Continental Drift & Plate Tectonics
• Continental Drift is the
idea that the continents
move around on Earth’s
surface.
• The surface of Earth is
broken into many pieces
like a giant jigsaw puzzle.
Plate Tectonics describes
how these pieces move
on Earth’s surface
Theory of Continental Drift
• The theory
that
continents
were once
connected,
but have
drifted apart
Alfred Wegener
• Alfred Wegener was a
German climatologist and
arctic explorer who
suggested the idea of
continental drift.
• He thought the
continents we know
today had once been part
of an earlier
“supercontinent”.
• He called this great
landmass Pangaea.
Evidence for Continental Drift
•
•
•
•
Coal beds stretch across the eastern U.S. and continue across southern Europe.
Matching plant fossils are found in South America, Africa, India, Australia, and
Antarctica.
Matching reptile fossils are found in South America and Africa.
Matching early mammal fossils are found in South America and Africa.
Fossil Evidence
• Impressions of organisms left in
rock layers that indicate the
organisms once lived in the area
Matching rock types and
mountain belts occur in
North America and the
British Isles, and Africa and
South America.
Evidence of glaciers is
present in regions with
warm, dry climates.
Continents that are close to
the equator today were
once closer to the South
Pole in the distant past
Map with
Fossil Locations
Plate Tectonic Theory
• Theory that the lithosphere is divided into
tectonic plates that slowly move on top of the
asthenosphere
Lithosphere
• Cool, rigid,
outermost layer of
Earth that is divided
into enormous
pieces called
tectonic plates;
consists of the crust
and the rigid
uppermost part of
the mantle
Sea Floor Spreading
• American geophysicist Harry Hess helped develop the
theory of plate tectonics.
• While a Navy officer, Hess helped map the ocean floor.
• Hess called his hypothesis sea-floor spreading.
Undersea Mountains discovered
• Naval maps showed undersea mountain chains that
formed a continuous chain down the centers of ocean
floors
• Hess wondered if new ocean floor was created at these
mid-ocean ridges.
Harry Hess’ Idea
• As new sea floor is made at mid-ocean ridges,
the continents are pushed away.
Sea Floor Spreading Evidence
• A parallel pattern of rock material found at identical locations on
each side of the Mid-Atlantic Ridge reveals rock of the same
geologic age and polarity
• The key was the discovery that there are “magnetic patterns” in the
rocks on either side of the mid-ocean ridges.
Reversed polarity
Normal polarity
Earth’s Tectonic (Lithospheric) Plates
• Earth’s crust is “broken” into many lithospheric (tectonic)
plates. There are 7 major plates, shown below.
Eurasian Plate
Plate
Nazca
Plate
Antarctic Plate
Moving pieces of the Lithosphere
• There are two kinds of lithospheric plates:
continental plates and oceanic plates
Andesite, Granite, and Basalt are all igneous rocks.
Basalt is a much denser igneous rock than Andesite
or Granite, so oceanic crust is more dense than
continental crust.
Mantle Convection
• Convection cells in Earth’s lower mantle drive the lithospheric
plates on the surface.
• Heated lower mantle material rises toward Earth’s surface.
• Then, cooling makes the nearby material denser and it sinks deeper
into the lower mantle. This sinking process is called subduction.
Hot Spots and Island Chains
• A single hot rising
plume, called a mantle
plume, can cause a
volcanic eruption in
the plate above it.
• If the eruption is
strong and lasts long
enough, the volcanic
eruption may form an
island on the plate.
Hot Spots and Island Chains
• After the island forms, the movement of the plate
carries it away from the mantle plume.
• Scientists determine the direction and speed of plate
movement by measuring these island chains.
Plate Boundaries
• There are 3 types of plate boundaries
.
*
*The words “grind” or
“shear” would be a
better description than
“slide”.
Divergent Boundary
Great Rift Valley in Africa
Mid-Ocean Ridge
• The boundary between two tectonic plates moving
away from each other; on land creates rift valleys,
on the sea floor creates new ocean crust
Divergent Plate Boundaries
• Divergent boundaries are found in the ocean as midocean ridges.
• A divergent boundary is the line between two plates
where they are moving apart.
• This type of boundary is found over the rising plume of
a mantle convection cell.
Divergent Boundaries
• Divergent
boundaries can also
be found on
continents as rift
valleys.
• When a rift valley
forms on land, it
may eventually split
the landmass.
Convergent Plate Boundaries
• Convergent Plate Boundaries are where two plates come together.
• When continental plates collide, mountains are formed.
• When an oceanic and continental plate collide, the denser oceanic
plate subducts under the less dense continental plate.
• When oceanic plates collide, one subducts under the other, forming
a trench.
Convergent Boundary with Mountain
Building
Continental plate
Crust
Continental plate
Crust
Lithosphere
Asthenosphere
• A major geological event; occurs when
continental plates of equal density converge
Convergent Boundaries and Mountains
• Mountain ranges are formed when
continents collide.
Formation of the
Himalaya Mountains
• The formation of the
Himalaya Mountains
is attributed to the
collision of the IndoAustralian Plate with
the Eurasian Plate
Convergent Boundary with Subduction
Volcanic mountain range
Some oceanic
crust melts and
forces its way
upward
• The boundary between two tectonic plates moving toward
each other resulting in volcanic activity when a denser
ocean plate subducts below a less dense continental plate
Subduction
Note the
formation of a
trench in the
Subduction
Zone
Subduction
Zone
• A plate is forced below when one plate is denser than
another as they converge; occurs at continental to
oceanic boundaries and oceanic to oceanic boundaries
Collision of Nazca and South American Plates
• As the oceanic Nazca
Plate collides with
the continental South
American Plate, the
denser Nazca Plate
subducts under the
South American
Plate.
• The resulting surface
feature is the Andes
Mountain range.
Subduction Zones &
Volcanic Activity
• Volcanoes often occur
at subduction zones
Transform Boundary
• The boundary between two plates that slide
past one another; the release of stored
energy result in sudden shifts often causing
major geological events such as earthquakes
Transform
Boundaries
• An edge of a lithospheric plate that slides by
another plate is called a transform plate
boundary, sometimes called a transform fault.
Transform Plate (Fault) Boundary
• A good clue for locating transform boundaries is
offsetting.
• When seen from above, the feature appears to make
a zig-zag.
Earth’s Largest Lithospheric Plates (again)
• Earth’s crust is “broken” into many lithospheric (tectonic)
plates. There are 7 major plates, shown below.
Eurasian Plate
Plate
Nazca
Plate
Antarctic Plate