Transcript Plate Tectonics

Plate Tectonics
8th grade science
• Crust - layer of rock that forms Earth’s outer layer,
composed of basalt and granite
• Mantle – below the crust, layer of hot rock, composed of
2 parts
• Lithosphere – rigid layer of upper part of mantle
• Asthenosphere – lower soft layer of mantle
• Core – center of earth, composed of 2 parts
• Continental drift – idea of continents moving slowly over
Earth’s surface
Continental Drift
•First hypothesised by Alfred Wegener – 1910
•His idea that all continents were joined mya
•Tens of millions ago landmass began to break apart – called
continental drift
• Looked at evidence from:
landforms- mountain ranges, other
features line up on other continents
fossils-same type of fossils found on continents
oceans apart
climate- fossils of tropical plants found on
continents that are now in polar
climates
Modified from USGS
Alfred Wegener
Born Nov. 1, 1880
Ph.D astronomy- University of Berlin- 1904
Made contributions in meteorology
Went on expedition to Greenland – studied
polar air circulation
During research reads paper linking fossils
of identical plants, animals, found on
opposite sides of Atlantic
Began looking for other cases where similar
organisms separated by oceans
Alfred Wegener cont.
Noticed close fit between coast of S. Africa and S. America
Needed large amt. of supporting evidence
Published 1915 – The Origin of Continents and Oceans
Claimed continents were a single mass 300mya
Called it Pangea
Hypothesized that over tens of millions of years Pangea broke
apart – known as continental drift
Looked at landforms (coastal mts. of S. Africa & S. America)
Originally was not believed
Died while on expedition to Greenland – 1930
http://www.ucmp.berkeley.edu/geology/tectonics.html
Earth’s Interior
• Study surface directly and use indirect evidence
to study interior (seismic waves)
• As you descend towards middle temperature
and pressure increase
• Have 3 basic layers - crust
mantle
core
http://volcano.und.edu/vwdocs/vwlessons/l
essons/Earths_layers/Earths_layers2.html
Crust
• 70% covered water
• 30% seven continental
continents
• Under oceans, dirt, & plants is
layer of rock
• Layer formed from cooled lava
4.5bya
• About 250mya land was
connected together -Pangea
• Oceanic crust under ocean,
mostly dark, dense rock
• Continents made of granite, is
less dense, has large crystals,
light color
Mantle
Below crust is mantle – layer of
hot rock
Upper part of mantle is rigid,
called lithosphere, is
about 100km
Below that is asthenosphere –
is like plastic, able to
flow smoothly, is
about 350km
Mantle extends to core, is
nearly 3000km thick
Core
Core contains 2 parts – inner
and outer
Made of iron and nickel
Outer layer is molten metal,
is like thick liquid even
though is under
extreme pressure
Inner layer acts like solid due
to pressure, is under
extreme heat
Both layers compose 15% of
earth’s volume but
only 1/3 of mass
Is just smaller than moon
Magnetic Field
Currents in outer core cause inner
core to spin
Inner core spins faster than rest of
planet
Movement causes planet to have
large magnetic field
Planet acts like large bar magnet
Magnetic field causes compass to
align to N pole
http://www.adlerplanetarium.org/cyberspace/planets/earth/#magnetic_field
Convection Currents in Mantle
Heat from core and from mantle
cause convection
currents in mantle
Heated magma rises and
spreads out and pushes
cooler magma out of the
way
Cooler material sinks to bottom
to be heated again,
which causes it to rise
thereby creating a
convection current
Currents have been moving
inside earth for more
than 4 billion years
Lab - Continental Drift
Question: How do the continental plates move?
Hypothesis:
Material: hot plate
scissors
baking pan
water
tongs
map
sheets of foam and cork
Procedure:
1. Using the scissors cut out 3 or 4 plates from the map. Then copy them
onto either a square of cork or colored foam. Cut them out.
2. Turn the hot plate on high. Place a baking pan filled with water on the ‘
hot plate.
3. Gently place the cut out continental plates on the surface of the water
Making sure they fit together.
4. As the water heats. Watch the action of the bubbles as they rise from the
bottom of the pan. Observe everything you can about what happens to them when
They rise under a piece of foam and a piece of cork. Notice any differences.
Lab - Continental Drift cont.
5. Once the water begins to boil, watch the pieces of foam and cork. How do they
move? In what direction do they move? Do they crash into other pieces? What
happens when cork and foam crash into each other? Record the answers to your
observations in the data table. Be sure to observe the boiling water for a while. It
may seem like there is no pattern to the action, but careful observation will reveal
certain movements in the boiling water.
6. When you have completed your observations turn the hot plate off and use the
tongs to remove the foam and cork pieces from the water.
7. In the data table record any changes in the foam or cork. Have any of the pieces
been damaged or melted?
Data
Actions of bubbles
Movement of foam in water
Movement of cork in water
Condition of foam /cork after
Observations
Lab - Continental Drift cont.
Results:
Conclusion:
1. How did you describe what happened to the bubbles as they gathered under
The foam?
The cork?
What happened at the sides of the foam? The cork?
2. What happened to the foam where it crashed into the cork? The cork when it
crashed into the foam?
3. What type of natural feature is similar to the action of the bubbles? Explain your
answer.
4. Describe the movement of the plastic pieces when the water started to boil.
Could you see a pattern?
