Plate Tectonics Earth has distinctive layers - Like an onion
Earth s Interior Core: Metallic (Iron, Nickel) Inner (hot, solid, dense, Iron, Nickel) Outer (cooler, liquid, less dense)
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
Lithosphere: Thin, Rigid, Outer Layer Oceanic Crust (basalt) ~10 km (6 mi) thick Continental Crust (granite) ~55 km (34 mi) thick Asthenosphere warmer, melted (flowing) layer of upper mantle Crust floats on melted layer Like an egg shell, Crust is brittle & can break
The Lithosphere is Moving! These rocks were under the sea! Moving plates can cause Earthquakes
Early Evidence: Da Vinci (1400 s) F. Bacon (1620) Saw continents fit together like a puzzle 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
oceanic trenches Pacific Basin: Zone of frequent volcanic eruptions & earthquakes
Patterns of 10,000 earthquakes = Earth is divided into sections
Echo Soundings reveal submerged, midoceanic mountain range 1960 - Seafloor Spreading hypothesis: Ridges = source of new sea floor Spreading Center Mid-Atlantic Ridge Conforms to edges of nearby continents
New Sea floor at ridges formed by Convection Hot material rises, cold material falls cool Hot
1. Hot magma upwells from mantle 3. Old crust is then pushed away from Mid-Ocean Ridge 2. New ocean crust forms 4. Cold (old) crust is subducted back into mantle at deep sea trenches Heated & recycled
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 plate boundary Lithospheric plates move away from each other. EX: Mid-Atlantic Ridge (Atlantic Ocean)
Divergent Boundary (constructive): Plates spread & new oceanic lithosphere forms at ocean ridges (Mid-Atlantic Ridge)
Pangea was separated by Divergence
Supercontinent Pangaea Eventually fragmented into continents we know today
Beginning to crack Basin forms as continents separate
Red Sea: Site of a new ocean basin Ethiopian volcano
Convergent plate boundary Lithospheric plates move toward each other Ex: Where oceanic Nazca Plate is pushed beneath continental portion of S. American Plate
Convergent Boundary (3 types): Plates collide together, Old oceanic crust destroyed -Many earthquakes -Subduction zone a. Oceanic Continental Dense Ocean plate subducts + gravity Magma powers volcano arcs
- (Deep) b. Oceanic Oceanic Old plate subducts beneath other 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 plate boundary Plates slide past each other. EX: San Andreas fault (California) Pacific Plate slides past North American Plate
Transform Boundary Lithosphere isn t created or destroyed Rock shatters Many shallow earthquakes Separates Pacific & North American Plates San Andreas Fault
1. Pacific Plate moves over hotspot Hot Spots: Small, Hot Spots long-lasting heat sources 2. Magma rises up &, forms active seamount 3. Grows to form island volcano 4. Plate movement cuts magma source = Volcanism ceases
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 ~ few 100,000 yrs. (N-S) Convection currents in core Earth s rotation Earth s field like a magnet Compass points N. today, Not always
As continents separate, they carry their magnetized rocks with them Rocks permanently magnetized in direction magnetic field pointed at that time (N or S)
Pole Direction North South