5.3 - Plate Tectonics

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The lithosphere (the crust and upper part of the mantle) is broken into 15 to 20 moving tectonic plates. The plates can be thought of like pieces of a cracked shell that rest on the hot, molten rock of Earth’s mantle and fit snugly against one another. The heat from convection within the asthenosphere causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.

Continental Drift

As soon as we had maps that showed the outline of the world's continents, people noticed that the continents looked like they fit together like the pieces of a jigsaw puzzle. South America's east coast clearly looks like it could snuggle right up to Africa's west coast. It wasn't until the early 1900's that science was applied to the the concept that the continents had all been connected at one point. In 1912 German meteorologist and geophysicist, Alfred Wegener developed the theory he called continental drift. 

Wegener cut out maps of the continents, stretching them to show how they might have looked before the landscape crumpled up into mountain ridges. Then he fit them together on a globe, like jigsaw-puzzle pieces, to form the supercontinent he called Pangaea. Next he assembled the evidence that plants and animals on opposite sides of the oceans were often strikingly similar: It wasn’t just that the marsupials in Australia and South America looked alike; so did the flatworms that parasitized them. Finally, he pointed out how layered geological formations often dropped off on one side of an ocean and picked up again on the other, as if someone had torn a newspaper page in two and yet you could read across the tear. He used clues to climate found in sedimentary strata, the fossil record, and  geologic features to support his theory.

Although Wegener got many things right in his theory of Continental Drift,  but he was missing some crucial pieces to the puzzle. His ideas were attacked by the scientists of his time. They believed that the crust was too rigid to move and Wegener was not able to explain what mechanism might allow the continents to do so. Wegener died in 1930 on an expedition to the Arctic and didn't live to see how his theory would  evolve and be reborn as the modern theory of plate tectonics.

Plate Tectonics

A whole series of new pieces to the puzzle came to light in the 1960's and 70's. A detailed map of the world's ocean was constructed based on thousands of depth soundings of the ocean floor. Scientists discovered undersea volcanoes (seamounts), narrow and deep oceanic trenches, and a continuous ridge system running across the floors of all the oceans. To understand this discovery, Watch this brief video Links to an external site.  A network of seismographs around the world was able to pinpoint the location of every earthquake and those earthquakes coincided with the the mid-oceanic ridges and the deep oceanic trenches.

Geoscientists postulated a theory to explain this and called it seafloor spreading. Now aware of the existence of the asthenosphere, their theory was that mid-oceanic ridges were formed by currents of magma rising up from the mantle; creating new basaltic ocean floor, that spread away from the ridges carried along by convective currents within the asthenosphere.  At the deep oceanic trenches the old portions of the ocean floor descend into the asthenosphere in a process called subduction and are recycled, resurfacing at the mid-oceanic ridges in about 220 million years. Rocks nearest the mid-oceanic ridges are the youngest rocks on our planet. As you move away from the ridges to the subduction zones, the rocks get older and older.

Today the theory of plate tectonics is the basis for our understanding of many geologic processes and landforms around the world. The lithosphere is thought to be broken into 15 to 20 plates "floating" over the underlying tar-like asthenosphere. The mechanism by which the plates move is thought to be convection within the Earth's mantle - specifically within the asthenosphere. These plates move slowly over the asthenosphere. The rates of seafloor spreading vary from less than 0.4 inches per year in parts of the Atlantic to 4 inches per year in the Pacific-Antarctic Ridge, with an average of about 2 inches per year. Sea floors are like a gigantic conveyor belt moving out from the mid-oceanic ridges. Sometimes the conveyor belt brings plates together and they collide and sometimes the slide by one another

Plate Boundaries

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1.  Divergent Plate Boundaries

At divergent (pull-apart) plate boundaries magma from the asthenosphere moves to the surface forming the mid-oceanic ridges. This upward flow of molten rock produces a continuous line of active volcanoes that eject basaltic lava, building the ocean floor. Earthquakes are common on the mid-oceanic ridges. Divergent plate boundaries can form on continents. The East African Rift Valley is an example. Most of the volcanic activity associated with these boundaries is beneath the oceans but occasionally  it breaks the surface as it does in Iceland.

2.  Convergent Plate Boundaries

At convergent plate boundaries plates collide. These plate boundaries are responsible for some of the most massive and spectacular of Earth's landforms: major mountain ranges, volcanoes, and deep oceanic trenches. There are three types of convergent plate boundaries:

• Oceanic-continental convergence: When oceanic crust and continental crust collide, the more dense, basaltic oceanic crust slides under the less dense continental crust in a process called subduction. These are very dynamic plate boundaries. Associated with them are: young and growing mountain ranges (Andes, Cascades), volcanoes, deep oceanic trenches and potentially,  very large earthquakes (1960, 9.5 earthquake Chile - largest in history).
• Oceanic-oceanic convergence: If the convergent boundary is between two oceanic plates, subduction also takes place. One of the oceanic plates will be subducted under the other. These too are dynamic plate boundaries and are associated with: deep oceanic trenches (Marianas Trench), volcanic island arcs (Philippines, Aleutians) and large earthquakes (2004, 9.2 earthquake Indian Ocean)
• Continental-continental convergence: Where two continental plates collide, no subduction takes place because continental crust is too buoyant to subduct. Instead large mountain ranges are pushed up by these collisions (Himalayas, Alps). There are no volcanoes associated with these boundaries but earthquakes are common (2015 Nepal earthquake 7.8)

3.  Transform Plate Boundaries

At a transform plate boundary two plates slip past one another laterally. This slippage occurs along vertical fractures called transform faults (the San Andreas fault). At transform plate boundaries new crust is neither formed nor destroyed. There are no volcanoes or deep oceanic trenches. But there is frequent earthquake activity. 

Hot Spots and the Islands of Hawaii

Around the world, most earthquakes and volcanoes correlate very well with plate boundaries. But there are exceptions: some we understand, some we don't. The Hawaiian Islands are one such exception but they provide more evidence supporting the theory of plate tectonics.

The Hawaiian Islands sit in the middle of the Pacific Plate - not at a boundary where you would expect to find a volcanic island chain. They have formed over one of a hundred hot spots or mantle plumes that exist around the world (other well know hot spots include: Yellowstone, Iceland and the Galapagos Islands). Mantle plumes are not fully understood but are thought to be stationary over long periods of time while plates continue their movement across them.  The Hawaiian Islands include not just the 5 main Hawaiian Islands we are familiar with,  but include Midway, Laysan, Gardner, Necker and other distant atolls. The Pacific Plate is moving to the northwest across the stationary mantle plume. An island will form as the plate moves across the mantle plume, then move with the plate off of the plume and another island will form and so on. The Hawaiian Islands get progressively older and more heavily eroded as you move to the northwest. Active volcanism occurs where the most southeasterly island in the chain (the Big Island) currently sits over the mantle plume.  The southeastern volcano on the Big Island, Kilauea, marks the location of the mantle plume today. A new seamount is forming to the the southeast of the Big Island (Loihi) and will be the next volcanic island in the chain.