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Plate Tectonics
Learn About the History and Principles of Plate Tectonics
By Amanda Briney
Plate tectonics is the scientific theory that attempts to
explain the movements of the Earth's lithosphere that have formed the landscape
features we see across the globe today.
By definition, the word "plate" in geologic terms
means a large slab of solid rock.
"Tectonics" is a part of the Greek root for "to
build" and together the terms define how the Earth's surface is built up
of moving plates.
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 Tectonics
Plate 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 seafloor
spreading caused by mantle convection to explain the movement of the Earth's
continents and plate tectonics.
Principles of Plate Tectonics Today
Scientists today have a better understanding of the make-up of
the 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 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, 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.
How Many Tectonic Plates Are on
Earth?
There are seven major tectonic 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).
To learn more about plate tectonics, visit the USGS
website This Dynamic Earth: The Story of Plate Tectonics.
Amanda Briney
Geography Expert
Education
M.A., Geography, California State
University - East Bay
B.A., English and Geography, California
State University - Sacramento
Introduction
Professional geographer, writer, and
scholar
Certificate of Advanced Study in
Geographic Information Systems (GIS)
More than 10 years of experience writing
about a broad array of geographical topics
Experience
Amanda Briney is a professional
geographer and writer who contributed to ThoughtCo for more than 10 years. She
wrote countless articles on a wide range of topics such as an introduction to
the subject of geography, reviews of ecotourism, discussions about
environmental determinism, and the structure of Latin American cities. The
scope of her work also includes other formats such as histories, guides, and
fact sheets about many parts of the world. An ultimate scholar, Amanda also
contributes work to academic venues and the GIS Lounge, an informational portal
about geography.
Amanda enjoys all aspects of geography
and mapping but is especially interested in examining natural landscapes
through spatial analysis. As such, she holds a certificate in Geographic
Information Systems (GIS) from California State University. She also attended
Diablo Valley College where she studied air photo interpretation and the
formation of the Earth's landscapes.
Education
Amanda Briney received a Master Arts
(M.A.) in Geography from California State University–East Bay. She also holds a
Bachelor Arts (B.A.) in English and Geography from California State
University–Sacramento and earned a Certificate of Advanced Study in Geographic
Information Systems (GIS) from California State University.
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