Slide 1

Chapter 17 Plate Tectonics

Slide 2

Drifting Continents Section 17.1

Slide 3

Continental Drift German scientist Alfred Wegener proposed the idea of continental drift which stated that Earths continents had once been joined as a single landmass Called this landmass Pangea, greek for all the Earth Wegener proposed Pangea broke apart 200 million years ago

Slide 4

Evidence from Rock Formations As Pangea broke apart, mountain ranges would have fractured So similar rock types should be found on opposite sides of the Atlantic Ocean Appalachian Mountains (US) share similar rocks in Greenland and Europe All older than 200 million years

Slide 5

Evidence from Fossils Similar fossils of different plants and animals have been found all over the world on widely separated continents Fossils are older than 200 million years

Slide 6

Ancient Climatic Evidence Rocks provide clues to past environments and climates Evidence of coal was found on Antarctica Coal forms from dead swamp plants Glacial deposits found on Africa, India, Antarctica

Slide 7

A Rejected Hypothesis Wegeners continental drift hypothesis was not widely accepted until the 1960s Wegener could not prove how or why the continents moved before his death in 1930

Slide 8

Seafloor Spreading Section 17.2

Slide 9

Help from Technology Most people thought oceanic crust was unchanging and older than continental crust Echo-sounding technology (SONAR) sent sound waves to the ocean floor to measure depth A magnetometer is a device that detects small changes in magnetic fields

Slide 10

Ocean Floor Topography Maps made from data showed vast, underwater mountain chains and ocean ridges Also discovered earthquakes and volcanoes along the ridges

Slide 11

Ocean Floor Topography Cont There are also deep- sea trenches, which are long, narrow depressions The deepest trench, the Mariana Trench, is in the Pacific ocean and is 11 km deep

Slide 12

Magnetism Rocks (like basalt) with iron provide record of Earths magnetic field The study of the magnetic record is paleomagnetism Magnetic Reversals or changes in Earths magnetic field were found in basalt flows Normal polarity is when the magnetic field is aligned with earths present field Opposite is called reversed

Slide 13

Magnetic Symmetry As data was collected, scientists noticed a series of positive and negative stripes parallel to ocean ridges The patter was identical on each side of the ridge

Slide 14

Seafloor Spreading Seafloor spreading states that the new ocean crust is formed at ocean ridges and destroyed at deep sea trenches Magma is less dense than oceanic crust so its forced up, fills the gap and hardens As spreading continues, magma forces up and hardens as well http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p _seafloorspreading.html http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p _seafloorspreading.html

Slide 15

Theory of Plate Tectonics Section 17.3

Slide 16

Theory of Plate Tectonics The theory states that Earths crust and rigid upper mantel are broken into enormous slabs called plates Plates move in different directions and at different rates

Slide 17

Plate Boundaries Tectonic plates interact at places called plate boundaries Plates either come together (converge), move away from each other (diverge) or slide past each other

Slide 18

Divergent Boundaries Places where two tectonic plates move apart from one another are called divergent boundaries Most are found on the seafloor where they form ocean ridges and where seafloor spreading occurs Over millions of years, seafloor spreading at divergent boundaries causes the ocean basin to grow wider

Slide 19

Convergent Boundaries Places where two tectonic plates are moving toward each other are convergent boundaries There are three types of convergent boundaries At oceanic-oceanic boundaries, one plate descends beneath the other in a process called subduction Arcs of volcanic islands often form around this boundary

Slide 20

Convergent Boundaries Cont At oceanic-continental convergence boundaries, subduction Creates a volcanic mountain range rather than a volcano island arc

Slide 21

Convergent Boundaries Cont The third convergent boundary is continental-continental Continental plates are pulled into subduction zones but are too buoyant to be forced under The colliding edges crumple and uplift the form a mountain range

Slide 22

Transform Boundaries Transform boundaries occur where two plates slide horizontally past each other, usually seen as long faults At divergent boundaries new crust is formed At convergent boundaries old crust is destroyed At transform boundaries crust is deformed or fractured *San Andreas Fault- Most studied fault in the world; produces CA earthquakes