Honors 1360 Planet EarthLast time:

Leading Up to Plate Tectonics…Obs: Mountain mass balanced by deep buoyancyHyp: “Solid” rock flows on long time scales

Obs: “Jigsaw Puzzle” fit of continental boundariesObs: Paleoclimate: Fossils, glacial striae at wrong latitudesObs: Similar mountain ranges, rock formations, fossils on either side of the AtlanticHyp: Continental Drift: Continents once were one large supercontinent (Pangea) & drifted to current locations

Obs: Ocean BathymetryHyp: ???

Today:• Plate Tectonics Cont’d: A Paradigm Shift

15 September 2008

Earth in the News…

Unusually long “earthquakes” are actually “icequakes” in actively moving glaciers… Greenland icequakes show significantly increasing frequency in the past three summers.

September 2008 “Physics Today”

Then (ca. 1950’s) came the Cold War…

Measured Ocean Bathymetry to facilitate submarine warfare;discovered mid-ocean ridges, trenches, and fracture zones

As part of an ultimately fruitless attempt to use magnetism to search for subs, measured Seafloor Magnetic Lineations

These were found to be Symmetric Across Ridges!

Now Here’s a Puzzle…

To understand, needed two pieces of information fromcontinental studies of rock magnetism:

(1) Rocks acquire the ambient magnetic field directionas they cool through the Curie temperature(called remanant magnetization)

(2) Magnetic dipole direction occasional Reverses

Magnetic Lineations (Symmetric Across Ridges!)

• Magnetite in basalt records field direction when solidified

• Field direction changes!

• Ocean floor slowly moves away from ridges

• “Seafloor Spreading”Testable Predictions?

Age of the Ocean Floor

Drilled/dredged rocks from seafloor and age-dated them!

Question: Why Are There Ridges Where The Ocean IsYoungest?

Wikipedia:

Wikipedia:

WRONG! Never trust a Wiki….Ocean crust is ~ constant 6 km thickness!

Always remember foundations of CRITICAL THINKING: Use multiple sources of information Evaluate the reliability of sources Think about what makes sense!

Recall thermal expansion & thermal

contraction?

HotRock

CoolRock

fast seismic velocity

slow seismic velocity