Transcript Plate boundaries - earthsciencewithmrb

Day 15- Plate movement
1. How are plated defined and identified?
2. How do convection cells drive the plates
around Earth’s surface?
3. Define and describe the three types of plate
boundaries.
4. What causes volcanic activity unassociated
with a plate boundary?
How are plated defined and identified?
What is a Plate?
• During the 1950s and early 1960s, scientists set up
seismograph networks to see if enemy nations were
testing atomic bombs. These seismographs also
recorded all of the earthquakes around the planet.
The seismic records were used to locate an
earthquake’s epicenter , the point on Earth’s
surface directly above the place where the
earthquake occurs.
• The lithosphere is divided into a dozen major and
several minor plates.
• A single plate can be made of all oceanic
lithosphere or all continental lithosphere, but nearly
all plates are made of a combination of both.
• The movement of the plates over Earth's surface is
termed plate tectonics . Plates move at a rate of a
few centimeters a year, about the same rate
fingernails grow.
How do convection cells drive the plates
around Earth’s surface?
In a convection cell, material deep beneath the
surface is heated so that its density is lowered and it
rises. Near the surface it becomes cooler and denser,
so it sinks.
How Plates Move
Steps of Mantle convection
1. Hot mantle from the two adjacent cells rises at the
ridge axis, creating new ocean crust.
2. The top limb of the convection cell moves horizontally
away from the ridge crest, as does the new seafloor.
3. The outer limbs of the convection cells plunge down
into the deeper mantle, dragging oceanic crust as
well. This takes place at the deep sea trenches.
4. The material sinks to the core and moves horizontally.
5. The material heats up and reaches the zone where it
rises again.
Define and describe the three types of
plate boundaries.
Plate Boundaries
• Plate boundaries are the edges where two plates meet. How can
two plates move relative to each other? Most geologic activities,
including volcanoes, earthquakes, and mountain building, take
place at plate boundaries. The features found at these plate
boundaries are the mid-ocean ridges, trenches, and large
transform faults.
• Divergent plate boundaries : the two plates move away from each
other.
• Convergent plate boundaries : the two plates move towards each
other.
• Transform plate boundaries : the two plates slip past each other.
• The type of plate boundary and the type of crust
found on each side of the boundary determines
what sort of geologic activity will be found there.
We can visit each of these types of plate boundaries
on land or at sea.
What causes volcanic activity unassociated
with a plate boundary?
Intraplate Activity
A small amount of geologic activity, known as
intraplate activity, does not take place at plate
boundaries but within a plate instead. Mantle plumes
are pipes of hot rock that rise through the mantle
from near the core. The release of pressure causes
melting near the surface to form a hotspot. Eruptions
at the hotspot create a volcano. Hotspot volcanoes
are found in a line
Summary
• The plate in plate tectonics is a large chunk of
lithosphere that can carry continental crust, oceanic
crust, or some of each. Plates can be identified by
the locations of earthquake epicenters. At the
boundaries of plates are mid-ocean ridges,
trenches, and large faults. Plates move by mantle
convection. Plates meet at plate boundaries. The
three types are divergent, convergent, and
transform.
PHET simulation
Day 16 –
topographic
features of
plate
boundaries
Is it easier to get to
the bottom of the
ocean or out into
space?
How many people
have been to “space”?
537
How many people
have been to the
deepest point on
earth?
3
Today’s Agenda:
1. Opening
2. Notes
3. Sea floor spreading activity
4. Finish Plate tectonics simulation
5. Independent research project
6. Call on students to review findings from
Phet activity.
7. Turn in PHET activity
1. How does the topography of the seafloor give
evidence for plate tectonics?
2. How does the pattern of magnetic stripes give
evidence for seafloor spreading?
3. How does the age and thickness of seafloor rocks
and sediment support the idea of plate tectonics?
How does the topography of the seafloor
give evidence for plate tectonics?
The people who first mapped the seafloor were
aboard military vessels during World War II. These
vessels used sound waves to search for submarines,
but also produced a map of seafloor depths. Depth
sounding continued in earnest after the war. Scientists
pieced together the ocean depths to produce
bathymetric maps of the seafloor.
Features of the Seafloor
• Although scientist expected an expanse of flat, featureless
plains, they were shocked to find tremendous features like
mountain ranges, rifts, and trenches. This work continues on
oceanographic research vessels as they sail across the seas
today.
• mid-ocean ridges: these features rise up high above the deep
seafloor as a long chain of mountains
• rift zones: in the middle of the mid-ocean ridges is a rift zone
that is lower in elevation than the mountains surrounding it.
• deep sea trenches: these features are found at the edges of
continents or in the sea near chains of active volcanoes
• abyssal plains: these features are flat areas, although many are
dotted with volcanic mountains
A modern
map of the
southeastern
Pacific and
Atlantic
Oceans made
with data from
several
decades.
How does the pattern of magnetic stripes
give evidence for seafloor spreading?
