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Plate tectonics, Earthquakes and Volcanoes
Key words: lithosphere, continental and oceanic plates, convective movements, plate boundary
The Structure of the Earth
The Earth’s engine: Convective movements
The surface of the Earth is covered by plates which are part of the lithosphere and are
about 100 km thick.
The oceanic plates are mainly composed of basalt (high density igneous rock) whereas the
continental plates are made of granite, which is less dense. The difference between the
plates density have an important effect on the results of the plates converging.
Inside the mantle there are convective movements (transfer of matter as a result of
differences in temperature. Hot material is less dense, so it rises and viceversa).
The Earth’s engine: the Planetary Heat Flux
Heat is constantly dispersed from the core trough the Earth’s surface. The quantity of energy
that escape from the Earth is called Planetary Heat Flux.
Mantle convection is the driving force that causes tectonic plates to move around the Earth's
surface and is the engine for the heat and mass transfer on the Earth.
The Earth’s engine: tectonic movements
The Mantle movements heat
and push the rocks up to the
Crust, which rise upward due
to the lower density.
As the molten rocks reach the
surface, they flow outward
with volcanic processes with
the formation of new Crust.
These movements pulls the
plates along until they meet
another plate.
The process trigger the movement of the plates known as the
Continental Drift.
The process is similar to the motion of the ice plates above the water.
Eventually the rocks cool, increase in density and sink down, back to
the Mantle.
The Earth’s engine: Plates boundaries
Earth's outer shell, the lithosphere is a fluid mosaic of many irregular rigid plates.
Composed of rocks, these enormous blocks vary in size and shape, and have definite borders that
cut through continents and oceans alike.
There are 9 large plates and a number of smaller plates and most plates are comprised of both
continental and oceanic crust. Of the nine major plates, six are named for the continents embedded
in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic.
The Earth’s engine: Plates boundaries
The interfaces between plates are called boundaries. There are three main types of
plate boundaries and they differ by the relative movement of the plate each other.
I - Diverging boundaries
Are formed when the plates move away from each other.
Magma from below rises up and forms a mid-oceanic-ridge, areas reach in volcanic activities. As
plates pull apart, hundred years by hundred years, a space between them called rift valley is created.
From an oceanographic point of view a rift valley is a young expanding ocean.
The Earth’s engine: Plates boundaries
Examples of diverging boundaries are found in the Mid-Atlantic Ridge and the East-Pacific
Ridge. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the
African Plate from the South American Plate in the South Atlantic.
Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough
elevation to extend above sea level as in the island of Iceland.
The Earth’s engine: Plates boundaries
The East African Rift Valley is a divergent boundary above sea level and
is a forming ocean created by the separation of two parts of the African Plate
The Earth’s engine: Plates boundaries
II - Converging boundaries
As two plates come together there is the formation of a converging boundary and
several scenarios can result (depending on the type of plates that collide):
1- continental vs continental plate
2- oceanic vs continental plate
3- oceanic vs oceanic plate
The Earth’s engine: Plates boundaries
Continental-continental plates collision
Both plates crumple upward, forming a single land mass. An example is the boundary between
China and India. India is moving north more than 5cm per year, creating the Himalayan
Mountains.
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Indian Plate
Asian Plate
The Earth’s engine: Plates boundaries
Oceanic-continental plates collision
When a continental plate collide with an oceanic plate, the second one, made of basalt (which
has a density of 3g/cm3) plunges downward, while the continental plate, made of granite (less
dense), stays at the surface. The oceanic plate is considered the subducting plate (which forms a
deep sea trench) while the continental one is called the overriding plate and forms mountain
chain. An example of this type of collision is the area along the Andes where the oceanic Nazca
Plate is being subducted beneath the continental South American Plate
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An
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Nazca Plate
(subducting)
South American Plate
(uplifting)
The Earth’s engine: Plates boundaries
Oceanic-oceanic plates collision
When two oceanic plates converge, they typically create an island arc as one plate is subducted
below the other. The arc is formed from volcanoes which erupt through the overriding plate as
the descending plate melts below it.
The Mariana Trench is the deepest part of the world's oceans and results from the convergence
of the Pacific Plate (oceanic-subducting) and the Mariana Plate (overriding).
Mariana Plate
(uplifting)
Pacific Plate
(subducting)
The Earth’s engine: Plates boundaries
III - Sliding boundaries
When the plates slide past each other, a sliding boundary is formed. This is also called
a Transform boundary. On the contrary of the other kinds of margins, at the
transform boundary there is no production of crust volumes but only the friction of
huge amounts of rocks, this is the reason why they are also called conservative
boundaries.
The Earth’s engine: Plates boundaries
III - Sliding boundaries
The most famous San Andreas Fault in California moves about 5 cm per year. This is
part of the larger sliding boundary between the Pacific and the North American
Plates.
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The Earth’s engine: Plates boundaries
The Continental Drift Theory
At the beginning of last century, Alfred Lothar Wegener, a German scientist, geophysicist
and meteorologist, proposed his theory of “Continental Drift” in 1912, which
hypothesized that the continents were slowly drifting around the Earth.
The Continental Drift Theory
Wegener found the same fossils in Brazil and South Africa (both dated back about 270 million
years ago), and also noted that there were similar rocks and mineral deposits found on both
these continents in corresponding locations.
The Continental Drift Theory
From 1912, Wegener advocated the theory of "continental drift", arguing that
all the continents were once joined together in a single landmass and have
drifted apart.
He supposed the cause might be the centrifugal force of the Earth's rotation
or the astronomical precession.
Unfortunately Wegener was unable to demonstrate a mechanism for
continental drift. It was not until the development of the theory of plate
tectonics in the 1960s, that a sufficient geological explanation of that
movement was found.
Evidences for the “Continental Drift”
1- Similar plant and animal fossils are found around different continent shores,
suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like
a small crocodile, found both in Brazil and South Africa, are one example
2- The complementary arrangement of the facing sides of continents as South America and
Africa
3- Widespread distribution of similar glacial sediments in South America, Africa, Madagascar,
Arabia, India, Antarctica and Australia. This was one of the major pieces of evidence for the
theory of continental drif
The Continental Drift Theory