How Plate Tectonics Works

How Plate Tectonics Works

Way back in 1912 a scientist by the name of Alfred Wegener came up with a crazy idea. He noticed that all of the continents seemed to fit together like the pieces of a giant puzzle. He thought, "Maybe they were once all joined together in a single, giant landmass that broke up and drifted apart over time?". He decided to give this supercontinent a name and called it Pangea, meaning, "all lands". At the time he presented his idea to the scientific community it came to be known as continental drift theory. Wegener was unable to find solid evidence to support his theory, so the other scientists laughed him off as a crackpot. One of his suggestions for the cause of continental drift was thatcentrifugal force from the rotation of the earth caused the continents to slide into each other and move around on the surface. They all calculated that there wasn't enough force generated by the earth's rotation to cause shifting of the crust and nobody took him seriously. They were all convinced the earth was rock-solid and immovable.

But then in 1929, along came a scientist named Arthur Holmes who didn't think that Wegener's theory of continental drift was too farfetched. "Now wait just a minute. Maybe he's got something here", he told them. He mentioned one of Wegener's other theories about the source of continental drift; the idea that the molten mantle beneath the earth's crust experiences thermal convectionand that the movement of these convection currents in the mantle could cause an upwelling beneath the crust, forcing it to break apart and move. Now, that sounded like a semi-reasonable explanation for the movement of the earth's crust. As a matter of fact, if you looked closely at this idea it explained a lot of things, not just the continental puzzle idea. It also explained how mountain ranges were formed - by continents crashing into each other and 'rumpling up rock'. Still, the other scientists just nodded and said, 'Yeah. Fine. Whatever'. And the theory was neatly tucked away and ignored.

Scientists are trained to be skeptical. They were all waiting for a preponderance of evidence that backed up this harebrained theory.

Over the next thirty years a lot of new and surprise discoveries were made as new technologies were developed for exploring the ocean floor . The discovery of volcanic activity on the ocean floor in the middle of the Antlantic that turned out to be part of a long, unbroken mountain chain of undersea volcanoes was the most ground-breaking discovery that supported the theory of continental drift. Scientists developed instruments for measuring earthquakeactivity around the world and began plotting the locations of earthquakes. They all got together and started drawing a new map of the world that showed volcanic and seismic (earthquake) activity was concentrated along certain areas that looked like the margins of huge crustal plates. During the 1960s several scientists published papers that reviewed the preponderance of evidence that had been gathered for the theory of continental drift and it soon came to be known as the theory of plate tectonics. The evidence that supports the theory consists of the following discoveries;

Mid-ocean Ridges
A spreading boundary is where the tectonic plates are separating. Some spreading boundaries are places where the crust is sinking downward as it is stretched thin - like in the East Rift Valley of Africa, where the Dead Sea is located. Many of the spreading boundaries are located deep in the ocean on the sea floor. These are places where volcanic activity is at a premium because the crust is being torn apart. New crust is forming when magma from the mantle deep down is forced upward out of the cracks where the plates are coming apart. Long chains of undersea mounts (the world's longest is the mid-Atlantic Ocean Ridge) and volcanic islands typically characterize these type of plate margins.

Geomagnetic Anomalies
New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cooled. By collecting and measuring samples of rock from various locations along the Mid-Atlantic Ridge, scientists have discovered that the newest, youngest crustal rocks are located in the center of the ridge, while the rocks get older as you move away from the ridge center. This supports the idea that oceanic crust continues to be pulled apart, while new crust is formed along the edges of the plates.

Deep Sea Trenches
At the same time, some of the oldest ocean crust occurs in deep sea trenches, which run parallel to continental mountain ranges. A lot of very large earthquakes have been plotted along deep ocean trenches, suggesting that these are seismically active areas (meaning the crust is moving). Scientists put two and two together, noting that the youngest oceanic crust was along the mid-ocean ridges and the oldest ocean crust was along the very bottoms of deep sea trenches. That neatly defined the edges of the tectonic plates and showed the direction of their movement. Where the deep sea trenches were found came to be called converging boundaries.

