Transcript Announcements: Read Chapter 5 for Thursday. Homework#3 due

Plate Tectonics
6.E.2.2 Explain how crustal plates and ocean basins are formed, move
and interact using earthquakes, heat flow and volcanoes to
reflect forces within the earth.
Looking at the world map, what do you notice about the shape of the
continents?
The world didn’t always look like that! It used to look like this:
Pangaea Supercontinent
How is this possible?!?!?
Plate Tectonics Theory
The lithosphere is divided into a number of large and
small plates and the plates are floating on the mantle
Lithosphere = the Earth’s crust plus the upper
portion of the mantle layer
How Plates Move
• A Canadian scientist, J. Tuzo Wilson, claimed the
lithosphere is broken into separate sections called
plates. These plates fit together along cracks in
the lithosphere.
• Scientists realized that the continental drift idea
could be explained by sea floor spreading. Wilson
took what these scientists knew and combined it
with his idea about Earth’s plates into a single
theory.
• A scientific theory is a well-tested concept that
explains a wide range of observations.
How Plates Move
(Cont.)
 Sea floor spreading provides the driving mechanism
for movement
 However, it is not the continents that are moving,
but the “plates” of lithosphere “floating” in effect on
the asthenosphere
 The lithosphere is made up of about 20 plates which
move relative to each other in several ways
 The theory of plate tectonics states that pieces
of Earth’s lithosphere are in slow, constant motion,
driven by convection currents in the mantle. The
theory of plate tectonics explains the formation,
movement, and subduction of Earth’s plates.
Evidence of Plate
Movement
Major Tectonic Plates of the World
“Plates” of lithosphere are moved around by the underlying hot mantle
convection cells
Convection
Convection is heat transfer by
the movement of currents within a
fluid. During convection, heated
particles of fluid begin to flow. This
flow transfers heat from one part
of the fluid to another.
Heat from the core and the mantle
itself causes convection
currents in the mantle.
Examples of Convection
Examples of Convection
(Cont.)
Convection Currents
Many geologists think that plumes of
mantle rock rise slowly from the bottom
of the mantle toward the top. The hot
rock eventually cools and sinks back
through the mantle. Over and over, the
cycle of rising and sinking takes place.
There are also convection currents in
the outer core. These convection
currents cause Earth’s magnetic field.
As the Earth’s plates move, they
collide, pull apart, or grind past
each other, producing spectacular
changes in Earth’s surface. These
changes include volcanoes,
mountain ranges, and deep-ocean
trenches.
Plate Boundaries
Types of Plate Boundaries
 Divergent boundaries (also called
spreading centers) are the place where two
plates move apart.
 Convergent boundaries form where two
plates move together.
 Transform fault boundaries are margins
where two plates grind past each other
without the production or destruction of the
lithosphere.
Types of Plate Boundaries
Divergent boundaries:
 Oceanic Ridges and Seafloor Spreading
• Oceanic ridges are continuous elevated
zones on the floor of all major ocean
basins. The rifts at the crest of ridges
represent divergent plate boundaries.
• Rift valleys are deep faulted structures
found along the axes of divergent plate
boundaries. They can develop on the
seafloor or on land.
• Seafloor spreading produces new oceanic
lithosphere.
Spreading Center
Convergent Boundaries
 A subduction zone occurs when one
oceanic plate is forced down into the
mantle beneath a second plate.
 Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic
activity caused by the subduction of oceanic
lithosphere beneath a continent.
• Examples include the Andes, Cascades, and
the Sierra Nevadas.
Oceanic-Continental
Convergent Boundary
Convergent Boundaries
(Cont.)
 Oceanic-Oceanic
• Two oceanic slabs converge and one descends
beneath the other.
• This kind of boundary often forms volcanoes on
the ocean floor.
• Volcanic island arcs form as volcanoes emerge
from the sea.
• Examples include the Aleutian, Mariana, and
Tonga islands.
Oceanic-Oceanic
Convergent Boundary
Convergent Boundaries
(Cont.)
 Continental-Continental
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new
mountain ranges, such as the Himalayas.
Continental-Continental
Convergent Boundary
Convergent
boundaries
Oceanic-Continental
Oceanic-Oceanic
Continental-Continental
Plate tectonics: predicting the future
Plate tectonics: predicting the future
(Cont.)
Australia will straddle the equator
E. Africa will form new continent
Mediterranean will close off
Atlantic Ocean will grow
Pacific Ocean will shrink
Questions
What theory states that the lithosphere
is divided into large and small plates?
A. Plate Tectonics Theory
B. Plain Tectonics Theory
C. Plain Technicians Theory
D. Plate Technicians Theory
What type of plate boundary is formed
when two plates move apart?
A. Transform fault boundary
B. Convergent boundary
C. Divergent boundary
D. None of the above
A _______________ Zone occurs when
one oceanic plate is forced down into
the mantle beneath a second plate.
A. Continental
B. Convection
C. Subtraction
D. Subduction
The End