are the source of most of the earthquakes of the world. In general, plate boundaries can be divided into three categories: mid-ocean ridges or divergent plate boundaries; submarine trenches or convergent plate boundaries; and transform faults.

Recall that the lithosphere includes the crust and a sticky part of the upper mantle. Oceanic lithosphere is produced spreading centers (oceanic ridges), formed as a result of the upward movement of magma from the asthenosphere. Interestingly, this is the primary mechanism by which heat escapes from the interior of the Earth, much like the "fissures" that form when cooking a pot of chili. As the new sea floor cools, it becomes more dense and thickens. As a result, a slab of oceanic lithosphere is formed from the ridges to trenches, with the coolest and heaviest crust occurring at the trenches. Because this slab is heavier at one end, it starts to sink at one end, and it is this sinking, called subduction, that pulls the crust away from the spreading centers, across the ocean floor, and back into the interior of the Earth. You might think of this process as being a kind of "crustal conveyor belt" that forms new crust at oceanic ridges and destroys old crust at submarine trenches (also called subduction zones).

Remember also that continental lithosphere can be part of this process, as lighter, continental crust is "taken along for the ride" with the movement of the plates. Thus, subduction can also happen at the edge of a continent, as we will see in greater detail below. One other interesting tidbit I turned up while researching this topic is that all of the continents appear to be moving towards the cooler parts of the mantle, except for Africa. Apparently, Africa is standing firm these days, being the "core" of Pangaea, which broke up 255 million years ago. There are also some interesting cratons in Africa, bits of "old" continental crust from pre-Pangaean times. (Fascinating, isn't it?)

Large-scale convection cells of hot magma circulate in the asthenosphere. Where these convection cells rise and magma breaks through the lithosphere, sea-floor spreading occurs. Where the crustal material of one plate is forced underneath another plate, crustal material is forced back into the interior of the Earth, as mentioned above.

This process gives rise to three types of plate boundaries:

Divergent boundaries, where plates move away from each other
Convergent boundaries, where plates move towards each other
Transform boundaries, where plates slide along next to each other
Divergent Plate Boundaries

We start our discussion with plate boundaries where new lithosphere (oceanic or continental crust) is being formed. These boundaries are known as divergent plate boundaries, or sea-floor spreading centers. At divergent boundaries, as discussed above, hot magma from the asthenosphere breaks through the crust and erupts onto the ocean floor as lava. As a result of these "eruptions", ridges (essentially underwater mountains) are built. Because the crust is being tugged at the other end (i.e. at subduction zones), the sea floor spreads and the ridge flattens and widens. Thus, formation of new sea floor is really a two-part process, whereby new material is introduced via magma flow, and then spreads laterally as a result of subduction.

The interaction of these two processes can lead to differences in the type of ridges that are formed. If the rate of flow of magma through the crust and onto the ocean floor is fast, then the ridges tend to be "peaky" with a well-defined top. On the other hand, if the rate of sea-floor spreading is faster than the rate of magma flow, then rift valleys will be formed, which appear as cracks of depressions in the top of the ridge. These rift valleys can be quite extensive along oceanic ridges.

A good example of an oceanic ridge with a rift valley is the Mid-Atlantic Ridge. The Mid-Atlantic Ridge is probably the most well-known ridge system, partly because scientists have been studying the Atlantic Ocean for longer than any other ocean, but also because the Mid-Atlantic Ridge is exposed on the island of Iceland. This volcanic island actually rests right on top of the Mid-Atlantic Ridge, and it offers a natural (and ocean-free!) environment for studying ridge processes. Between 1975 and 1984, a series of "rifting" events, cracking of the surface, took place around the Krafla Volcano in Iceland. In some places, the ground actually rose 3 -6 feet and dropped again during these events.