A converging boundary is the opposite of a spreading boundary. Typically you will see a converging boundary on a tectonic plate that is on the opposite side of a spreading boundary. As a plate moves in one direction it collides with the adjacent plate on its "front" end in a deep sea trench, while the trailing end of the plate is being pulled and stretched (spreading) from the plate on the other end at a mid-ocean ridge. For example, look at the Pacific plate. The entire plate is moving north and westward as the top edge converges with the North American and European plates. You can see the left side of the Pacific plate is converging with the Indian plate. Then if you look at the bottom and right edges of the plate you can see it's spreading apart from the Antarctic and Nazca plates.

Sometimes you'll see volcanic activity at converging boundaries where plates are crashing into each other. When one plate (usually the lighter continental crust) rides up over the top of the other it's called a subduction zone - because one plate margin is being subducted under the other.

A good example of this type of plate margin is where the Nazca and South American plates are crashing into each other. The lighter continental South American plate is riding up over the heavier oceanic Nazca plate. Deep down where the leading edge of the Nazca plate is diving down under the South American plate it's making contact with the molten magma of the earth's mantle. The long cordillera, or chord-like chain of volcanic mountains known as the Andes, are a result of the rumpling of the South American plate where the Nazca plate crashes into it, and the volcanoes that have formed from the increased seismic activity on the Nazca plate margin deep down.

In other converging boundaries, there is no volcanic activity because the tectonic plates are both continental plates, weighing the same. No subduction happens along these margins, just massive deformation of the edges of the plates. A good example of this is the Himalayan Mountains where the European and Indian plates meet. The two plates have continued ramming into each other, causing the crust to buckle, wrinkle, and uplift into thehighest mountain range on earth.

The few transverse boundaries are places where the two plates are just sliding past each other. In many of these boundaries there is a lot of tension and strain where the two plates are sliding and scraping past each other. The resulting strain from the sliding action of the plates causes cracks in the crust called faults. As the larger plates move past each other some chunks of crust and overlying rock are broken into fault blocks. When there is a big enough movement along the cracks or faults in the earth's crust we feel it in the form of earthquakes.

One of the most famous faults is the San Andreas, which runs along the west coast of California. It's famous for generating many of the larger quakes in California, including the world-renowned San Francisco earthquake of 1906. Funny thing is, the 1906 earthquake itself didn't do nearly as much damage as the fires that burned the city afterwards - all the water mains had burst and broken during the 'quake so there was no water to put out the fires!

Hot Spots

About 30 years ago a Geophysicist named J. Tuzo Wilson came up with an idea to explain why there was volcanic activity out in the middle of the Pacific Ocean, in the middle of the huge Pacific Plate. At the time, scientists thought that volcanoes only happened at plate boundaries, but nobody could explain why they were happening out in the middle of a tectonic plate. Dr. Wilson said that there are "hot spots", under the earth's crust in some places. These are called hot spots because they are places where a lot of heat is concentrated in a small area. The heat causes the overlying rock to melt. Since the magma is liquid and is lighter than the surrounding rock it "floats" to the surface and forces its way out of fissures in the crust. once magma erupts through the crust it is known as lava. Over time, the continual outpouring of lava can form a sea mount or island volcano if the hot spot is under the ocean floor, as in the case of the Hawaiian Islands. There is just one hot spot that never moves. But the Pacific Plate continually (and slowly) moves north over the hot spot, forming a new volcano on the overlying plate each time.

Doing the Science

Scientists had a lot of questions about why there were volcanic islands way out in the middle of the Pacific plate. It just didn't seem to fit in with their theory of plate tectonics. Dr. Wilson's idea of hot spots helped the island volcanoes to fit into the theory of plate tectonics. If the Pacific plate was moving over a hot spot, then that would explain why a chain of sea mounts and volcanoes had formed as the plate moved. If this was true, then the volcanoes should be of different ages, from oldest to youngest in a single direction.

In order to test his theory, Dr. Wilson took samples of volcanic rock from each of the volcanic islands in the Hawaiian chain and tested them to see how old they were on a geologic time scale. He found that the oldest rocks were from the northernmost island of Kauai, which also had the most weathering of rock. He also found that progressively younger rocks were found on the Hawaiian islands the further south he went (see map). The youngest rocks of all were found on the big island of Hawaii, the southernmost island. In fact, new "rocks" are still forming on the island of Hawaii, making it the youngest volcano in the island chain. There is even more evidence to support his theory because there is a new volcano forming on the sea floor south of Hawaii, called Loihi. Right now it's just a sea mount, but if the lava continues to build up on its slopes, someday it will be a new island.

Read about a scientist who "chases" volcanoes