9.4 and 9.5 notes

Review/Preview (True or False?)

1. The main criticism of Wegener’s hypothesis was his lack of a mechanism.

2. Scientists now know the mechanism of plate motion.

3. The mantle consists of liquid magma.

4. Scientists cannot effectively study the mantle because they cannot experiment with it directly.

Theory of Plate Tectonics

• 1968: updated version on Continental Drift based on new evidence

• Contients and seafloor move

New Evidence for Plate Tectonics:

9.4 The Evidence for Plate Tectonics

Strongest new evidence came from ocean drilling (new technology) in the 1960’s

Geochronology = seafloor age

New seafloor at crust gets older on both sides as you move away shows plate movement.

No oceanic crust older than 180 million years shows it gets recycled during subduction.

Paleomagnetism Preserved in Lava Flows

Paleomagnetism

• Iron-rich minerals act as tiny magnets

• As they cool, they line up parallel to earth’s magnetic field

• Earth’s magnetic field periodically reverses (N becomes S)

Evidence for PT: strips of alternating polarity in a mirror image across the

ocean ridge

Hot Spot – mantle plume rises to surface -- not at a plate boundary -- shows that plates move Ex. Hawaiian Islands

9.5 Mechanisms of Plate Motion

Driving force for plate movement is the unequal distribution of heat in earth.

Convection – method of heat transfer in fluids where hot material rises and cold material sinks

Movement is due to differences in density.

But isn’t the mantle made of solid rock?

“On a human time scale, rocks are solid, but if you wait for a long time – millions of years – rocks slowly change their shape. They flow.”

-- Peter Bunge

Asst. Professor of Geophysics

Princeton University

Mantle Convection Models

Heat sources:

1. radioactive decay of elements in mantle

2. residual heat from formation

How do scientists study mantle convection?Geophysicists use computer modeling to simulate

mantle movement.

Burkavici http://www.columbia.edu/itc/ldeo/mutter/jcm/Topic3/Topic3.html

Mantle convection model animations

http://homepage.usask.ca/~sab248/Mtemp28.WMVhttp://www.gps.caltech.edu/~gurnis/Movies/movies-more.html

Proposed Mechanisms :

1. Ridge push

2. Slab pull

At present, none is able to explain all facets of plate motion.

There are still many unanswered questions about the details of Plate Tectonics.

Pangea Ultima

• http://www.nytimes.com/packages/html/science/20070109_PALEO_GRAPHIC/