Plate Tectonics I. 1. The Continental Drift Hypothesis . Proposed by Alfred Wegener in 1915. Supercontinent Pangaea started to break up about 200 million years ago. Continents "drifted " to their present positions. Continents "plowed" through the ocean crust.
a. Geographic fit of South America and Africa
b. Fossils match across oceans
a. Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land.
Mason & Raff, 1961
Black: normal polarity
White: reversed polarity
Both: very magnetic
b. Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field.
c. To test this hypothesis, scientists determined the eruptive ages AND the polarity of young basalts using the newly developed technique of K-Arradiometric dating.
The study validated the reversal hypothesis...
d. And then (1962-1963) geologists realized that the patterns are SYMMETRICAL across oceanic ridges.
e. The K-Ar dates show the youngest rocks at the ridge. (Center)