Plate Tectonics

Chapter 3 (p 61-82)

A New Understanding of Earth

• Earth has a geologically active surface

• How do internal layering and heat contribute to mountain building, the arrangement of continents, the nature of the seafloors, and the wealth of seemingly randomly distributed geological features found everywhere?

• Patterns?

The Age Debate• Know to be 4.6 billion years old

• Late 1700s most scientists believed that earth was about 6,000 years old– Based on interpretation of the Bible

• Uniformitarianism – earth processes happening today are identical to those in the past– Going on for a very long time based on

current rates

The Age Debate

• Catastrophism – biblical catastrophic events shaped the young earth– Flood– Explained mountain-top fossils

• Natural selection– Time was needed for many species to exist

Age-of-earth arguments led to discoveries during the 1800s ancient earth

http://images.google.com/imgres?imgurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/beam.ap.jpg&imgrefurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/index.html&h=168&w=220&sz=12&hl=en&start=15&tbnid=bA5UiDTi_m_xCM:&tbnh=82&tbnw=107&prev=/images%3Fq%3Dgreat%2Bflood%26gbv%3D2%26svnum%3D10%26hl%3Den

A Puzzling Fit

• Fit of South Atlantic continents

– Evidence for single large landmass

– Fossil evidence

– Fit of continental shelves

• All by chance?

• No mechanism for continental movement suggested

Continental Drift• 1912 – Alfred Wegener

– Pangaea (pan = all, gaea = earth)• Single supercontinent

–Pieces broke apart 200 million years ago

–Still moving today• Evidence

–Fit of shorelines–Fossils of tropical plants

in Antarctica

Seismic Events

Seafloor Spreading

• 1960 - Mid-Atlantic ridge suggested as origin of new seafloor (spreading center)– Explains “fit” of continents– Mechanism for movement – convection

currents in mantle– Then ridges should be hot they are– New crust should become more dense over

time it does– Crust furthest from ridge should be oldest it

is

Where does the old crust go?• Subduction zones

– Crust plunges into the mantle in the Pacific– Creates a balanced system

Plate Tectonics

• 1965 - John Tuzo Wilson suggests that there are 12 plates that make up Earth’s lithosphere (crust)– These plates float on the asthenosphere– Moved by hot mantle becoming less dense

rising• Lifts and cracks the crust = plate edges• Avg. movement ~ 2 in per year

Plate Tectonics

• Plate movement caused by two forces:– Plates form and slide off the raised ridges of

the spreading centers– Plates are pulled downward into the mantle by

their cool, dense leading edges

• This theory explains many previously unanswered questions

Major Plates

Plate Boundaries

Divergent Plate Boundary

• Line along which 2 plates are moving apart– Mid-Atlantic Ridge– Cooling magma creates new crust

Convergent Plate Boundary• Areas of violent geologic activity where

plates are pushed together– Oceanic crust is destroyed

Convergent Plate Boundary

• What happens when 2 convergent plates are of equal density?

Transform Plate Boundary

• Plates moving laterally past one another– Necessary since

Earth is a sphere– Does not create or

destroy crust– Creates

earthquakes

Transform Plate Boundary