The planet's surface is shaped by the movement of Earth's tectonic plates. Sections of the Earth's crust, for example, can come together and crash (a "convergent" plate border), spread apart (a "divergent" plate boundary), or slide past one another (a "transform" plate boundary). These movements create mountains, volcanoes, and other geologic features.
Earth's plate boundaries are very important to scientists because they play a role in redistributing Earth's mass and energy through time. They also provide some of the most active places on our planet: The mid-ocean ridges rise up where two plates meet and slowly move away from each other; the trenches are where two plates collide and suddenly change direction. Trench warfare—two plates battling it out at the bottom of the ocean floor! —has been observed off Japan and South America.
Plate tectonics was first proposed in 1945 by Harry Hess and John O'Sullivan who noticed that many large mountain ranges formed near major oceans. They suggested that these must be results of huge amounts of water being removed from the oceans, which allowed the rocks beneath the waters to become exposed instead. This provided evidence that Earth's surface is dynamic and that it changes over time.
In 1970, new data from around the world showed that there were more than one type of plate boundary.
The movement of the plates causes three types of tectonic borders: convergent boundaries, where plates move into one another, divergent boundaries, where plates move apart, and transformational boundaries, where plates shift sideways in regard to one another. Transformational boundaries can be further divided into oblique (slope) boundaries and horizontal boundaries.
Convergent boundaries occur where two plates collide and grind away at each other's edges until they are completely merged together. The result is a mountain range or deep ocean trench. An example of this type of boundary is the Mid-Atlantic Ridge, which runs between Europe and Africa. Here, water is escaping from the interior of the Earth through these new plate margins.
Divergent boundaries occur where two plates drift away from each other. The result is that islands get pushed up into the middle of oceans. Examples include Hawaii and Madagascar. These islands were once part of a continent but now exist as separate entities within their own plates.
Transformational boundaries can also occur where one plate moves over another, causing the upper plate to slip with respect to the lower plate. This can cause mountains to rise up without any involvement by continental landmasses. Examples include Taiwan and parts of India. Here, an island arc system forms.
Oblique transformational boundaries occur where one plate shifts sideways in relation to another.
They migrate one to two inches (three to five cm) every year. Transform faults occur when two plates collide and then slide past one another, causing earthquakes as the land behind the fault is pushed upward.
Convergent boundaries cause mountains to be formed as they squeeze the earth's crust together. Diversive boundaries cause plains to be formed as they spread the earth's crust away from each other.
Transform faults are where two plates collide and then slide past one another, causing earthquakes as the land behind the fault is pushed upward. This type of boundary forms valleys where the plates collide/intersect.
Ridge boundaries are areas on a continent where two plates meet and push up against one another, creating highlands that extend like fingers into the ocean. These islands were once part of a larger piece of land that was torn away by gravity and thrown up onto the surface of the globe. Today, ridge lines contain the highest density of civilization on Earth.
Subduction zones are where one plate is forced under another. This occurs when one plate dives down beneath another, causing earthquakes as the earth's core pulls it deeper into itself.
Magma rises through and into the opposite plate, hardening into granite, the rock that forms the continents. As a result of convergent borders, continental crust is formed while oceanic crust is destroyed. A transformed plate boundary is produced by two plates moving past one other. At these places, the lower plate subducts under the upper plate.
The word "plate" refers to a large section of Earth's surface that moves as a single unit. The major plates consist of an outer shell with deep cracks called tectonic fissures inside it. These shells are made up of lithosphere - the rigid part of the Earth's mantle - and asthenosphere - the weak layer between the lithosphere and the more ductile plasma within the planet.
Lithosphere is the rigid portion of the earth's mantle that lies above its core. It consists mainly of mafic (high magnesium content) silicate rocks such as gneiss and marble. Mantle material is rich in iron and silicon; therefore, lithosphere is also rich in those elements. Because of its rigidity, lithosphere does not deform under its own weight but instead resists against the soft asthenosphere below it.
Asthenosphere is the weak layer between the lithosphere and the more ductile plasma within the planet.