Plate tectonics
Continental Drift
Alfred Wegener (1910)
 Hypothesized that all the
continents were once joined
together in a single landmass
call Pangaea and
have since drifted
(like rafts in water)
○ This became known
as CONTINENTAL
DRIFT
○ Pangaea-
supercontinent
(350-200 million
years ago) (Pangaea
means “all lands” in latin)
 Others had similar ideas, however Wegener was the first to have scientific
evidence to support this idea
Plate tectonics
Evidence for Continental drift
1.)Geologic features
 Coastlines of some continents seemed to fit
together like a jig saw puzzle
 Mountain ranges in N. America line up with those in
Europe
Plate tectonics
2.) Fossils- any trace of an ancient organism that is preserved
in rock
 Glossopteris- fern like
plant with leaves the
shape of a tongue,
live 250 million years ago
○ Found in Africa, South
America, Australia, India
and Antarctica
 These land masses are
now separated by oceans
indicating that Pangea once existed
 Mesosaurus and Lystrosaurus- fresh
water reptiles
○ Fossils found in places separated by
oceans
 Neither reptile could have swum that great of a
distance in salt water
- These reptiles lived on a single landmass that
split apart
Plate tectonics
3.) Climate
 Fossils of tropical plants found on an island in the
arctic ocean

Scratches in rocks
made by glaciers
found in South Africa

Coal fields in Europe and North America lined up

Coal only forms in tropical swamp areas
****When continents move, north or south, they take the fossils
and rocks that formed in the past with them
Plate tectonics
Continental Drift—REJECTED
 Wegener provide supporting evidence to the
continental drift theory
BUT….
Could not identify the mechanism that cause the plates
to move (the force that caused the push or pull)
because of this, most geologist rejected his theory
Plate tectonics
The theory of Plate Tectonics

Plate- A section of the
lithosphere that slowly moves
over the asthenosphere,
carrying pieces of continental
and oceanic crust

Theory of Plate TectonicsEarth’s plates are in slow,
constant motion, driven by
convection currents in the
mantle.

Convection current –A flow of
material caused by temperature
differences

Faults-breaks in the Earth’s crust
where rocks have slipped past each
other, form along these boundaries
Convection Currents

Occurs in the asthenosphere of
the mantle
How Convection Works:
 Heat from Earth’s core causes
the material of the mantle to
become a plastic like solid (it
can stretch and “flow”)

Once heated the material
becomes less dense so it rises
toward the lithosphere

At the lithosphere it cannot move
upward any more so it moves to
the right or the left

As it moves, it drags the
lithospheric plate with it and
cools.


The material becomes less dense
and it sinks back toward the core
The above process is repeated.
Convection Currents
Plate tectonics
Measurement of plate motion over time

Satellites are used to measure plate motion precisely.
 Plates move very slowly between 1-12 cm per year
○ North American and Eurasian plate move about 2.5 cm/year
 This is about the same speed in which your fingernails grow
○ Although very slow the plates have been moving for hundreds of millions
of years so they have moved great distances.
Plate Motion
Since the breakup of Pangaea, the continents have taken about 200 million years to move
to their present location.
200 million years ago

115 million years go
Earth Today
As the plates move, they create earthquakes, volcanoes, mountain ranges and
deep ocean trenches
Plate tectonics
3 Types of Plate Boundaries
1.) Divergent Boundariestwo plates slowly move away
from each other
 In the ocean = Mid Ocean
Ridge (MOR)
○ Underwater mountain ranges
○ new crust is added during
process of
sea floor spreading
 ex. Mid Atlantic Ridge,
Atlantic Ocean
 On land = Rift Valley
○ ex. East African Rift System,
Africa
Plate tectonics
Sea Floor Spreading- the process by which molten
material adds new oceanic crust to the ocean floor at
DIVERGENT BOUNDARIES
 This occurs at mid-ocean ridges (MOR)

How sea floor spreading works:
 MOR form crack in the oceanic crust, called central valley (see
pic. on previous slide)
 Along the ridge, molten material from inside Earth rises,
erupts, cools and hardens to form a solid strip of rock
○ Sea Floor spreading adds more crust to the ocean floor while older
strips of rock move outward from either side of the ridge
Plate tectonics
Evidence of Sea
Floor Spreading
1.) Drilling samples
 Scientists drilled
into the ocean
floor
○ Farther away the
sample was taken
from the MOR, the
older the rock was
○ Youngest rocks were
found at the center of
the MOR
○ The rocks age
shows that sea floor
spreading occurs
**As distance from the ridge
increases, age increases
Plate Tectonics
2.) Magnetic Stripes

Lava contains iron
 Iron is magnetic (points to
magnetic north)

When the lava solidifies, the
iron will be “frozen” in the
direction of the magnetic
north at the time of the
eruption

Direction of the iron at the
seafloor is not always the
same.

