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Ever wondered what our earth
is made of? Think of it as an apple. An apple constitutes the skin,
the pulp and the core in the middle. Similarly, the earth is
made up of the thin outermost layer called the crust, the innermost
part called the core, and the part in between them called the mantle.
At the beginning, the
early earth seemed to have heated up, the center became molten, and convection
currents
developed as the lighter compounds tended to rise towards the surface forming
the brittle crust. Together with the top part of the mantle, it formed the
hard slabs known as lithosphere. The continents are embedded in these
slabs. The lithosphere is divided into oceanic and continental crusts.
Oceanic crust (sima), the floor of the
deep oceans, is thin, about 7km thick, and made of relatively dense rocks
like basalt. Continental crust (sial) is much thicker, averaging 33km,
and is composed of relatively light material such as granite.
The denser materials such
as iron sank to form the core. It is partly solid. Temperatures are
extremely high, at about 3000oC.
Between the core
and the crust, the intermediate zones form the mantle, which is
mainly solid rocks but there is also a layer of molten rock called magma
nearer the core. Temperatures are high, at about 2000oC.
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German scientist Alfred Wegener formed this idea of Continental Drift. He argued that todays continents once formed a single landmass, which he named Pangaea (Greek for "all land"). It broke into pieces due to the weaknesses in the earth's crust as they were made up of less dense materials, which drifted centimeter by centimeter over millions of years until they arrived at where they are now. Figure 1.5 shows how the Pangaea split up into plates and drifted over the millions of years.
(Fig 1.5) The plates drifted through the years to where
they are now.
It is possible for the plates to move due to the convection currents in the mantle. The movement of these currents is similar to the movement of convection currents in a beaker of boiling water.
(Fig 1.6) Convection currents and plate movements.
(Fig 1.7) Magma heats up and rises.
Water that is heated expands and rises to the surface of the beaker. Similarly, the magma nearer the core expands and rises.
(Fig 1.8) Magma spreads out, plates
move apart.
Water that has risen spreads out across the surface, cools and sinks to the bottom. Similarly, the magma that has risen spreads out beneath the plates. As the magma spreads out, the plates are dragged along and they move away from each other. This is the process of sea-floor spreading.
(Fig 1.9) Magma sinks, plates
dragged towards each other.
When the magma sinks, the plates are dragged towards each other. This is the process of subduction. The repeated heating and rising of the magma sets up continuous convection currents in the mantle, causing the plates to move.
It is believed that the crust, beneath the oceans as well as the continents, together with the upper part of the mantle is divided into huge slabs called plates. The movement of the plates is explained by the earlier theory of Continental Drift.
There are eight identified major plates plus an assortment of smaller ones. The major plates include the Eurasian plate, the African plate, the North American plate, the South American plate, the Antarctic plate, the Indoaustralian plate, the Pacific plate and the Nazca plate.
(Fig 1.10) Major plates of the world.
Figure 1.10 shows the different plates on the world map, which shows that the plates are capped by both the oceanic and continental crust. Most volcanoes are found along and around the plate edges.
Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano.
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As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called an oceanic ridge. The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. The ocean floor gradually extends and thus the size of these plates increases. As these plates get bigger, others become smaller as they melt back into the Earth in the process called subduction.
The new rock at the edge has no sediments like the sand or mud, since it is formed only recently. Farther away from the ridge, sand and mud gradually settle on it, in an ever-thickening blanket. The oldest rocks may have 14,000 feet of sand and other sediments resting on top of it.
An example of an oceanic ridge is the Mid-Atlantic Ridge. It is one part of a system of mid-oceanic ridges that stretches for 50,000 miles through the world's oceans. The underwater mountains of the ridge may not be more than two miles higher than the surrounding sea floor.
On the whole, sea-floor spreading is basically volcanic, but it is a slow and regular process, without the explosive outbursts of the volcanoes on land.
Subduction is the process in which one plate is pushed downward beneath another plate into the underlying mantle when plates move towards each other. The plate that is denser will slide under the thicker, less dense plate. Faulting occurs in the process. It is the process in which rocks break and move or are displaced along the fractures. The subducted plate usually moves in jerks, resulting in earthquakes. The area where the subduction occurs is the subduction zone. A long, narrow, deep depression forms in this area. It is called an oceanic trench.
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The jerky movement, as well as the friction between the plates causes much heat, and together with the heat from the mantle and from radioactive decay, causes the subducted plate to melt. Magma is produced by the melting plate. It rises through fractures in the crust and reaches the surface to form volcanoes. In the end, the heat may be so intense that large areas of the crust are melted, forming granites just below the surface. The boundary of such plate movements is 'destructive' as part of the plate melts during subduction and is destroyed.
As plates move towards each other, the opposing force between them is so great that the massive folding or the bending of rocks also occurs at the edges of the less dense plate. A range of fold mountains, such as the Himalayas, is formed.
