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Plate tectonics
5.1 Drifting Continents
 1910 Alfred Wegener
 Hypothesized that all the continents were once joined
together in a single landmass call Pangea and have
since drifted
○ This became known as CONTINENTAL DRIFT
○ Pangaea- supercontinent (350-200 million years ago)
(Pangaea means “all lands” in latin)
 1915 published book about this called The Origin of
Continents and Oceans
 Others had similar ideas, however Wegner was the first
to have scientific evidence to support this idea
Plate tectonics
Evidence for Continental drift
1.) Land features
 Coastlines of some continents seemed to fit
together like a jig saw puzzle
 Mountain ranges in Africa line up with those in S.
America
 Coal fields in Europe and North America lined up
Piecing It All
Together
The coastlines of
some continents
seem to fit
together like a
jigsaw puzzle.
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
 Continents near the equator have a warm
climate
 Continents near the poles have a colder
climate
 When these continents move, north or south, they
take the fossils and rocks that formed in the past
with them
○ Fossils of tropical plants found on an island in the
arctic ocean
○ Scratches in rocks made by glaciers found in South
Africa
Pangaea and
Continental Drift
Many types of
evidence suggest
that Earth’s
landmasses were
once joined
together.
Plate tectonics

Continental Drift—REJECTED
 Wegner 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
5.2 and 5.3 Sea floor Spreading and 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 Tectonics- Earth’s plates are in slow, constant
motion, driven by convection currents in the mantle.
 Explains formation, movement and subduction of Earth’s plates
○ Subduction-the process by which gravity pulls denser plate edges downward into the
mantle.
Earth’s Plates
Plate boundaries divide the lithosphere into
large plates.
Faults-breaks in the Earth’s crust where
rocks have slipped past each other, form
along these boundaries
Plate tectonics
Plate motion over time

Satellites are used to measure plate motion precisely.
 Plates move very slowly – 1-12 cm per year
○ North American and Eurasian plate move 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, 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 ago
As the plates move, they
create earthquakes, volcanoes,
mountain ranges and deep ocean
trenches
Earth Today
Plate tectonics
Plate Boundaries
 Divergent Boundaries-two plate slowly move away from
each other
 Most occur at mid ocean ridges (divergent boundary in ocean)-new
crust is added during sea floor spreading
○ ex. Mid Atlantic Ridge, Atlantic Ocean
 Rift Valley- divergent boundary on land
○ ex. East African Rift System, Africa
○ Mid Ocean Ridge (MOR)- An undersea mountain chain where new
ocean floor is being produced, created at divergent boundaries

Mid 1900’s sonar was used to map ocean floor
 Sonar uses sound waves to measure the distance to an object
○ MOR lie under thousands of meters of water
○ MOR extend into all oceans
○ Steep sided valley, called the Central Valley, splits the top of some
MOR
○ MOR make up the longest mountain ranges on Earth
Plate tectonics
Sea Floor Spreading- the process by which molten material adds new
oceanic crust to the ocean floor
 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
Does the ocean floor just keep growing and growing?
NO!
On the opposite side, the ocean floor plunges into deep ocean
trenches, sinking back into the mantle
This is called SUBDUCTION
Plate tectonics
Evidence of Sea Floor Spreading
 Ocean Material
 Pillow Basalt--In central valley of MOR, scientists have
found pillow shaped rocks—this happens only when
molten material hardens very quickly after erupting under
water

Magnetic Strips
 As molten material cools, magnetic minerals inside rock
line up in direction of the Earth’s magnetic poles
○ Forms “unseen” magnetic stripes on ocean floor.
○ Pattern of magnetic stripes is mirror image on opposite side
of MOR—this shows that two sides of ridge spread out at
the same time and rate
○ Magnetic Poles reverse themselves—each stripe shows a
period when molten material erupted and hardened and
magnetic poles did not change
Plate tectonics
Evidence (cont.)
 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
Ocean Floors
Mid-ocean ridges rise from the
sea floor like stitches on the
seams of a baseball.
Mid-Ocean Ridges
Plate Tectonics
Convergent
Boundary-boundary where two plates collide or come together
Density of plates determine which one comes out on top
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
-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
Deep ocean trenches-A deep valley along the ocean floor beneath
which oceanic crust slowly sinks toward the mantle
***Trenches generally run parallel to volcanic island arc’s
Plate tectonics

