Earth Science
Chapter 4
Section 5
A. The Theory of Plate Tectonics:
Plate Tectonics is the geological theory that states that pieces
of the Earth’s lithosphere are in constant, slow motion, driven
by convection currents in the mantle.
As the plates move, they collide, pull apart, or grind past each
other, producing changes in the Earth’s surface.
The changes include volcanoes, mountain ranges and deep-sea
trenches.
B. Plate Boundaries:
The Earth’s lithosphere is broken into pieces called plates.
These plates fit together along cracks in the lithosphere.
The places where plates meet are called Plate Boundaries.
Breaks in the Earth’s crust where rock have slipped past one
another are called Faults.
1. Transform Plate Boundaries:
Here crust is neither created nor destroyed. This is a place
where two plates slip past one another in opposite directions.
2. Divergent Plate Boundaries:
A place where two plates move apart or diverge, such as at a
mid-ocean ridge. When this occurs on the land surface a rift
valley is formed.
3. Convergent Plate Boundaries:
The place where two plates come together, or converge.
a. Ocean to Ocean – subduction occurs and a trench is
formed at the subduction zone and a Island arc is formed
on the opposite side of the trench.
b. Ocean to Continent – The more dense ocean plate is
subducted under the less dense continental plate. A
volcanic mountain range is formed inland.
c. Continent to Continent – Two continental plates collide,
no subduction occurs, the crust buckles and mountain
ranges form.
C. Continental Movement:
The plates move at very slow rates, about 1 to 10 cm per year.
The North American plate moves about 2.5 cm each year.
It is believed that Pangea began to break apart about 225
million years ago.