Plate tectonics

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Tectonic plates are massive, rigid pieces of the Earth’s crust; they form the majority of the geological foundation of the surface features of the earth. These plates slowly travel across the Earth, moving entire sections of continental and oceanic crust along with them. The boundaries between the plates are areas where significant earthquakes and volcanic activity occur. [1] The theory of plate tectonics, which describes the movement of tectonic plates, is relatively young. It was only accepted into the scientific community in the late 1900’s. [2]

A Brief history of the theory of plate tectonics

Map of the World's Tectonic Plates. Credit: USGS [5]

The possibility of continental movement was first suggested in the late 1600’s, but it was not accepted as a scientific hypothesis until 1912, when German meteorologist Alfred Wegener published articles describing his idea of continental drift. [3] His hypothesis proposed that the ancient supercontinent Pangaea broke apart around 200 million years ago, eventually forming the outline of the continents as they exist today. Wegener derived his idea from a couple of different observations, namely that the edge of the continental shelves of Africa and South America seemed to fit together and the fossil record indicated that Antarctica was once located in a temperate region with swampy vegetation. However, Wegener’s hypothesis was generally dismissed because he could not explain how the continents moved. [4] It was not until ocean mapping revealed previously unknown features of the ocean floor that a geological mechanism supporting Wegener’s idea became apparent. The expansion of ocean exploration in the 1950’s discovered the mid ocean ridge (an underwater mountain range meandering around the earth) and evidence of magnetic striping (stripes of rock which show that the earth’s magnetic poles switched), which led to the discovery of the process known as seafloor spreading, a viable mechanism to explain Wegener’s idea of continental drift. [5]

How tectonic plates move

How Tectonic Plates Move. Credit: USGS [6]

Although the exact details of how the tectonic plates move are still unknown, scientists generally agree that the movement is caused by motion in the layers of the Earth. The crust and upper mantle form the lithosphere, which is divided into the Earth’s tectonic plates. The layer just below the lithosphere is the asthenosphere. This portion of the Earth is molten and moves slowly in a boiling motion, with hotter material near the outer core of the Earth heating, then slowly rising toward the crust, moving along the bottom of the lithosphere as it cools, and sinking back toward the outer core to repeat the process. Scientists believe that it is this motion that provides much of the force for tectonic plate movement. As the asthenosphere moves beneath the lithosphere, some of the molten rock breaks through the crust, forming ridges of intense volcanic activity along tectonic plate boundaries. Other parts of the crust are pulled deeper into the mantle, eventually melting in the intense heat. [6]

Hotspots

A hotspot creating an island chain. Credit: USGS [7]

Hotspots, also known as mantle plumes, are areas where magma can push through the middle of a tectonic plate rather than the edge, which is more common. Often, the magma erupting from hotspots is located below oceanic crust and forms an island over-top of the hotspot. Why hotspots occur is a mystery to scientists, as they often occur far from any plate boundary where most volcanic activity occurs. However, the effect of hotspots is visible in the island chains they create. One example of islands formed by hotspots is the Hawaiian Island chain. The reason hotspots form island chains rather than one large island is because of the movement of tectonic plates. Currently, scientists believe that a hotspot is fixed in one place, and as the tectonic plate above the hotspot moves, the island over-top of the hotspot moves with the plate, eventually traveling off of the hotspot. Without the hotspot erupting and adding to its landmass, the island will eventually erode as the hotspot starts making a new island. This is why the western Hawaiian islands are smaller than Main Island furthest east: the western islands are no longer gaining landmass from the erupting mantle plume and are thus eroding. [7]


Types of tectonic plate boundaries

The boundaries between tectonic plates have different characteristics depending upon a different variables: primarily the type of crust and the direction of the plate movement. The basic types of plate boundaries are divergent boundaries, convergent boundaries, and transform boundaries.

Divergent boundaries

Divergent boundaries occur where two plates are moving away from each other. The Mid-Atlantic Ridge is one such boundary; located close to the center of the modern-day Atlantic Ocean, it extends from Iceland to the southern tip of Africa. Most of the volcanic activity on earth occurs at divergent boundaries. As the plates pull apart, magma from the mantle seeps through and gases and heat are released. Hydrothermal vents and submarine volcanoes occur frequently along divergent boundaries.

Convergent boundaries

Convergent boundaries occur when two plate collide. The characteristics of convergent boundaries depend upon the composition of the crusts that comprise the colliding crusts. Ocean crust is dense and heavy while continental crust is thicker and less dense. How the plates react when they meet depends upon if it’s an ocean and a continental crust meeting, two ocean crusts, or two continental crusts.

  • when an ocean plate and a continental plate collide, the ocean plate sinks into the mantle underneath the continental plate. The process of a plate sinking into the mantle is known as subduction.
  • when two ocean plates collide they both subduct (sink into the mantle), creating a deep trench. The deepest trench in the world, the Mariana Trench, is in the western Pacific Ocean between the Philippine and Pacific plates.
  • when two continental crusts collide, they buckle upward and form mountain chains because they are too light and buoyant to subduct. The Himalayan Mountains are a good example of mountains created by two plates colliding, in this case the Indian Plate and the Eurasian Plate.

Transform boundaries

Transform boundaries do not collide, rather, they slide past each other in opposite directions without pulling apart like divergent boundaries. For example, the San Andreas Fault, which is the source of most earthquakes in San Francisco, is a transform boundary between the Pacific Plate and the North American Plate. The North American Plate is moving approximately southeast, while the Pacific plate is moving in a northwesterly direction. [8]

Effects of tectonic plates

The Ring of Fire in the Pacific: an area with frequent earthquakes and volcanic activity. Credit: USGS [8]

Plate tectonics affects people all over the world. The locations of mountains and other geological formations can be attributed to plate tectonics. Plate tectonics also affects people living near plate boundaries. Many earthquakes occur at plate boundaries, caused by the movement of the two plates. Volcanic activity is also much higher near plate boundaries. People in Iceland are able to generate most of their electricity through geothermal energy because of the hot springs and other heat sources created by their location on top of the Mid-Atlantic Ridge. Earthquakes in California are caused by the San Andreas Fault, where the Pacific Plate and the North American Plate meet. As a final example, the area around the Pacific and Philippine plates is known as the Ring of Fire because of the high rate of earthquakes and volcanic activity around that area, which can also cause tsunamis and create islands. On a more positive note, the volcanic activity caused by plate tectonics also creates incredibly fertile soil, is a source of geothermal energy, and help to produce the fossil fuels humans rely on so heavily in their current society. [9]

References

  1. United States Geological Survey (May 5, 1999) What is a tectonic plate? Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/tectonic.html
  2. United States Geological Survey (August 7, 2012) Historical Perspective. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/historical.html
  3. United States Geological Survey (August 7, 2012) Historical Perspective. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/historical.html
  4. United States Geological Survey (August 7, 2012) Historical Perspective. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/historical.html
  5. United States Geological Survey (May 5, 1999) Developing the theory. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/developing.html
  6. United States Geological Survey (May 5, 1999) Some unanswered questions. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/unanswered.html#anchor19928310
  7. United States Geological Survey (May 5, 1999) “Hotspots”: Mantle thermal plumes. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/hotspots.html#anchor19316266
  8. United States Geological Survey (September 15, 2014) Understanding plate motions. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/understanding.html
  9. United States Geological Survey (May 5, 1999) Plate tectonics and people. Retrieved June 3, 2015, from http://pubs.usgs.gov/gip/dynamic/tectonics.html#anchor19989073

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