PLATE TECTONICS

A LITTLE BACKGROUND INFO: EARTH: A UNIQUE PLANET

Earth is covered by 71 percent water and about 97 percent of that water is in the salty oceans.

We have learned about Earth’s interior by studying seismic waves, which are vibrations that travel through the earth. These can be caused by earthquakes or explosions near the surface.

Zones of the Earth:

• Crust (1% of mass)• Thin, solid outermost covering• Both oceanic and continental

crust• Mantle (2/3 of mass)

• Upper part is solid, lower part is plastic-like

Zones of the Earth:

• Core (1/3 of mass)• Outer core is liquid and inner is

solid• Made mostly of iron

http://scienceblogs.com/startswithabang/upload/2011/03/why_physics_gives_us_earthquak/Layers%20of%20Earth.jpg

Another way to look at it:

• Lithosphere

• Rigid layer• Crust and solid portion of the mantle

• Asthenosphere:

• Just below lithosphere• Due to extreme heat & pressure, the solid

rock has the ability to flow, known as plasticity.

http://mediatheek.thinkquest.nl/~ll125/images/struct.jpg

DISCOVERING EARTH’S LAYERS

In 1909, a Croatian seismologist, Andrija Mohorovicic, used the information from seismic waves to determine that all layers of Earth weren’t the same. He is credited with finding the Moho, which is a boundary between the crust and the mantle.

PLATE TECTONICS

The idea that the continents have changed locations throughout history is known as continental drift.

•Pangaea - Single landmass, means “all lands”

•Panthalassa – One huge ocean, means “all seas”

Evidence of Continental Drift:

Alfred Wegener proposed the theory continental drift.

In addition to the puzzle piece fit, Wegener soon found other evidence to support his hypothesis.

http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/Images_specific/sa_af.gif

Evidence of Continental Drift:

1. Fossil Evidence:

• Mesosaurus, a small, extinct land reptile was found on both South America and Africa.

• There was no evidence of land bridges and the animal could not have swam across the ocean.

Evidence of Continental Drift

2. Geologic Evidence

• The age and types of rock in coastal regions of widely separated areas and Africa and S. America matched closely.

• Mountain chains (ex. The Appalachains) that ended on one coastline seemed to continue on the coastline of other landmasses.

Evidence of Continental Drift

3. Climatic Changes

• Layers of debris from glaciers were found in southern Africa and South America.

• Coal deposits were found in eastern US, Europe, and Siberia.

Many scientists rejected Wegener’s ideas because it lacked an explanation of the force that caused continental drift.

Seafloor Spreading:

•The Mid-Atlantic Ridge is an undersea mountain range, with a steep, narrow valley running down its center.

•Scientists in the 1950s discovered that the ocean floor was younger than the continental rocks. (!!!)

•Harry Hess and Robert Dietz explained this movement, known as seafloor spreading.

Paleomagnetism:

• When magma cools and solidifies, iron-bearing minerals become magnetized.

• The magnetic orientation of these minerals becomes permanent and points towards the north.

• The validity of this idea grew when scientists discovered the difference of the magnetism of the ocean floor.

Paleomagnetism:

• In 1965 scientists discovered a reversal in the earth’s magnetic orientation.

• This was the evidence that Wegener was looking for to support his theory of continental drift.

PLATE TECTONIC THEORY

•The theory of plate tectonics not only describes continental movement, but also presents a possible explanation of why and how the continents move.

•Two types of crust: oceanic and continental. To date 30 plates have been identified.

• Oceanic and continental crust AND the rigid upper mantle make up the lithosphere.

• Under this lies the asthenosphere, which is solid rock that flows like melted silly putty due to extreme heat and pressure.

3 TYPES OF LITHOSPHERIC PLATE BOUNDARIES

1.) Divergent Boundary

•Two plates moving away from each other

•Forms a rift valley

•A narrow valley that forms as two plates separate. (ex: at center of a mid-ocean ridge)

•Ex. Mid-ocean ridge

http://cmspugliano.wikispaces.com/file/view/divergent4.jpg/100083335/divergent4.jpg

3 TYPES OF LITHOSPEHERIC PLATE BOUNDARIES

2.) Convergent boundary (warning: there are three variations of this one, based on the types of crust involved)

•Two plates colliding with each other•Subduction zone - area where one plate slides under another plate

•Ocean trench - this is the feature that you see at a subduction zone

THREE TYPES OF CONVERGENT BOUNDARIES

A.Continental-continental:

•2 plates push together•fold upward to form folded mountains

•Ex. Himalayas and Appalachains

THREE TYPES OF CONVERGENT BOUNDARIES

B. Oceanic-oceanic:

•2 plates push together•the older oceanic plate is pushed down (subducts)

•forms volcanic island arc (chain of volcanic islands)

•ex. Aleutian Islands and Japanese Islands

THREE TYPES OF CONVERGENT BOUNDARIES

C.Continental-oceanic:

•2 plates push together•the oceanic plate subducts•volcanic mountains are formed on the continent’s edge