The patterns are mirror
images on either side

Width of stripe indicates how
long the pole was in the
normal or reversed position
 Wider = longer
Ocean Floors
Mid-ocean ridges rise
from the sea floor like
stitches on the seams
of a baseball.
Diagram of MOR where
two plates move apart
Plate Tectonics
2.) Convergent Boundary-boundary where two plates
collide or come together
Density of plates determine which one comes out on top

Subduction- the process by which oceanic crust sinks beneath a
deep-ocean trench and back into the mantle at a convergent
boundary
 New crust is hot—less dense
 Old crust is cool—more dense
 Deep ocean trenches-A deep valley along the ocean floor
beneath which oceanic crust slowly sinks toward the
mantle
○ Gravity pulls the older, more dense oceanic crust down
beneath the trench into the mantle
Plate tectonics
Three types of convergent boundaries
1.) Oceanic-Oceanic-the one that is more dense will
sink beneath the one that is less dense
often forms island arc-volcanoes which erupt
through the overriding plate as the descending
plate melts below it
Ex. Aleutian Islands of Alaska
2.) Oceanic-Continental-oceanic crust is more dense
so it will sink below continental
Mountains (Andes) and volcanoes can
form-Volcanic Mountain Range
-Water leaves sinking crust, rises into wedge of mantle
above it, the water lowers the melting point of the
mantle, mantle partially melts and volcano forms
-Ex. Andes Mountains
***Trenches generally run parallel to volcanic island arc’s
Plate tectonics
Types of convergent boundaries cont.
3.) Continental-Continental-neither is dense enough to sink
into mantle, no subduction, no trench, mountains
form
Ex Indian Plate and Eurasian plate collide to
form Himalayans
Plate tectonics
 Sea Floor spreading and subduction:
○ work together
○ change the size and shape of the oceans
○ They move the ocean floor like a large conveyor belt.
 Size determined by how fast new crust is being created and old crust being
swallowed
 Ocean surrounded by many trenches—may shrink
 Ocean with few trenches—will probably grow larger
Plate tectonics
3.) Transform Boundary-place where two plates slip past
each other, moving in opposite directions.
Plates are rocky and jagged, and get “stuck”, forces inside Earth cause
plates to become “unstuck”, sudden release of energy in the form of an
earthquake
○Crust is neither created nor destroyed.
○Ex. San Andreas Fault in California
Subduction
Oceanic crust created along a mid-ocean ridge is destroyed at a deep-ocean
trench. During the process of subduction, oceanic crust sinks down beneath the
trench into the mantle.
Plate tectonics
Convergent Boundaries (cont.)
3.) Continental-Continental-neither is dense enough to
sink into mantle, no trenches, no subduction, mountains
form
Ex Indian Plate and Eurasian plate collide to form Himalayans
Plate Tectonics
Earthquake and Volcanic Activity

Most earthquakes and volcanoes occur because of the
movement of the plates, especially as plates interact at their
edges or boundaries.
Plate Tectonics
Volcanoes, Earthquakes and Converging Boundaries
Volcanoes often form where two plates collide.
Hot Spots

Sometimes, volcanoes do not occur at plate
boundaries BUT in the middle of tectonic plates
 This is called a HOT SPOT
 Examples: Hawaii, Yellowstone Hot Spot
The Hawaiian Islands have formed one by one
as the Pacific plate drifts slowly over a hot spot.
This process has taken millions of years
 The island over the mantle plume is the youngest and the
island farthest from the mantle plume is the oldest
The Ring of Fire

The world's most active
volcanoes lie along what's
called the Ring of Fire.

Lines the Pacific Ocean

The "ring" stretches along
a 25,000-mile (40,000kilometer) arc from the
boundary of the Pacific
Plate, to smaller plates
such as the Philippine Sea
plate, to the Cocos and
Nazca Plates

Results in 90% of the
world’s earthquakes

Home to 75% of the
planets volcanoes