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Review Questions: Plate Tectonics NOTE: Each time, the
questions will change when you reload your browser! Top of Form 1 Your Fullname (required) Your Email Address (NOT
required) 1.What is the Earth made
of? 2.Which of the following
statement is true about the continental crust? 3.Which of the following
statements is false about magma? 4.The lithosphere
consists of: 5.Subduction does not
involve 6.Earth's core is 7.Subduction causes all
of the following except 8.Which of the following
is not part of the process of sea-floor spreading? 9.The process subduction
does not include 10.The word Pangaea is: Bottom of Form 1 |
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Results 1. "The crust, the
mantle and the core" ... CORRECT You got 5 out of 10 correct |
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A volcano constitutes a vent, a pipe, a crater,
and a cone. The vent is an
opening at the Earth's surface. The pipe is a
passageway in the volcano in which the magma rises through to the surface during an eruption. The crater is a bowl-shaped
depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released. Solidified lava, ashes, and cinder form the cone. Layers of lava, alternate with layers of ash to build the steep sided cone higher and higher. |
In the theory of plate tectonics most of the geological characteristics are explained in consequence of tension among plates. However, some of most spectacular formations are caused by "hot spots" which are situated far from the edges of tectonic plates.
Hot spots are fixed places within the mantle or oceanic lithosphere, where rocks melt to generate magma. When a hot spot is situated in the oceanic lithosphere a class of volcanoes known as shield volcanoes is built. These are constructed on the deep ocean floor and may be build high enough to rise above sea level as volcanic islands. The Hawaiian hot spot, for example, has been active at least 70 million years, producing a volcanic chain (of shield volcanoes) that extends 3,750 miles (6000 km) across the northwest Pacific Ocean.
Another one, the Yellowstone hot spot has been active for at least 15 million years, producing a chain of calderas and other volcanic features along the Snake River Plain (US) that extends 400 miles (650 km) westward from northwest Wyoming to the Idaho-Oregon border.
Where a hot spot lies beneath a continental plate the hot spot may generate enormous volumes of lava that accumulate layer upon layer. Thousands of square kilometers may be covered, these accumulations are called flood basalts.
An important example of a flood basalt is found in the Columbia Plateau
region of southeastern Washington, northeastern Oregon and westernmost Idaho;
about 130000 sq. km is covered by flood basalts, this is the same area
as the state of New York!
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. Introduction |
Plate Tectonics, theory that the outer shell of
the earth is made up of thin, rigid plates that move relative to each other.
The theory of plate tectonics was formulated during the early 1960s, and it
revolutionized the field of geology. Scientists have successfully used it to
explain many geological events, such as earthquakes and volcanic eruptions as
well as mountain building and the formation of the oceans and continents.
Plate tectonics arose from
an earlier theory proposed by German scientist Alfred Wegener in 1912. Looking at the shapes of
the continents, Wegener found that they fit together like a jigsaw puzzle.
Using this observation, along with geological evidence he found on different
continents, he developed the theory of continental drift, which states that
today's continents were once joined together into one large landmass.
Geologists of the 1950s and
1960s found evidence supporting the idea of tectonic plates and their movement.
They applied Wegener's theory to various aspects of the changing earth and used
this evidence to confirm continental drift. By 1968 scientists integrated most
geologic activities into a theory called the New Global Tectonics, or more
commonly, Plate Tectonics.
Tectonic plates
Tectonic plates are made
of either oceanic or continental crust and the very top part of the mantle, a
layer of rock inside the earth. This crust and upper mantle form what is called
the lithosphere. Under the lithosphere lies a fluid rock layer called the
asthenosphere. The rocks in the asthenosphere move in a fluid manner because of
the high temperatures and pressures found there. Tectonic plates are able to
float upon the fluid asthenosphere because they are made of rigid lithosphere. See
also Earth: Composition.
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#A. Continental
Crust |
The earth's solid surface is
about 40 percent continental crust. Continental crust is much older, thicker
and less dense than oceanic crust. The thinnest continental crust, between
plates that are moving apart, is about 15 km (about 9 mi) thick. In other
places, such as mountain ranges, the crust may be as much as 75 km (47 mi)
thick. Near the surface, it is composed of rocks that are felsic (made up of minerals
including feldspar and silica). Deeper in the continental crust, the
composition is mafic (made of magnesium, iron, and other minerals).
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#B. Oceanic Crust |
Oceanic crust makes up the
other 60 percent of the earth's solid surface. Oceanic crust is, in general,
thin and dense. It is constantly being produced at the bottom of the oceans in
places called mid-ocean ridges—undersea volcanic mountain chains formed at
plate boundaries where there is a build-up of ocean crust. This production of
crust does not increase the physical size of the earth, so the material
produced at mid-ocean ridges must be recycled, or consumed, somewhere else.
Geologists believe it is recycled back into the earth in areas called
subduction zones, where one plate sinks underneath another and the crust of the
sinking plate melts back down into the earth. Oceanic crust is continually
recycled so that its age is generally not greater than 200 million years.
Oceanic crust averages between 5 and 10 km (between 3 and 6 mi) thick. It is
composed of a top layer of sediment, a middle layer of rock called basalt, and
a bottom layer of rock called gabbro. Both basalt and gabbro are dark-colored
igneous, or volcanic, rocks.
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#C. Plate Sizes |
Currently, there are seven
large and several small plates.
Continue article...
Ring of Fire
Tsunami, Japanese word, meaning
"harbor wave" and used as the scientific term for seismic sea wave
generated by an undersea earthquake or possibly an undersea landslide
or volcanic eruption. When the ocean floor is tilted or offset during an
earthquake, a set of waves is created similar to the concentric waves generated
by an object dropped into the water. Most tsunamis originate along the Ring of
Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long,
that encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the
Hawaiian Islands.