Unit #2 Review: Plate Tectonics
The lithosphere is the solid outermost shell of a rocky planet.
The Lithosphere can be divided up into layers:
- crust (the rocks of the continents and the ocean floor)
- uppermost mantle (upper mantle or lower lithosphere) (joined to the
crust)
Plate tectonics is an important theory developed in the 1960s to explain how the
continents move across the Earth's surface.
The lithosphere is solid rock, as rigid and stiff as steel. Beneath it is a softer,
hotter layer of solid rock called the asthenosphere ("es-THEEN-osphere") that
extends down to around 220 kilometers depth. Because it's at red-hot
temperatures the rock of the asthenosphere is weak ("astheno-" means weak in
scientific Greek), and can bend in a plastic way. As a result, the lithosphere
floats on the asthenosphere.
The plates make up Earth's outer shell, called the lithosphere.
Early 20th century geologist Alfred Wegener realized that the puzzle-like fit of the
continents was more than a coincidence, but he couldn't correctly explain what
powered their movement.
Geologists now know that the Earth's outermost layer, the lithosphere, is divided
into independently moving plates into which the continents are embedded. The
plates "float" on a layer called the asthenosphere.
The geo-scientific community accepted the theory after the concepts of seafloor
spreading were developed in the late 1950s and early 1960s.
On Earth, there are seven or eight major plates (depending on how they are
defined) and many minor plates.
Where plates meet, their relative motion determines the type of boundary:
convergent
Divergent
transform.
Earthquakes, volcanic activity, mountain-building, and oceanic trench formation
occur along these plate boundaries.
The lateral relative movement of the plates typically varies from zero to 100 mm
annually.
Tectonic plates are able to move because the Earth's lithosphere has a higher
strength and lower density than the underlying asthenosphere.
Convergent Boundaries
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2 plates are running into each other
Where plates serving landmasses collide, the crust crumples and
buckles into mountain ranges. India and Asia crashed about 55 million
years ago, slowly giving rise to the Himalaya, the highest mountain
system on Earth.
As the mash-up continues, the mountains get higher.
convergent boundaries also occur where a plate of ocean dives, in a
process called subduction, under a landmass.
As the overlying plate lifts up, it also forms mountain ranges. In
addition, the diving plate melts and is often spewed out in volcanic
eruptions such as those that formed some of the mountains in the
Andes of South America.
At ocean-ocean convergences, one plate usually dives beneath the
other, forming deep trenches like the Mariana Trench in the North
Pacific Ocean, the deepest point on Earth. These types of collisions
can also lead to underwater volcanoes that eventually build up into
island arcs like Japan.
Divergent Boundaries
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At divergent boundaries in the oceans, magma from deep in the
Earth's mantle rises toward the surface and pushes apart two or more
plates. Mountains and volcanoes rise along the seam.
The process renews the ocean floor and widens the giant basins.
On land, giant troughs such as the Great Rift Valley in Africa form
where plates are tugged apart. If the plates there continue to diverge,
millions of years from now eastern Africa will split from the continent to
form a new landmass. A mid-ocean ridge would then mark the
boundary between the plates.
Transform Boundaries
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two plates grind past each other along what are called strike-slip faults.
Example: San Andreas Fault in California
These boundaries don't produce spectacular features like mountains or
oceans, but the halting motion often triggers large earthquakes, such
as the 1906 one that devastated San Francisco.
Earthquakes
• The shaking of the Earth’s crust caused by a release of energy.
• The major cause is the build up of stress between two lithospheric plates.
• Sometimes plates moving past each other will lock, and over time stress
builds. Plates begin to change shape and deform as well.
• Eventually, the stress becomes too much and the plate slips. This movement
is what causes the earthquake.
• The plates that had become deformed snap back into their original shape.
(This process is known as the Elastic Rebound Theory).
Folding and Faulting
• When lithospheric plates are put under stress, they react by folding or
faulting.
• A fold occurs due to compressional stress or plates moving towards each
other.
• 2 Basic Folds:
• Anticline
• Syncline
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A fault is a crack or break in the Earth’s crust where movement has occurred.
