The theory of plate tectonics itself says that the Earth's lithosphere is made up individual plates that are broken down into over a dozen large and small pieces of solid rock. These fragmented plates ride next to each other on top of the Earth's more fluid lower mantle to create different types of plate boundaries that have shaped the Earth's landscape over millions of years.
History of Plate TectonicsPlate tectonics grew out of a theory that was first developed in the early 20th century by the meteorologist Alfred Wegener. In 1912, Wegener noticed that the coastlines of the east coast of South America and the west coast of Africa seemed to fit together like a jigsaw puzzle. Further examination of the globe revealed that all of the Earth's continents fit together somehow and Wegener proposed an idea that all of the continents had at one time been connected in a single supercontinent called Pangaea. He believed that the continents gradually began to drift apart around 300 million years ago - this was his theory that became known as continental drift.
The main problem with Wegener's initial theory was that he was unsure of how the continents moved apart from one another. Throughout his research to find a mechanism for continental drift, Wegener came across fossil evidence that gave support to his initial theory of Pangaea. In addition he came up with ideas as to how continental drift worked in the building of the world's mountain ranges. Wegener claimed that the leading edges of the Earth's continents collided with each other as they moved causing the land to bunch up and form mountain ranges. He used India moving into the Asian continent to form the Himalayas as an example.
Eventually Wegener came up with an idea that cited the Earth's rotation and its centrifugal force toward the equator as the mechanism for continental drift. He said that Pangaea started at the South Pole and the Earth's rotation eventually caused it to break up, sending the continents toward the equator. This idea was rejected by the scientific community and his theory of continental drift was dismissed as well.
In 1929 Arthur Holmes, a British geologist, introduced a theory of thermal convection to explain the movement of the Earth's continents. He said that as a substance is heated its density decreases and it rises until it cools sufficiently to sink again. According to Holmes it was this heating and cooling cycle of the Earth's mantle that caused the continents to move. This idea gained very little attention at the time.
By the 1960s Holmes' idea began to gain more credibility as scientists increased their understanding of the ocean floor via mapping, discovered its mid-ocean ridges and learned more about its age. In 1961 and 1962 scientists proposed the process of sea floor spreading caused by mantle convection to explain the movement of the Earth's continents and plate tectonics.
Principles of Plate Tectonics TodayScientists today have a better understanding of the make-up of the Earth's tectonic plates, the driving forces of their movement, and the ways in which they interact with one another. A tectonic plate itself is defined as a rigid segment of the Earth's lithosphere that moves separately from those surrounding it. There are seven major plates (North America, South America, Eurasia, Africa, Indo-Australian, Pacific and Antarctica) as well as many smaller, microplates such as the Juan de Fuca plate near the United States' state of Washington (map of plates).
There are three main driving forces for the movement of the Earth's tectonic plates. They are mantle convection, gravity and the Earth's rotation. Mantle convection is the most widely studied method of tectonic plate movement and it is very similar to the theory developed by Holmes in 1929. There are large convection currents of molten material in the Earth's upper mantle. As these currents transmit energy to the Earth's asthenosphere (the fluid portion of the Earth's lower mantle below the lithosphere) new lithospheric material is pushed up toward the Earth's crust. Evidence of this is shown at mid-ocean ridges where younger land is pushed up through the ridge, causing the older land to move out and away from the ridge, thus moving the tectonic plates.
Gravity is a secondary driving force for the movement of the Earth's tectonic plates. At mid-ocean ridges the elevation is higher than the surrounding ocean floor. As the convection currents within the Earth cause new lithospheric material to rise and spread away from the ridge, gravity causes the older material to sink toward the ocean floor and aid in the movement of the plates. The Earth's rotation is the final mechanism for the movement of the Earth's plates but it is minor in comparison to mantle convection and gravity.
As the Earth's tectonic plates move they interact in a number of different ways and they form different types of plate boundaries. Divergent boundaries are where the plates move away from each other and new crust is created. Mid-ocean ridges are an example of divergent boundaries. Convergent boundaries are where the plates collide with one another causing the subduction of one plate beneath the other. Transform boundaries are the final type of plate boundary and at these locations the no new crust is created and none is destroyed. Instead the plates slide horizontally past one another. No matter the type of boundary though, the movement of the Earth's tectonic plates is essential in the formation of the various landscape features we see across the globe today.
To learn more about plate tectonics, visit the USGS website This Dynamic Earth: The Story of Plate Tectonics.