Plate Tectonics Table of Contents Drifting Continents SeaFloor

Plate Tectonics Table of Contents Drifting Continents Sea-Floor Spreading Theory of Plate Tectonics

What was Wegener’s Hypothesis About the Continents? • Wegener’s hypothesis was that all the continents were once joined together in a single landmass and have since drifted apart. • Wegener’s idea that the continents slowly moved over Earth’s surface became known as continental drift. • Pangaea- a supercontinent 300 million years ago

Evidence from Land Features • Pieced together maps of Africa and South America • Mountain ranges lined up and coal fields in Europe and North America also line up.

Drifting Continents Piecing It All Together The coastlines of some continents seem to fit together like a jigsaw puzzle.

Evidence from Fossils • A fossil is any trace of an ancient organism that has been preserved in rock. • The occurrence of Glossopteris on landmasses that are now separated by oceans indicates that Pangaea once existed. • Wegener inferred that other fossils of reptiles had lived on a single landmass that had since split apart.

Drifting Continents Pangaea and Continental Drift Many types of evidence suggest that Earth’s landmasses were once joined together.

Evidence from Climate • As a continent moves toward the equator, its climate gets warmer. • Continents carry with it fossils and rocks that formed at all of its previous locations. • If you find fossils/rocks from warmer climates than it is currently in, then the location must have changed.

Wegener’s Hypothesis Rejected • Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents, which resulted in geologists at his time rejecting his hypothesis.

What Are Mid-Ocean Ridges? • Mid-ocean ridges- seams formed mountain ranges that ran along the middle of some ocean floors. • Mid-ocean ridges form long chains of mountains that rise up from the ocean floor. • Used sonar – a device that uses sound waves to measure the distance to an object- to map mid-ocean ridges. • Most lie under thousands of meters of water and steepsided valley splits the tops of some mid-ocean ridges.

Sea-Floor Spreading Ocean Floors Mid-ocean ridges rise from the sea floor like stitches on the seams of a baseball.

Sea-Floor Spreading Vertical Scale Exaggerated

What is sea-floor spreading? • Mid-ocean ridges continually add new material to the ocean floor called sea-floor spreading. • Along the ridge, new molten material from inside Earth rises, erupts, cools, and hardens to form a solid strip of rock. • Sea-floor spreading adds more crust to the ocean floor. At the same time, older strips of rock move outward from either side of the ridge.

Evidence for Sea-floor spreading • Rocks shaped like pillows –pillow lava- which only form when magma material hardens quickly underwater. • As magnetic material in the magma hardens, it lines up in the direction of Earth’s magnetic poles. Each stripe defines a period when molten material erupted and hardened while Earth’s magnetic poles did not change. • Mirror patterns on both side of the ridge indicate that the crust on the two sides of the ridge spread from the ridge at the same time and same rate. • Drilled for rock samples and found the farther away from a ridge a rock sample was taken, the older the rock was. Youngest rocks always found close to the ridge.

Sea-Floor Spreading Some mid-ocean ridges have a valley that runs along their center. Evidence shows that molten material erupts through this valley and then hardens to form the ocean floor.

What Happens at Deep-Ocean Trenches? • Deep-ocean trenches- eventually the ocean floor plunges into deep underwater canyons. • In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.

The Process of Subduction • Changes in density affect the ocean floor. • New oceanic crust is hot, but as it moves away from the mid-ocean ridge it cools and becomes denser. Eventually the cool dense crust might collide with the edge of a continent and then gravity pulls it down back into the mantle. • Subduction- the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle again. • Sea-floor spreading and subduction work together. They move the ocean floor as if it were on a giant conveyor belt.

Sea-Floor Spreading Subduction Oceanic crust created along a mid-ocean ridge is destroyed at a deep-ocean trench. During the process of subduction, oceanic crust sinks down beneath the trench into the mantle.

Subduction and Earth’s Oceans • The processes of subduction and sea-floor spreading can change the size and shape of the oceans. • The ocean floor is renewed every 200 million years, which is the time it takes for new rock to form at the midocean ridge, move across the ocean, and sink into a trench. • Atlantic Ocean is expanding.

Sea-Floor Spreading Deep-Ocean Trenches The deepest part of the ocean is along the Mariana Trench. Several trenches in the Pacific Ocean are shown in yellow.

The Theory of Plate Tectonics Slip-Sliding Away In 30 million years, it may take an hour longer to fly from New York to London because the cities are moving slowly apart.

What is the Theory of Plate Tectonics • Lithosphere- earth’s solid outer shell • Plates- The pieces of Earth’s broken lithosphere • Earth’s plates meet at one of three types of boundaries: • Divergent boundary-plates move apart from each other • Convergent boundary- plates move together • Transform boundary- plates slip/slide past each other • Plate tectonics- the combined ideas of sea-floor spreading, Earth’s plates, and plate motions. • The theory of plate tectonics states that Earth’s plates are in slow, constant motion, driven by convection currents in the mantle. • This explains the formation, movement, and subduction of Earth’s plates.

The Theory of Plate Tectonics How do the plates move? Divergent Boundaries Convergent Boundaries Transform Boundaries

Mantle Convections and Plate Motion • Earth’s plates move because they are the top part of the large convection currents in Earth’s mantle. • During subduction, gravity pulls denser plate edges downward. The rest of the plate also moves.

The Theory of Plate Tectonics Earth’s Plate boundaries divide the lithosphere into large plates.

Plate Motions Over Time • Plates move very slowly, about 1 to 12 centimeters per year. • Over time, the movement of Earth’s plates has greatly changed the location of the continents and the size and shape of the oceans. • As they move, they produce earthquakes, volcanoes, mountain ranges, and deep-ocean trenches.

Plate Boundaries • Faults- breaks in Earth’s crust where rocks have slipped past each other- form along plate boundaries.

Divergent Boundaries • Most occur along the mid-ocean ridges. • In a few places the mid-ocean ridges rise above sea level. • Rift valley- where pieces of Earth’s crust diverge on land, a deep valley occurs.

Convergent Boundaries • The density of the plates determines which one comes out on top when they collide. • Oceanic vs. oceanic- the denser one is pulled underneath (subduction) • Oceanic vs continental – oceanic is denser, which means it can push up the less dense crust, and the denser one sinks, water eventually leaves the sinking crust and rises into the wedge of the mantle above it. Results in volcanoes. • Continental vs. continental – neither piece is dense enough to go into the mantle, so they squeeze the crust into high mountain ranges.

Transform Boundaries • Beneath the surface of a transform boundary the plates are rocky and jagged. • The plates can grab hold of each other and lock in place. • Earthquakes often occur when the plates suddenly slip along the boundary that they form. • San Andreas Fault

The Theory of Plate Tectonics Earth's Changing Crust As plates move, they produce mountains, volcanoes, and valleys as well as midocean ridges and deep-ocean trenches. Use the terms from the list to label the diagram.