•ex. Andes Mountains and Cascade Mountains

http://www.daukas.com/Geoscience/MAtour/images/convergent.jpg

http://rst.gsfc.nasa.gov/Sect2/boundaries.jpg

TYPES OF PLATE BOUNDARIES

3. Transform Boundary

•neither destructive nor constructive•2 plates are grinding past each other•usually move in a series of sudden movement separated by periods of no motion

•Ex. San Andreas fault

http://www.gweaver.net/techhigh/projects/period1_2/Yellowstone/Images/transform.jpeg

http://conceptactivityresearchvault.files.wordpress.com/2011/03/usn-research-laboratory-plate-boundary-depiction.gif

CAUSES OF PLATE MOTION

Convection currents - the transfer of heat thru the movement of heated material, it is a cycle of warm rising currents and cold currents sinking

Scientists have measured the amount of heat leaving rocks at various points in the lithosphere. Hot convection currents are rising along these boundaries, thus explaining the temperature difference.

CAUSES OF PLATE MOTION

SUSPECT TERRANES

Theory of Suspect Terranes - theory that continents are patches of land that have different distinct geologic histories

Blocks of terranes are carried along the ocean floor by the action of seafloor spreading. When they meet a plate boundary, subduction occurs.

Geologists have found evidence to support this theory in Northern California.

ROCK DEFORMATION

• Deformation - the bending, breaking, and tilting of the earth’s crust.

• Isostatic Adjustments - the up-and-down movements of the crust in an attempt to reach

isostasy – which is the balance of the upward and downward forces that act on the earth’s crust

Isostatic adjustment and plate movement cause stress in the rocks that make up the earth’s crust. Stress is a force that causes pressure in the rocks of the crust.

This stress causes strain in crustal rocks. Strain is a change in the shape or volume of rocks that results from being squeezed, twisted or pulled apart.

ROCK DEFORMATION

• Three types of Stress:

1. Tensional•

2. Compressional•

3. Shear•

http://www.eas.purdue.edu/mesozoic/Lab_12/Stress_Types.jpg

Stress – 3 Types 1. Compression - when crustal rocks are squeezed together

  2. Tension - when crustal rocks are pulled apart

 3. Shearing - when crustal rocks are pushed in opposite horizontal directions

FOLDING - flat layers of rock are under severe compression and are squeezed from the sides. The layers moved into folded positions without breaking (although some cracks may appear).

Three types of Folds:

1. Anticlines - upward folds

2. Synclines – downward folds

3. Monoclines – gently dipping bends in horizontal rock layers

http://www.cr.nps.gov/history/online_books/geology/publications/bul/1508/images/fig27.jpg

A break in the rock layers along which there is no movement is called a fracture.

Once movement occurs, the fracture is known as a fault.

The type of fault is determined by how the blocks of rock on either side of the fault move.

Hanging Wall – the block of rock above the fault Foot Wall – the block of rock below the fault

Hanging Wall

Foot Wall

Three Main Types of Faults:

1. Normal Fault • occurs along divergent boundaries

(tension force present)

• The hanging wall drops down below the foot wall.

2. Reverse Fault

•occurs along convergent boundaries (compression force)

•The hanging wall moves up above the foot wall

•Thrust Fault ospecial type of reverse fault where the

hanging wall is pushed at such a low angle (not steep) that it moves UP AND OVER the foot wall

3. Strike-Slip Fault

•occurs at transform boundaries (shearing force)

• The rocks along either side of the fault slide horizontally

Ex. San Andreas Fault

Four major types of faults:

1. Normal• Tension• Hanging wall goes down

2. Reverse• Compression• Hanging wall goes up

Thrust (special type of reverse)• Compression• Hanging wall goes up and over

3. Strike-slip• Shearing• Side-to-side

http://www.geol.umd.edu/~jmerck/geol100/images/20/faults.gif

JUST FOR REVIEW

Mountain Hierarchy Single Mountain Mountain Range(ex. Mt. Mitchell) (ex. Smoky Mts.)  Mountain Belt Mountain System(Ex. Eurasian Melanesian) (Ex. Appalachian Mts.) Mountain Belts:1.) Eurasian-Melanesian Belt2.) circum-Pacific Belt (aka Ring of Fire)

FOUR TYPES OF MOUNTAINS1. Folded Mountains - Found where continents have

collided. Ex. Appalachian Mts., Himalayan Mts.• Plateaus - areas of flat-topped land high above sea

level. Ex. Tibetan Plateau (part of Himalayas) 2. Fault-Block Mountains - Formed by faults, where part of the earth’s crust have been broken into large blocks and then were lifted above the surrounding crust. Ex. Sierra Nevada Mts. in California

• Graben - long, narrow valleys formed when blocks of rock slipped downward Ex. Death Valley in CA

http://upload.wikimedia.org/wikipedia/commons/d/d8/Fault_block_mountain.JPG

http://martianchronicles.files.wordpress.com/2010/04/horstgraben.gif

3. Volcanic Mountains - Form when molten rock erupts onto earth’s surface. Ex. Cascade Mts. in Pacific NW and

the mid-Ocean ridges 4. Dome Mountains - An unusual type that is formed when molten rock rises but does not break through the surface.

Ex. Adirondacks, Black Hills of SD