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: The continents were once all together in one place forming a supercontinent, Pangea. 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.

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

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.....................

Evidence for continental drift. - Copyright © 2010 Ryan P. Murphy

The shapes do match. Copyright © 2010 Ryan P. Murphy

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.

Same fossils found 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

“Darn you continental Drift.” 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

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.

The speed at which the plates move is about the speed at which your fingernails grow. Copyright © 2010 Ryan P. Murphy

Behold the Supercontinent! Pangea ? Copyright © 2010 Ryan P. Murphy

Applause! Behold the Supercontinent! Pangea Copyright © 2010 Ryan P. Murphy

End Applause! Behold the Supercontinent! Pangea Copyright © 2010 Ryan P. Murphy

Gondwondaland and Laurasia were two mega continents before Pangea. Copyright © 2010 Ryan P. Murphy

Current Day + or – 4mm

Continental drift is not a theory. It’s a fact. Let’s review the evidence for continental drift................................ 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......?

BRAIN BREAK!

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 con- tinents 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.

Plate Tectonics Lithosphere (the crust and the solid upper part of the mantle) is not one solid mass but is broken into small sections called tectonic plates Each plate floats independently on the asthenosphere (the part of the mantle that acts like a liquid) Explains why earthquakes and volcanoes happen; where they occur; and, why mountain ranges, ocean trenches and faults have formed.

Boundaries The location where two plates are floating side by side Plates interact with each other at this boundary. Convergent boundary plates are running into each other Divergent boundary plates are moving away from each other Transform boundary one plate is sliding past the other plate

Divergent Plates

Continental Convergence The Himalayan mountain range provides a spectacular example of continent vs. continent collision. When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant.

Oceanic Plate Convergence When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink. Volcanoes rise out of the ocean volcanic mountain chains island arcs. The Aleutian Peninsula of Alaska is an excellent example of a very volcanically-active island arc.

Transform Plates

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! Divergent plates Copyright © 2010 Ryan P. Murphy

Answer! Convergent 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

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. http://www.teachertube.com/viewVideo.php?video_id=192589 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.