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Geological Phenomena
Energy from below…
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Below the surface, planet Earth hides strong internal
forces.
These forces raise, fold and fracture the crust.
The crust is always transforming due to these forces.
Earth has its own energy source: its interior with its
intense heat.
This energy source is responsible for multiple geological
phenomena: volcanic eruptions, mountains and
earthquakes.
…and from above
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There is another energy source on which we depend:
the sun.
The sun is responsible for many other phenomena like
erosion, ocean currents and wind.
Continental Drift
Continental Drift
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At first glance, the soil seems to be immovable. We
have thought for many years that the continents,
mountains and oceans had never changed since the
birth of the planet.
At the beginning of the 20th century, Alfred Wegener,
studied a number of clues that led him to hypothesize
that the continents are moving.
He observed many maps and noticed that South
America and Africa fit together like a puzzle piece. In
fact, many of the other continents seem to fit together,
as well.
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Wegener also studied the rock compositions of multiple
mountain chains on several continents.
He discovered that the rocks on these different
mountains are of the same age and of the same
composition. Were they once a part of the same
mountain chain?
This same discovery was made among fossils. The
mesosaurus, for example, has been discovered in Brazil
and in the south of Africa.
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After his discoveries, Wegener advanced the idea that
the continents once formed one single continent called
Pangaea.
It would have existed about 220 million years ago.
It was in a single ocean called Panthalassa.
This supercontinent eventually became fragmented
until they attained their current locations.
Wegener called this phenomenon Continental Drift.
The problem? He didn’t know how it happened!
Tectonic Plates
Tectonic Plates
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In the 1960’s, sonar (radar) was invented.
With this device, geologists made many underwater
discoveries.
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The Tectonic Plates
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Plates of crust that fit inside one another, better than the
continents.
The Ocean Ridges
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A long, underwater mountain chain The Mid-Atlantic Ridge seems
to separate the Atlantic ocean in two.
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During this same time, scientists also discovered the
existence of the upper mantle.
This discovery revealed that the continents float on a
layer of partially-molten rock. It is then easy to imagine
that the continents are in movement.
John Tuzo Wilson, a geophysician from Canada,
modified Wegener’s theory. He suggested that all of
the Earth’s Crust is divided into rigid plates that are
always in motion.
Convection: The Driving Force of Plate
Tectonics
Convection Currents
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Tectonic plate movement is explained by convection.
Scientists are convinced that there are convection
currents in the upper mantle.
A convection current results when a liquid is subjected
to extreme heat.
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Example: When water is boiled on the stove, water is
moving. This is convection. The same thing occurs with
magma in the upper mantle.
Magma receives its heat from the core, (the hottest
part of the planet).
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A convection current forces hot rocks nearest the core
upwards towards the crust. When they attain the crust,
they are forced horizontally.
This movement is what forces plate tectonics to move.
How do the Plates Move?
How do the Plates Move?
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Due to convection currents, the plate tectonics are
constantly in motion.
These plates move anywhere from 1 to 20 cm per year.
Slowly but surely, the plates are either moving closer
together, are separating or are sliding past one
another.
These movements create an enormous amount of
pressure on the crust. When that pressure reaches it
maximum, the crust folds, fractures or lifts.
Divergent Boundaries
Two plates that separate from one
another. Also called a rift.
Convergent Boundaries
Two plates that approach one another. Also
called a subduction zone.
Transform Boundaries
Two plates that slide past one another. Also
called a transform fault.
Divergent Boundaries
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Once two plates separate from one another, a fault,
(crack), appears in the crust.
Magma infiltrates this fault and forms new crust.
This is what produces the oceanic ridges.
Convergent Boundaries
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If the Earth is making new crust in some places, crust
disappears in others. This disappearance occurs when
two plates converge or collide.
When two plates collide, one of two things occurs:
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Continental-continental – because both plates are equally
dense, they will push each other upwards, forming
mountains.
Continental-oceanic – the oceanic plate will sink under the
continental plate and will become magma.
