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Plate Tectonics
The Driving Force of Earthquakes
• Before we can
understand
earthquakes and their
effects, we need to
have a basic
understanding of the
driving force behind
them, plate tectonics.
Bent rails from 1976 Guatemalan earthquake.
What is Plate Tectonics ?
• Plate tectonics is a
theory whereby the
earth’s lithosphere is
broken up into
numerous fragments,
or plates, each of
which independently
rides or drifts on the
plastic asthenosphere.
• The vast majority of earthquakes occur along the boundaries of
these tectonic plates. The above diagram shows the epicenters
for recent large earthquakes throughout the world. Notice how
the epicenters tend to fall in long belts, with few outside these
regions.
• If we look at a map of the Earth’s surface, one
of the features which is immediately apparent is
the apparent jigsaw puzzle fit of the continents
of South America and Africa.
Alfred Wegener
• In fact, based on this and
other lines of evidence,
the German meteorologist
Alfred Wegener theorized
that approximately 200
million years ago all of the
present day continents
were joined together into a
single landmass or
“supercontinent” he
termed Pangea.
• According to his
theory, over the past
200 million years, the
continents had drifted
over the surface to
their present positions.
But how was this
possible? To
understand this we
must examine the
Earth’s interior.
What Does the Interior Look
Like?
• First, we must examine
what we mean by the
terms lithosphere and
asthenosphere. If we look
at a cross-section through
the Earth’s interior, we see
that it is layered, with a
crust, mantle and a 2-part
core (an outer liquid core,
and a solid inner core).
How we know this we will
examine at a later time.
The Crust
• As you can see from
the previous diagram,
Earth has 2 distinct
forms of crust, oceanic
crust which underlies
all of our ocean basins
and continental crust
which makes up our
continents.
Oceanic Crust
• Oceanic crust is made
entirely of the rock basalt,
and is between 5 and 10
km thick. Basalt is very
rich in both of the
elements iron (Fe) and
magnesium (Mg). As a
result of its’ composition,
oceanic crust tends to be
quite dense.
Continental Crust
• Continental crust by
contrast is largely granitic
in composition and is
approximately 35 to 70
km thick. It is
compositionally rich in
elements such as silicon
(Si), sodium (Na), and
potassium (K). As a
result, it tends to be much
less dense than the oceanic
crust.
The Mantle
• Contrary to popular belief,
the Earth’s mantle is not a
liquid, but is instead a
solid composed largely of
the rock peridotite. As
pressure and temperature
increase as you go further
towards the core, the
peridotite metamorphoses,
or changes, into new rock
types.
The Asthenosphere
• Within the upper mantle,
pressures and
temperatures are high
enough to cause the solid
rock to become a plastic.
The zone where this
occurs is called the
Asthenosphere. Silly
Putty would be a perfect
analogy to what the
mantle is like here.
The Lithosphere
• The solid crust, plus
the solid upper mantle
above the plastic
asthenosphere act as a
single package, which
we call the
Lithosphere.
The Core
• As mentioned earlier,
Earth has a 2-part iron
core, an outer core that is
liquid, and an inner core
which is solid. At first
that may seem backwards,
but near the center of the
Earth, pressures are so
high that temperatures
cannot rise high enough to
cause the inner core to
melt.
Harry Hess and Seafloor
Spreading
Harry Hess
• In the period following
World War II,
considerable research was
done on the ocean floor
and some remarkable
discoveries were made by
Harry Hess, a naval
commander and geology
professor at Princeton
University.
Hess had determined several important facts about the
seafloor:
1. The existence of a mid-ocean ridge system (an
approximately 10,000+ foot high volcanic mountain
chain that bisects the oceans).
2. The ocean floor is young (less than 200 million years
old, compared to the 4 billion age of the continents).
3. Age progression on sea floor (oldest adjacent to the
continents and youngest at mid-ocean ridges).
• Based on his findings, he
formulated his theory of
“seafloor spreading”.
According to his theory,
because it it less dense, hot
asthenosphere rises upwards
in certain areas of the Earth’s
mantle. This is similar to the
idea of a hot air balloon
rising. As the air is heated it
becomes less dense and
begins to rise. The rising
asthenosphere then causes the
overlying lithosphere to bend
upwards. The rising
asthenosphere also as it rises,
begins to partially melt.
• As the solid lithosphere bends upwards, it begins to crack and break. The
rising molten asthenosphere now can reach the surface, where it can cool and
solidify forming new seafloor. As the magma solidifies, its’ density increases
and as a result, starts to sink. However, since hot asthenosphere continues to
rise upwards, it cannot. Instead of sinking, the cooled magma spreads
outwards to the sides. The cycle then repeats and older cooling magma is
replaced by younger. These areas are called “divergent boundaries”. In this
way, continents will “drift” over the Earth’s surface.
•
If the Earth’s lithosphere
is separating at
“divergent boundaries”, it
must be pushed together
in other regions. These
are termed “convergent
boundaries”. Since there
are 2 distinct forms of
crust, 3 possible types of
convergent boundaries
are possible:
1. Ocean-Continent
2. Ocean-Ocean
3. Continent-Continent
Ocean-Continent Convergence
• Convergence between oceanic
and continental lithospheric
plates generally results in the
denser oceanic lithosphere
being pushed under, or
subducted beneath the less
dense continental lithosphere.
This is the case in South
America where the Nazca Plate
is being subducted beneath the
South American Plate, and also
in the Pacific Northwest where
the Juan DeFuca Plate is
subducted beneath the North
American Plate
Ocean-Ocean Convergence
• In the case of
convergence between
2 oceanic plates, the
older, colder and
therefore more dense
oceanic plate will
preferentially be
subducted beneath
the younger oceanic
plate. This is the
case in the region
around Japan. Notice
in the lower figure
how seismicity in
Japan is tied to the
tectonic boundaries.
Continent-Continent Convergence
• The final possibility
involves the convergence
between two continental
plates. In this case,
continental lithosphere,
which is low in density,
cannot be subducted.
Instead, the lithosphere is
pushed upwards, forming
tremendous mountains,
such as the Himalayas and
the Alps.
Hot Spot Volcanism
• At both divergent and
convergent boundaries, we
have volcanic activity. But
what about areas such as
Hawaii, and Yellowstone
National Park, where
volcanic activity occurs in
the middle of a tectonic
plate, not associated with
either of these boundaries?
These areas are called “Hot
Spots”.
• In these situations, hot
material rising from the
lower mantle rises
upwards. As it rises, it
partially melts forming a
magma. It then “punches
through” the lithosphere
and at the surface forms a
volcano. As the
lithosphere moves over
this mantle “plume”, a
chain of volcanoes will
form.
Transform Plate Boundaries
• The final situation that
needs to be addressed is
that where two tectonic
plates simply slide past
one another. As one can
observe, the mid-ocean
ridge is not a continuous
straight line, but instead
consists of short
segments offset from
one another. Connecting
these offset segments are
transform faults.
• Quite possibly, the most
famous example of a
transform fault is the San
Andreas Fault in
California. People often
think that with a large
earthquake on this fault,
California will fall into the
ocean. However, the two
sides are simply sliding
past one another.
Pacific Plate
North
American Plate
• In summary, we have seen how that due to motions within the
Earth’s mantle, several different types of tectonic boundaries will
develop: divergent, convergent and transform. In addition to
these, areas where material is rising from the lower mantle will
form hot spot volcanism. In the next section, we will look at
faulting and see how it is related to tectonism.
Activities
• Activity #1: How Tectonic Forces
Affect Faults
Determine a fault's slip by knowing the
tectonic environment around it -- and
vice versa.