12. Plate
Tectonics/Earthquakes/Volcanoes
1.
The Theory of Continental
Drift
The
shapes and positions of continents may seems fixed at
the time of human experience.
But
at the geologic time scale, measured in millions or tens of millions of years,
continents are quite mobile.
In
1912, a German meteorologist Alfred Wegener proposed
the theory of continental drift. He assumed
(1) about 250 million b.p., only one super continent on the earth, he called
“ Pangaea” (Greek for “ whole land”). And only have a single ocean.
(2) The super continent then
broke up into two massive pieces: Laurasia in
the North Hemisphere and Gondwanaland in the southern Hemisphere.
(3) Futhermore, those two massive lands
broke into a number of smaller continents.
(4)
His
theory was scoffed by the scientific community as fanciful at beginning.
However, gradually, more and more evidence support his theory:
Evidence:
(1) The continents fit together
pretty well. E.g. the bulges of
(2) Different continents (like
(3) they show distinctive glacial
scouring marks on rocks of the same age, suggesting that formerly the
continents were united and covered by a glacial.
(4) They possess many of the
same types of rare fossils.
However,
those evidence still not convinces the scientific
community to accept the theory:
(1) Earth’s crust was believed
to be too rigid to permit such large-scale motions;
(2) No suitable mechanism could
explain the energy needed to displace such large masses for a long journey.
The
questionable validity of continental drift theory attract
continuing research about crustal mechnics.
Since
1950s, more and more data have been collected by geologists, geophysicists,
oceanographers from the ocean floors and the underlying crust. The theory of
plate tectonics was proposed which further support the continental drift
theory.
12.2 Plate Tectonics
Plate Tectonic study the motion of plates
(1)
Modern lithospheric plates
Plate
tectonics theory divided the lithosphere into seven major plates which
correspond to major continents or ocean basins:
-Pacific
plate
-North
American plate
-South
American plate
-Eurasian
plate
-African
plate
-Indo-Australian
plate
-Antarctic
plate
(2)
Mechanisms of plate movement
The
process that drives plate movement is not fully understood, but it is clear
that involves geothermal energy and convective currents in the mantle.
Presumably, plumes of molten material from deep in the mantle rise to the asthenosphere and initiate plate movement.
The
plates move at the very slow rate of about 1-4 inches per year. Comparable to the rate of fingernail growth.
(3) Types of plate boundries
A. Spreading centers. sites of
diverging lithospheric plates
Along
this kind of boundaries,
-Tensional
force is induced by convective currents of magma which is the molten material
in mantle. As a result, plates being pulled apart, or separate along a rupture.
-magma
material rises from the rupture, push plates move towards both sides, new ocean
is formed. Like
-those
rising magma as underwater volcano eruption, and develop a linear string of
mid-ocean ridges.
The
validity of the theory of seafloor spreading has been confirmed by two sets of
evidence: paleomagnetism and core sampling.
Paleomagnetism: when any rock containing iron grains is formed, it is magnetized so
that the iron grains become oriented toward Earth’s magnetic pole. This
orientation then becomes a permanent record of the polarity of Earth’s magnetic
field at the time the rock solidified.
During
the last 100 million years, Earth’s magnetic field is known to have reversed
itself, with the north and south magnetic poles changing places more than 170
times.
Thus
if the seafloor has spread laterally by the addition of new crust at the
oceanic ridges, there should be a relatively symmetrical pattern of magnetic
orientation on both sides of the ridges. The measurement of the magnetic
orientation proved that this is the case.
Core
sampling: several thousand core samples were collected from the holes drilled
into the sea-bottom floor by a research ship. Analysis of
those sediments indicate that sediment thickness and age increase with
increasing distance from the oceanic ridges.
This
indicates that sediments farthest from the ridges are oldest. Sediments near
ridges are thinner and younger, and right at the ridges the materials is almost
all igneous, with little accumulation of sediment.
Thus
the seafloors can be likened to gigantic conveyor belts, moving ever outward
from the oceanic ridges and toward the trenches.
B. Subduction zone. Sites of converging
lithosphere plates.
Since
the earth remains virtually constant in diameter, addition of new material to lithospheric plates along spreading centers must be
balanced by the loss of materials from lithospheric
plates where they collide with one another.
(
I)oceanic plate versus continent-bearing plate; where a oceanic plate collides
with a continent plate, Once the oceanic plate collide with continental plate,
as we talked last time, because the oceanic is more dense than continental
plate, so it plunge or subduct downward into mantle.
As
the crustal materials melted in deep, because they
are lower in density than the mantle materials, they eventually will rise back to the surface as molten magma (intrusive igneous
rocks, like granite) or lava (volcanoes).
