Plate Tectonic Theory
Plate Motion
• The theory of plate
tectonics states that
Earth’s crust is broken
into rigid plates that
move slowly over
Earth’s surface.
• The rigid plates are
called tectonic plates.
• North America is part of
the North American Plate.
Plate Motion (cont.)
• The movement of one plate is described
as either moving away from or toward
another plate, or sliding past another
plate.
• Plates move at speeds of only a few
centimeters per year.
• At this rate, it takes moving plates
millions of years to make new continents,
new mountain ranges, or other
landforms.
Plate Motion (cont.)
Long before
geologists proposed
the theory of plate
tectonics, they
discovered evidence
of continental
movement.
Photo by G.K. Gilbert, U.S. Geological Survey
Plate Motion (cont.)
• In 1912, Alfred
Wegener developed
the hypothesis that
continents move,
called continental
drift.
• One piece of
evidence is the
shape of Earth’s continents.