Transcript convection currents in the mantle.

Pg. 96
Dec. 7, 2012
Focus:
Pangaea and Continental Drift
Objective: 6.E.2.2- I will be able to model Pangaea
and analyze the evidence for continental drift.
HW: review / practice sheet
Warm Up: Student Checklist
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!
• How did you decide where each of the continents or land masses went?
• Where did you place North America? Why? Where did you place South
America? Why? What about Africa? Australia? India? Antarctica?
• What evidence or clues did you have to help you place the land masses
in the correct spot?
• Based on what you know about the continents and Pangea, how do you
think that the fossils of the same animals were found in different
continents? What do you think happened?
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.
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) Same rocks from opposite shores.
3) Same 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.

Continental Drift: The gradual movement
of the continents across the earth.
Copyright © 2010 Ryan P. Murphy
• The speed at which the plates move is about
the speed at which your fingernails grow.
Copyright © 2010 Ryan P. Murphy
• Video! Plate Tectonics.
– http://www.youtube.com/watch?v=ryrXAGY1dmE
• In 1915, The German geologist Alfred
Wegener (1880-1930) proposed continental
drift.
Copyright © 2010 Ryan P. Murphy
• In 1915, The German geologist Alfred
Wegener (1880-1930) proposed continental
drift. Not accepted until the 1950’s!
Copyright © 2010 Ryan P. Murphy
• In 1915, The German geologist Alfred
Wegener (1880-1930) proposed continental
drift. Not accepted until 1950’s!
“I say.” “Africa and South
America fit strangely like
two puzzle pieces.”
Copyright © 2010 Ryan P. Murphy

Evidence for continental drift.
-
Copyright © 2010 Ryan P. Murphy

The shapes match.
Copyright © 2010 Ryan P. Murphy

Same fossils found on different continents
 These
are the pictures on the puzzle pieces.
Copyright © 2010 Ryan P. Murphy
• Same fossils found on different continents
– These are the pictures on the puzzle pieces.
Copyright © 2010 Ryan P. Murphy

The Same rock structures on different
continents
Copyright © 2010 Ryan P. Murphy
• What is this a fossil of?
– Where would you expect to find a specimen
like this on the planet?
Copyright © 2010 Ryan P. Murphy
• Answer! This is a fossilized tropical plant
found on Antarctica.
Copyright © 2010 Ryan P. Murphy
• Answer! This is a fossilized tropical plant
found on Antarctica.
– Remember, the continents have moved and
Antarctica use to be in a warmer climate.
Copyright © 2010 Ryan P. Murphy
• Answer! This is a fossilized tropical plant
found on Antarctica.
– Remember, the continents have moved and
Antarctica use to be in a warmer climate.
“Darn you
continental Drift.”
“Darn you.”
Copyright © 2010 Ryan P. Murphy

Fossils of plants and animals in Antarctica
Copyright © 2010 Ryan P. Murphy
• Behold the Supercontinent! Pangea
?
Copyright © 2010 Ryan P. Murphy
• Behold the Supercontinent! Pangea
Applause!
Copyright © 2010 Ryan P. Murphy
• Behold the Supercontinent! Pangea
End Applause!
Copyright © 2010 Ryan P. Murphy

Pangea: The “Super Continent”
 All
of the plates were once together.
Copyright © 2010 Ryan P. Murphy

Pangea: The “Super Continent”
 All
of the plates were once together.
Copyright © 2010 Ryan P. Murphy
 Gondwondaland
and Laurasia were two mega
continents before Pangea.
Copyright © 2010 Ryan P. Murphy
• Current Day + or – 4mm
• What causes continental drift and plate
tectonics?
Copyright © 2010 Ryan P. Murphy
• Answer!
– Convection currents (Remember heat rises)
move the plates
Copyright © 2010 Ryan P. Murphy
• Answer!
– Convection currents (Remember heat rises)
move the plates
Copyright © 2010 Ryan P. Murphy
• Answer!
– Convection currents (Remember heat rises)
move the plates
Copyright © 2010 Ryan P. Murphy
• Answer!
– Convection currents (Remember heat rises)
move the plates
Copyright © 2010 Ryan P. Murphy
• Which colored arrows are incorrect based
on the convection current patterns and
plate movements below?
Copyright © 2010 Ryan P. Murphy
• Answer! The blue arrows. The plates
should be moving toward each other.
Copyright © 2010 Ryan P. Murphy
• Answer! The blue arrows. The plates
should be moving toward each other.
Copyright © 2010 Ryan P. Murphy
“Oh-no!” “We
are trying it
again.”
• Try Again! Which colored arrows are
incorrect based on the convection current
patterns and plate movements below?
Copyright © 2010 Ryan P. Murphy
• Answer! The purple arrows should be
diverging instead of converging.
Copyright © 2010 Ryan P. Murphy
• Answer! The purple arrows should be
diverging instead of converging.
Copyright © 2010 Ryan P. Murphy
“Oh-no!” “We
are trying it one
more time.”
• Try Again! Which colored arrows are
incorrect based on the convection current
patterns and plate movements below?
Copyright © 2010 Ryan P. Murphy
• Answer! The light blue arrows should be
diverging instead of converging.
Copyright © 2010 Ryan P. Murphy
• Answer! The light blue arrows should be
diverging instead of converging.
Copyright © 2010 Ryan P. Murphy
“Oh-no!” “We
are trying it one
more time.”
• Try Again! Which colored arrows are the
only correct on in the entire group.
Copyright © 2010 Ryan P. Murphy
• Try Again! Which colored arrows are the
only correct on in the entire group.
Copyright © 2010 Ryan P. Murphy
“Oh-No!”
“He’s Dead.”
• Try and figure out the picture beneath the
boxes. Raise your hand when you think
you know.
– You only get one guess.
Copyright © 2010 Ryan P. Murphy
Pangea
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.