Ch 7: Plate Tectonics

A work in progress …

Animation of Continental Drift• pangea moving• Pangea w/ labels

• PANGAEA:– Name of a supercontinent.– All continents were once combined.– Broke apart to form our current continents.

Sect 1: Continental Drift

• Alfred Wegener came up w/ Continental Drift Hypothesis– Continents are constantly moving (on the surface

of Earth). – So slowly we don’t feel it.

• No one believed him … WHY? – Couldn’t explain HOW the continents moved.

Evidence FOR Cont. Drift (4 Ways)1) Fossil Clues: Fossils found on the edge of

continents that were once joined MATCH. • Ex. Glossopteris (plant) – found on different continents.• Other fossils of similar organisms found on several

continents – separated by oceans. (Organisms that cannot swim!)

2) Rock Clues: Same rocks on the edge of continents that were once joined MATCH.

• Ex. West Africa rocks match East South America• Look at Chemistry and age of rocks.• Mountain ranges in N. Europe match eastern N. America

(Caledonian Mts to Appalachian Mts.)

Evidence Cont.3) Climate Changes: Certain areas have evidence

that the continents once had different climates … AGAIN: Continents match!– Ex.) Sediments left from glaciers …

-Same in S. America & Africa-Same in India & Australia

*Beneath sediments – grooves (marks left by glaciers)

4) Continents look like puzzles pieces that fit together.

Sect. 2: Seafloor Spreading• Process by which:

– New oceanic crust forms along mid-ocean ridges– Older oceanic crust moves away from the ridge.

• Began to explore ocean floor after WWII –led to new discoveries – led to this process.– Started using Sound Waves to map ocean floor. – Created detailed maps.– Discovered:

• Mountain Ranges, Valleys, etc. like those found on Continents.

• Mid-Ocean Ridges = mountain ranges in the middle of the oceans. (Much longer than on land.)

Theory states:(Helped explain how continents move):

• Seafloor spreads = plates are moving apart.• Mantle (crust) melts and forms magma (like lava

only inside Earth).• Magma is less dense than mantle (crust) so it…

rises through cracks along the mid-ocean ridges.• Seeps through = lava … cools quickly forms a rock

type called basalt. (Forms shape like a pillow so called pillow lava.)

• **Means: young rocks are by ridges and rocks get older as you move away from ridges.

Why this discovery was important:

• Wegener was RIGHT!• This explains HOW the continents are

moving. • Continents do NOT move through the solid

mantle or seafloor. • Continents move as the seafloor moves.

iv) Evidence supporting Seafloor Spreading …

• Ages of Rocks: younger at ridges and increasingly older as you move away from center of ridges.

• Magnetic Pole Reversal: found in rocks – showed a pattern parallel to ridges. (Basalt contains iron-rich minerals –magnetic)– Normal Polarity: magnetized objects point North.– Magnetic Reversal: changes direction.– Reversed Polarity: magnetized objects point SOUTH

instead of North.

How ?• Studied magnetic minerals in rocks on

seafloor.• Found parallel magnetic strips on either side

of M-O ridges. (Same composition, age and magnetic character.)

• Patterns = line up a certain way during periods of normal or reversed polarity.

• Sediment from seafloor can be dated – next to m-o. ridges younger than farther away. (Sediment thickness increases too.)

• What does this image show?• Seafloor spreading, stripes of basaltic rock• Which stripes are the oldest? Newest?• Further outside, further inside• How can you tell? New magma rises from mid-

ocean ridge

Sect. 3: Plate Tectonics

PLATES: Rigid SECTIONS OF THE EARTH’S CRUST THAT MOVE. (picture P. 234)

PLATE TECTONICS THEORY:- Theory that combines Cont. Drift and Seafloor Spreading.- Crust is broken into sections that move.

-Layer below is thick liquid like material that moves.

Video

Force Behind Plate Techtonics

• CONVECTION CURRENT!!! – OCCURS THROUGHOUT MANTLE

– CYCLE OF HEATING, RISING, COOLING, SINKING.