• WWII navy ships towed magnetometers to search
for enemy submarines. Shipboard magnetometers
also revealed the magnetic polarity of the rock
beneath them. When scientists plotted the points
of normal and reversed polarity on a seafloor map
they made an astonishing discovery: the normal
and reversed magnetic polarity of seafloor basalts
creates a pattern.
Observations of Patterns:
• Stripes of normal polarity and reversed polarity
alternate across the ocean bottom.
• Stripes form mirror images on either side of the midocean ridges
• Stripes end abruptly at the edges of continents,
sometimes at a deep sea trench.
How does the age and thickness of seafloor
rocks and sediment support the idea of
plate tectonics?
Additional Observations
• The scientists noticed that the rocks got older with
distance from the mid-ocean ridges.
• The youngest rocks were located at the ridge crest and
the oldest rocks were located the farthest away,
abutting continents.
• Scientists also noticed that the characteristics of the
rocks and sediments changed with distance from the
ridge axis Away from the ridge crest, sediment
becomes older and thicker, and the seafloor becomes
thicker. Heat flow, which indicates the warmth of a
region, is highest at the ridge crest.
• Seafloor is youngest at the mid-ocean ridges and becomes
progressively older with distance from the ridge.
Seafloor Spreading
• The features of the seafloor and the patterns of magnetic
polarity symmetrically about the mid-ocean ridges were
the noticed by Harry Hess.
• Hess wrote that hot magma rose up into the rift valley at
the mid-ocean ridges. The lava oozed up and forced the
existing seafloor away from the rift in opposite directions.
Since magnetite crystals point in the direction of the
magnetic north pole as the lava cools, the different stripes
of magnetic polarity revealed the different ages of the
seafloor. Hess called this idea seafloor spreading.
• Since new oceanic crust is created at the mid-ocean ridges,
either
– Earth is getting bigger (which it is not)
Or
– Oceanic crust must be destroyed somewhere.
• Since the oldest oceanic crust was found at the edges of
the trenches, Hess hypothesized that the seafloor subducts
into Earth’s interior at the trenches to be recycled in the
mantle.
Summary
• Data from magnetometers dragged behind ships
looking for enemy submarines in WWII discovered
amazing magnetic patterns on the seafloor.
• Rocks of normal and reversed polarity are found in
stripes symmetrically about the mid-ocean ridge axis.
• The age of seafloor rocks increases from the ridge crest
to rocks the farthest from the ridges. Still, the rocks of
the ocean basins are much younger than most of the
rocks of the continents.
Magnetic Banding Activity
Independent research project
• Get 5 pieces of copier paper for this. You may also use your
phone to research this if you’re so inclined.
•
•
•
•
•
Cascade mountains
Hawaii
Appalachian mountains
Himalayan mountains
The Mariana Trench
• Describe the location of the geologic formation.
• Describe the tectonic movements that caused them to be
formed.
• Draw and label a diagram showing how they formed.
• Find at least one interesting fact about them.
Earthquakes
1. How does the elastic rebound theory explain
earthquakes?
2. Describe how the center of an earthquake is
located and described, including focus and
epicenter.
3. Describe the scales used to measure
earthquakes.
This photo shows the Mission District of San Francisco burning after the 1906 earthquake.
How does the elastic rebound theory
explain earthquakes?
Earthquake!
• An earthquake is rapid ground movement caused by
the sudden release of energy stored in rocks.
Earthquakes happen when so much stress builds up
in the rocks that the rocks rupture. The energy is
transmitted by seismic waves.
Elastic rebound theory.
Stresses build on both sides of a fault, causing the
rocks to deform (Time 2). When the stresses become
too great, the rocks break and end up in a different
location (Time 3). This releases the built up energy
and creates an earthquake.
Describe how the center of an earthquake
is located and described, including focus
and epicenter.
In an earthquake, the initial point where the rocks
rupture in the crust is called the focus. The epicenter is
the point on the land surface that is directly above the
focus.
Earthquake Zones
• About 80% of all earthquakes strike around the Pacific Ocean
basin because it is lined with convergent and transform
boundaries
• About 15% take place in the Mediterranean-Asiatic Belt, where
convergence is causing the Indian Plate to run into the Eurasian
Plate.
• The remaining 5% are scattered around other plate boundaries
or are intraplate earthquakes.
A seismograph produces a graph-like representation of
the seismic waves it receives and records them onto a
seismogram. Seismograms contain information that can
be used to determine how strong an earthquake was,
how long it lasted, and how far away it was
Interpreting a Seismogram
A seismogram shows:
• foreshocks.
• the arrival of the P-waves.
• the arrival of the S-waves.
• the arrival of the surface waves
• P-waves (primary waves) are fastest, traveling at
about 6 to 7 kilometers per second, so they arrive
first at the seismometer.
• P-waves move in a compression/expansion type
motion, squeezing and unsqueezing Earth materials
as they travel.
