Reading: Ch. 2

Continental Drift:
In 1600s, similarity of Atlantic coastlines was noted.
In 1912, Wegener proposed that all the continents had been united into a supercontinent, Pangaea.

Besides “fit”, other evidence:

• continuity of rocks and mountain ranges
• glacial
• fossil evidence

Paleomagnetism indicates that continents have moved.
• In magmas (molten rock), magnetic minerals orient themselves with the magnetic field (magnetic inclination).
 Once the rock is solid, this magnetic orientation is preserved.  Points to magnetic north pole.

Paleomagnetism helped test the theory of seafloor spreading.
 Magnetic reversals: North and South poles have switched many times in past.
Magnetic studies of ocean floor showed:

• symmetrical pattern across ridges.
• Ocean floor gets older as it moves away from ridges

• Lithosphere is broken up into rigid plates which move over the weaker asthenosphere.
• The interior of plates are rigid and most stress and deformation is concentrated at the boundaries between plates.

Plate Boundaries:

1.Divergent: plates move apart. Shallow earthquakes, volcanoes.
2.Convergent: plates come together
        a. subduction zone: ocean crust sinks  Creates earthquakes, volcanoes (such as “Ring of Fire” around    Pacific Ocean).
b. collision (continent- continent): continental crust not dense enough to be subducted. Creates earthquakes
3. Transform boundary: plates slide past one another