A. The theory that pieces of the Earth’s lithosphere, called plates, move slowly on top of the asthenosphere B. Explains the formation and movement of the Earth’s plates

1. Wegener hypothesized that the continents were once joined in a single supercontinent (Pangaea), which then broke into pieces that moved apart by continental drift.

2. Problems with the theory: a. Unable to explain how continents plow through solid rock sea floor b. What force could move the continents?

1. The mid-ocean ridge- chain of underwater mountains 2. Formation of Oceanic Crust by sea- floor spreading- Process by which new oceanic crust is created at mid-ocean ridges as older crust moves away

a. Subduction- As sea-floor spreading occurs, old oceanic plates sink into the mantle due to differences in density b. Trench- As a plate sinks through the subduction zone, it bends, forming a depression in the ocean floor c. Sea- floor spreading creates new oceanic crust at mid-ocean ridges. Subduction destroys old oceanic crust at subduction zones.

a. Rock sampling at mid-ocean ridges showed matching stripes on either side of the ridge b. Radioactive dating- rocks near mid- ocean ridge are younger than rocks further away

1. Plate motions are the visible part of the process of mantle convection 2. Figure 28- Heat flows from Earth’s hot interior toward the cooler surface through large convection currents in the mantle

1. Dozen major tectonic plates 2. Plates move slowly- 0.1 to 10 cm per year

a. Divergent- Plates move away from each other, forming new rock b. Convergent- Plates come together or collide, rock is destroyed c. Transform- Plates slide past one another moving in opposite directions, rock is neither created nor destroyed

1. Geologists found that most mountains form along plate boundaries 2. Example: Himalayas formed from collision of Indo-Australian and Eurasian plate and Iceland formed at a mid-ocean ridge of diverging plates