Transcript Unit 4- Plate Tectonics

Unit 4- Plate Tectonics
Lesson 2- The Center of the Earth
-The Earth is made up of many layers
-The outer layer is the crust and is made
of mostly rock
-It includes the oceans, ocean floor, and
continents
◦ 2 types of crust-Oceanic- thinner, 5-10 km thick, made
of dense rocks like Basalt
-Continental- thicker 30-40 km thick,
made mostly of Granite
What do you think?
From where you are now, about how
many miles is it to the center of the
Earth?
About how long would it take you to
walk to the center of the Earth with no
stops?
A) 8,000 miles
A) 2 years
B) 4,000 miles
B) 3 months
C) 10,000 miles
C) 1 year
D) 2,000 miles
D) 6 months
What do you think?
From where you are now, about how
many miles is it to the center of the
Earth?
About how long would it take you to
walk to the center of the Earth with no
stops?
A) 8,000 miles
A) 2 years
B) 4,000 miles
B) 3 months
C) 10,000 miles
C) 1 year
D) 2,000 miles
D) 6 months
The Mantle
-Layer below the crust 3,000 km thick, made of
hot rock
-Upper Mantle has the following layers-Lithosphere-the rocky outer layer of the solid
earth, averaging about 100 km in depth; the
lithosphere includes the continents, islands, and
the entire ocean floor the
-Asthenosphere-the upper part of the earth´s
mantle. Rock here is soft and at it’s melting point.
It is capable of being molded (plasticity)
-The Lower Mantle-In this layer, pressure and
heat make the rock flow very slowly, like a thick
liquid
The Core
Outer Core- Made mostly of metals iron and
nickel, and is a liquid
Inner Core- Deepest layer of the Earth, made
of solid iron and nickel, hotter than the Outer
Core
Lesson 3- Continental Drift
-In the 1500’s, Dutch mapmaker, Abraham
Ortelius thought that the continents at one
time all fit together like a puzzle to make one
“supercontinent”.
-In 1915, Alfred Wegener, a German
meteorologist, believed this also and named
the supercontinent Pangaea.
-He believed that over 200 million years, the
continents drifted apart into 2
supercontinents:
◦ Gondwanaland- made up of modern-day Africa,
South America, Australia, Antarctica, and India
◦ Laurasia- made up of North America, Europe,
and northern Asia
This theory was the Continental
Drift!
Alfred Wegener investigated the Continental Drift
theory and came up with evidence to convince others.
1) Fossils- Similar fossils were found on
southern continents that are now separated
by oceans.
2) Mountain chains and Rock Layersmountain layers seemed to match up where
they are now separated by bodies of water.
3) ClimateSouth America, southern Africa, India, and
Australia have very warm climates today. All
are far apart from each other. But, landforms
in these areas indicate the presence of glaciers
and that they were once together under a
huge ice sheet in the colder areas of Pangaea.
Other places reveal evidence of dramatic
climate change. Tropical swamps once existed
in areas that are now much colder, such as
parts of North America, Europe, and northern
Asia.
Did scientists believe Wegener’s theory?
- Many did not believe this theory of Continental Drift
- Scientists wanted to know- how did the continents drift apart?
- Wegener suggested that perhaps they moved across oceanic crust, like a ship breaks through
ice
-More data was needed because there was no proof of this movement!
Lesson 4- Seafloor Geography
-Many scientists did not believe Wegener’s theory of the continents drifting apart
-Then, in the 1950s, the simple activity of mapping the ocean floor began providing evidence to
support Wegener’s theory.
-Scientists wanted to know what was on the ocean floor so they used sound waves (sonar)
-Sonar waves travel to the bottom of the ocean, and an echo travels back to the ship
-The time the sound takes to travel to the ocean floor and back is used to figure out the distance
it traveled
-These depth measurements are combined to create a map of the ocean floor in particular
areas, called bathymetric maps.
Under the Sea!
