The World as it Turns Changes in the Earth’s Crust…Plate Tectonics

The Theory of Continental Drift Alfred Wegener, 1912

Continental Drift… proposed – continents slowly drift in relation to one another; continents were once joined, single super-continent (Pangaea) Interactive Map of Pangaea Alfred Wegener

Continental Drift: The Evidence… shape of the continents - puzzle-like fit same animal fossils – fresh water fossils-opposite shores of Atlantic Ocean same sequence of rock layers - found on opposite shores of Atlantic Ocean along coastlines fossils of tropical plants - found in polar areas glacial deposits - found in tropical areas More on the evidence

Continental Drift: Lack of Support… Wegener’s theory did not go over well in the scientific community… –no explanation how the continents “drift” –continents plow through sea floor, really??? –What strong force moves huge mass-large distance??? To read an excellent historical account of Continental Drift with informative graphics, visit the U.S. Geological Survey.U.S. Geological Survey

Crustal History Confirmation A.Describe the evidence Wegener used to support his theory of Continental Drift. B.Why did the scientific community not believe in Wegener’s theory? In your journal

Lithosphere. This layer varies in thickness from 1 to 250 miles. The lithosphere is a cool, strong and rigid layer. Its uppermost part is called the crust and is divided into oceanic and continental-type crusts discussed in the next section. Asthenosphere. The upper reaches of the mantle are not solid; they are considered plastic and flow very slowly. This is due to the reduction in pressure as we approach the top of the mantle- the rocks are more likely to begin to melt. Mantle. About 1860 miles thick, the mantle is made of iron and magnesium-rich silicate rocks and has a density of about 4.5 g/cm3. The mantle is hot, but mostly solid because the minerals are under pressure. Outer Core. About 1240 miles thick, the outer core is also made mostly of iron with a density of about 10 g/cm3. However, a reduction in pressure makes this hot layer a liquid. Inner Core. About 800 miles in diameter and composed of more than 90% iron with a maximum density of 13 g/cm3. This iron in this layer, although very very hot, is solid due to the immense pressure at the center of the Earth.

Parts of the Earth… There are 5 main layers, from surface to center: –Lithosphere (crust): solid outermost layer, about 5-50km deep; –Asthenosphere: the lower layer of the earth's crust; less rigid than the lithosphere—more slushy/flexible –Mantle: region below crust extending down to Earth's core; some molten rock; less flexible –Liquid Outer Core: made of molten iron/nickel; flows & creates Earth’s magnetic field –Solid Inner Core: solid iron/nickel center of the Earth; high temperatures & pressure; like a solid How do we know?  Earthquakes 

Sea Floor Spreading More Evidence is Found

Sea Floor Spreading 1940’s, use sonar to map ocean floor Discovery of ridges (underwater mountains), middle Atlantic Ocean

Sea Floor Spreading: The Evidence… 1968, scientists drill cores of rock from ocean floor –youngest rock at the mid-ocean rift; oldest at continental coasts –Earth’s magnetic field constantly shifting and is recorded in the rocks – this provides further evidence of sea floor spreading On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to fingernail growth Polarity reversals same distance from rift on either side N N N N N N N S S S S S S S S

Sea Floor Spreading: the How… Occurs at MID-OCEAN RIDGES CONVECTION – Hot, less dense magma rises, flows sideways, cools & sinks Magma/crust friction pulls crust; rift fills with magma/lava, cools - creating NEW crust Ocean lithosphere (crust) is youngest NEAR mid-ocean ridge Mid Ocean Ridge Convection Currents Oceanic Crust (lithosphere) Rising Magma youngest oldest

Sea Floor Spreading: The How… Sea Floor Spreading animation Remember: convection currents CAUSE sea floor spreading—gravity causes convection currents Convection Currents and Plate Movement animation Sea Floor Spreading from BBC documentary film "Earth The Power Of The Planet "Sea Floor Spreading

Things to Ponder about Sea Floor Spreading… Use Newton’s Laws of Motion to explain the movement. What force is causing the motion? What happens when the plates hit each other? What type of force does there have to be to cause the movement? Remember Flip Videos for tomorrow!

The Plate Tectonics Theory Plate Tectonics Theory combines continental drift & seafloor spreading Lithosphere floats on denser, liquid rock in mantle called asthenosphere. Lithosphere broken into tectonic plates Tectonic plates move about 1-8 cm/yr This movement is the “continental drift” referred to by Wegener

There are three types of plate boundaries: –Divergent Boundaries - Boundary between two plates that are moving apart. - Convergent Boundaries - Boundary between two plates that are moving apart. - Transform Boundaries - Boundary between two plates that are sliding past one another. –animationsanimations

Convergent – oceanic to continental Continental vs. oceanic –A subduction zone is the area where a more dense oceanic plate dives under less dense continental plate. –The place where the continental crust is pulled down by the oceanic crust is a deep, v shaped called called a trench. –Where the oceanic plate subducts and melts under continental plate, the magma and hot gasses are forced upwards creating a volcanic mountain range on the continent.

Mt. St. Helens Lassen peak

Convergent: oceanic to oceanic Oceanic vs. Oceanic If it is 2 oceanic crusts that meet, then a underwater volcanic range is formed in the ocean. As they erupt and the rock cools they will form islands.

Convergent: continental and continental Continental vs. Continental 2 continental plates converging will form a mountain range like the Himalayan mountain range.

Himalayan Mtns. Taken by Satellite, 2004… India (Indian Continental Plate) Nepal Tibet ( \Eurasian Continental Plate) Himalayan Mountain Range * Himalayan mountain range is the perfect example of how the highest mountains in the world continues to grow

India Is still moving into Asia Today

Divergent boundaries…. When oceanic moves away from oceanic it forms a mid ocean ridge. When continental moves away from continental if forms a rift valley. Mid Atlantic ocean ridge

Examples of Divergent Boundaries: Mid- Atlantic Ocean Ridge, East African Rift, and Red Sea

Spreading Centers

Transform… At transform plate boundaries, plates slide/grind past each other. A famous example of a transform boundary is found in California. It is called the San Andreas Fault.

Plate Tectonics

Identify the Plate Boundaries below A. B. C. D. E.

Review… Alfred Wegener created a theory of Continental Drift based on fossil, rock formation, and puzzle-like fit of the continents The advent of sonar allows us to map the ocean floor where mid- ocean ridges are discovered Rock samples taken show the age of rock on either side of a mid-ocean ridge gets older the further away from the ridge Sea floor spreading is explained by rock age and magnetic reversals being the same on either side of the ridge theory of plate tectonics based on previous theories, sea floor spreading, and other observations such as volcanoes and earthquakes Plate boundaries can move apart (diverge), move together or collide (converge), or move sideways past each other (transform) Convergence can cause the creation of volcanic mountain ranges, volcanic island arcs, or continental mountains Transform boundaries cause major earthquakes