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4- PLATE TECTONICS AND RUPTURING

Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide
over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell
compared to Earth's mantle. This strong outer layer is called the lithosphere, which is 100 km (60
miles) thick, according to Encyclopedia Britannica. The lithosphere includes the crust and outer
part of the mantle. Below the lithosphere is the asthenosphere, which is malleable or partially
malleable, allowing the lithosphere to move around. How it moves around is an evolving idea.
Tectonic comes from a Greek word “tektonicos” which means builder. Plate tectonics
refers to the total system of plate motion and deformation. According to the theory of plate
tectonics, the lithosphere is divided into large and small plates. Tectonic plates are made of either
oceanic or continental crust. They constitute the very top part of the mantle, a layer of rock
inside the earth. This crust and the upper part of the mantle form what is called the
asthenosphere. The rocks in the asthenosphere move in a fluid manner because of the high
temperature and pressure found there. Tectonic plates are able to float upon the fluid
asthenosphere because they are made of rigid lithosphere.

Developed from the 1950s through the 1970s, plate tectonics is the modern version of
continental drift, a theory first proposed by scientist Alfred Wegener in 1912. Wegener didn't
have an explanation for how continents could move around the planet, but researchers do now.
Plate tectonics is the unifying theory of geology, said Nicholas van der Elst, a seismologist at
Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.

"Before plate tectonics, people had to come up with explanations of the geologic features in their
region that were unique to that particular region," Van der Elst said. "Plate tectonics unified all
these descriptions and said that you should be able to describe all geologic features as though
driven by the relative motion of these tectonic plates."

A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate

Geologists study how tectonic plates move relative to a fixed spot in the earth’s mantle and how
they move relative to each other. The first type of motion is called absolute motion (which
entails vertical movement on the spot) and can lead to a string of volcanoes. The second kind of
motion is called relative motion (which entails horizontal movement) and can lead to different
types of plate boundaries.

Currently, there are seven large and several small plates. The largest plates include;

 The North American Plate

 The Eurasian Plate

 The African Plate

 The south American Plate

 The Antarctic Plate

 The Pacific Plate

 The Indo-Australian Plate

The smaller plates include the Nazca plate, the Cocos plate, the Caribbean plate, the scotia plate,
the Philippine plate etc

Areas that separate two different tectonic plates are called Margins or boundaries.
When plates collide or push against each other, regions called convergent plate margins are
formed. Along these margins, one plate is usually forced to dive below the other. As that plate
dives, it triggers melting of the surrounding lithosphere and a region just below it known as the
asthenosphere.
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These pockets of molten crust rise behind the margin through the overlying plate, creating
curved chains of volcanoes known as arcs. This process is called subduction. (see Diagram on
destructive Margin)

It is worthwhile noting that convergence can be between the

a. continental crust and oceanic crust,

b. between continental crust and continental crust.

c. between oceanic crust and oceanic crust

In the first case the denser oceanic crust dives below the continental crust. In the second case,
either of them may be subducted depending on which one has a greater density.

In the last case, the contact may continue for a long time but eventual subduction of the less
buoyant and dense continent, subduction continues but eventually ends in a while. 80% of the
world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. In cases
like the meeting of the pacific plate and the Philippine Plates, a trench (like the Mariana trench)
can be formed. On the other hand, when two continental plates collide, mountain building
occurs. For instance the collision of the Indo-Australian plate with the Eurasiatic plate produced
the Himalayan Mts. This collision resulted in the creation of the highest spot on earth-the Mt
Everest.

A simplified model of a convection system in the mantle causing motion of the overlying
lithospheric plate can be seen in the diagram below.

In some respects this model of convection currents in the mantle resembles the atmospheric
circulation at low latitudes within the Hardley cell. Unequal heating of mantle rock leads to
differences in rock density and this in turn may set in motion very slow rising and sinking
motion.

Areas where plates pull apart are called divergent or constructive margins. When plates pull
apart, two types of phenomena occur depending on whether the movement takes place in the
ocean or on land.
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On the other hand, this can give birth to rift valleys like the Great Rift Valley which extends
from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the ocean long
sinuous chains of volcanic Mountains called mid-oceanic ridges form, and as new seafloor is
created at the sites of these ridges. Rift valleys are also present along the crests of the mid-
oceanic ridges. It is very important to note that constructive margins lead to the formation of
continents.

Some plates neither collide nor pull apart. These plates may instead slide pass each other. These
areas are called conservative or transformed margins.
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The number of volcanoes that occur around these regions are few. Earthquakes are however
highly frequent (abundant). Great faults such as the San Andreas Fault in California are a known
example of conservative margin. Other features that are attributed to plate tectonics include; fault
blocks, block Mountains, Volcanic island arcs, Fold Mountains and volcanic mountains (Alpine
chains).