Plate Tectonics
The Dynamic Interior of the Earth
Review of Earth’s Interior:
1. Name, in order going from the center of the
Earth to where we live, the four major layers of
the Earth.
2. What composes the lithosphere?
3. How do the two layers closest to the center of
the Earth differ in their composition?
4. Describe how scientists determine the
composition of the different layers of the Earth.
If you look at a map of the world, you may notice that some of
the continents could fit together like pieces of a puzzle.
Plate Tectonics
• The Earth’s crust is divided into 12 major
plates which are moved in various
directions.
• This plate motion causes them to collide,
pull apart, or scrape against each other.
• Each type of interaction causes a
characteristic set of Earth structures or
“tectonic” features.
• The word, tectonic, refers to the
deformation of the crust as a consequence
of plate interaction.
The World’s Plates
What are tectonic plates made
of?
• Plates are made of
rigid lithosphere.
• Remember - The
lithosphere is
made up of the
crust and the
upper part of the
mantle.
Theory of Continental Drift
• Proposed by Alfred
Wegener in early
1900’s.
• He hypothesized that
the continents were
once joined together in
a single large land mass
he called Pangea
• Pangea split apart and
the continents moved
gradually to their
present positions
• Process is known as
Continental Drift
Pangaea, about 200 million years ago, before it began breaking up.
Wegener named the southern portion of Pangaea Gondwana, and
the northern portion Laurasia.
The continents about 70 million years ago. Notice that the breakup
of Pangea formed the Atlantic Ocean. India’s eventual collision
with Eurasia would form the Himalayan Mountains.
Learning Check
• What layer of the Earth composes the
tectonic plates?
• How many “major” plates make up the
Earth?
• Who develop the theory of plate
tectonics?
• Describe Pangea.
Evidence of Continental Drift
• Continents fit
together like a
puzzle.
• Ex. - The Atlantic
coastlines of
Africa and South
America.
More Evidence…
• Fossils of several
plants and
animals of the
same species
found on
different
continents.
Some More Evidence
• Rock sequences (the
order of rock layers) in
South America, Africa,
India, Antarctica, and
Australia show
remarkable
similarities.
• Wegener showed that
the same three layers
occur at each of these
places.
Cont’
•The same three layers are in the same
order in areas now separated by oceans.
• Wegener proposed that the rock layers
were made when all the continents were
part of Pangaea.
•He proposed that they formed in a
smaller small joined land mass that was
later broken and drifted apart.
Another Learning Check…
1. Explain 3 pieces of evidence used to
support Wegener’s theory of continental
drift.
Sea Floor Spreading
• Everyone agreed that
Wegener’s evidence was
compelling. But wouldn’t
we feel the movement?
• Also, wouldn’t there be
evidence to show that
the continents were still
moving today?
• Wegener was a
meteorologist and his
theory was not well
accepted. (He died on an
expedition in Greenland
collecting ice samples)
Sea Floor Spreading
• One reason scientists
had a hard time with
Wegener’s theory is
that there was no
mechanism for the
continents motion.
Sea Floor Spreading
• Henry Hess
• 1960’s
• Using new
technology, radar, he
discovered that the
seafloor has both
trenches and midocean ridges.
• Proposed the seafloor spreading
theory.
Sea Floor Spreading
• Hess proposed that hot, less dense material below
Earth’s crust rises toward the surface at the mid-ocean
ridges.
• Then, it flows sideways, carrying the seafloor away from
the ridge in both directions.
Sea Floor Spreading
• As the seafloor spreads apart at a midocean ridge, new seafloor is created.
• The older seafloor moves away from the
ridge in opposite directions.
• This helped explain how the crust could
move—something that the continental
drift hypothesis could not do.
Evidence of Seafloor Spreading
• In 1968, scientists
aboard the research
ship Glomar
Challenger began
gathering
information about
the rocks on the
seafloor.
• Scientists found that
the youngest rocks
are located at the
mid-ocean ridges.