PLATE TECTONICSChapter 9

The Continental Puzzle

When looking at globe-does it look like continents used to fit together?

Continental Drift-Hypothesis1915—Alfred Wegener

Continents used to form a single land mass-Pangaea

Surrounded by ocean-Panthalassa Started breaking up 200 million years

ago Crumpled crust in some places and

mountains formed

Evidence for Continental Drift Matching Fossils

Several fossils organisms found on different landmasses

Mesosaurus found in South America and Africa

No proof that land bridges ever existed

Evidence for Continental Drift Rock Types

Mountain belts that are separated by oceans have rocks that match in age and composition.

Appalachians match range in Newfoundland

Evidence for Continental Drift Ancient Climates

Deposits of glacial till can be found in areas with temperate or tropical climates○ South Africa, South America, India and

AustraliaCoal deposits of Pennsylvania have plants

that grow in tropical swamp climates

Evidence for Continental Drift

Rejection of Wegener’s Hypothesis He couldn’t precisely explain how the

continents moved He was made fun of in the science community Tried to explain it with different ideas

Tidal influence of moonContinents acting like ice breakers through oceanic

crust Died before technology to explain everything

was available

Section 1 Review

What is the hypothesis of continental drift?

List the evidence that supported the hypothesis of continental drift.

What was Pangaea? How does the occurrence of the same

plant fossils in South America and Africa support continental drift?

9.2 Sea-Floor Spreading

No one believed Wegener Years passed and few people cared

about continental drift Interest renewed when we began to

study the ocean floor

Exploring The Ocean Floor In 1800s scientists tried to map the ocean

floor Thought Mid-Atlantic would be deep but found

undersea mountains instead Sonar – Sound Navigation and Ranging

Uses waves to

determine distance to

an object

Deep Ocean Trenches Deepest part of the ocean Marianas Trench

In Pacific Ocean11km deepThat’s a little over

22 miles Most trenches are

around edges of

PacificThere are just a few in the Atlantic and Indian

Oceans

Mid-Ocean Ridges

Long chains of mountains on ocean floor Longest feature on Earth’s surface 70,000km through all ocean basins Not narrow ridge 1000-4000km wide Iceland – ridge is above surface Rift Valley

Deep central valley running down center of ridge

Composition of Ocean Floor Igneous rocks made of mostly basalt Forms when lava reaches surface and

hardens Most of floor covered with thick layer of

sedimentBecomes thinner the closer you get to a

mid-ocean ridgeNo sediment at ridge

The Process of Sea Floor Spreading 1963 Henry Hess

Navy submarine caption during WWII New ocean floor forms along Earth’s mid-

ocean ridges, slowly moves outward across ocean basins and sinks back into the mantle beneath deep-ocean trenches

Eruptions Along Mid-Ocean Ridges Form because of volcanic activity Fracture in ridge fills with magma from

below Can also happen on

land between

continental plates Red Sea getting

wider

Movement of Ocean Floor Sea floor spreads an average of 5cm

per year If we keep adding to ocean floor why

isn’t Earth getting bigger? Subduction Zones

Older parts of floor return to mantle as it sinks beneath deep ocean trench

Evidence for Sea Floor Spreading

Unlike Wegener, Hess was able to show evidenceMagnetic stripes in ocean-floor

rockEarthquake patternsMeasurements of the ages of

ocean floor rocks

Magnetic Strips Paleomagnetism

When lava cools it

becomes magnetized

parallel to the field

present at that timeOnce rock has

cooled the

magnetism doesn’t

change

Earthquake Patterns

Plot depth of earthquake in relation to their distance from deep ocean trenches

Shallow focus earthquakesHappen in and around a trench

Intermediate and Deep focus earthquakesOccur in belt 50km thickDeeper the earthquake the farther away it is from

the trenchNone happen below 700km because rocks soften

Earthquake Patterns

The Age of the Ocean Floor Youngest along central valley of ocean

ridge Oldest in subduction zones or near the

edges of continents far from ridge Oldest rock is only 180 million years

9.2 Review Describe mid-ocean ridges and deep ocean

trenches Explain what occurs during sea-floor spreading List the evidence for sea-floor spreading Why are the oldest parts of the ocean floor less

than 200 million years old? How do strips of magnetized rock on the ocean

floor provide evidence of sea-floor spreading? Would earthquakes occur at a depth of over

700km? Why or why not?

9.3 Theory of Plate Tectonics Earth’s lithospheric plates move slowly relative

to each other, driven by convection currents in the mantle.

Wegener never could say why – J. Tuzo Wilson figured out is was convection currents within Earth that made plates moveHot material rises while cool material sinks

Plates move 5cm/yearThat’s as fast as your fingernails grow

Earth’s Tectonic Plates

Types of Boundaries

DivergentPlates moving apart

ConvergentPlates moving together

Transform FaultPlates grind past each other

Plates can shrink or growPhilippine plate being subducted, but has no

area where new lithosphere can be added

Divergent Boundaries Spreading centers along crests of mid-

ocean ridgesCan happen on continent

ConstructiveNew lithosphere produced

On land rift

valleys form

East African Rift Valley

Convergent Boundaries Plates collide Produces trenches, volcanoes and mountain

ranges Destructive

Older portions return to mantle Three types

Ocean-OceanOcean-ContinentContinent-Continent

Convergent Collisions Oceanic crust & Continental crust

Oceanic crust is more dense so it is forced under – Subducted

Deep ocean trenches form hereCrust melts and becomes part of mantle,

rises to surface through continental crust and forms volcanic mountains

Convergent Collisions

Two continental crustsSame density—no subductionColliding edges crumpled and uplifted Produce large mountain ranges

○ Himalayas

Convergent Collisions

Two Oceanic CrustsOne plate subducted, melts, rises to the

surface right along the trenchForms island arc

Transform Fault Boundaries Plates grind past each other without destroying

lithosphereAKA – strike-slip fault

Mostly found along mid-ocean ridges San Andreas Fault

Pacific plate moving past North American PlateEventually California will become island off West

Coast and move towards Alaska

9.3 Review

Briefly explain the theory of plate tectonics List the three types of plate boundaries Why is a divergent boundary considered a

constructive plate margin? Suppose you could view the Great Rift Valley

in Africa millions of years from now. How might the region have changed?

What forms when oceanic lithosphere collides with continental lithosphere at a convergent boundary? Explain.

9.4 Mechanisms of Plate Motion

Convection CurrentsContinuous flowWarm material rises, cool material sinks

Heat provided by release of energy from radioactive isotopesUranium, thorium and potassiumSome heat also comes from core

What Causes Plate Motions Plates are top part of mantle convection

current Stiff lithosphere slides across weaker

asthenosphere At top, ocean plates cool and become denser

than mantle rock beneath them So ocean plate sinks to base of mantle Rock from lower mantle rises into upper

mantle at mid-ocean ridges Process takes half-billion years

Whole Mantle Convection

Plate Motion Mechanisms Slab Pull

Force of gravity pulls old ocean lithosphere into mantle

Ridge PushStiff ocean lithosphere slides down

asthenosphere that is elevated near mid-ocean ridge

Scientists argueDoes upwelling occur at mantle plumes? ORIs lithosphere replaced through a slow, broad rise

of rock throughout the mantle?

9.4 Review

How are the forces of slab-pull and ridge-push related to plate motions?

What is the ultimate source of heat that moves the plates?

If the earth did not form with very much uranium, thorium, or potassium, how might it have been different than it is today?

What characteristic of old, oceanic lithosphere in a subduction zone contributes to slab-pull?