Chapter 2: Plate Tectonics

Tectonic Plates of Earth

Pangaea Pangaea:

Pan = all Gaea = Earth

Panthalassa Thalassa =

sea AKA Tethys

Sea

Continental Drift

Progressive breakup of Pangaea into modern continents

Similar to sea ice

Wegner’s Evidence Present-day

shorelines fit like puzzle pieces

Better fit from continental shelves

Fossil Evidence Identical fossils

present in S. Amer. & Africa

e.g. Mesosaurus

e.g. Glossopteris

e.g. Marsupials

Geologic Evidence Identical rocks on

different continents e.g. 2.2Ga igneous

rocks in Brazil & Africa

Similar Mountain Ranges e.g. Appalachian Mts

~ Caledonian Mts

Mesozoic Supergroup Identical

package of rocks & fossils found in S.Amer., Africa, Australia & Antarctica

Paleoclimatic

Evidence Paleo = ancient Climate =

weather conditions

Glacially transported sediments

Glacial striations

Rejection of Continental Drift Hypothesis

No evidence of continents “breaking through” oceanic crust

Tidal forces necessary would halt Earth’s rotation

Danish scientists found no astronomical evidence of drift from 1927-1948

Earth’s Magnetic

Field Similar to

bar magnet Magnetic

materials align themselves to magnetic field

N-S Orientation & Dip Magnetic

orientation has 2 dimensions North-South Dip angle

(Inclination) Curie point (T) Fossil Magnetism /

Paleomagnetism

Magnetic Inclination Magnetizatio

n = degrees from N pole

Magnetization + Latitude = 90°

e.g. Lavas from Puerto Rico show 75 ° from N pole & Puerto Rico = 15 ° from equator

Apparent Polar Wander

Seafloor Spreading

Paleomagnetism

Progression of Seafloor Spreading

Plate Boundaries

Plate Boundarie

s Correspond

s to Earthquakes & Volcanoes

Plate Boundarie

s Three Types

Divergent AKA

Rift Convergent

AKA Subduction

Transform AKA Strike-

slip

Divergent Boundaries AKA

Spreading Centers

AKA Rifts Largest

mountain chains

Plates move apart due to eruption of lava New lava =

new oceanic crust

Oldest oceanic crust 180Ma

Pillow Basalts Form when

lava extruded under water

Immediately outer layer freezes

New material pushes through like toothpaste

Continental Rifting Rifts also can form in

continental settings Linear depressions

Lakes, valleys, etc. Asthenosphere thins

due to tension e.g. East Africa Rift

Zone, Mt. Kilimanjaro

Continental Rifting (con’t) If tension

continues, eventually continental rift develops into oceanic spreading

e.g. Red Sea, Sea of Cortez

Convergent Boundaries Old oceanic crust

dense & heavy Heavy vs. light

=> subduction AKA destructive

margins Large earthquake

& explosive volcanoes

Melting triggered at ~100km depth

Oceanic-Oceanic Convergence

e.g. Virgin Islands, Japan, Philippines

Oceanic-Continental

e.g. Andes, MesoAmerica, Italy

Continental-Continental

e.g. India

Hot Spots

Caused by mantle plumes

Plumes do not move, plates do

Bend at 40Ma Major

change in plate motion

Relative Plate Motions Relative to

Hot Spots & other plates

Measure motions with Paleo-magnetism

Forces Driving Plate Motions Convectio

n of Mantle

Upwelling Mantle Ridge-

push Slab

Suction AKA

Slab-pull

Layer-Cake Model Two zones of

convection, above & below ~660km

Explains why mid-ocean ridge basalt different than hot spot basalt

Whole Mantle Convection Cold oceanic

crust descend to bottom of mantle, “stirring” it

Hot plumes rise from core-mantle boundary Bring “primitive”

mantle to surface Not popular b/c

complete mixing in 100s Ma

Deep Layer Model Heat from Earth’s

interior causes two layers to shrink & swell Similar to lava lamp

Small amt of material rises to surface to create hot spots

Little seismic evidence to support this model

Importance of Plate Tectonics

First theory to provide comprehensive view & explain: Earth’s major surficial processes Geologic distribution of earthquakes,

volcanoes & mountain ranges Distribution of mineral resources &

ancient organisms