Ch 10 Plate Tectonics

• Section 10.2 Seafloor Spreading

After Wegener’s hypothesis of Continental Drift was rejected…

New technology & discoveries rekindled interest in his idea

• SONAR (Sound, Navigation and Ranging) was used to study the ocean floor in 1940’s – 1950’s

• 1947 – rocks on ocean floor found to be younger than continental rocks

Columbia University’s Vema Instrumental in discovering the Mid-Atlantic Ridge in 1950’s

Theory of Sea Floor Spreading

Proposed in 1960 by Harry Hess of Princeton U.

if seafloor is moving, continents may also be moving

Rift zone

Glomar Challenger sampling ocean floor rock in 1968

Ages of rock get older as the distance from rift zones increases.

Rift zones

Deep sea vents found in 1976

Woods Hole Oceanographic Institute’s submarine, Alvin

Hydrothermal Vents

• mid-1960’s – paleomagnetic reversal time scale developed for continental rock matches that of rocks found on sea floor

Magnetic evidence for Theory of Sea Floor Spreading

Earth’s magnetic field has reversed many times!

When lava or magma solidifies, it provides us with a record of Earth’s

magnetic field

Lava contains iron crystals which orient themselves to Earth’s magnetic field before solidifying

Magnetic Reversals also show that new rock forms at rift zones

Seafloor Spreading

Geomagnetic Reversal Time

Scale

Summing up 10.2:

• Theory of Seafloor Spreading supports Wegener’s hypothesis of Continental Drift

• Evidence from technology, repeated observations:– Sonar – mid-ocean ridges – Drilling ocean floor – youngest rock at center– Rift zones – visible eruptions of lava– Lava rock records magnetic field – rocks on ocean floor

agree with radioactive dating used for determining age of rock samples in 1940’s – 1970’s