Sometimes, the heat and tension of upward-rising magma can cause the crust to thin at that spot. This continued heat and pressure cause a plate to stretch and weaken, such that cracks and rifts (called grabens) may appear. These features may represent areas where new plate boundaries (and spreading centers) are formed. Eventually, the magma breaks through the crust and a new ridge system is formed. It is thought that processes that form grabens lead to the formation of new ocean basins, such as is happening in the Red Sea today. If the spreading continues at the Red Sea, which occurs at the boundaries of three plates, the easternmost corner of Africa may become a large island.

One feature of the sea floor that we talked about earlier was the phenomenon of magnetic striping. As the new sea floor cools, the iron particles within it align themselves with the magnetic field of the Earth. Today the magnetic field is oriented towards the north, as it has been for the past 710,000 years. Under these circumstances, the iron particles in the new sea floor align towards the north. However, this hasn't always been the case. Occasionally, the Earth undergoes what are known as polar reversals, when the magnetic field is oriented towards the south. At least 170 polar reversals have occurred in the past 76 million years. During polar reversals, the iron particles in the sea floor orient towards the south. Thus, the spreading sea floor keeps a kind of historical record of polar reversals, and this leads to the phenomenon of magnetic striping, that provided the strongest evidence that sea-floor spreading was occurring.

As all things are formed, so they must be destroyed for everything to remain in balance. Because the size of the Earth doesn't appear to have changed in the past 4.56 billion years, it is surmised that oceanic crust is being destroyed (or recycled) at the same rate as it is being created. This process whereby crust is recycled occurs at convergent plate boundaries, or subduction zones.

At convergent boundaries, two (or sometimes more) plates actually collide with each other. These collisions, played out over millions of years, can have spectacular results. Just such a collision caused the formation of the Himalayas and the highest peak on Earth, Mount Everest.

Essentially, three types of convergence can occur:

Convergence between an oceanic plate and a continental plate
Convergence between two oceanic plates
Convergence between two continental plates

Even without telling you, it should be obvious by now just what happens when oceanic crust (basalts) meet continental crusts (granite). Because oceanic crust is heavier, it tends to dive down where it meets continental crust. The submarine trenches that we looked at along the western edge of the north Pacific Plate, such as the Japan-Kuril Trench, are a good example of subduction of an oceanic plate underneath a continental plate.

Another good example is provided along the Peru-Chile coast, where the oceanic Nazca Plate is diving underneath the South American Plate. As a result of the forces that are generated by this collision, the Andes mountains were formed. The Andes are a truly incredible and beautiful stretch of mountains and, if you ever get a chance to fly to southern Chile, be sure to book a window seat. You will see many fine examples of frozen lakes on mountaintop calderas and lots of glaciers.

Another effect of this convergence between the oceanic Nazca Plate and the continental South American plate is the occurrence of large earthquakes. Some of the most devastating earthquakes in the world occur here. Most recently, a magnitude-8.3 earthquake hit La Paz, Bolivia, on June 9, 1994. The depth of this earthquake, 636 km (395 miles), provided an excellent example of the extent to which subduction occurs. This was one of the most powerful and deepest earthquakes ever recorded in South America. Fortunately, because it was so deep, there was no major damage.

The subduction of oceanic plates beneath continental plates is also believed to be responsible for creating the "Ring of Fire," a circle of active volcanoes, including Mt. Saint Helens and Mammoth Mountain, that span the boundaries of the Pacific Plate. Whether these volcanoes are produced from melting oceanic crust, melting continental crust, or a little of both, is a topic of great debate!

Where two oceanic plates converge, they both go deep (although one usually always ends up underneath the other). The deepest spot in the ocean, the Challenger Deep, is located where the fast-moving Pacific Plate slides underneath the slow-moving Philippine Plate, forming the Marianas Trench. These type of convergences can also lead to the formation of island arcs, chains of island volcanoes. These island chains typically parallel the trenches along which they are formed. It appears that the subduction of ocean sediments causes them to melt, creating a source of magma. Eventually, this magma is forced upwards creating lava flows that build undersea mountains that eventually grow high enough to emerge above the surface of the water to become islands. Because these ocean sediments are comprised largely of diatoms that have "shells" made of silica, these magmas tend to be highly silicic and explosive. That's why many of the volcanoes in this regions, such as Mt. Pinatubo in the Philippines, tend to be so explosive. The Marianas and Aleutian Islands are a good example of this process.

