TB Chapter 13
In order to study Plate Tectonics, we must first
reorganize our layering system for the Earth:
Old System – 4 layers
Crust (rigid)
(rigid)
Mantle (putty like)
(semi rigid)
Outer core (liquid)
Inner core (solid)
New System – 5 layers
Lithosphere
Asthenosphere
Mantle
Outer Core
Inner Core
ESRT’s Page 10 Cross Section of the
Layers of the Earth/
Temperature Graph/
Pressure Graph
What is Plate Tectonics?
• Earth’s surface is made up of moving, solid
pieces called plates
– (Reference Tables pg 5)
• Plate tectonics is the study of the formation
and movement of these plates
• Earth’s plates are part of the lithosphere
– The lithosphere is the crust and the upper part of
the mantle
Properties of the Crust
• Oceanic crust is mainly made of the
igneous rock basalt
• Continental crust is mainly made of
the igneous rock granite
• Oceanic crust is denser than
continental crust (b/c basalt is
MAFIC…remember?...ROCKS!)
How Plates Move…
• Asthenosphere - middle part of the mantle
• Similar composition to the rigid upper
mantle, but it is partially melted
• This makes the asthenosphere like a very
thick “liquid” (viscous)
– (think VERY THICK MUD or SILLY PUTTY)
• The “liquid” property of the asthenosphere
allows it to flow very slowly
How Plates Move…cont’d
• You can think of the lithospheric plates as
“floating” on the asthenosphere
• The heat from inside the Earth causes
convection currents to form in the
asthenosphere
• Where convection currents rise, molten
rock forces the plates apart
• Where the convection currents are sinking,
the plates are moving together
DIVERGENT BOUNDARY
NEW ROCK
LIQUID HOT MAGMA
CONVERGENT BOUNDARY
MOUNTAIN
BUIDLING
LIQUID HOT MAGMA
Alfred Wegener’s Theory of
Continental Drift
• In 1915, German geologist and
meteorologist, Alfred Wegener, first
proposed the theory of continental drift,
which states that parts of the Earth's crust
slowly drift atop a liquid core
• Wegener hypothesized that there was a
gigantic supercontinent 200 MYA, which
he named Pangaea, meaning "All-earth“
• Wegener published this theory in his book,
On the Origin of Continents & Oceans
Wegener
OK, so we’re saying that the
lithospheric plates move
around on top of the
asthenosphere, but what
evidence do we have of this
Plate Movement?
Evidence of Plate Movement
1) “Puzzle-like” fit
• The shape of the west coast of Africa and
the east coast of South America seem to
fit together like a puzzle
• It is believed that they were once
together, but moved apart over time
(Theory of Continental Drift)
Evidence of Plate Movement
2) Geologic Evidence
• Similar fossil remains in Africa and South
America of a reptile that is found nowhere
else in the world
• Distinctive rocks were also found in Africa
and South America in regions where the two
continents were most likely joined together
Evidence of Plate Movement
3) The majority of earthquakes and
volcanoes are found in belts that occur
along plate boundaries (where the plates
are moving together or apart)
– Ex. the Pacific Ring of Fire
Pacific Plate
RING OF FIRE – PACIFIC PLATE
Evidence of Plate Movement
4) Magnetic polarity reversals are recorded in
ocean floor rocks as the sea floor spreads apart
• During certain periods in Earth’s history, the
magnetic fields have reversed
• Iron in the igneous rocks on the ocean floor
would shift before the magma cooled and
hardened
• If the polarity changed, new magma would cool
with minerals shifted in the opposite direction
Evidence of Plate Movement
5) Heat flowing out of the rocks
(temperature) is greatest where the rocks
are spreading apart (rising convection
current)
• Heat (temperature) decreases where the
rocks are moving together
(sinking convection current)
ESRT’s Page 5 Plate Tectonic Boundaries
3 Main Types of Plate Boundaries
I Divergent Boundaries
II Transform Boundaries
III Convergent Boundaries
A. Collision (C-C)
B. Subduction (O-C) & (O-O)
Plate Boundaries
• Divergent Boundaries- plates are moving
apart at spreading centers
• These boundaries form mid-ocean ridges or
rises (like underwater mountain ranges)
• In between the 2 peaks of the ridge are
valleys called rift valleys
– Ex. mid-Atlantic Ridge, East Pacific Ridge
(Overhead View)
Mid-Atlantic
Ridge
Plate Boundaries
• Transform (Sliding) Boundary - 2 plates slide
past each other
• The sliding movement often causes earthquakes
to occur along faults
• A fault is nothing more than a crack in the
Earth’s crust where movement has occurred
– Ex. North American Plate and the Pacific Plate are
sliding past each other along the San Andreas Fault in
California
Someday, Los Angeles will be North of
San Francisco!!!!!
Convergent Boundaries
- two plates are converging or coming together
(2 main types)
Type 1
Collision Boundary
• (continental-continental) (C – C) - the
two plates moving together are both
continental crust plates
• The collision causes the plates to form a
single, larger continent and the crust is
pushed upward into a mountain range
–
Ex. Himalayan Mountains (including Mt.
Everest). The Indian Plate and the Eurasian
Plate are still moving together, so Mt. Everest
is still slowly getting taller!
Continental crust
Continental crust
Type 2
Subduction Boundary - one of the plates plunges
under (subducts) the other
• Occurs between two oceanic plates
(O-O), or an oceanic and continental (O-C)
plate
• Most common characteristic is a
deep-sea trench VIF!!!!
•
•
These trenches are the deepest spots in the ocean
Ex - the Marianas Trench is approx. 35,000 feet
deep! Mt. Everest could be put into the trench
and the peak would still be about 1 mile below
the ocean’s surface!
Continental crust
Oceanic crust
Subduction Boundaries cont’d
• Because oceanic crust is denser than continental crust,
the oceanic crust will always subduct underneath the
continental
• Volcanoes always seem to form at subduction
boundaries
– If two oceanic plates converge, volcanic islands will
form (ex - Aleutian Islands in Alaska)
– If one plate is continental and the other oceanic, the
volcanoes will form along the edge of the continental
crust (ex - Cascade Mountain Range on Western US)
Subduction Boundaries cont’d
• When the oceanic crust gets buried deeper
and deeper under the Earth, it begins to melt
into “liquid hot” magma.
• Since “liquid hot” magma is less dense than
the solid rock surrounding it, it has to rise.
• When it rises, it comes to the surface of the
Earth and forms volcanoes.