Plate Tectonics Chapter 8

What Is Plate Tectonics? Section 1 What Is Plate Tectonics? Theory that describes the formation, movements, and interactions of Earth’s lithospheric plates. Lithosphere is broken into large “plates” that move (“float”) on top of the asthenosphere. 3 ways Away from each other (divergent plate boundaries) Toward each other (convergent plate boundaries) Slide past each other (transform plate boundaries)

Early Ideas About Plate Movements Have you ever noticed that the continents look like they would fit together like the pieces of a puzzle??? If you have, you’re not alone…. 1912 Alfred Wegener hypothesis  continental drift Continents have “drifted” over time Observations to support hypothesis Shape of continents Fossils ~Mesosaurus *South America & Africa Distinctive rock formations Climate change evidence ~coal deposits Continental Drift Video

Evidence for Joined Continents Interaction

The Theory of Plate Tectonics Continents (and ocean basins) are part of lithospheric plates. Plate movement = continent movement Evidence/Explanations of many geologic processes Location of volcanoes & earthquakes Formation of new crust (ocean floor)

Locations of Earthquakes & Volcanoes Occur in concentrated areas Mark locations of plate boundaries Where plates move apart, together, or past each other Strain builds up  releases causing EQs Molten rock rises  volcanic activity Pacific Ocean Ring of Fire Plates & Volcanoes Video

Locations of Earthquakes & Volcanoes Ring of Fire

Magnetism & the Age of the Ocean Floor Magnetic properties & ages of igneous rocks on the ocean floor provide evidence for theory of plate tectonics. Some igneous rocks contain magnetic minerals. Provide record of direction of Earth’s magnetic field when rock formed

Magnetism & the Age of the Ocean Floor Records of Earth’s magnetic field Some rocks recorded reversals in magnetic field normal polarity what we know today as N & S magnetic poles reversed polarity present N magnetic pole became the S magnetic pole present S magnetic pole became the N magnetic pole A # of magnetic reversals have taken place at different times over millions of years

Magnetism on the Ocean Floor

Magnetism & the Age of the Ocean Floor Mid-ocean ridge: long chain of volcanic mountains on the ocean floor with a deep central valley Magnetic reversals are recorded in bands of rock on opposite sides of the ridge Mirror image Center of ridge shows current orientation of Earth’s magnetic field (normal or reversed) Rocks at center of ridge are youngest (most newly formed) As move further from center rocks get older Magnetic Polarity Recorded in Rocks Animation

Magnetism & the Age of the Ocean Floor Mid-ocean ridges  boundaries between lithospheric plates New rock along ridge is formed by hot, molten rock which rises up between the plates (because it is less dense) As new rock forms plates spread Older rocks pushed away from ridge (both sides) Rock at center of ridge hottest Temperature decreases as move out from center

HW Read pgs 172-175 Answer #s 1, 2, 3

Types of Plate Boundaries Section 2 Types of Plate Boundaries Earth’s lithosphere is broken in to large plates move in 3 ways Divergent plate boundaries Convergent plate boundaries Transform plate boundaries

Divergent Boundaries Plates move apart Most are along ocean floor Sometimes called spreading centers Most are along ocean floor Creates mid-ocean ridge

Divergent Boundaries Rift valley Center of mid-ocean ridge Border between 2 diverging plates Sea-floor spreading  molten rock forces through cracks (rifts) in valley Oceanic crust forms as rock cools older crust moves away from mid-ocean ridge as the new crust is formed

Divergent Boundaries Rift valley (continued) Broken into sections offset from each other by breaks (fracture zones) Perpendicular (right angle) to ridge Source of earthquakes at mid-ocean ridges Divergent Plate Boundary Animation

Mid-Ocean Ridges Mid-Atlantic Ridge East Pacific Rise

Convergent Boundaries 2 plates move towards each other 2 broad classifications Subduction boundaries Collision boundaries

Subduction Boundaries Oceanic plate subducts (plunges) below another plate Boundary between the 2 plates is called a subduction boundary Features of subduction boundaries Deep-sea (or ocean) trenches Deepest part of the ocean Volcanic activity Island arcs Along the coast of continents Earthquakes