Transform Boundaries
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Two plates may also slide past one another.
This slip causes severe earthquakes and are marked by
fault lines.
Mountain Building
Orogenesis
Orogenesis
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Sometimes, two plates collide and neither one sub
sinks under the other. In this case, the crust folds and
mountains result. Examples – the Himalayas.
The Himalayan mountains are fairly young. Because
their plates, (Indian and Eurasian), are still colliding,
these mountains continue to grow in height by a few
centimetres each year.
The same effect is occurring with the Alps, where the
Eurasian and the African plates are colliding.
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On the other hand, there are certain mountain chains
that were formed in other ways. Instead of growing,
they are eroding each year and getting smaller.
The Laurentians and the Appalachian mountains were
formed after a sudden intrusion of magma. These are
very old mountains and have no chance of becoming
taller.
Erosion is progressively causing these mountains to
disappear.
Volcanoes
The Formation of Volcanoes
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There are several types of volcanoes, but were formed in
one of two ways in the different plate boundary zones.
Convergent Boundary:
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Occurs where an oceanic plate penetrates the mantle below a
continental plate.
When the oceanic plate enters the mantle, it melts and becomes
magma. This magma is trying to escape due to pressure.
Magma is trying to escape through the cracks in the crust. Usually,
the magma is blocked and accumulates in a magmatic chamber.
Once the pressure becomes too high, the magma escapes in a
violent explosion.
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Divergent Boundary:
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Other volcanoes form in areas where plates are separating.
They are less violent because magma escapes with less
pressure.
The lava contributes to the formation of new soil.
Structure of a Volcano
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Magmatic Chamber: Where the magma accumulates
until an eruption.
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Chimney:
The throat.
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Crater:
The mouth.
Earthquakes
When the Earth Shakes…
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An earthquake is caused when two plates rub against
each other.
This rubbing causes shockwaves that are more or less
intense.
Some earthquakes are devastatingly strong, while
others are so weak that they are not even felt.
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All tectonic plate movement causes earthquakes,
whether in a convergent, divergent or transform zone.
Divergent Boundary
Weak earthquakes.
Convergent Boundary
Strong earthquakes.
Transform Boundary
Extreme earthquakes.
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Three quarters (3/4) of the planet is covered in water.
Therefore it stands to reason that earthquakes occur
often in water.
If the earthquakes occur in shallow water, a tidal wave
is possible. This is also called a tsunami.
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The waves created by an
earthquake grow quickly.
Once they attain shallow
waters, the waves can be
as high as 30 m.
Danger Zones
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Certain regions of the planet are more prone to
earthquakes and volcanic eruptions than others.
These regions are located near the boundaries of
tectonic plates.
The regions most
affected by tectonic
activity is called the
Pacific Ring of Fire,
marking the
boundaries of the
Pacific plate.
Erosion
Erosion
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The Earth seems to never change, but there are small,
slight changes that occur everyday.
Continental drift causes some of these changes to the
relief of the Earth, but another is the effect of erosion.
Rocks, despite their hardness, are not everlasting.
Under the effects of water, freezing and wind, rocks
degrade over time. This is erosion.
Erosion diminishes the heights of mountains and carves
out valleys.
3 Stages of Erosion
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Weathering
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Transport
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The freeze-thaw cycle fragments rocks and soil on the
surface of the Earth.
These rock and soil fragments are transported by wind and
water.
Sedimentation
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After being deposited, the sediments accumulate and
compact, forming new rock at the bottom of oceans or in
valleys.
Weathering
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Weathering occurs in many ways.
Biological Weathering
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Mechanical Weathering
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This type is performed by living things. The roots from
plants and trees infiltrate the bedrock and break it.
This type is caused by variations in temperature, by wind or
by water. This is a physical process that doesn’t alter the
chemical structure of the rock.
Chemical Weathering
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Rain is often somewhat acidic due to pollutants the air.
These acids degrade the rocks.