Plate Tectonics

Evolution of Ocean Basins

Earth Structure - Layered Earth

- 3 layers - composition

a) Crust - Silicon & Oxygen

- continental versus ocean

b) Mantle - Magnesium & Iron

- lower versus upper

c) Core - Iron Alloy

- inner versus outer parts

Earth Structure - Layered Earth

- Upper mantle divided based on properties

- Lithosphere = brittle and rigid crust and outer mantle

- Asthenosphere = plastic and flowable

- Isostasy - gravitational balance

Earth Structure - Layered Earth

- Lithosphere floats on and glides across the Asthenosphere

- convection in lower mantle pushes and pulls lithosphere in different directions and caused it to break into plates

- moving plates = Plate Tectonics

Earth Structure - How Do We Know

How is it that we know so much about the Interior of the Earth?

* deepest wells ever drilled into the Earth only penetrate around 12 km (7.5 mi)

=> Seismic Waves (body waves = P & S) behave in predictable ways as they pass through earth materials => and we can follow their path

Earth Structure - How Do We Know

• seismic waves

P-waves = primary waves

Þ compression....=> highest velocity

S-waves = secondary or shear waves

=> shear => slower

Earth Structure - How Do We Know

• Seismic Wave Behavior

• refraction and reflection

• velocity

• propagation

• shadow zones

Continental Drift

Alfred Wegener - 1915 => continents have changed position on the Earth’s surface through time.

=> Continental Drift

- Pangea -300 m.y. old Supercontinent

Continental Drift

Wegener’s evidence:

a. Jigsaw fit of the continent’s edges

b. Paleoclimate indicators at wrong latitudes

c. Fossils & rocks on seperated continents

Continental Drift

• Problems with Wegener’s hypothesis

• an inadequate mechanism

- floating continents

- movement by centrifugal force (earth’s rotation)

Sea Floor Spreading

Wegner’s Pangaea did exist…….

- continents didn’t drift apart

moved apart by sea floor spreading

Historical development of Plate Tectonics

• For 30 years - stationary continent model (geosynclinal theory)

• Continued study of the earth eventually led to discovery of plate tectonics

• Sea Floor Topography

• Earth’s Magnetic Field

The earth’s magnetic field

• Movement of iron alloy in the outer core = magnetic field

• The earth’s dipole - an arrow that points from the north pole towards the south

– How does a compass work?

• Magnetic declination

• Magnetic inclination

Earth’s past magnetic field preserved in rocks

• Rocks that contain iron-rich minerals can preserve the magnetic field of earth that exists at the time that they form

– dipole

• Basaltic lava flows

• Ocean crust

• Direction & inclination

Magnetic Anomalies and Reversals - the final straw

• Magnetic anomalies discovered on the ocean floor

– Imaginary stripes of + and - parallel to the mid-ocean ridges

• Paleomagnetism demonstrates that the polarity of the earth’s magnetic field changes over time = magnetic reversals

Magnetic anomalies and reversals = related

• Oceanic crust recording magnetic reversals

• As new oceanic crust added at the mid-ocean ridges -material acquires polarity that exists at the time

• Illustrates sea-floor spreading

Sea Floor Spreading=> oceans = natural strip recorders

• Magnetism, age, etc.

Plate Boundaries

• Divergent

• Convergent

• Transform

What Drives Plate Motion

• Mantle Convection

• Ridge-push

• Slab-pull

Divergent Boundaries = Formation of the Oceans........

• Sea Floor Spreading

• Divergent Boundaries

• ocean-ocean divergence => mid-ocean ridges form....

• ex.: Mid-Atlantic Ridge

How do Divergent Boundaries Form?

• continental rifting

• continent-continent divergence

• most oceans began this way - proto MOR’s

• ex.: Arabian Shield & East African Rift

Transform Boundaries

• shear forces

• plates slide past one another

• transform faults

• form perpendicular to MOR’s

• ex.: San Andreas Fault

Convergent Boundaries

• ocean - continent convergent boundaries (subduction zones)

• Andean Arcs

• subduction of denser crust at trench

• causes melting and volcanism

• ex.: Cascades and Andes

• ocean - ocean convergent boundaries (subduction zones)

- Island Arcs

- Ex.: Japan

Other Convergent Boundaries

• continent -continent => continental crust clogs a subduction zone

• continental collision

• Suture Zone

• ex.: Himalayas, Alps, Appalachians