Drifting Continents

Continental Drift• Earth was once a super continent called

Pangaea (over 200 mya)• Proposed by Alfred Wegener

Evidence1. Rock Formation• Africa & South America

2. Fossils• Glossopteris found in cold

climates

3. Ancient Climates• Coal deposits in

Antarctica

Rejected Hypothesis• Wegener could not explain how continents moved

– Rotation of Earth (centrifugal force) disproved– Wegener died in 1930

• 1960’s evidence revealed the process Wegener could not explain

Sea Floor Spreading

Technology mapped sea floor• Fathometer (echo sounding device):

found mountains, ridges and trenches in sea floor topography

• Magnetometer (measures changes in magnetic field): magnetism mirror image on each side of ridge (magnetic symmetry)– Paleomagnetism: study of magnetic records– Magnetic reversal: change in Earth's

magnetic field

Harry Hess• Seafloor spreading: crust is formed at

ridges and destroyed at deep-sea trenches– Oceanic topography (mountains, valleys,

trenches)– Magnetic symmetry (reversal of Earth’s

magnetic field)

• Isochron: Line on map connects point of same age

Missing link to Wagner's theory

Theory of Plate Tectonics

• Earth’s crust and ridged upper mantle broken into slabs called plates

Three types of plate boundaries1. Divergent

boundaries• Two plates move

away from each other• Form:

– oceanic ridges – rift valleys – new crust

• Have: – volcanism– earthquakes

2. Convergent boundaries• Two plates moving toward each other

Three types:

A. Oceanic-Oceanic convergence

• Form: – Trench (subduction) – island arcs

• Have:– Volcanism– Earthquakes

• Crust is destroyed

B. Oceanic-Continental convergence

• Form: – Trench (subduction) – volcanic mountain range

• Have:– Volcanism– Earthquakes

• Crust is destroyed

C. Continental-Continental convergence• Form:

– Folded mountain range • tallest mountains • no subduction

• Have:– earthquakes

• Crust is deformed

Island Arcs

Volcanic Mountains

Folded Mountains

oceanicoceanic

oceaniccontinental

continentalcontinental

3. Transform Boundaries• Two plates slide

horizontally past each other– Crust is only deformed or

fractured– Characterized by

earthquakes• San Andreas best

example

Gorda Plate

Juan de FucaPlate

Convergent Divergent

Earths layers:

1. Lithosphere (crust): Solid, ridge mass;– Oceanic (basaltic, more dense, thinner), – Continental (granitic, less dense thicker)

2. Asthenosphere (mantle): liquid molten area of convection

3. Core: – Outer: more liquid like

than inner core (still very dense due to pressure) made of iron & some nickel, temp 7200 - 9032 ºF (4000-5000ºC).

– Inner: solid (due to pressure) made of iron & nickel, temp 9032 - 10832 ºF (5000-6000 ºC).

– The outer core and the inner core together cause the earth's magnetism. Because the earth rotates, the outer core spins, the inner core doesn't spin because it's solid.

Outer core

Inner core

Causes of Plate MotionMantle Convection• Convection currents in mantle thought to drive plate

movement• Creates

– Ridge push: weight of uplifted ridge push plate toward trench

– Slab pull: weight of subducting plate pulls plate into trench

Slab pull

Ridge push

Hot spots: areas of volcanic and earthquake activity away from plate boundaries