Continental Shelf- a shelf of undersea land reaching a depth of about 200 meters (656 feet),
extending out from the shoreline
Continental Slope- the relatively steeply sloping undersea land extending from the outer edge of
the continental shelf
Submarine Canyon- a deep channel on the ocean floor, in the outer continental shelf, in the
continental slope, or in the continental rise
Continental Rise- a region of gentle slope between the continental slope and the main ocean
floor
Abyssal Plain- a flat expanse of ocean floor Guyot- a seamount with a flattened top
-Data gathered by ocean surveys showed that a great mountain range on the
ocean surrounds the entire earth, running between continents like the seam on
a baseball
-This huge, global mountain chain rises an average of 4 kilometers above the sea
floor! That’s taller than most mountains!
Lesson 5- Seafloor Spreading
-In 1952, geologist Marie Thorpe was
trying to measure the depth of the
ocean floor
-She discovered a deep valley running
through the mid-oceanic mountain
chain which became known as the
Mid-Atlantic Ridge.
-The walls along the valley appeared to
be slowly spreading apart.
Seafloor Spreading- occurs along midocean ridges, molten material from
below earth's surface moves upward,
cools, and forms new oceanic crust.
The older crust will then move farther
away from the rift in either direction,
and new crust forms in its place.
How is this happening?
Magnetism!
-Scientists think that currents in the
earth’s iron core produce a giant
magnetic field all around the planet.
-This effect causes earth to behave like
a giant bar magnet, with a north end
(the magnetic north pole) and a south
end (the magnetic south pole).
-These poles are located near the
geographic north and south poles, but
not in the exact same spots.
-Many rocks on the ocean floor contain iron
-As these rocks cool, they change from a liquid to
a solid
-As a liquid, magnetic particles can float freely
and like tiny compasses, they line up with earth’s
magnetic field and point north.
-When the rock cools, the mineral grains lock in
to place, recording the direction of north at the
time the solid rock formed.
-The magnetic minerals in rocks became useful
markers for earth’s history, and they showed
- Iron-rich minerals in rock pointed in one
direction, and then switched to the exact
opposite direction. This seemed to support the
idea that magnetic poles have switched their
locations.
-The record of earth’s magnetic field is called
paleomagnetism. This reveals when the earth’s
magnetic poles switched and how rocks have
moved over time.
-As spreading pulled new oceanic crust apart,
a pattern of magnetic “stripes” would have
formed.
-They would be about the same size and
carried away from ocean ridges on each side.
-the pattern looks a lot like magnetic “stripes”
in rock, switching back and forth from normal
pole positions to reversed polepositions.
-Stripes occurred in rock extending out on
both sides of the ridge
-This showed that seafloor spreading does
occur!
How old is the seafloor?
-The age of the seafloor is also important.
-Using rock-dating techniques, geologists can
find out the ages of rocks on the ocean floor.
-Then, knowing how far the rocks are from the
mid-ocean ridge, they can figure out the rate
at which the seafloor is spreading.
-If the seafloor is spreading, the youngest
oceanic crust would be found at the ridges and
older crust would be found as you move
outward.
Is that what is happening in the picture?
Lesson 6- Plate Tectonics
-Scientists concluded that earth is
composed of many different-sized
plates.
-This information led to a new theory
that explained movement of
continents and geologic activity such
as earthquakes and volcanoes.
-The population on Earth is increasing,
but the land is not!
Earthquakes
-rocks on both sides of the mid-oceanic ridges
convinced scientists that earth's lithosphere
moves, carrying continents with it.
-Once scientists knew that pieces of earth’s
crust could move, they looked to explain how
and why.
-Data from earthquake recording instruments,
called seismographs, showed scientists where
earthquakes were occurring on the ocean floor
-Information from seismographs was used to
map out several earthquake zones
-At earthquake zones, scientists could
demonstrate where ocean crust was
being created and old crust was sinking
back into earth’s mantle
-Scientists concluded that the surface
of the planet is divided into a number
of moving sections, called tectonic
plates.
-Where these plates interact, there are
numerous earthquakes and volcanoes!
Plate Movement
Earth’s plates move against one another in
different ways, producing some interesting
results!
3 kinds of Plate Movement1) Divergent Plate Boundary- a plate
boundary where two plates move away from
each other
2) Convergent Plate Boundary-a boundary at
which tectonic plates are moving toward one
another or colliding
3) Transform Plate Boundary- a plate
boundary where two plates move in opposite
directions alongside one another
Plate motion is due to 2
forces1) Gravity
2) Convection- the transfer
of heat by the circulation or
movement of the heated
parts of a liquid or gas
Lesson 7- Energy of Convection
-The Earth’s surface is in constant movement!