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
○ Cool, dense crust of ocean might collide with edge of continent
 Gravity pulls the older, more dense oceanic crust down beneath the trench
into the mantle
 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 (Atlantic p. 173)
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.)
○Continental-Continental-neither is dense enough to
sink into mantle, mountains form
Ex Indian Plate and Eurasian plate collide to form Himalayans
Transform Boundaries-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
Plate tectonics
Forces in Earth’s Crust
Stress- A force that acts on a rock to change its shape
Type of stress
Definition
Boundary
Type of
Fault
Tension
Stretches rock so that it becomes
thinner in the middle
Divergent
Normal Fault
Compression
Squeezes rock until it folds or
breaks
Convergent
Reverse
Fault or
Thrust Fault
Shearing
Pushes rock in opposite directions,
sideways movement, causes a
rock to slip or break apart
Transform
Strike Slip
Plate Tectonics
Stress in Earth’s Crust
Stress can push, pull, or
squeeze rock in Earth’s crust.
Three kinds of stress can
occur in the crust.
Plate Tectonics
Fault-A break in Earth’s crust along which rocks move.
Types of Faults
Normal Fault-due to
tensional stress,
hanging wall moves
down, footwall moves
up
Reverse Fault-due to
compressional stress,
hanging wall moves up, footwall
moves down
Strike Slip Fault-due to
shear stress, no hanging
wall or footwall, rocks on
either side move past each
other
sideway with little up or
down motion
**A Thrust Fault is a Reverse Fault with a low angle
Plate Tectonics
Folding of Earth’s Crust
 Fold = Bends in Earth’s crust due to compression
 causes crust to become shorter and thicker
Anticline-fold that bends upward into an arch
 Syncline-fold that bends downward to form V-shape

 Appalachian Mountains in PA are anticlines and synclines

The collision of two plates causes this folding which
creates FOLDED MOUNTAINS such as
 Himalayas (Asia), Alps (Europe)
Plate Tectonics
Anticlines and Synclines
Compression can cause folds in the crust. Two types of
folding are anticlines, which arch up, and synclines, which
dip down.
Plate Tectonics
Stretching Earth’s Crust
Fault Block Mountains
1.) Two plates move away from each other
2.) Normal Faults created on either side
3.) Hanging wall on each side drops down
4.) Block in between stands above surrounding valleys.
Tension and Normal Faults
As tension forces pull the crust apart, two normal faults can form a fault-block mountain range.
Plate Tectonics
Tension and Normal Faults
What are the hanging wall and
the two footwalls in diagram A?
What is the new position of the
hanging wall after movement
occurs in diagram B?
Plate Tectonics
Uplifting Earth’s Crust
Plateau-A large landform that has high elevation and a
more or less level surface above sea level
 Cause by the same forces (uplift) that causes
mountains
**it is wider than it is tall
The Kaibab Plateau
Look at the sequence of drawings. In your own words, describe what happens in the last two
diagrams.
Plate Tectonics
Earthquakes (EQ)
Earthquake-vibration of Earth produced by the rapid release of
energy
**caused by a slippage along a fault
Focus-The point within the Earth
where the EQ starts
-energy radiates in all directions in the form
of waves
Epicenter- location on the surface of the
Earth directly above the focus
Seismologist-person who studies EQ
Seismograph- instrument that records EQ
waves
Plate Tectonics
Earthquakes produce two types of waves
 Surface waves-move along Earth’s surface, move up and down and side to
side, most destructive waves
 Body waves
1.) P (primary) waves Longitudinal or compression waves
(or push pull waves)
 particles move in the same direction
(parallel to) direction the wave is traveling
 like slinky
 Can travel through solids and liquids
2.) S (shear) waves transverse waves move particles at right
angles (perpendicular) to material
 Like jump rope
CANNOT Travel through liquids or gases,
only through solids

Travel time:
 In order from fastest to slowest
○ P wave, S wave, surface wave
Plate Tectonics
Locating an Earthquake:
1.) EQ DistanceCompare the difference in speed of P and S wave
to find the distance traveled
 The greater the difference in arrival time, the greater the
distance to the epicenter
○ Use a travel-time graph to find how far the epicenter is
 First, find the time interval between the arrival of the first P and the
first S wave
 Second, find the time spread between the P and S wave curve
- This will tell you distance from the epicenter to the seismograph
for that location
- BUT we do not know in what direction!
Plate Tectonics

Travel-time graph can tell you distance to the
epicenter but it does not tell us in what direction

To find the exact location, travel-time graphs
from three or more seismographs are needed
Plate Tectonics
We now know distance but the EQ could be in any
direction from the seismic station.
2.) EQ direction
To find the exact location, time-travel graphs from
three or more seismographs are needed
 Draw a circle around each seismic station
 each circle represents the distance of the epicenter to
each station

The point where the three circles intersect is the
epicenter of quake.
Plate Tectonics
Locating the epicenter with data from three seismographs
1st circle –
tells you ALL
possible epicenters
for that city
2nd circlenarrows
down possible
location of
epicenter to
TWO locations
(where the two
circles intersect)
3rd circlepinpoints the
location of the
epicenter
(where all three circles
intersect)
Plate Tectonics
Measuring Earthquakes
Scientists have used two different types of measurements to describe the size of an
earthquake
1.) intensity-(qualitative) measure of amount of shaking based on amount of damage for
a given location
2.) magnitude-(quantitative) rely on calculations using seismograms, measures size of
seismic waves or amount of energy released.
○
Richter Scale-size of seismic waves



○
based amplitude of largest seismic wave recorded on a seismogram
useful for small shallow EQ within 500 km of epicenter
Familiar but outdated-used on news reports but not by scientists
Moment Magnitude-estimates energy released