5. How does this experiment model the moving tectonic plates?
6. How is this experiment different from the real world in terms of tectonic plates?
(Hint: What were your foam pieces like after the experiment?)
7. Predict what would happen if the convection currents of the molten magma
changed direction or stopped altogether?
Plate Tectonics continued
Mid-ocean ridge - longest mt. chain in world, found underwater
Sea-floor spreading – way new floor is added to ocean floor
Deep-ocean trench – formed where ocean crust bends downward
Subduction – process in which ocean crust sinks back into mantle
Transform boundary – place where 2 plates slip past each other
Divergent boundary – place where 2 plates move apart
Convergent boundary – place where 2 plates come together
Rift valley – a divergent boundary on land
Theory of Plate Tectonics
•Canadian scientist observe cracks in crust – J. Tuzo Wilson
•Called them plates
•Developed theory of plate tectonics – idea that pieces of Earth’s
crust broken up into pieces
•Explains formation, movement, and subduction of plates
•As plates move they collide, pull apart, grind past each other
•Where breaks occur in crust along boundary called faults
Have 3 types of boundaries:
Transform boundary- plates slipping past each other, crust neither
created nor destroyed, is happening in California
Convergent boundary – where 2 plates are coming together, called a collision,
may bring oceanic crust with oceanic crust (more dense will go under creating a
trench),
continental with continental (both low dense will squeeze crust into
mountain range) or
or oceanic with continental (ocean will go under continental back into mantle),
density will determine which type of crust winds up on top, oceanic crust made of
basalt, is more dense so is usually subducted
http://www.pbs.org/wgbh/aso/tryit/tectonics/#
Divergent boundary – where 2 plates moving apart, found on land and water
most occur at mid-ocean ridge, on land called rift, have rift in New Mexico
called Rio Grande Rift, also in Africa where someday may pull eastern part
away from rest of continent
in ocean have series of volcanoes along ridge, is evidence of sea floor
spreading
http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html
Sea-Floor Spreading
• mid 1900’s able to use sonar to map ocean
•
discover mid-ocean ridge, place where material from mantle rises,
spreads out, pushes old material (rock) to both sides of ridge
• called sea-floor spreading by American geologist Harry Hess
• new material being added on regular basis (Atlantic Ocean)
• when rock pattern studied realized evidence that poles have reversed
themselves, last happened 780,000 yrs ago
• rocks from sea floor able to be gotten due to sea-floor drilling, age able
to be determined
• if sea spreading then else where needs to be shrinking (Pacific Ocean)
• found at deep-ocean trenches, where ocean floor being subducted
down to mantle
• result of this constant movement is that about every 200 m.y. ocean is
renewed
http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html
Vocabulary
Stress- force that adds potential energy to rock until it changes shape
or breaks and moves
Deformation – change in rock’s shape or volume
Strike-slip fault- rocks on either side slip past each other in a
sideways motion, have little up/down motion
Normal fault – fault is at an angle
Hanging wall - half of fault that is above block of rock
Footwall – half of fault that is below block of rock
Reverse fault – blocks move in opposite directions of a normal fault
Fault block mountain – mountains created by crust moving along
normal fault lines
Faults and Fault Movement
•Movement of plates produces changes in crust
•Movement produces powerful forces that push, pull, bend, twist
lithosphere
•A change in the rock’s shape or volume is called deformation
•Deformation takes place over long period of time
•Have 3 types:
shearing
tension
compression
•If stress on plate strong enough it breaks creating a fault
•When stress builds up along fault line the fault may suddenly break
and slide causing an earthquake
Deformation caused by stress
Shearing - pushes mass of rock in 2 opposite directions
Compression pushes rock together
Tension stretches the rock.
Fault and Fault Movements - have 3 types
Strike-slip fault – rocks on each side of the
fault slip past each other sideways
Normal fault –fault is at an angle, one
block of rock lies above fault while other
block lies below fault, movement means
hanging wall slips downward
Reverse faults – produced by compression, same structure as normal fault
blocks move in opposite directions
http://www.uoregon.edu/~millerm/faults.html
Plate movement causes folding and faulting which results in mountain building.
When continental plates collide slowly the layers of rock in the plate fold, and
the edges are pushed towards each other.
Sometimes the movement causes tension which causes the crust to break forming
a normal fault. This faulting may cause mountains to form.
Some mountains are the result of volcanic activity. Volcanoes may be found along
plate boundaries. There are currently about 600 active volcanoes on land. Many are
found along the Pacific Rim or what is called the ”Ring of Fire”. A few develop over wha
is called a “hot spot”. This is an area where the magma melts through the crust. These
are often in the middle of the continental crust or oceanic plate. When on an oceanic
plate may form a series of volcanic mountains such as the Hawaiian Islands.
Land subsidence occurs when the land surface sinks, or subsides, as a result of
geological movement or human activities. Diverging boundaries can lead to rift valleys
or ocean basins. Sometimes one part of crust will rise and the other side will subside.
The crust may sink below sea level.
Humans may also cause subsidence by with drawing large amounts of water
from the ground. As the water is lost underground the rocks and soil settle closer
together which means less space used. This will cause the land above to sink or subside.
If the land sinks to far flooding may occur during heavy rains.
http://ga.water.usgs.gov/edu/earthgwsinkholes.html