Download October 10, 2011

V. The Theory of Plate Tectonics
A. The Earth’s lithosphere is broken into many pieces known as plates.
1. In 1965 J. Tuzo Wilson, a Canadian scientist, proposed that the entire surface of the Earth is
broken into plates.
a. A Scientific Theory is a well tested concept that explains a wide range of
observations.
B. How Plates Move
1. Plate Tectonics Theory states that the pieces of Earth’s lithosphere are in slow, constant
motion, driven by convection in the mantle.
a. The Theory of PT explains the formation, movement, and subduction of Earth’s plates.
2. Gravity helps pull a subducting plate down into the mantle.
3. As plates move they collide, pull apart, and grind past each other creating changes in Earth’s
surface including volcanoes, mountain ranges, and deep ocean trenches.
C. Plate Boundaries—the edges of Earth’s plates meet at plate boundaries.
1. Faults—breaks in Earth’s crust where rocks slip past each other.
2. There are 3 kinds of plate boundary:
a. Divergent—a place where two plates move apart, or diverge. These occur at mid-ocean
ridges on oceanic crust and rift valleys in continental crust. New crust is created here.
b. Convergent—a place where two plates collide, or converge. When 2 plates collide, the
less dense one will always come out on top. Crust is destroyed here. When continent and
ocean crust collide, subduction can occur. When two continents collide, mountains form.
c. Transform—a place where two plates slide past each other, moving in opposite
directions. Crust is neither created nor destroyed at transform boundaries. Earthquakes
often occur at transform boundaries.
3. Plate Motion Over Time—the movement of plates has greatly changed the Earth’s surface.
a. Scientists believe that other supercontinents existed before Pangaea.
b. Pangaea formed about 260 million years ago, and began to break apart about 225
million years ago.