J. Tuzo Wilson was a Canadian scientist. He observed that there were cracks in the continental crust similar to those on the ocean floor. Wilson proposed that the lithosphere is broken into separate sections called plates and that they fit together closely along cracks in the crust.

Plate Tectonics Wilson’s theory came to be known as the theory of Plate Tectonics. This theory explains the formation, movement and subduction of the Earth’s crust. No plate can move without affecting the other plates surrounding it. As the plates move, they collide, pull apart, or grind past each other, causing changes in the Earth’s surface.

Plate Boundaries A boundary is a place where tectonic plates touch. There are three types of plate boundaries: Divergent Convergent Transform

The type of boundary depends on how the plates move in relation to one another. They can collide, pull apart, or slide past each other. Earthquakes can occur at all three types of plate boundaries.

Convergent Boundaries Plates collide at a convergent boundary. What happens next depends on what type of crust is involved in the collision. There are three types of collisions: Continental and Oceanic crust Oceanic and Oceanic crust Continental and Continental crust

Oceanic – Oceanic Collision When two pieces of oceanic crust meet at a deep-ocean trench, the one that is farthest away from the mid-ocean ridge, and therefore denser, dives, or subducts under the less dense plate and returns to the mantle. This can trigger volcanic activity. If the activity continues the volcanoes will emerge as islands in the ocean. This newly formed land consists of an arc-shaped chain of small volcanic islands is called a volcanic island arc. The Aleutian Islands are examples of volcanic island arcs.

Oceanic – Continental Collision When there is a collision between continental and oceanic crust, the less dense continental crust cannot sink under the denser oceanic crust. Instead, the denser oceanic crust begins to sink and plunges underneath the continental plate, back into the asthenosphere where it is recycled. This area is called a subduction zone. An example of this is the western part of South America where the Andes Mountains are.

Continental – Continental Collision When two pieces of continental crust collide, subduction does not take place. Since both plates are mostly low-density granite, they squeeze together forming mountain ranges, such as the Himalayas.

Divergent Boundaries The place where two plates move apart, or diverge, is called a divergent boundary. Most divergent boundaries occur at the Mid-Ocean Ridge, but they also can occur on land. A deep valley, called a rift valley forms along the divergent boundary as the land moves apart. The Great Rift Valley in East Africa is an example of a divergent plate boundary.

Transform Boundaries Along transform boundaries, crust is neither created, nor destroyed. A transform boundary is a place where two plates slip past each other, moving in opposite directions. Earthquakes frequently occur along these boundaries. The San Andreas Fault in California is an example of a transform boundary.

Remember! Faults are breaks in the Earth’s crust where rocks have slipped past each other.