LECTURE NOTES:
OCEANOGRAPHY (MS 100)
SNYDER, L
Ch. 3: Plate Tectonics
Earth has distinctive layers like an onion
Earth’s Interior:
Core: Metallic (Iron, Nickel)Inner: 1070 km (hot, solid, dense, Iron, Nickle)
Outer: 2400 km (less dense, cooler, liquid)
Earth’s Crust (outermost layer): rigid & very thin
Mantle: dense, hot layer of semi-solid rock Core ~ twice as dense as mantle b/c its
metallic, not stony
Earth's crust is literally only skin deep
Oceanic Crust (basalt) ~10 km (6 mi) thick
Continental Crust (granite) ~55 km (34 mi) thick
Like an egg shell, Earth's crust is brittle & can break
Lithosphere: outer rigid shell of EarthAsthenosphere: warmer, melted (flowing) layer of
upper mantle
Continental crust floats on melted layers
The Lithosphere is Moving! Moving plates can cause Earthquakes
Early Evidence:
•Some saw that continents fit together like a puzzle
•Da Vinci (1400’s)
F. Bacon (1620)
•How did they move to their present locations?
Continental Drift
•Alfred Wegener (1912)
•Continents once joined as super continent (Pangea)
•One world ocean (Panthalassa)
•Continents broke up 200 mya
•Still moving!
Evidence for Continental Drift
1. Paleontology:
•Fossils - same age & species (not swimmers) on both sides of Atlantic
•Tropical species in Antarctica
2. Geologic: Desert sands under rain forest
3. Pacific Ring of Fire (1935-1940): Earthquakes & volcanoes in Pacific associated with
C. Drift
Pacific Basin: Zone of frequent earthquakes & volcanic eruptions, oceanic
trenches
Patterns of 10,000 earthquakes = Earth is divided into sectionsEcho Soundings
revealed a submerged, mid-oceanic mountain range•1960 - Seafloor Spreading
hypothesis:
•Ridges = source of new sea floor
Mid-Atlantic Ridge (a spreading center): Conforms to edges of nearby continentsNew
Sea floor at ridges formed by Convection: Hot material rises, cold material falls•Cold
(old) crust is subducted back into mantle at deep sea trenches
•Heated & recycled
1. Hot magma upwells from mantle 2. New ocean crust forms3. Old crust is
then pushed away from Mid-Ocean Ridge Continental Drift & Sea Floor Spreading
integrated into Plate Tectonics (1965)
•Plates move relative to one another
•3 different types of boundaries:
1. Divergent Boundary (apart)
2. Convergent Boundary (together)
3. Transform Boundary (shear)
Divergent Boundary:
 Plates spread & new oceanic lithosphere forms at ocean ridges (Mid-Atlantic
Ridge) Supercontinent Pangea was separated by Divergence: Eventually
fragmented into continents we know today
 Ocean Basins form as continents separate
 Red Sea: Site of a new ocean basinConvergent Boundary (3 types): Plates collide
together, Old oceanic crust destroyedMany earthquakes, Subduction zone
a. Oceanic – Continental Dense Ocean plate subducts + gravity Magma powers
volcano arcs
b. Oceanic–Oceanic
 Old plate subducts beneath other (trench)Heats, & partially melts
(magma)Less dense than surrounding crust = rises to form Volcanic Island
Arcs (Japan, Philippines, Caribbean)
c. Continental – Continental: plates collide, fold, & uplift
 Indian & Asian plates -> HIMALAYAS One plate may move beneath
Transform Boundary: Plates slide past each other
•Lithosphere isn’t created or destroyed
•Rock shatters
•Many shallow earthquakes (San Andreas Fault)
•Separates Pacific & North American Plates
Paleomagnetism Confirms Plate Tectonics•Magnetite: Iron bearing, magnetic mineral
present in magma
•When magma erupts at ridges,magnetite solidifies in cooled rock•Spreading centers symmetrical magnetic bands
 Earth’s magnetic field changes direction (N-S) ~ few 100,000 yrs.Due to
Convection currents in core & Earth’s rotationCompass points N. today, Not
alwaysEarth’s field like a magnet•As continents separate, they carry their
magnetized rocks with them
•Rocks permanently magnetized in direction magnetic field pointed at that time (N or
S)
Hot Spots: Small, long-lasting heat sources 1. Pacific Plate moves over hotspot2.
Magma rises up &, forms active seamount
3. Grows to form island volcano
4. Plate movement cuts magma source = Volcanism ceases