Presentation on theme: "Plate Tectonics The theory of plate tectonics was first proposed by"— Presentation transcript:
1 Plate Tectonics The theory of plate tectonics was first proposed by Alfred Wegener in Wegener noticed that theshorelines 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 oneplace forming a supercontinent, Pangea.The continents broke apart and drifted to theirpresent locations.Wegener’s theory was not taken seriouslybecause no one could believe that things aslarge as continents could move and becauseWegener could not propose a mechanismwhich could explain such motion.
2 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 thecontinents do fit together like the pieces of a puzzle.But that could just be acoincidence. For a theoryto be accepted as fact, theremust be more
3 If the continents were once together, we ought to be able to find similar kinds of rocks in places that were once together but havesince drifted apart.If we believe that area A andarea B were once together, thenwe should find similar rock typesin these two locations even thoughtoday they are thousands of milesapart.Rocks are a lot like fingerprints. The mineral content and structureof 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.
4 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 evidencethat these two areas were once together forming a single habitat.It is important to choose ourfossils carefully. Finding fossilsof organisms that lived all overthe world would mean nothing.But if we could find the same fossils in areas A and B, fossils thatexist nowhere else because they had a very limited range, then thisis even more powerful evidence that areas A and B were oncetogether.Correlating fossils from opposite shores is a third piece of evidence.
5 Sea Floor SpreadingIf the continents are moving apart then the seas between them must begetting wider. This is called sea floor spreading. How do we know thatthis is true?First, a series of under-water mountains called mid-ocean ridges is found throughout the world. These mountains areformed as new sea floor (basalt) is created from magma that wellsup from the mantle below.
6 As new sea floor is added it pushes the old sea floorapart.Proof that this occurs can befound by looking at the ageof the sea floor itself.New sea floor is found at the mid ocean ridge where material isconstantly being added. As distance from the ridge in eitherdirection increases, so does the age of the rock.
7 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.
8 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 magneticproperties of rocks) is towed across the ocean it indicates that there are alternating zones of rock with either normal orreversed polarity.In the green zones bits of magnetic iron in the rocks are pointing towhere north is today. They act like countless little magnets trappedin the rock when it solidified from magma.But in the orange zones the bits of iron are pointing in the oppositedirection. Why?
9 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 ofnormal polarity. As it cools that polarity is locked into the rock.But as can be seen indiagrams ‘b’ and ‘c’, asnew rock emerges alongthe ridge it pushes theold rock away. The newrock may record reversedor normal polarity depend-on conditions when itformed.
10 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 wasspreading apart as the earth’s magnetic field reversed itself overlong periods of geologic time.So together, studies of the age of the sea floor and of paleomagnetismprove that sea floor spreading has occurred. This is the forth piece ofevidence that continental drift is fact, not theory.
11 The Ultimate Proof This is the Laser Geodynamics Satellite (Lageos). It was launched in 1976 andwas joined in 1992 by Laegos II. Theyorbit the earth at an altitude of 3,700 milesand carry no electronic equipment.Laser beams are bounced from one point on the earth, off thesatellite, to a second point on the ground. Scientists can thenmeasure the distance between the two points with great accuracy.Geo positioning satellites enable scientists touse GPS devices to make accurate measurementsof positions on the earth.These systems show conclusively that thecontinents are still drifting at a rate of a fewcentimeters a year.
12 Continental drift is not a theory. It’s a fact. Let’s review the evidence for continental driftThe continents seem to fit together like the pieces of a jig sawpuzzle.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) Paleomagnetism5) Direct satellite measurement of the drifting continents.Continental drift is not a theory. It’s a fact.Wegener was right! But why......?
13 How could continents, giant slabs of rock, possibly move? The key is to understand the internal structure of the earthWe live on a very thin, coolcrust which averages about50 km thick under the con-tinents and 10 km under theoceans. Beneath the crustis the mantle. Rock in themantle is very hot and isunder high pressure. Rockhere is described as plasticwhich means that, likesilly putty, it can flow veryslowly over inimaginablylong periods of time.
14 Because temperatureincreases with depth,the inside of the earthis very hot. Rock nearthe core is heated andbecomes less dense. Itslowly rises whilecooler rock nearer thesurface is more denseand sinks. This formsconvection currentsjust like those in ouratmosphere. As these convection currents in the mantle circulate, theycause the continents above them to move. What Wegener had no wayof knowing is that the force that is driving plate tectonics is convectioncurrents in the mantle.
15 Let's sum up everything we've learned: 1) The earth's crust is divided into many pieces called tectonicplates 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 ridgesand oldest near the continents).e) Paleomagnetism: zones of oppositely magnetizedrock on opposite sides of the mid ocean ridge.f) Direct satellite and GPS measurement of continentalmovement.3) The force driving the plates are convection currents (densitydifferences) in the earth's mantle.