Based on the amount of displacement that occur along fault zone
- The average movement along the fault
- The area of the surface break (Surface area of fault)
- strength of rock broken (rigidity of rock)
Earthquake Hazards
Damage depends on intensity and duration of vibrations, material on which buildings are constructed,
design of the building
○
○
○
○
Liquefaction-soils and other unconsolidated materials saturated with water are turned into a liquid that is
not able to support buildings.
Tsunamis-Japanese word for seismic sea wave
Landslides
Fire
Plate
Tectonics
Volcanoes and Plate Tectonics
Volcano-a weak spot in the crust where magma has come to the surface
Magma-molten mixture of rock-forming substances, gases and water from the mantle
Lava-liquid magma that reaches the surface

Volcanoes form along boundaries of Earth’s plates
 Divergent Boundaries-fractures allow magma to reach Earth’s surface
 Convergent Boundary○ Oceanic/oceanic-subduction occurs, water from crust leaves and rises in wedge of
mantle causing melting point of mantle to decrease resulting in melting. Melted
magma breaks through ocean floor creating volcanoes
 Island Arc is formed-string of volcanoes that form as the result of subduction of one oceanic
plate beneath another oceanic plate
- Aleutian Islands of Alaska
○ Oceanic/continental- oceanic plates subducts beneath continental plate
 Andes Mountains, Mt. St. Helens, Mount Rainer
Hot spots-


A volcano forms above a hot spot when magma erupts through the crust and reaches the surface
○
Hawaiian Islands
Plate Tectonics
The Ring of Fire
The Ring of Fire
is a belt of
volcanoes that
circles the Pacific
Ocean. As with
most of Earth’s
volcanoes, these
volcanoes form
along boundaries
of tectonic plates.
Volcanoes and
Converging
Boundaries
Volcanoes often
form where two
plates collide.
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.
Plate Tectonics
Volcanic Eruptions
What happens?
 Magma forms in asthenosphere just below lithosphere
 Magma is less dense (its hot) so it rises into tiny cracks, if it reaches surface, a volcano can
form
Inside a volcano
 Magma chamber-pocket beneath a
volcano where magma collects
 Pipe-long tube through which magma
moves from the magma chamber to
Earth’s surface
 Vent-opening where molten rock and gas
leave
 Lava flow-area covered by lava
as it pours out of a volcano’s
vent
 Crater-bowl shaped area that
forms around a volcano’s
central opening
Inside a Volcano
A volcano is made up of many different parts.
Plate Tectonics
Volcanic Eruption

When a volcano erupts, the force of the expanding gases pushes the magma from magma chamber 
pipe  vent
Two types of eruptions (determined by silica content)
Silica-material found in magma, composed of oxygen and silicon, primary substance of crust and
mantle

Quiet-magma is hot or low in silica, gases escape slowly, lava oozes quietly from vent, can flow for many
Km
 Forms two types of rock
○ Pahoehoe- from fast moving, hot lava that is thin and runny
○ Aa-from cooler, thicker lava that is slower moving
 Quiet eruptions are responsible for building Hawaii
 HAZARD—sets fire to and burns everything in its path, can cover large areas with a thick layer of lava


Explosive-thick, sticky magma that is high in silica
 Magma can build up in pipe and get stuck like a cork, pressure builds up from gases and eventually
there is an explosion, gases push magma out with great force
 Lava is thrown powerfully into air, breaks into pieces that cool and harden quickly
○ Ash- smallest particles, fine, like dust
○ Cinders-pebble sized particles
○ Bombs-golf ball to car sized
 Pyroclastic flow- The flow of ash, cinders, bombs, and gases down the side of a volcano during an
explosive eruption.
HAZARD—landslides of mud, melted snow and rock
Plate Tectonics
Types of Volcanoes

Shield




built almost entirely of fluid lava flows
gently sloping cone of flat, domical shape, with a profile much like that
of a warrior's shield
They are built up slowly by the accretion of thousands of highly fluid
lava flows called basalt lava that spread widely over great distances,
and then cool as thin, gently dipping sheets.
Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea
and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes.
Stratovolcano- (strata = layers)






steep-sided, symmetrical cone shape
HUGE- may rise as much as 8,000 feet above their bases
built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs
tend to erupt explosively and pose considerable danger to nearby life and property.
Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California,
Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.
Cinder Cones-






simplest type of volcano
built from particles and blobs of congealed lava
ejected from a single vent
As the gas-charged lava is blown violently into the air, it
breaks into small fragments that solidify and fall as cinders
around the vent to form a circular or oval cone.
Most have a bowl-shaped crater at the summit and rarely
rise more than a thousand feet or so above their
surroundings.
western North America
Plate Tectonics
Stages of Volcanic Activity
 Active-live volcano, is erupting or has shown signs
it will erupt in the near future

Dormant- sleeping volcano, not currently active but
able to become active

Extinct- dead volcano, no longer active and is
unlikely to erupt again

Prediction- not possible but Geologists monitor
○
○
○
○
Surface changes in elevation and tilt with tiltmeter
Amount of escaping gases
Small earthquakes that occur around volcano
Rising temperatures in ground water