3 Basic faults :
• Normal
• Reverse
• Strike-slip
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Anticline- An upwards fold.
Syncline- A downward fold.
Normal Fault- caused by tensional stress (crust is being pulled apart) and
one rock plane drops down on one side of the fault.
Reverse Fault- caused by compression stress (crust is being pushed
together) and one rock plane rides up on one side of the fault.
Strike-slip- caused by shear stress and the rocks on either side moved
horizontally past each other.
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2 Major Types of Volcanoes:
1. Composite (or Stratovolcanoes):
• Mainly found along subduction zones.
• Examples: Andes and the Cascades.
• These volcanoes are explosive and present a great threat to human life.
• Cone shaped
• Magma contains higher amounts of silica (making it stickier or more
viscous).
• The stickiness of the magma plugs up the volcano, which builds pressure, and
causes an explosive eruption.
2. Shield:
• Mainly found where sea floor spreading occurs.
• Example: Hawaiian Islands
• Quiet flowing eruptions with large amounts of lava.
• Found around Iceland and hotspots- little is known about them, but appear
to be areas where magma rises from the asthenosphere and burns through
the plate.
• Broad shape with gentle slopes.
• Lava is runnier (less viscous), thus travels further from the crater before it
cools. This builds up the layers of the volcano over time.
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Hotspots:
There are a couple of theories to explain this process
One suggestion is that hotspots occur due to hot mantle plumes that rise
from the core mantle boundary. (high temperatures cause it).
Another hypothesis is that lithospheric extension permits the passive rising
of melted mantle from deep below the surface. (spreading plates allow
material from below to rise).
Examples: Hawaii and Yellowstone
Intrusive Volcanic Features:
• DIKE – the path that igneous rock makes through rock layers on its way to
the surface
• SILL – when the igneous rock intrudes between rock layers and is unable to
penetrate, separating the layers
• LACCOLITH – pooling of magma between rock layers. Normally sills occur
here, but instead the magma pushes the rock layers up and creates a domelike feature
• VOLCANIC NECK – when overlaying layers of soil are eroded away from an
ancient volcano.
• BATHOLITH – largest feature created as a result of erosion exposing this
hardened magma. Coast Range in B.C. is the largest batholith in North
America
Extrusive Volcanic Features:
• Lava – igneous rock that has reached the surface.
• Pyroclastics – any material blasted out of a volcano (cinders, ash, gases, and
rocks).
• Caldera – huge volcanic crater formed in two ways 1)the mountain looses
support and collapses in on itself due to lack of lava. 2) the entire top of the
mountain is blown off.
• Lahars – heat melts ice and snow forming mud flows
• Nuees ardentes – pyroclastic flows of super heated denser than air gas (1000
degrees C and 160k/hr. Because it is denser, it hugs the ground and destroys
everything in its path.
Hot springs: Occur when magma is close to the earth's surface, water comes
into contact and becomes heated.
This water finds a path back to the surface along a fault line and can reach
200 C.
Geysers: Same as hotsprings, but if there is an obstruction, the water pools
become superheated. As pressure builds, it reaches a point where it is
expelled forcefully.
Review Questions:
1. What is the theory of plate tectonics?
2. Identify the layer below the lithosphere, and describe the characteristic of
this layer.
__________________
Characteristics:
3. Label the intrusive and extrusive volcanic features:
4. Identify which photo represents a convergent plate boundary and which
photo is a divergent plate boundary. Also include a physical feature that
would be found at this particular plate boundary.
a. _____________________
b. ___________________
c. ____________________
5. Identify what type of fault is represented in each photo.
a. __________________
b. __________________
c. _________________
6. Define the following terms about volcanoes…
Lahars=
Pyroclastic Flow=
Composite Volcano=
Shield Volcano=
Dike=
Sill=
Laccolith=
Batholith=
Caldera=
7. An earthquake has occurred at point “a.” Label both point “a” and “b” on
the diagram.
8. What is a tsunami? Name a location on the globe that would be at risk of
experiencing a tsunami. Why would a location like the lower mainland of
British Columbia not be at risk of experiencing an extremely devastating
tsunami?