Subduction Boundaries Ocean-Ocean subduction 2 Oceanic plates collide Deep-sea trench formed accompanied by chain of volcanic islands (volcanic island arc) on the overriding plate Example: Pacific Plate subducts under Phillippine Plate Pacific Plate pulled down Forms Mariana Trench Phillippine Plate overrides Pacific Plate Forms Mariana Islands (volcanic island chain) Subduction and Collision Boundary Animations

Ocean-Ocean Subduction Boundary

Ocean-Continent Subduction Boundary The denser oceanic plate subducts below the less-dense continental plate Ocean trenches form. Volcanoes & mountains are formed on the continental plate. Example: Nazca Plate (off west coast of S. America) subducts under the South American Plate Nazca Plate pulled down Forms Peru-Chile Trench South American Plate overrides Nazca Plate Forms Andes Mountains and volcanoes (along western edge of S. America) Subduction and Collision Boundary Animations

Ocean-Continent Subduction Boundary

Collision Boundaries Continent-continent collision Example: Neither plate subducts, so the crust piles up (because the continents join to form a single larger continent) Mountains are formed. Example: Indian subcontinent colliding into Eurasian Plate Formed Himalaya Mountains Subduction and Collision Boundary Animations

Continent-Continent Collision

Transform Plate Boundary 2 plates slide past each other at a fault. Stress is released as an earthquake. Example California—San Andreas Fault Pacific Plate (Southwestern Cali.) is moving NW N. American Plate (rest of N. Amer.) moving SE

Transform Plate Boundary Transform Boundary--San Andreas Fault Animation N

HW Read pgs 176-179 Answer #s 1, 2, 3, 4, 6

Causes Of Plate Movement Ch. 8.3 Convection cells (caused by density differences) in the asthenosphere (mantle) cause hot material to rise, move outward (pushing the plates/ridge push), & sink as the material cools (pulling plates down/slab pull). Convection in the Mantle Animation

The Big Picture

HW Read pgs 180-181 Answer # 1

Plate Movements & Continental Growth Sec 4 Reconstructing the Past Many different kinds of evidence give clues as to what Earth looked like in the past Rocks in Ural & Appalachian Mountain evidence of past subduction  formed at convergent boundary, but neither mountain range is located near a plate boundary today Ages of rocks that form in ocean basins Magnetic record of igneous rocks Can reveal latitude of formation Fossils Organisms that lived in shallow seas  fossils found on high mountaintops Rocks that show evidence of having been covered by glaciers, but now are in tropical areas

Plate Movements & Continental Growth Plate Tectonics & Pangaea Evidence suggests 250 million years ago (mya) that all the continents were welded together into one land mass  Pangaea

Plate Movements & Continental Growth Formation of Pangaea Use data from continents to make models of what Earth may have looked like before Pangaea A large continental mass stretched between the south pole & the equator  Gondwana made of smaller landmasses (S. America, southern Europe, Africa, the Near East, India, Australia, New Zealand, & Antarctica) Other small landmasses ranged across the globe Gondwana moved northward and converged w/ the other landmasses to form Pangaea

Plate Movements & Continental Growth Break up of Pangaea 2 separate landmasses Gondwana Laurasia Over time both broke into smaller landmasses which began to resemble the continents today Pangaea is still breaking up… The formation & break up may have happened many times in the past Break up of Pangaea animation

Plate Movements & Continental Growth Plate Tectonics & Continental Growth P.T. affects shape of continents as well as position Added rock materials to the margins of ancient continent cores (cratons) ~2.5 billion years ago (bya) core continental material stabilized Ex. North American craton at surface in most of eastern Canada  Canadian Shield Some of the oldest rock material (~4 billion years old) Rest of the craton is buried under a platform of sediments Remainder of the continent is material added to the craton

Plate Movements & Continental Growth Sources of Growth Material Deep-sea sediments Added when an oceanic plate plunges under a continental plate at a subduction boundary Sediments from ocean floor scraped off & left behind on edge of continent Igneous Rock Magma rises beneath surface & cools  pluton Volcanoes at subduction boundaries add to edges

Plate Movements & Continental Growth River Sediments Weathered & eroded materials deposited at edges Terrane large block of lithospheric plate that has been moved & attached to continent 3 characteristics to ID a terrane 1. bounded on all sides by faults 2. rocks & fossils found in terrane do not match those of neighboring terranes 3. magnetic record of terrane does not match surround terranes Growth of a Continent Animation

HW Read pgs 182-187 Answer # 3