Remember that 2 forces are responsible for
plate movement- gravity and convection
-Convection -describes the movement of gases
or liquids due to differing temperatures.
-Gravity- is the force of attraction between any
two masses such as earth and everything on it,
including giant tectonic plates.
-When Pangaea broke apart about 200 million
years ago, large pieces of land shifted,
eventually forming the arrangement of
continents and oceans we have today.
-Scientists call these pieces tectonic plates and
have given them names such as Pacific Plate,
Cocos Plate, and South American Plate.
-Even today, these plates are always moving!
-Earth’s plates rest on top of the upper mantle,
the asthenosphere.
-It is made up of molten rock so hot that it
flows
-When convection currents occur here, the
plates move around
-Divergent plates move AWAY from each other
-Convergent plates move TOWARDS each
other, colliding
-Subduction is when one plate goes under
another
-convection is a transfer of heat in a circular
motion
-When cool areas are heated up, they rise,
then they cool, sink and repeat!
-Gravity also can affect plate movement
-Heat from rising magma pushes rocks upward
from the ridge. As they cool, they become
denser and gravity acts on these raised edges,
pulling them downward and away from one
another. This create
So, what causes the Earth’s crust to move?
1) Convection causes material in the mantle
to flow.
2) Plates may move toward, away
and under one another or continue to
collide, fold and crumple resulting in
mountains.
3) Gravity pulls on plate edges creating
ridge push and slab pull and making room
for new rock to form.
Lesson 8- Plate Boundaries
Lithosphere
Asthenosphere
-tectonic plates are pieces of the lithosphere;
the solid, rigid top layer of the earth
-Tectonic plates float on a weaker layer of the
mantle called the asthenosphere
-The lithosphere includes the crust and the
uppermost part of the mantle.
-
-Geologists group the crust and uppermost
portion of the mantle together because they
behave similarly.
-The lithosphere is solid and rigid but when it
is put under stress, it will break.
Oceanic Crust
Continental Crust
-lies
-forms
in ocean basins
-made of dense rock such as
Basalt
-about 35 km thick
the shallow sea beds near
the shores and continents
-about 5-7 km thick
Plate boundary- border around two plates
Divergent plate boundary- a plate boundary
where two plates move away from each other
Convergent plate boundary- plates that are
moving toward one another
Transform plate boundary- a plate boundary
where two plates move in opposite directions
alongside one another
Lesson 9- Landforms
-Earthquakes and Volcanoes tend to
occur near plate boundaries
-Remember there are 3 types of
boundaries- convergent, transform,
divergent
-Different landforms occur at each
type of boundary
Convergent Plate Boundaries
-plates that are moving toward each other
-Most tectonic plates contain both oceanic
crust and continental crust.
-when oceanic and continental crusts come
together, the oceanic crust is denser, so it sinks
into the asthenosphere
-new magma pushes up through cracks in the
crust, creating volcanic activity.
-that’s why the terrain near convergent
boundaries is so rugged
Continental Collision
Underwater Collision
-ocean trenches, volcanoes, and mountains
may occur at convergent boundaries between
oceanic crust and continental crust
-other changes can take place at a convergent
boundary between two pieces of oceanic crust
-2 pieces of continental crust can move
towards each other also
-when this happens, they crunch up, folding
and bending to form tall, rugged mountains.
-The collision of convergent boundaries
between continental crusts takes place
gradually.
-One plate may sink, slowly sliding beneath
the other, creating underwater volcanoes.
- The tops of the volcanoes build up and may
extend above the water’s surface to form
islands.
Divergent Plate Boundaries
-a plate boundary where two plates move
away from each other
-Divergent boundaries are most often
found underwater between oceanic
plates.
-As the plates separate, they create a
space that may fill with magma and create
a ridge, or cause underwater volcanoes.
-When divergent boundaries occur in
continental crust, they may cause it to
split apart.
Transform Plate Boundaries
-a plate boundary where two plates move
in opposite directions alongside one
another
-Transform boundaries do not change the
amount or type of crust, but they can have
effects on the landforms near them.