The bad boy of all plate convergences is continental-continental convergence. Because continental lithosphere is relatively light, these plate boundaries tend to "reach for the sky." As one continent pushes against the other, the buoyant forces of both tend to cause the continents to push upwards. Eventually, one continent may override the other, but this type of convergence produces the highest mountains in the world.

Like rams butting heads, the formation of the Himalayas resulted from a head-on collision between the Indian Plate and the Eurasian Plate. The slow and continuous battle between these two plates lifted the Himalayas and the Tibetan Plateau to heights greater than the Swiss Alps, and higher than any other continental mountains in the world.

The third type of plate boundary, and the one that we experience here in California with great frequency, is the transform plate boundary. Transform boundaries represent a region where two plates slide horizontally past one another. A smaller type of sliding boundary, where faults or fractures slide past each other, are called transform faults.

Transform boundaries tend to connect divergent plates, allowing spreading regions to slide past each other. Most transform faults occur on the ocean floor, and they tend to produce a zig-zag pattern along plate margins. Transform faults, occur quite frequently along oceanic ridges, and appear as cracks parallel to the main direction of the ridge.

The most infamous transform fault is the San Andreas fault. As you all know, this fault is caused from the movement of the Pacific Plate alongside the North American Plate, which has been happening at a rate of about 5 centimeters per year for the past 10 million years. As a transform boundary, it connects the East Pacific Rise, a divergent boundary south of us, with the Juan de Fuca Ridge, another divergent ridge, which is off the coast of Washington. The San Andreas is actually one of the few transform faults that is exposed on land, and extends approximately 1,300 kilometers (~807 miles) from southern California through San Francisco out to the Pacific Ocean along the coast of Washington.

How fast are these plate boundaries moving? One way to tell is to measure their rate of movement, which scientists can now do with some accuracy using satellite global positioning systems, or GPS. Because GPS systems can detect movements in the range of millimeters, scientists can now quite easily measure the movement of plates in just about any region of the world. Of course, it's still a bit more difficult within the oceans, but oceanic islands often provide a good platform.

The slowest rates of plate movement appear to be happening in the Arctic, along the Arctic Ridge. Plate movements here have been estimated at 2.5 cm per year, much like the average rate for the Mid-Atlantic Ridge. On the other hand, the rate of movement of Easter Island in the South Pacific Ocean about 2000 miles off the coast of Chile, have been estimated at a whopping 15 cm per year. Easter Island, you will recall, is the site of the amazing hand-carved monoliths (probably put there by ancient astronauts to mark the spot where the fastest rate of plate movement is occurring).

One other feature of continental plates that I would like to mention in relation to their movement is the "trail" they leaves as they move. Any of us who have been to the beach on the Atlantic coast have probably noticed how smooth the beaches are and how gradually they get deeper as you wade offshore. In contrast, beaches on our coast tend to get deep very quickly. Because the North American plate is moving generally westward, the east coast of our continent leaves a kind of trail. For this reason, the side of continents closest to ocean spreading centers are often known as trailing margins, or trailing edges. The opposite side of continents facing the direction of plate movement is known as the leading margin or leading edge.

Not all of the plate boundaries we find across the globe fit into these three simple categories. As with all natural processes, there are always exceptions to the rule, and there are always other factors that complicate or alter the ideal models we create to describe them. However, I hope you have gained some understanding of the awesome power and incredible journey that the crust of our earth contains. The height and direction of mountains, the size and shape of beaches, the strength and frequency of earthquakes are just a few of the phenomena that can be explained by the theory of plate tectonics.


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