Because
of the high heat and pressure produced by collision, the rocks around the subduction zone may change their properties, becoming
metamorphic rocks.
Most
subduction zones occur as collisions between an
oceanic and continental plate.
Some
subduction zones form where two oceanic plates
collide. This form:
-distinctive volcanic island arcs (such as
-deep sea trenches (like the
the
oceanic plate is driven beneath the continental plate. This plate drags down
with it crustal materials and forms an ocean trench.
(ii)
continental plate versus continental . Rarely, two
continental plates will collide along a subduction zone.
Since continental plates are thick, their collision unleashes tremondous mountain building forces, in the form of folding
and faulting.
The typical examples of this
type is the ongoing ramming of
C. transform boundaries. Sites where two
plates are sliding laterally with respect to more another.
1. shearing, a lateral, or sideways,
force is exerted on two plates, so that they fracture to produce faults and
associated earthquakes.
2. Most transform plate
boundaries occur in ocean basins, but there are some exceptions which occurs beneath continental plates.
Questions ?
Despite
the scientific enrichment we have received from plate tectonics theory, we
still have many unanswered questions.
For
example, why are some plates so much larger than others ?
What
determines the zones of crustal weakness where plate
boundaries occur ?
What
is the ultimate cause of plate movement ?
Those questions still waiting for answers. If you are interested, you
may continue the research efforts in your future to answer those unsolved
puzzles.
12.3 Earthquakes associated
with plate tectonics
The
majority of earthquakes are explainable based on plate tectonic theory, The lithosphere is broken into rigid plates that
move away from, past, and into other rigid plates.
(a)The
pull-apart motion at spreading centers causes rocks to fail in tension. This
process yields mainly smaller earthquakes.
(b)
the transform motion occurs as the rigid plates slide
past each other in the dominantly horizontal movement of transform faults. This
process creates large earthquakes as the irregular plate boundaries retard slip
because of irregularities along the faults. It takes a lot of stored energy to
overcome the rough surface, nonslippery rocks, and
bends in faults. When they are finally overcome, a large amount of seismic
energy is released.
Like the
So,
parts of
A
number of communities and buildings in that area just sit on the the fault or nearby, like Like
the football stadium for the UC Berkeley directly sit astride the fault zone.
This
transform motion causes the major earthquakes in
( c)
The collision boundaries are also associated with violent earthquakes. Let’s
see the distribution of earthquakes in the world, we
can see there is highly consistent between plate boundaries and earthquake
belts.
12.4. Volcanoes
What
is volcanoes ?
Volcanoes
are conduits in the Earth’s crust through which gas-enriched, molten silicate
rock-magma-reaches the surface from beneath the crust.
3.2. The
causes of Volcano eruption
In
order to understand the volcano eruption, we need to know more about magma.
Though the origin of magma is still debated, it is generally believed that the
mantle is partially liquefied 75-300 km below the Earth’s surface. The melting
of the rock generates magma. Rock may melt by (1) raising its temperature, (2)
lowering the pressure on it, or (3) increasing its water content.
If temperature in deep increases, some
rock melts with a resultant increase in volume that causes overlying rocks to
fracture. The fractures allow more material to rise to lower pressure levels,
causing more rock to liquefy.
Magma
at depth does not contain gas bubbles because the high pressure at depth keeps
gas dissolved in solution. But as magma rises toward the surface, pressure
continually decreases, and gases begin to come out of solution, forming bubbles
that expand with decreasing pressure.
When
gas-bubble volume reaches around 75 percent, gas can fragment the magma into
pieces which are carried up and out by a powerful gas jet. Upon escape from the
volcano, the gas jet draws in air which adds to buoyancy in the turbulent,
rising plume.
There
are three types of magma: basalt, andesite, and Rhyolite (see table
6.6)
The
fliuidity of a liquid is measured by its viscosity,
its internal resistance to flow. The lower the viscosity of a magma, the more
fluid is its behavior. The viscosity of
magma increases with the content of silica. Basalt has highest temperature
which causes atoms to spread farther apart, thus decreasing density and
increasing fluidity, so more of it reaches the surface (80%) while Rhyolite is more viscous and they tend to be traped deep below the surface. In addition, gases in basalt is relatively
easy to escape and therefore the basaltic volcano eruption is relatively
peaceful, whereas the gas is very difficult of escape from the Rhyolite volcano eruption, and also the magma is very
sticky and resistant to flow because of high viscosity. So how the entrapped
gas escapes from the sticky magma ? explosion.
3.4. types of volcanoes
While
most classification schemes refer to a specific historical event characterizing
an eruption sequence, it should be realized that each volcanic eruption is
unique, and may over time take on the characteristics of more than one type.
based on the behavior of volcano eruption, there are two major types: