- glaciation evidence
- paleoclimatic eveidence

In the 1960's, continental drift was expanded to form the theory called Plate Tectonics based on two new technologies at the time:

Using geophysics, maps of the ocean floor revealed a central ridge down the centers of the oceans with a 1-3 km deep valley in the center.

- Here magma was welling up.
- 1960: Hess proposed sea-floor spreading to account for continental drift.
- mantle convection driving the spreading
- magma forms new oceanic crust

Paleomagnetism:

- Basalts are rich in Fe, become weakly magnetised by the Earth's magnetic field as they xlize and cool.
- Becomes a fossil magnet, oriented w/respect to the Earth's magnetic field at the time of xlization
- Records where magnetic north is on the Earth
- magnetic north is different than geographic north
- Rocks record that the magnetic pole has wandered "moved" over time.
- these wandering "paths", discovered in Europe and NA are parallel to each other.

Magnetic field reverses polarity periodically (1-3 my). N becomes S, etc.

- Can be detected in basalts on ocean floor.
- Produces magnetic "stripes" on either side of a spreading center which are symmtrical and record the Earth's magnetic polarity at the time of the basalt's formation.

Eventually discovered that the Earth's lithosphere is divided into 10-12 large tectonic plates and that these plates move around on the mantle and interact in 3 ways:

Divergent Plate Boundaries.

- rift valleys then floored by basaltic rocks

- Mississippi is an old rift zone
- Champlain Valley
- St Lawrence River Valley (show map)

Convergent Plate Boundaries

- cont-cont: like the Himalayas
- cont-ocean: Like the Cascades
- ocean-ocean: Island arcs like Japan

Transform Boundaries

- friction between plates stops the movement until the pressure builds up enough to break the piece, causing an earthquake.

Mantle plumes: