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Plate Tectonics
Table of Contents
Earth’s Interior
Convection and the Mantle
Drifting Continents
Sea-Floor Spreading
The Theory of Plate Tectonics
Plate Tectonics - Earth’s Interior
Exploring Inside the Earth
• Earth’s surface is constantly
changing. It looks very
different today than it did
millions of years ago.
• Geologists (scientists who
study rocks) would have to
dig down 6,000 kilometers to
get to the Earth’s core.
• Because this would be
impossible, geologists use
two main types of evidence to
learn about Earth’s interior
Plate Tectonics - Earth’s Interior
Exploring Inside the Earth
– Direct evidence – from rock samples
– Indirect evidence – from seismic
waves.
Plate Tectonics - Earth’s Interior
Exploring Inside the Earth
• Seismic waves – vibrations
that travel through Earth
carrying the energy released
during an earthquake.
– Can detect the structure of
the planet by studying the
speed and path of the
waves
– Determined that the Earth is
made up of many layers
Plate Tectonics - Earth’s Interior
A Journey to the Center of Earth
• The three main layers of Earth:
– crust, the mantle, and the core.
– Layers vary greatly in size,
composition, temperature, and
pressure.
• Temperature – the heat is a result
of the formation of the planet and
radio active materials.
– Increases by about 1°C every 40
meters down
Plate Tectonics - Earth’s Interior
A Journey to the Center of Earth
• Pressure – force exerted on a surface
divided by the area over which the force is
exerted.
– Increases as you go deeper into the Earth
Pressure
increases the
deeper you go,
like that of a
swimming pool
Plate Tectonics - Earth’s Interior
The Crust
• Crust – layer of
rock that forms
Earth’s outer skin
– Includes both dry
land and the
ocean floor.
– Thinnest at
ocean floors and
thickest at
mountains
Plate Tectonics - Earth’s Interior
The Crust
• Two types of crust:
– Oceanic crust – the crust that
makes up the ocean floor
• Made mostly of basalts –
dark rock with a fine grained
texture
– Continental crust – the crust
that makes up the continents
• Made mostly of granites –
light rock with a coarse
grained texture
Plate Tectonics - Earth’s Interior
The Mantle
• Mantle – layer of
hot semi-solid rock
under the crust
– Divided into
layers based on
the physical
characteristics
– Total about
3,000 km thick
Plate Tectonics - Earth’s Interior
The Mantle
• Two parts to the upper mantle:
• Lithosphere –where the
upper most part of the
mantel and the crust merge
– “lithos” Greek for “stone”
– About 100 km thick
• Asthenosphere – under
the lithosphere where rock
is heated causing it to be
flexible (like plastic)
– “asthenes” Greek for “weak”
– Still considered a solid
Plate Tectonics - Earth’s Interior
The Core
• Lower mantle – beneath the asthenosphere,
very hot solid material
• Core – in the center of Earth and made mostly
of iron and nickel
– Consists of two parts: outer and inner
– Total 3,486 km thick
• Outer core – molten liquid metal
– Scientists think that movements in the liquid
outer core create Earth’s magnetic field.
• Inner core – dense ball of iron and nickel atoms
– So much pressure that it remains a solid
Plate Tectonics - Earth’s Interior
The Core
• The core is made mostly of the metals iron
and nickel. It consists of two parts–a liquid
outer core and a solid inner core.
Plate Tectonics - Earth’s Interior
Temperature Inside the Earth
• The graph shows how
temperatures change
between Earth’s
surface and the bottom
of the mantle. On this
graph the temperature
at the Earth’s surface is
0oC. Study the graph
carefully and then
answer the questions.
Plate Tectonics - Earth’s Interior
Temperature Inside the Earth
• Reading Graphs:
As you move from left
to right on the x-axis,
how does depth
inside the Earth
change?
The depth increases.
Plate Tectonics - Earth’s Interior
Temperature Inside the Earth
• Estimating:
• What is the
temperature at the
boundary between
the lithosphere and
the asthenosphere?
About 1,600oC
Plate Tectonics - Earth’s Interior
Temperature Inside the Earth
• Estimating:
What is the temperature
at the boundary
between the lower
mantle and the core?
About 3,200oC
Plate Tectonics - Earth’s Interior
Temperature Inside the Earth
• Interpreting Data:
How does temperature
change with depth in
Earth’s interior?
It generally
increases with
depth.
Plate Tectonics - Earth’s Interior
Links on the Structure of Earth
• Click the SciLinks button for links on the
structure of Earth.
Plate Tectonics - Convection and the Mantle
Types of Heat Transfer
• Heat always moves
from a warmer
substance to a cooler
substance.
– Ex: holding an ice cube
in your hand
• There are three types
of heat transfer:
radiation, conduction,
and convection.
Plate Tectonics - Convection and the Mantle
Radiation
• Radiation – the transfer of energy through
space
– takes place with no direct contact between a
heat source and an object
– Ex: Sunlight
Plate Tectonics - Convection and the Mantle
Conduction
• Conduction – the transfer of heat within a
material or between materials that are
touching
– Ex: spoon in a pot of hot soup
In conduction, the heated
particles of a substance
transfer heat through
contact with other particles
in the substance.
Plate Tectonics - Convection and the Mantle
Convection
• Convection is caused by differences of
temperature and density within a fluid.
• Density – measure of how much mass there is
in a volume of a substance.
– Ex: A rock is more dense than the same
volume of water.
• As a substance is heated, molecules spread
apart, causing it to be less dense, and allowing it
to rise
• As it cools the molecules get closer together,
increasing density, and causing gravity to pull it
back down.
Plate Tectonics - Convection and the Mantle
Convection Currents
• Heating and cooling of the fluid, changes
in the fluid’s density, and the force of
gravity combine to set convection
currents in motion.
Plate Tectonics - Convection and the Mantle
Convection Currents
• Convection current – movement of a
fluid, caused by differences in
temperature, that transfers heat from one
part of the fluid to another.
– Continue as long as heat is added
– Without heat, convection currents eventually
stop.
Plate Tectonics - Convection and the Mantle
Convection Currents in Earth
• Heat from the core and the mantle itself
causes convection currents in the mantle.
Plate Tectonics - Convection and the Mantle
Mantle Convection
• Click the Video button to watch a movie
about mantle convections.
Plate Tectonics - Convection and the Mantle
More on Convection Currents in the
Mantle
• Click the PHSchool.com button for an
activity about convection currents in the
mantle.
Plate Tectonics - Drifting Continents
Continental Drift
• Throughout centuries people have been
studying maps. During this time many people
have noticed the similarities between the
coastlines of the continents.
• The continents on each side of the Atlantic
Ocean looked as though they could fit together
like a puzzle piece.
Plate Tectonics - Drifting Continents
Continental Drift
• Alfred Wegener – (1910) German
scientist who hypothesized that all of
the continents were once joined
together in a giant continent that he
named Pangaea.
– Continental drift theory - idea
that the continents slowly move
across Earth’s surface
– Pangaea was believed to exist
about 300 million years ago when
reptiles and winged insects first
appeared
Plate Tectonics - Drifting Continents
Continental Drift
• Wegener’s hypothesis was that all the
continents were once joined together in a
single landmass.
Plate Tectonics - Drifting Continents
Continental Drift
• Wegener gathered three types of
evidence to support his ideas about
continental drift.
– They included: land features, fossils,
and evidence of climate change
• Evidence from land features
– Wegener found that the mountains of
eastern South America matched that
of western Africa
– Coal in England matched that of
eastern North America
Plate Tectonics - Drifting Continents
Continental Drift
• Evidence From Fossils
• Wegener also used fossils to support
his idea.
• Fossil – any trace of an ancient
organism that has been preserved in
rock.
– Ex: Glossopteris (250 mill yr old fern) –
found in rocks of Africa and South
America
Plate Tectonics - Drifting Continents
Continental Drift
– Ex: Mesosaurus and Lystrosaurus (ancient
freshwater reptiles) now separated by an ocean
• Evidence From Climate
– Wegener found fossil evidence of ancient
tropical plants and animals in polar regions
– He also found evidence of ancient arctic
species in areas that are now close to the
equator.
Plate Tectonics
Evidence for Continental Drift
Plate Tectonics - Drifting Continents
Evidence for Continental Drift
Plate Tectonics - Drifting Continents
Wegener’s Hypothesis Rejected
• Many scientists opposed Wagener’s
hypothesis, despite the evidence, because
he was unable to explain what the force
was that caused this movement.
Plate Tectonics - Drifting Continents
Links on Continental Drift
• Click the SciLinks button for links on
continental drift.
Plate Tectonics - Sea-Floor Spreading
Mid-Ocean Ridges
• The East
Pacific Rise
is just one of
the many
mid-ocean
ridges that
wind
beneath
Earth’s
oceans.
Plate Tectonics - Sea-Floor Spreading
Mid-Ocean Ridges
• Mid-ocean ridge – undersea mountain chain
where new ocean floor is produced
– Discovered by using sonar (a device that
bounces sound waves off underwater objects
and then records the echoes)
• Harry Hess – (American) first
geologists to study mid-ocean ridges
– Found evidence to back up
Wegener’s Continental Drift
Theory
– Called this seafloor spreading
Plate Tectonics - Sea-Floor Spreading
What Is Sea-Floor Spreading?
• Sea-floor spreading –
sea floor spreads apart
along both sides of a
mid-ocean ridge as
new crust is added
– Ocean floors move
like conveyor belts,
carrying the
continents along with
them.
Plate Tectonics - Sea-Floor Spreading
Evidence for Sea-Floor Spreading
• Hess found evidence to support his theory of seafloor spreading, which included:
– eruptions of molten material, magnetic stripes in
the rock of the ocean floor, and the ages of the
rocks themselves.
Plate Tectonics - Sea-Floor Spreading
Evidence for Sea-Floor Spreading
• Evidence From Molten Material
– 1960’s – submarine Alvin found rocks
along the ocean floor running parallel with
the ridge
– These type of rocks only form when
molten material hardens quickly after
erupting under water
• Evidence From Magnetic Stripes
– rock that makes up the ocean floor lies in
a pattern of magnetized “stripes.”
– record of reversals in Earth’s magnetic
field.
Plate Tectonics
Evidence for Sea-Floor Spreading
• Evidence From Drilling Samples
– Glomar Challenger – (1968) drilling
ship that obtained rock samples from
the ocean crust
– Found that the rocks closest to the midocean ridge were “younger.”
– The farther away from a ridge the
samples were taken, the older the
rocks were
Plate Tectonics
Subduction at Trenches
• Deep-ocean trench – deep
valley along the ocean floor
beneath which oceanic crust
slowly sinks toward the mantle.
– The oceanic crust bends
downward into the mantle
• Subduction – process by
which oceanic crust sinks
beneath a deep-ocean trench
and back into the mantle.
– Occurs at a convergent plate
boundary
Trench
Subduction
Plate Tectonics - Sea-Floor Spreading
Subduction at Trenches
• In a process taking tens of millions of years, part
of the ocean floor sinks back into the mantle
through deep-ocean trenches.
Plate Tectonics - Sea-Floor Spreading
Growing an Ocean
• Because of seafloor spreading,
the distance
between Europe
and North
America is
increasing by a
few centimeters
per year.
Plate Tectonics - Sea-Floor Spreading
More on Sea-Floor Spreading
• Click the PHSchool.com button for an
activity about
sea-floor spreading.
Plate Tectonics - Sea-Floor Spreading
Sea-Floor Spreading
• Click the Video button to watch a movie
about sea-floor spreading.
Plate Tectonics
How Plates Move
• Plate tectonics – theory that pieces of
Earth’s lithosphere are in constant motion,
driven by convection currents in the
mantle.
– As the plates move, they collide, pull apart, or
grind past each other, producing spectacular
changes in Earth’s surface.
– Changes include volcanoes, mountain
ranges, and deep-ocean trenches.
Plate Tectonics - The Theory of Plate Tectonics
How Plates Move
• The theory of plate tectonics explains the
formation, movement, and subduction of Earth’s
plates.
Plate Tectonics
Plate Boundaries
• The edges of Earth’s plates meet at plate
boundaries that extend deep into the
lithosphere.
• Fault – break in Earth’s crust where masses of
rock slip past each other.
Plate Tectonics - The Theory of Plate Tectonics
Plate Boundaries
• There are three kinds
of plate boundaries:
– divergent
boundaries,
convergent
boundaries, and
transform
boundaries.
– A different type of
plate movement
occurs along each
type of boundary.
Plate Tectonics - The Theory of Plate Tectonics
Calculating a Rate
• To calculate the rate of plate motion,
divide the distance the plate moves by
the time it takes to move that distance.
Rate = distance/time
• For example, a plate takes two million
years to move 156 km. Calculate its rate
of motion.
• 156 km/2,000,000 years = 7.8 cm per
year
Plate Tectonics - The Theory of Plate Tectonics
Calculating a Rate
• Practice Problem
• The Pacific plate is sliding past the North
American plate. It has take ten million
years for the plate to move 600 km. What
is the Pacific plate’s rate of motion?
• 60,000,000cm ÷ 10,000,000years = 6 cm/yr
Plate Tectonics - The Theory of Plate Tectonics
Plate Boundaries
• Divergent boundary – plate
boundary where two plates
move away from each other.
– Most divergent boundaries
occur along the mid-ocean
ridges where sea-floor
spreading occurs.
– Rift valley – deep valley that
forms where two plates move
apart.
• Ex: Great Rift Valley in Africa
Plate Tectonics - The Theory of Plate Tectonics
Plate Boundaries
• Convergent boundary – plate boundary where two
plates move toward each other.
– The density of the plates determines which one
comes out on top. – Ex: Oceanic – Continental
collision:
• Oceanic crust is more dense,
therefore subduction occurs
– Ex: Continental – Continental
collision:
• Neither is more dense,
therefore mountains form
Ex: Appalachia Mountains
Plate Tectonics - The Theory of Plate Tectonics
Plate Boundaries
• Transform boundary – plate boundary
where two plates move past each other in
opposite directions.
– Earthquakes often occur along transform
boundaries, but crust is neither created nor
destroyed.
Ex: San Andreas
Fault, California
Plate Tectonics - The Theory of Plate Tectonics
Continental Drift
• It has taken the continents about 225 million
years since the breakup of Pangaea to move to
their present locations.
Plate Tectonics - The Theory of Plate Tectonics
Continental Drift Activity
• Click the Active Art button to open a
browser window and access Active Art
about continental drift.
Plate Tectonics
Graphic Organizer
Type of
Boundary
Transform
boundary
Convergent
boundary
Divergent
boundary
Type of
Motion
Plates slide
past each
other.
Plates
move
together.
Plates
move
apart.
Effect on
Crust
Crust is
sheared.
Subduction
or
mountain
Feature(s)
Formed
Strike-slip
fault
Mountains,
volcanoes
Mid-ocean
Crust
pulled apart ridge,
ocean floor