Transcript Plate Tectonics

Plate Tectonics
The theory of plate tectonics was first proposed by
Alfred Wegener in 1910. Wegener noticed that the
shorelines of the continents seemed to ‘fit together’
like the pieces of a giant jig saw puzzle.
Wegener’s theory stated:
1) The continents were once all together in one
place forming a supercontinent, Pangea.
2) The continents broke apart and drifted to their
present locations.
Wegener’s theory was not taken seriously
because no one could believe that things as
large as continents could move and because
Wegener could not propose a mechanism
which could explain such motion.
Today plate tectonics and continental drift are accepted as facts.
Why? Let’s see the evidence!
First, there’s Wegener’s original observation. The shorelines of the
continents do fit together like the pieces of a puzzle.
But that could just be a
coincidence. For a theory
to be accepted as fact, there
must be more.....................
If the continents were once together, we ought to be able to find
similar kinds of rocks in places that were once together but have
since drifted apart.
If we believe that area A and
area B were once together, then
we should find similar rock types
in these two locations even though
today they are thousands of miles
apart.
Rocks are a lot like fingerprints. The mineral content and structure
of rock outcrops often identify them as unique.
Wegener himself investigated this possibility and found that, yes,
he was able to correlate rock types from opposite shorelines.
Correlating rock types is a second piece of evidence.
If we examined rocks from area A and found in them fossils that are
also found in area B but nowhere else, this would be powerful evidence
that these two areas were once together forming a single habitat.
It is important to choose our
fossils carefully. Finding fossils
of organisms that lived all over
the world would mean nothing.
But if we could find the same fossils in areas A and B, fossils that
exist nowhere else because they had a very limited range, then this
is even more powerful evidence that areas A and B were once
together.
Correlating fossils from opposite shores is a third piece of evidence.
Sea Floor Spreading
If the continents are moving apart then the seas between them must be
getting wider. This is called sea floor spreading. How do we know that
this is true?
First, a series of under-water
mountains called mid-ocean
ridges is found throughout the
world. These mountains are
formed as new sea floor (basalt)
is created from magma that wells
up from the mantle below.
As new sea floor is added
it pushes the old sea floor
apart.
Proof that this occurs can be
found by looking at the age
of the sea floor itself.
New sea floor is found at the mid ocean ridge where material is
constantly being added. As distance from the ridge in either
direction increases, so does the age of the rock.
Because the sea floor is spreading away from the center, rocks which are equidistant
but on opposite sides of the ridge are the same age. Rock B is the same age as rock D.
Rock A is the same age as rock E. The oldest rocks are found at the edges of the
continents.
More evidence to support sea floor spreading comes from the study of
paleomagnetism, the magnetic properties of ancient rocks.
When a magnetometer (a
device that detects the magnetic
properties of rocks) is towed
across the ocean it indicates that
there are alternating zones of
rock with either normal or
reversed polarity.
In the green zones bits of magnetic iron in the rocks are pointing to
where north is today. They act like countless little magnets trapped
in the rock when it solidified from magma.
But in the orange zones the bits of iron are pointing in the opposite
direction. Why?
This is because the earths magnetic field flip-flops on average about
every 250,000 years. North becomes south and south becomes north.
The rocks of the sea floor record these reversals.
In diagram ‘a’ the rock along the ridge emerges during a period of
normal polarity. As it cools that polarity is locked into the rock.
But as can be seen in
diagrams ‘b’ and ‘c’, as
new rock emerges along
the ridge it pushes the
old rock away. The new
rock may record reversed
or normal polarity dependon conditions when it
formed.
The result of these reversals can be seen as alternating bands on the
sea floor. These could only have been produced if the sea floor was
spreading apart as the earth’s magnetic field reversed itself over
long periods of geologic time.
So together, studies of the age of the sea floor and of paleomagnetism
prove that sea floor spreading has occurred. This is the forth piece of
evidence that continental drift is fact, not theory.
The Ultimate Proof
This is the Laser Geodynamics Satellite
(Lageos). It was launched in 1976 and
was joined in 1992 by Laegos II. They
orbit the earth at an altitude of 3,700 miles
and carry no electronic equipment.
Laser beams are bounced from one point on the earth, off the
satellite, to a second point on the ground. Scientists can then
measure the distance between the two points with great accuracy.
Geo positioning satellites enable scientists to
use GPS devices to make accurate measurements
of positions on the earth.
These systems show conclusively that the
continents are still drifting at a rate of a few
centimeters a year.
Let’s review the evidence for continental drift................................
1) The continents seem to fit together like the pieces of a jig saw
puzzle.
2) Correlation of rocks from opposite shores.
3) Correlation of fossils from opposite shores.
4) Sea floor spreading.....
a) The existance of mid ocean ridges (mountain ranges)
b) Age of the sea floor.....
c) Paleomagnetism
5) Direct satellite measurement of the drifting continents.
Continental drift is not a theory. It’s a fact.
Wegener was right! But why......?
How could continents, giant slabs of rock, possibly move? The key
is to understand the internal structure of the earth
We live on a very thin, cool
crust which averages about
50 km thick under the continents and 10 km under the
oceans. Beneath the crust
is the mantle. Rock in the
mantle is very hot and is
under high pressure. Rock
here is described as plastic
which means that, like
silly putty, it can flow very
slowly over inimaginably
long periods of time.
Because temperature
increases with depth,
the inside of the earth
is very hot. Rock near
the core is heated and
becomes less dense. It
slowly rises while
cooler rock nearer the
surface is more dense
and sinks. This forms
convection currents
just like those in our
atmosphere. As these convection currents in the mantle circulate, they
cause the continents above them to move. What Wegener had no way
of knowing is that the force that is driving plate tectonics is convection
currents in the mantle.
Let's sum up everything we've learned:
1) The earth's crust is divided into many pieces called tectonic
plates which move around over long periods of time.
2) Evidence for crustal movement includes:
a) The 'fit' of the continents.
b) Correlation of rock types from opposite shores.
c) Correlation of fossils from opposite shores.
d) Age of the sea floor (youngest near mid-ocean ridges
and oldest near the continents).
e) Paleomagnetism: zones of oppositely magnetized
rock on opposite sides of the mid ocean ridge.
f) Direct satellite and GPS measurement of continental
movement.
3) The force driving the plates are convection currents (density
differences) in the earth's mantle.