Closer look at Crust

• asthenosphere and lithosphere

Top Layer Vocab:

• LITHOSPHERE: Includes the CRUST and part of the Upper Mantle.– RIGID!! (HARD)

• ASTHENOSPHERE: movable layer below the Lithosphere. Where convection current IS!!– MOVABLE !!! (Thick liquid like material.)

3 WAYS PLATES MOVE• 1)Convergent: move TOWARDS each other.

• 2.) Divergent: move AWAY from each other or pull apart.

• 3.) Transform: – Slide by each other.– Different directions OR– Same direction but one plate is moving slower.

Divergent Boundary

• Almost all the Earth's new crust forms at divergent boundaries.

• Most are not well known because they lie deep beneath the oceans.

• These are zones where two plates move away from each other, allowing magma from the mantle to rise up and solidify as new crust.

Divergent: http://geomaps.wr.usgs.gov/parks

/pltec/pltec3.html

Convergent Boundaries• Crust is “disappearing” when 2 plates move

together. • Converging … Ocean to Ocean Plates:

Continent to Continent Plates

Ocean to Continent Plates

The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a subduction zone is born!

Remember

• Ocean Plate is DENSER than Continental Plate!!

• Earth is balanced … where crust is “disappearing” , somewhere else it is forming new crust.

Animations of Plate Movements

http://www.classzone.com/books/earth_science/terc/content/visualizations/es0804/es0804page01.cfm?chapter_no=visualization

Plate Motion

• Convection Current – circulation of material caused by diff. in temp. and density.

• Forces:– Basal Drag = conveyor belt movement

(lithosphere moves)– Ridge Push = force causes plates to move away

from m-o ridges in opposite directions.– Slab Pull = force pulls slab (harder, older crust)

down into mantle

Features on Earth made by Plate Tectonics …

1) AT Divergent Plate Boundaries: –Normal FAULTS (Tension forces pull

crust apart and cause rocks to break and move = fault line.)

–Rift Valleys: continent & continent pulling apart (ex. Africa)

–Mid Ocean Ridges

2) AT Convergent Boundaries• Continent to Continent = Mountain

Ranges: forces cause massive folding and faulting.

(Ex. Himalaya Mountains are still forming.)

• Ocean to Ocean = Volcanoes …–Island Arcs: Curved chain of volcanic

islands – form above the sinking plate. –Trenches

Convergent Cont.

• Ocean to Continent = Volcanic Mountain ranges: continent is again folding and faulting.–Volcanoes form w/in the mountains.

• (Along coasts)–Of course, Trenches

3) Transform Boundaries

• Strike - Slip FAULTS: – the plates try to move past each other they

stick and then slide. – When they stick and then suddenly move ,

this motion causes vibrations = earthquakes.

– Ex. San Andreas Fault

Rate of Plate Movement

• Varies … 1 cm to 12 cm / yr.• Use lasers and satellites to figure out how

much.• Hawaii is moving TOWARD Japan = 8.3 cm / yr.• Maryland is moving AWAY from England

= 1.7 cm/yr

Flow Chart for Features

Landforms

CompressionTension Shear Stress (Converge) (Diverge) (Transform)

Mts. Vol. M-O Cont. Trans Fracture Fault Arcs Ridges Rift Fault Zones Zones

(Rift Trenches Valleys)

Ch 8 Vocab

• Transform Fault: – Plates slide past each other – Break in Earth’s crust

• Fault Zone: – Area of MANY fractured pieces BY a large fault.

Mountains

• 1) Fault Block Mountains– Tension forces WITHIN a continent.– Some blocks rise or fall – Parallel Ridges.

• 2) Uplifted Mountains– Rise VERTICALLY– Little Deformation – Don’t totally understand … think sections break

off = mantle sinks and the other side rises.

Mountains Cont. • 3) Folded Mts.

– Happens deeper in crust.– Therefore: sections are warmer & under pressure**that is why they FOLD instead of break.

• HOW do Mts. change over time? – Collisions & Rifting– Weathering

• Ex) Appalachians – old, not growing so not as high or rugged as Rockies.

– Uplift • BALANCE (Erodes – wears down – new crust

rises to replace it.)