-As the plates slide past one another, the
pressure on those pieces increases, and
when they separate, energy is suddenly
released and earthquakes occur on the
surface above the plate boundary.
Hot Spots
-Although most volcanic activity occurs at
plate boundaries, volcanoes can appear within
a plate.
-Sometimes, hot, molten material rises from
the mantle, melting the crust above it to form
volcanoes. These areas are called hot spots.
-As a plate moves over a hot spot, a chain of
volcanoes may be built up.
- The Hawaiian Islands are an example of how
hot spots formed volcanoes within the
boundaries of the Pacific plate. The big island
of Hawaii, with an active volcano, is now
directly over a hot spot.
Did you know….?
-Mount St. Helens volcano formed at a
-Mount Whitney formed at a
convergent boundary in California!
convergent boundary in Washington
State where the Pacific Plate and North
-The Mid-Atlantic Ridge formed at
American plate meet!
divergent boundaries that run along
-The San Andreas Fault formed at a
the North and South American plates
transform boundary in California!
and the Eurasian and African plates!
-The Hawaiian Islands formed as a
result of hot spots in the Pacific Ocean!
Lesson 12- Earthquakes
-there are about 70-80 major earthquakes per year and thousands more that are minor and
unfelt
-as tectonic plates move against each other, they build up a lot of pressure and this stress can
cause rocks to bend and fold
-Sometimes the stress builds to the point that the rocks break or slip. The sudden release of
energy creates waves that can travel far from the break, move the ground, and cause structures
to sway and even fall down.
-Forces can change rocks position drastically, causing them to bend or fold
-Stress on rock does not always cause it to fold and bend. Sometimes, the rock breaks, forming a
fault-a break along which the surrounding rock moves. However, the rock does not move
smoothly.
3 kinds of faults
Normal fault- forms when the pieces of crust
move away from each other as they are pulled
apart. One piece ends up lower than the other,
forming a slope or cliff.
Reverse fault- forms when tectonic plates
move toward one another, compressing the
crust. One piece pushes over the other.
Strike-Slip fault- forms when two pieces slide
alongside one another, in opposite directions.
If you look at the rocks of an exposed fault,
you can see how much force was needed to
move the crust.
Focus- the zone within the earth
where rock displacement produces an
earthquake
Seismic Waves- compression waves
caused by movements in the earth´s
crust; seismic waves radiate outward
from the source of an earthquake
Epicenter- the location on the surface
of the earth directly above the focus of
an earthquake
What's the difference between a
seismograph and a seismogram?
Seismograph- an instrument used to
record earthquake waves
Seismogram- the record of an
earthquake tremor, as recorded by a
seismograph; by reading a
seismogram, we can learn how
powerful an earthquake is
Making Waves!
-there are two types of seismic waves, which
travel in different ways.
◦ -Pressure or Primary waves (P waves)- They
contract and release as they move very quickly.
They can move through Earth like they do
through a spring or a Slinky
◦ - Secondary waves (S waves)- S waves move
more like a rope. That means they move
sideways and back and forth compared to the
direction the wave is traveling. They do not
travel as fast as P waves.
-Scientists find out where the focus and the
epicenter of the earthquake are located by
comparing the length of time that it takes for
the two kinds of waves to travel to
seismographs.
-The P waves arrive first, and the S waves
follow them
- By timing the arrival of the waves, scientists
can tell how far their station is from the
earthquake.
-Studying how seismic waves travel to distant
places has informed us more about the inner
structure of the earth.
- Scientists have found that P waves can travel
through solids, liquids, and gases. Using this
information, scientists are able to figure out
the composition of the earth’s interior.
- Studying these waves also helps scientists
figure out the depths and the densities of the
layers.
-Seismic waves travel faster through denser
material
More Resources!
www.happyscientist.com
Bill Nye, the Science Guy videos-
User name- LVS
Earthquakes-
Password- LVS
http://www.youtube.com/watch?v=24R_yGDg
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www.sciencespot.net
Earth’s Crust
http://www.youtube.com/watch?v=7NJQKs9NM0
Volcanoes
http://www.youtube.com/watch?v=oiNWdZuf
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