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_Seafl
oor_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/convecti
on.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