Plate TectonicsPlate 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 WegenerAlfred WegenerBorn Nov. 1, 1880Ph.D astronomy- University of Berlin- 1904Made contributions in meteorologyWent on expedition to Greenland – studied

polar air circulationDuring research reads paper linking fossils

of identical plants, animals, found on opposite sides of Atlantic

Began looking for other cases where similarorganisms separated by oceans

Alfred Wegener cont.

Noticed close fit between coast of S. Africa and S. AmericaNeeded large amt. of supporting evidencePublished 1915 – The Origin of Continents and OceansClaimed continents were a single mass 300myaCalled it PangeaHypothesized that over tens of millions of years Pangea broke

apart – known as continental driftLooked at landforms (coastal mts. of S. Africa & S. America)Originally was not believedDied while on expedition to Greenland – 1930

http://www.ucmp.berkeley.edu/geology/tectonics.html

Earth’s InteriorEarth’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/lessons/Earths_layers/Earths_layers2.html

CrustCrust• 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 350kmMantle extends to core, is nearly 3000km thick

CoreCore contains 2 parts – inner

and outerMade of iron and nickelOuter layer is molten metal,

is like thick liquid even though is under

extreme pressureInner layer acts like solid due

to pressure, is under extreme heat

Both layers compose 15% of earth’s volume but

only 1/3 of massIs 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 fieldPlanet acts like large bar magnetMagnetic 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 mantleHeated magma rises and spreads out and pushes cooler magma out of the

wayCooler material sinks to bottom

to be heated again, which causes it to rise thereby creating a

convection currentCurrents 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 panwater tongs mapsheets of foam and cork

Procedure: 1. Using the scissors cut out 3 or 4 plates from the map. Then copy themonto 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 waterMaking 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 whenThey 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? Whathappens when cork and foam crash into each other? Record the answers to yourobservations 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 revealcertain 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 piecesbeen damaged or melted?

Data ObservationsActions of bubbles

Movement of foam in water

Movement of cork in water

Condition of foam /cork after

Results:

Conclusion: 1. How did you describe what happened to the bubbles as they gathered underThe 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 itcrashed 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 magmachanged direction or stopped altogether?

Lab - Continental Drift cont.

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

VocabularyStress- 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:

shearingtension 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

Tension stretches the rock.

Compression pushes rock together

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 forminga normal fault. This faulting may cause mountains to form.

Some mountains are the result of volcanic activity. Volcanoes may be found alongplate 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 whatis called a “hot spot”. This is an area where the magma melts through the crust. Theseare often in the middle of the continental crust or oceanic plate. When on an oceanicplate 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 ofgeological movement or human activities. Diverging boundaries can lead to rift valleysor 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 waterfrom the ground. As the water is lost underground the rocks and soil settle closertogether 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