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PLATES TECTONICS
A world to explain
and some strange connections
What is plate tectonics?
Plate tectonics is a scientific theory developed during the 1960s and
1970s, that describes the large-scale motion of Earth's lithosphere.
This theoretical model builds on the concept of continental drift, a
theory first proposed by scientist Alfred Wegener in 1912.
Plate tectonics is the unifying theory of geology, capable to explain
seismic activity, volcanism and mountain formation.
Prior to the 1960s, most geologists held the view that the ocean basins
and continents had fixed geographic positions and were of great
antiquity.
Less than a decade later researchers came to realize that Earth’s
continents are not static; instead, they gradually migrate across the
globe.
Three Parts of Earth’s Interior
A knowledge of earth's interior is essential for understanding plate tectonics.
The Earth is composed of three layers: the crust, mantle and the core.
Earth’s crust
There are two different types of crust:
thin oceanic crust, that underlies the ocean basins, and thicker continental
crust, that underlies the continents.
These two different types of crust are made up of different types of rock.
The thin oceanic crust is composed primarily of basalt and the thicker
continental crust is composed primarily of granite.
The low density of the thick continental crust allows it to "float" in high relief on
the much higher density mantle below
Earth’s mantle
Earth's mantle is thought to be composed mainly of ultrabasic rock. It has
different temperatures at different depths. The temperature is lowest
immediately beneath the crust and increases with depth. The highest
temperatures occur where the mantle material is in contact with the heatproducing core. This steady increase of temperature with depth is known as the
geothermal gradient. The geothermal gradient is responsible for different rock
behaviors and the different rock behaviors are used to divide the mantle into
two different zones. Rocks in the upper mantle are cool and brittle, while rocks
in the lower mantle are hot and soft (but not molten). Rocks in the upper mantle
are brittle enough to break under stress and produce earthquakes. However,
rocks in the lower mantle are soft and flow when subjected to forces instead of
breaking.
Earth’s core
Earth's Core is thought to be composed mainly of an iron and nickel alloy.
The core is earth's source of internal heat because it contains radioactive
materials which release heat as they break down into more stable substances.
The core is divided into two different zones. The outer core is a liquid because
the temperatures there are adequate to melt the iron-nickel alloy. However, the
inner core is a solid even though its temperature is higher than the outer core.
Here, tremendous pressure, produced by the weight of the overlying rocks is
strong enough to crowd the atoms tightly together and prevents the liquid state.
Lithosphere
The crust and the rigid, outer zone of the mantle make up a layer that is called
the lithosphere.
The zone directly under the lithosphere is made of a flowing, denser layer
called the asthenosphere.
Lithospheric plates
The lithosphere is composed of about two dozen segments having irregular
sizes and shapes called lithospheric plates or tectonic plates that are in
constant motion with respect to one another. Seven major lithospheric plates
are recognized. These plates, which account for 94 percent of Earth’s surface
area, include the North American, South American, Pacific, African, Eurasian,
Australian-Indian, and Antarctic plates. Plates are bounded by three distinct
types of boundaries, which are differentiated by the type of movement they
exhibit: divergent boundary, convergent boundary, transform boundary.
Convergent boundaries 1
Convergent plate boundaries are locations where lithospheric plates are
moving towards one another. The plate collisions that occur in these
areas can produce earthquakes, volcanic activity and crustal
deformation.
When continental and oceanic plates collide the thinner and more
dense oceanic plate is overridden by the thicker and less dense
continental plate. The oceanic plate is forced down into the mantle in a
process known as "subduction". As the oceanic plate descends it is
forced into higher temperature environments. At a depth of about 100
miles (160 km) materials in the subducting plate begin to approach their
melting temperatures and a process of
partial melting begins. If a magma
chamber rises to the surface without
solidifying the magma will break through
in the form of a volcanic eruption.
The Andes Mountain Range of western
South America is an example of a
convergent boundary between an oceanic
and continental plate. Here the Nazca
Plate is subducting beneath the South
American plate.
Convergent boundaries 2
When two thick continental plates collide neither of them will subduce
because both of them have a density that is much lower than the mantle.
Effects found at a convergent boundary between continental plates include:
intense folding and faulting, the formation of a broad folded mountain range
(orogenesis), shallow earthquake activity, shortening and thickening of
the plates within the collision zone. The Himalaya Mountain Range and Alps
are the best active example of this type of plate boundary.
faulting
Folding and mountain building
Divergent boundaries
Divergent plate boundaries are locations where plates are moving away from
one another. This occurs above rising convection currents. The rising current
pushes up on the bottom of the lithosphere, lifting it and flowing laterally
beneath it. This lateral flow causes the plate material above to be dragged
along in the direction of flow. At the crest of the uplift, the overlying plate is
stretched thin, breaks and pulls apart (rifting). Magma from the mantle
flows into the rift forming new oceanic crust. The Mid-Atlantic Ridge and the
East African Rift Valley are a classic examples of this type of plate boundary.
rift
convection currents
new oceanic crust
Transform boundaries
Transform Plate Boundaries are locations where two plates slide past one
another. The fracture zone that forms a transform plate boundary is known
as a transform fault. Most transform faults are found in the ocean basin
and connect offsets in the mid-ocean ridges. The most famous example of
this is the San Andreas Fault Zone of western North America. The
earthquakes are usually shallow because they occur within and between
plates that are not involved in subduction. Volcanic activity is normally not
present because the typical magma sources of an upwelling convection
current or a melting subducting plate are not present.
Evidence for Plate Tectonics
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Evidence based upon fossil record
Evidence based upon earthquake patterns
Evidence based upon sea-floor spreading
Evidence based upon “hot spot” tracks
Evidence based upon fossil record indicates
that once continents were united
Evidence based upon earthquake patterns
shows location of plate boundaries
Evidence based upon sea-floor spreading
support the separations of continents and the
creation of new oceanic crust
Evidence of plate movement from Hot-Spot
tracks
What Drives Plate Motions?
Scientists still continue to study and debate the mechanisms that move the plates.
They believe, however, that a major driving force behind plate tectonics is
thermal convection in the mantle:
1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust.
2. The top limb of the convection cell moves horizontally away from the ridge crest,
as does the new seafloor (sea-floor spreading).
3. The outer limbs of the convection cells plunge down into the deeper mantle,
dragging oceanic crust as well. This takes place at the deep sea trenches.
4. The material sinks to the core and moves horizontally.
5. The material heats up and reaches the zone where it rises again.
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A closer look at the Mediterranean
Lesson Summary
• Most of the Earth’s geologic activity takes place at plate boundaries.
• Plates interact at three types of plate boundaries: divergent, convergent
and transform.
• At a convergent boundary with at least one oceanic plate, an ocean trench,
a chain of volcanoes develops and many earthquakes occur.
• At a convergent boundary where both plates are continental, mountain ranges
grow and earthquakes are common.
• At a divergent boundary, rifting occurrs and volcanic activity produces a mid ocean ridge
and small earthquakes.
• At a transform boundary, there is a transform fault and massive earthquakes
occur but there are no volcanoes.
• Plate tectonics is based on numerous evidence, including sea-floor spreading, fossil
distribution and pattern of earthquake epicenters.
• Plates of lithosphere move because of convection currents in the mantle.