Ch 10-Plate Tectonics

• Objectives– Wegener’s hypothesis of continental drift– Process of sea floor spreading– Identify how paleomagnetism provides support for the idea of

sea floor spreading– Explain how sea floor spreading provides a mechanism for

continental drift– Theory of plate tectonics– Types of plate boundaries– Causes of plate movements– Identify how movements of tectonic plates change Earth’s

surface– Summarize how movements of tectonic plates have influenced

climates and life on Earth– Describe the supercontinent cycle

Continental Drift

• Alfred Wegener-German scientist– Hypothesized continents once formed part of

single landmass-supercontinent– Broke up into smaller continents 200 million

yrs ago during Mesozoic Era– Proposed crumpling of crust produced mt

ranges– Concepts in Motion

Evidence for Continental drift

• Fossil Evidence– Mesosaurus-small extinct reptile found in S America and W

Africa• Rock formations

– Age and types of rock in W Africa and S America matched closely

– Appalachian Mts similar to age and structure to mts in Greenland, Scotland, and N Europe

• Climatic Evidence– Ancient glaciers in S Africa and S America – Fossils in tropical swamps covered areas that now have much

colder climates• Wegener’s missing mechanism

– He believed continents plowed through rock of ocean floor, rejected by scientists

– Disproved by geologic evidence and he died in 1930

Mid Ocean Ridges

• Long, undersea mt chain w/ steep, narrow valley at center

• Forms as magma rises from asthenosphere-creates new oceanic lithosphere as tectonic plates move– Rift- crack in ocean floor through which

magma rises

• Oceanic rocks younger than land rocks

Sea Floor Spreading

• Process by which new oceanic lithosphere forms as magma rises to Earth’s surface and solidifies at mid ocean ridge

• Harry Hess-Geologist suggested ocean floor moving as well as continents in 1950

• Paleomagnetism-study of the magnetic properties of rock – Provided more evidence

• http://glencoe.com/sec/science/earthscience/2007/concept_motion/NGS/Visualizing_Seafloor_Spreading_17.swf

Paleomagnetism

• Magma solidifies to form rock• Iron rich minerals in magma align w/ Earth’s

magnetic field, same way compass needle does• Rock hardens, magnetic orientation of minerals

becomes permanent• Normal polarity=all rocks w/ magnetic fields that

point north, classified in same time period• Reversed polarity=rocks point south• Information supported Hess’s idea of sea floor

spreading

Sea floor spreading and Continental drift

• Scientists reasoned that sea floor spreading provides a way for continents to move other Earth’s surface

• Continents are carried by widening sea floor

• Wegener redeemed

Sec 2-Theory of Plate Tectonics

• The theory that explains why and how continents move and is the study of the formation of features in Earth’s crust– Explains how large pieces of lithosphere

called plates, move and change shape

• Crust- thin, solid outermost zone of Earth, 1% of Earth’s mass

• Continental and oceanic crust

• Mantle-beneath crust, 2,900 km thick, 2/3rd of Earth’s mass

• Core-center of Earth, sphere w/ radius of 3500 km, composed mainly of iron and nickel

How Continents Move

• Lithosphere-solid, outer layer of Earth. Consist of crust and rigid upper part of mantle. Made up of tectonic plates

• Asthenosphere-solid, plastic layer of mantle beneath lithosphere. Made up of mantle rock that flows very slowly. Allows tectonic plates to move on top of it– Plastic rock-solid rock that is under great deal of pressure and

that flows very slowly

• Types of Earth’s Crust– Oceanic-dense and made of rock rich in iron and magnesium– Continental-low density and made of rock rich in silica – Tectonic plates may be made of both

• Earths Plates

Tectonic Plates

• 15 identifiable tectonic plates• Some border major surface features• Some located far from mt ranges• Earthquakes-sudden movements that occur

along boundaries of tectonic plates– Frequent earthquakes=2 or more plates meet

• Volcanoes-form when plate motions generate magma, erupts on Earth’s surface

• Pacific Ring of Fire

Types of Plate Boundaries

• Divergent-boundary between tectonic plates that are moving away from each other– Magma rises as plates move apart, cools and forms new

oceanic lithosphere– Rift valley-narrow valley that forms where plates separate– Most found on oceanic floor– Red Sea– http://www.wwnorton.com/college/geo/egeo/flash/2_11.swf

• Convergent boundary-between tectonic plates that are colliding– Oceanic collides w/ continental lithosphere-oceanic subducts

which leads to volcanic mts– http://www.wwnorton.com/college/geo/egeo/flash/2_6.swf– 2 continental lithosphere collides-formed Himalayas– 2 oceanic lithospheres collide-forms island arc-chain of volcanic

islands-Japan

• Transform-between tectonic plates that are sliding past each other horizontally, not smoothly– San Andreas Fault– N American plate and Pacific plate– Occurs along mid ocean ridges– Fracture zones-short segments of mid ocean

ridge that are connected by transform boundaries

Causes of Plate Movement

• What is plate motion? Movement of tectonic plates that is a part of the mantle convection system– Convection-movement of heated materials due to

differences in density that are caused by differences in temps

• Mantle convection– Heated mantle material rises, cooler denser

material sinks– Drags overlying tectonic plates along w/ it– http://education.sdsc.edu/optiputer/flash/

convection.htm

• Ridge Push– New rocks at mid ocean ridge cool and slope

downward away from ridge– Asthenosphere exerts force on rest of plate=ridge

push– Force pushes rest of plate away from mid ocean ridge

• Slab Pull– Cooler lithosphere subducts into asthenosphere– Pulls rest of plate, force is called slab pull– Driving force for tectonic plate motion

Sec 3-Changing Continents

• Cratons-large areas of stable rock that are older than 540 million yrs

• Shields-rocks w/in cratons that have been exposed at Earth’s surface

• Rifting-process by which Earth’s crust breaks apart– Oceanic or continental– Forms in zone of weakness and continent

breaks apart– East African Rift

Effect of Continental Change

• Continent’s location in relation to equator and poles- affects overall climate– Along w/ oceans and other continents– Air flow, wind patterns, moisture

• Continents separate- new species– Madagascar

Pangaea

• Supercontinent cycle- process by which super continents form and break apart over millions of yrs

• Formed 300 million yrs ago

• Appalachian and Ural Mts formed when continents collided

• Panthalassa was single ocean surrounding

• Breakup of Pangaea-200 million yrs ago– Broke up into 2 continents-Laurasia and Gondwanaland

– Laurasia broke into N. America and Eurasia

– Gondwanaland broke into Africa and S America as one continent and India, Australia, and Antarctica as the other

Geography of Future

• 150 million yrs from now=Africa colliding w/ Euasia=No Mediterranean Sea

• N and S America will move east

• No Atlantic Ocean

• N and S America will collide w/ Africa

• Los Angeles will move north of San Francisco