• S-waves (secondary waves) are about half as fast as
P-waves, traveling at about 3.5 km per second, and
arrive second at seismographs.
• S-waves move in an up and down motion
perpendicular to the direction of wave travel.
• P-waves bend slightly when they travel from one layer into
another. Seismic waves move faster through denser or more
rigid material. As P-waves encounter the liquid outer core,
which is less rigid than the mantle, they slow down. This
makes the P-waves arrive later and further away than would
be expected. The result is a P-wave shadow zone. No P-waves
are picked up at seismographs 104o to 140o from the
earthquakes focus.
• S-waves are only able to propagate through solids.
S-waves cannot travel through liquid.
Describe the scales used to measure
earthquakes.
Richter Magnitude Scale
• Developed in 1935 by Charles Richter, this scale
uses a seismometer to measure the magnitude of
the largest jolt of energy released by an earthquake.
• The Richter scale and the moment magnitude scale
are logarithmic scales.
• The amplitude of the largest wave increases ten
times from one integer to the next.
• An increase in one integer means that thirty times
more energy was released.
Mercalli Intensity Scale
• Earthquakes are described in terms of what nearby
residents felt and the damage that was done to
nearby structures.
• Quantified by post-earthquake observations
• What factors would go into determining the
damage that was done and what the residents felt
in a region?
Japanese earthquake videos
Volcanoes
• Compare magma and lava. Locate volcanoes
and relate back to plate boundaries. Explain
volcanic effects on the lithosphere and relate
back to plate boundaries (convergent,
divergent, transform) including lahar (mud)
flows and ash in the atmosphere.
Volcanoes
1. Where do most volcanoes occur? Why?
2. How does the composition of magma
affect the behavior of the volcano?
3. How can we predict volcanic activity?
Magma chamber
• A magma chamber is a large underground pool of
liquid rock found beneath the surface of the Earth.
The molten rock in such a chamber is under great
pressure, and given enough time, that pressure can
gradually fracture the rock around it creating
outlets for the magma.
Volcanoes
• A volcano is a vent from which the material from a
magma chamber escapes. Volcanic eruptions can
come from peaky volcanic cones, fractured domes,
a vent in the ground, or many other types of
structures.
Where They Are
• Volcanoes are common along convergent and
divergent plate boundaries.
• Volcanoes are also found within lithospheric plates
away from plate boundaries. Wherever mantle is
able to melt, volcanoes may be the result.
Convergent Plate Boundaries
• Converging plates can be oceanic, continental, or
one of each. If both are continental they will smash
together and form a mountain range. If at least
one is oceanic, it will subduct. A subducting plate
creates volcanoes.
Pacific Rim
• Volcanoes at convergent plate boundaries are found
all along the Pacific Ocean basin. Trenches mark
subduction zones.
• The Cascade Range is
formed by volcanoes
created from
subduction of oceanic
crust beneath the North
American continent.
• The volcanoes are
located just above
where the subducting
plate is at the right
depth in the mantle for
there to be melting.
Creating Magma
• Volcanoes erupt because mantle rock melts forming
magma. This is the first stage in creating a volcano.
• Mantle may melt if temperature rises, pressure
lowers, or water is added.
• The type of material melted impacts the type of
eruption.
• Factors Affecting
Eruptions
– Viscosity
• Def: A substances
resistance to flow
• More viscous = More
violent eruptions
• Higher temperature =
Less viscous
• More silica (a mineral) =
More viscous
• Factors Affecting
Eruptions
– Dissolved Gases
• Trapped gases provide
force to eject magma from
a vent
– Vent = opening to the
surface
• More viscous = Harder for
gases to escape = More
violent eruption
TEPHRA
• Tephra is a general term
for fragments of
volcanic rock and lava
that are blasted into
the air by explosions or
carried upward by hot
gases in eruption
columns or lava
fountains.
• Magma is called lava
once it reaches the
surface
Pyroclastic Material
• Scorching hot tephra, ash,
and gas may speed down
the volcano’s slopes at
700 km/h (450 mph) as a
pyroclastic flow.
Pyroclastic means fire
rock
Person from the city of Pompeii
buried in a pyroclastic flow from Vesuvius’ eruption
• Pyroclastic flows knock down everything in their
path. The temperature inside a pyroclastic flow may
be as high as 1,000°C (1,800°F).
Trees knocked down by mount St. Helens’ explosion.
Predicting Volcanic Eruptions
• A volcano’s history —
how long since its last
eruption and the time
span between its
previous eruptions is a
good first step to
predicting eruptions
Predicting Volcanic Eruptions
• Earthquakes- Moving magma shakes the ground, so
the number and size of earthquakes increases
before an eruption.
• Slope Deformation
• Magma and gas can push
the volcano’s slope
upward. Most ground
deformation is subtle and
hard to detect.
• Gas Emissions
• Gases may be able to escape a volcano before
magma reaches the surface. Scientists measure gas
emissions in vents on or around the volcano to help
predict eruptions.