Physical Geology

Plate Tectonics

Plate Tectonics - the theory that the outer layer of the Earth (lithosphere = crust + upper mantle) is divided into a series of plates that move around the surface of the earth through time.

Plate Tectonics had its birth in the early 1960âs and is now considered to be the single greatest unifying idea in geology - a theory that allows us to make consistent sense of our observations of the geology of the earth.

Its first earnest scientific proponent was a german meteorologist Alfred Wegner, who in 1912 proposed that the continents of the modern world were once connected together into a large Îsupercontinentâ that he called ÎPangeaâ that had broken apart and drifted to place the continents in their present positions.

Wegener supported his proposal of continental drift with a number of observations:

As had been known by map readers for centuries, the coastlines of the two sides of the Atlantic seem to fit together like jigsaw puzzle pieces.

Furthermore, many of the rock formations of the west coast of Africa were known to be similar to those of the east coast of South America.

Fossils from the time just prior to the appearance of the dinosaurs were known from all of the southern hemisphere continents, including Antarctica. Many species of fossil land animals and plants were the same on all four continents, even though they are separated by thousands of miles of ocean today.

Other supporters of Wegener, including the South African geologist Alexander DuToit, added to his observations.

There is a genus of earthworm that is only found at the southern tips of Africa and South America. Another earhtworm genus is only known from India and southern Australia. How did the same earthworm get to these very distant places without being found anywhere in between?

All of these observations were very difficult to explain without admitting that these continents used to be connected.

However, is spite of much circumstantial evidence in favor, the theory of continental drift was resoundingly rejected by almost all geologists. Why?

The problem was that the process itself seemed impossible. In order for the continents to be moving, it was thought that they would have to plow through the solid rock of the ocean floor. This was a physical impossibility.

Revolution in the 1960âs

The information that geologists needed to invent the theory of plate tectonics was not available in Wegenerâs time. It lay unattainable at the bottom of the ocean.

Prior to WWII, we knew more about the surface of the moon than we knew about the bottom of the deep ocean.

However, during and after the war the U.S. and British governments began to explore the deep ocean in earnest.

The rise of submarine warfare in WWII and the invention of deep diving, long submersion nuclear submarines in the 1950âs rapidly made mapping the ocean floor a top priority. Using sonar and seismic waves ships began to explore the topography of the deep ocean.

Sonar relies on echo patterns from sound waves bounding off the sea floor

Seismic waves is a remote sensing using shock waves generated by explosions

The ocean floor prior to the 1950âs

Although the lines of undersea mountains called mid-ocean ridges had been know for years, most geologists had assumed that the deep sea floor was an essentially featureless plain that stretched from the edges of the continents to the mid-ocean ridges.

The Înewâ ocean floor

As the sonar images of the seafloor began to accumulate, geologists began to discover that all of their assumptions were wrong. What did they find?

Mid-ocean ridge systems - huge undersea mountain chains with a steep chasm running down their centers. Most interesting was the fact that these mountain chains always seemed to lie in the center of ocean basins. It was also noted that they were areas of high heat flow coming up from the mantle.

Undersea volcanoes and flat-topped mountains - Chains of undersea volcanoes and flat-topped, undersea mountains or guyotes were seen in scattered throughout the deep oceans. The guyotes were particularly puzzling because they were clearly volcanoes that had been eroded flat by waves at the surface of the sea. However, the flat tops of the guyotes are now thousands of feet beneath the sea.

Deep trenches - these, elongate, narrow trenches can be almost 5 miles deep! They are the deepest parts of the ocean and are found lying close to island chains such as Japan and Indonesia, as well as continents such as South America. A particularly spectacular line of trenches follows the rim of the Pacific ocean. This rim is also know for its abundant volcanoes and earthquakes - the ring of fire.

Thin layers of sediment - instead of thick piles of mud and other sediment, the ocean floor was covered with a relatively thin layer, thickest near the continents but thinning to nothing toward the mid-ocean ridges.

In 1962 an American geologist named Harry Hess published a paper in which he proposed a theory to explain all of the features of the seafloor. Hessâs proposal was admittedly speculative, , but it did suggest a single explanation for all of the features of the deep ocean, as well as for continental drift.

Hess suggested that instead of plowing through the seafloor, the continents were riding along with the seafloor which itself was moving. The key components of Hessâs theory of seafloor spreading:

New ocean floor is welling up as molten rock from the mantle below along the center of the mid-ocean ridges. As the ocean floor moves away from either side of the ridge, molten rock moves up from the mantle to replace it. This is why the ridges show high heat flow, and it is why they stand up as a ridge - the new, warm seafloor is less dense and floats high on the underlying mantle. As the seafloor moves away from the ridge system it cools and sinks.

If the ocean floor is created at the mid-ocean ridges and moves along like a giant conveyer belt away from the ridges then it must go somewhere where it is eventually destroyed. Hess proposed that the deep ocean trenches are where the seafloor finally descends back into the mantle. The trenches themselves are a result of the downward bending of the seafloor as it begins its descent.

Finally, Hess reasoned that if the ocean floor is continually being remelted into the mantle and created anew at the ridges then it shouldnât be surprising that the ocean floor is not very ancient. Furthermore, the reason that the mud and sediments deposited on the ocean floor thin toward the mid-ocean ridges is because the age of the seafloor gets younger and it has had less and less time to accumulate sediments.

When it came out in print, Hessâs work generated little widespread interest. However, for a few people it was just the explanation they were looking for the help them make sense of their own work.

One year later, the evidence that would eventually prove Hess right began coming in.

