Plate Tectonics by Blake Harris - 12/16/12

Have you ever stood on a beach in California wondering why Asia is coming towards you so fast? Probably not, because continents don’t move nearly this quickly. The rate of continental drift is only five centimeters per year. Even if you stood on that beach for your whole life, you wouldn notice anything.

The plate tectonics theory is the theory that Earth’s lithosphere (the top of the mantle and bottom of the crust) is divided up into separate pieces, called plates, that slowly move around on top of the asthenosphere. There are 13 plates, each a different size and shape, and each carrying either continents, seafloor, or both. Earth’s plates are constantly in motion, being pushed apart from each other or towards each other. Plates are moved by convection currents located in the mantle. Evidence that supports the theory of plate tectonics are volcanoes, mountains, trenches and faults. Other evidence that supports this theory are eruptions and earthquakes.   

At all convergent plate boundaries, two plates will move towards each other, causing the two plates to collide. If the two plates are both continental crusts, and since they have the same density, neither one will subduct under the other and they will crumple up to form a mountain range. This collision also will cause an earthquake to occur. An example of this type of convergent boundary is the Indian Plate pushing up against the Eurasian plate, which is forming the Himalayas.
-Feature: Mountains

-Event: Earthquakes

    Convergent Plate Boundary (Continental-Continental)                                        Himalayas in Asia

At convergent plate boundaries involving continental crust and oceanic crust, the denser, heavier oceanic crust, made up of basalt, will subduct under the less dense, lighter continental crust made up of granite. This subduction will form a deep-ocean trench, along with earthquakes. When the oceanic crust subducts into the mantle, it will heat up, melt, and rise up to the continental crust and form a volcano along with eruptions. An example of oceanic crust subducting underneath continental crust would be the Andes Mountains located in South America.

-Features: Volcanoes and Trenches

-Events: Eruptions and Earthquakes

   Convergent Plate Boundary (Oceanic and Continental)                                 Andes Mountains in South America

Convergent plate boundaries consisting of two oceanic crusts, the denser oceanic crust will subduct underneath the less dense crust.This subduction will form a deep-ocean trench, and the melting crust from the mantle will rise up to form a volcanic island arc. The collision of the two plates will also cause an earthquake to occur and will cause eruptions from the volcanic island arc. These island arcs have formed part of Alaska, the Aleutian islands, and they have been formed on the Philippines and Japan.

-Features: Volcanoes, Trench

-Events: Earthquakes, Eruptions

       Convergent Plate Boundary (Oceanic-Oceanic)                                        Aleutian Islands in Alaska

                      Mayon Volcano, Philippines                                                                 Mount Fuji, Japan


Plate motion at divergent boundaries is the opposite of convergent boundaries-- two plates will move apart instead of together. What happens at one of these boundaries is that one plate splits in two, and then those two plates begin to move apart. When one plate begins to be pulled apart, it starts to rift. This will then form a rift basin. When the basin becomes deep and wide enough, an ocean will form there and magma from the upper mantle will erupt from the mid-ocean ridge-- the place where the single plate had split. Magma will continue to erupt from the mid-ocean ridge and push the two plates apart. This type of plate boundary can be found in the middle of the Atlantic Ocean and also in other oceans.
Features: Rift Valleys, Mid-Ocean Ridges

Events: Eruptions

                                Divergent Plate Boundary                                                            Mid-Atlantic Ridge

Two plates at transform plate boundaries move neither towards each other nor away from each other. The two plates move in opposite directions alongside each other. The line where the two plates are separated is called the fault. The two plates moving against each other along the fault will cause earthquakes. One place where this is happening, located in North America, is the San Andreas fault in California.

Features: Faults

Events: Earthquakes

                         Transform Plate Boundary                                                      San Andreas Fault in California


As I mentioned in the first paragraph, earthquakes and volcanoes are both evidence for the theory of plate tectonics. Earthquakes are caused by two plates grinding against each other, like in transform and convergent plate boundaries. Two plates can only make these vibrations (the earthquake) if they are both moving, therefore proving the theory of plate motion. Volcanoes are also evidence. They are evidence of subduction zones. When a plate subducts, it enters the mantle and melts because of the heat. The now melted plate rises to the surface, and the lava then comes out of the crust through a volcano.
Both of these pieces of evidence can be found around the Ring of Fire. The Ring of Fire is the “ring” of volcanoes and earthquakes located around the Pacific Ocean. There is such a large number of these around the ocean because of the type of plate boundary-- Convergent Plate Boundary consisting of an oceanic crust and continental crust or two oceanic crusts. At each of these boundaries, subduction occurs. When the denser plate subducts, it heats up as it enters the mantle. The plate then melts and rises up towards the crust. The rising magma will form volcanoes, and since there are so many plates subducting around the Pacific, volcanoes are in abundance.
Earthquakes are also caused by these boundaries. The collision of the two plates, as you might imagine, would cause quite a vibration. Also, when the plate begins to subduct, the two plates grinding together have a similar effect. Again, since collisions and subduction happen all around the coast of the Pacific, earthquakes occur all around the Ring of Fire.

So, if you live to see the day when Asia and North America join, you’ll know why it happened.