ch 7: plate tectonics a work in progress …. animation of continental drift pangea moving pangea w/...
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Ch 7: Plate Tectonics
A work in progress …
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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.
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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.
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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.)
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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.
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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.)
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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.
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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.
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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.
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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.)
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• 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
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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
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Force Behind Plate Techtonics
• CONVECTION CURRENT!!! – OCCURS THROUGHOUT MANTLE
– CYCLE OF HEATING, RISING, COOLING, SINKING.
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Closer look at Crust
• asthenosphere and lithosphere
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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.)
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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.
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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.
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Divergent: http://geomaps.wr.usgs.gov/parks
/pltec/pltec3.html
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Convergent Boundaries• Crust is “disappearing” when 2 plates move
together. • Converging … Ocean to Ocean Plates:
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Continent to Continent Plates
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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!
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Remember
• Ocean Plate is DENSER than Continental Plate!!
• Earth is balanced … where crust is “disappearing” , somewhere else it is forming new crust.
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Animations of Plate Movements
http://www.classzone.com/books/earth_science/terc/content/visualizations/es0804/es0804page01.cfm?chapter_no=visualization
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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
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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
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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
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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
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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
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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
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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)
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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.
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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.
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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.)