Geologists know that when a rock cools from a molten state, the tiny particles of iron mineral in the rock align with the direction and polarity (N-S) of the Earthâs magnetic field. Geologists had also discovered that rocks of different ages have different polarities, apparently because the Earthâs magnetic field occasionally changes polarity from N-S to S-N and then eventually back again.

Two british geophysicists named Fred Vine and Drummond Matthews had been making magnetic maps of the seafloor along the Indian Ocean ridge. using technology developed to detect submarines in WWII (more submarines!), Vine and Matthews discovered that the volcanic rock of the seafloor is magnetized in stripes of opposite polarity. Furthermore, the pattern of stripes - their width and polarity - is symmetrical on opposite sides of the mid-ocean ridge.

Vine and Matthews realized that the best explanation for the pattern of magnetic stripes in the seafloor was Hessâs theory of seafloor spreading. As the seafloor was created at the ridge it picked up the current magnetic polarity of the Earth. As the seafloor spread, half of each stripe would be carried in one direction while half went in the other direction. If the magnetic polarity of the earth changed then a new stripe would be created between the previous stripes.

In 1968, a program was started to drill into the seafloor at many places around the world and retrieve samples of the sediments lying on top of the ocean basement as well as samples of the basement itself. A special ship was built and outfitted by the the National Science Foundation to be able to send drilling pipe thousands of feet down to and into the seafloor. The DSDP (Deep Sea Drilling Project) provided the evidence that clinched Hessâs theory.

Samples of the sediments from just above the basement of the seafloor could be age dated based on their fossils of tiny sea creatures. The DSDP proved beyond a doubt that the ocean floor did get younger toward the mid-ocean ridges. Furthermore, as Hess predicted, nowhere is the seafloor extremely ancient. As of 1993, the oldest region of seafloor discovered, a portion near one of the Pacific trenches, dates back only about 180 million years. Considering that the Earth is 4.6 billion years old, this is not very old.

Seismologists also contributed to proving Hessâs theory by showing that the distribution of earthquakes around the world made sense if what Hess said was true. Earthquakes are concentrated along the mid-ocean ridges and trenches. The mid-ocean ridge earthquakes are all shallow, less than 60 km deep. Earthquakes along trenches are both shallow and deep.

Hess rescued the idea of continental drift by giving it a plausible mechanism. Instead of the continents plowing through the ocean floor they are carried along with the seafloor as it moves from mid-ocean ridge to ocean trench.

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The Plate Tectonic Revolution II (Modern Plate Tectonic Models)

But why are there continents? What happens to the continents when the ocean floor returns to the mantle? What pushes the seafloor along? As currently constructed, our theory of plate tectonics provides answers to these questions.

Hot Spots and Rift Zones

Heat does not rise through the mantle evenly. There appear to be regions of upwelling mantle material and corresponding regions of sinking mantle material. These regions may be tied together as mantle convection cells. In any case, in areas where the hot mantle is rising part of the aesthenosphere melts and flows upward to the surface.

Isolated zones of upwelling are scattered over the globe and are called hot spots. Hot spots often seem to remain stationary within the mantle for long periods of time. Two hot spots familiar to you are located under the island of Hawaii (the Big Island) and under Yellowstone National Park. Each of these hot spots has left a trail of volcanism as the crustal plate has moved over the stationary hot spot.

Long, linear zones of upwelling seem to be responsible for the mid-ocean ridges. These are called rift zones because they are essentially large cracks or rifts in the crust of the earth. As we have already seen, when a rift zone forms under the ocean a ridge develops and the seafloor on both sides of the ridge begins to move away from the ridge.

In Iceland, the mid-atlantic ridge is exposed above sea-level and the great mid-ocean rift can be seen slowly pushing the island apart.

Sometimes, rift zones form beneath continents. When this happens, the continent will begin to pull apart and then eventually separate along the rift. A new ocean basin will form between the two fragments of the separating continent. This is how Wegenerâs supercontinent of Pangea was split apart some 200 million years ago.

We can see this process happening today in places like the Arabian Peninsula, the great African rift valley, and the Baja Peninsula.

Where plates are moving apart, we call the edge where the plates meet a divergent margin.

Convergent margins

When two plates come together the edges of the plates form a convergent margin. There are three main types of convergent margins because there are two types of crust.

As the plate is subducted it forms a deep trench where it bends down into the mantle. As the plate moves downward it heats up. Part of the seafloor and the sediments on the seafloor melt and move upward to the surface, erupting out to form a chain of volcanic islands.

We can see this happening today in the western Pacific ocean. All of the large island chains such as Japan and Indonesia are island arcs lying next to trenches over subduction zones. All of these places are known for their active volcanoes and frequent earthquakes.

Ocean to continent convergence

When a plate margin composed of oceanic crust meets a continent the ocean floor is always subducted beneath the continent. This is because continents can never be subducted. SiAlic crust is simply not dense enough to sink into the mantle. .

An excellent example of this kind of collision can be seen on the west coast of South America where the Nazca plate is subducting beneath the South American plate. The Andes mountain range runs down the coast of the continent above the subduction zone. The Andes are a rugged combination of folded rock and immense volcanoes.

Likewise, in the North American northwest, the small, remaining portion of the Juan de Fuca plate is being subducted. As it melts magma rises to form the Cascade Mountain Range which includes the highest mountains in the continental United States. The most famous mountain in the Cascades is Mount Saint Helens.

The most famous transform margin is located in southern California. The Baja Peninsula and a slice of coastal California are moving northwest as they are carried by the Pacific plate. As this portion of the continent moves it slides past the rest of California and North America along a rupture know as the San Andreas Fault. Los Angeles is thus slowly travelling north to meet San Francisco.