• defn: refers to large-scale movement and deformation of Earth's crust

• proposed by Alfred Wegner (1915) - the idea that the continents migrate

• (the continents do move at speeds up to 6 cm/year)

• all landmasses were once united in a giant "supercontinent" - "pangaea" ~ 225 million years ago

....Fit of the Continents....

• the similarity between the coastlines on oposite sides of the South Atlantic was noted by Wegner
• a better fit between the continents was obtained by using the edges of the continental shelf

• Wegner documented cases of several fossil organisms that had been found on different landmasses but could not have crossed the present-day oceans which separate the continents

• E.g., Mesosaurus - a "snaggle-toothed" aquatic reptile whose fossil remains are limited to eastern South America and southern Africa - why not found elsewhere if it could swim?

....Rock Types & Structures Match....

• if continents were once joined, rocks and structures on the margins of each continent should match in terms of age and type

• E.g., the Applachians trend northeastward through the eastern U.S. and disappear off the coast of Newfoundland - comparable mountains (in age & strucure) are found in the British Isles and Scandinavia

....Ancient Climates (Paleoclimates)....

• E.g., near the end of the Paleozoic era (220-300 million years ago), ice sheets covered extensive areas of the Southern Hemisphere - layers of glacial till were discovered in southern Africa, South America, Australia and India - in present-day subtropical and tropical climates

    Paleoclimatic Data (Earth 300 million years ago & Today)

....Paleomagnetism....

• the most persuasive evidence for continental drift/plate tectonics comes from the study of the Earth's magnetic field

• recall that the Earth's magnetic field has a north pole and a south pole - which align closely with the geographic poles - (think of a giant bar magnet placed at Earth's center)

• invisible lines of force pass through the Earth, from one pole to the other

• any "freely moving" magnetized object would become aligned with these lines of force & would point toward the magnetic poles - e.g., a compass needle, which iself is magnetized

• some of Earth's early iron-rich minerals (found in basalts), became magnetized in the direction parallel to the Earth's existing magnetic field - once cooled, their magnetism was frozen in place - act like a compass, pointing toward the Earth's magnetic poles

• discovery: the magnetic alignment of these rocks possessing fossil magnetism (paleomagnetism) has changed with time

• two explanations: (1) the poles had wandered; or (2) the lava flows had moved over time

• this movement resulted from continental drift - plate tectonics

• the earth's magnetic field periodically reverses polarity; ("normal"=today's; "reverse"=opposite of today's)
• magnetometers revealed alternating strips of seafloor having different intensities of magnetism; low intensity strips corresponded to reverse polarity; high intensity strips correspond to normal polarity
• provides further evidence of plate tectonics

Magnetic Reversals & Seafloor Spreading

(1) Pre-Pangaea

• 465 M.Y. ago - mid-Ordovician - S.A., Africa, India, Australia, & S. Europe are part of Gondwanaland in the S. Hemisphere; Greenland & N.A. are part of Laurentia

• 225-200 M.Y. ago - Triassic-Jurassic - a "supercontinent" with areas of N.A., N. Africa, the Middle East, & Eurasia near the equator & covered w/abundant vegetation

• the Atlantic Ocean was non existant

• by 135 M.Y. ago - beginning of Cretacious - N.A. was spreading away from the landmasses to the east as new sea floor was forming in the Atlantic; India was sliding northward

• the leading edge of the the India plate was diving under Eurasia ("subduction"); while new sea floor was forming along the trailing edge of the plate ("sea-floor spreading")

• the leading edge of the India plate has pushed under the southern portion of Asia (subduction) & formed the Himalayas

• plate motions continue today, & will continue in the future

Break-Up of Pangaea

D. Sea-Floor Spreading

• on the sea floor there exists an interconnected worldwide mountain chain - 64,000 km long & 1,000 km wide - called "mid-ocean ridges"

Sea-Floor Spreading

• they result from upwelling flows of magma from hot areas in the upper mantle & asthenosphere

• mantle convection brings the material to the crust, the crust is fractured & the magma cools to form new sea floor

• the youngest crust is near the ridges; the crust gets older w/increasing distance from the ridge

• are areas where the more dense oceanic crust (basalt ~3.0 g/cm 3 ) collides with the less dense continental crust (~2.7 g/cm 3 )

• the oceanic crust dives beneath the continental crust forming a "subduction zone" (oceanic trenches - "Mariana Trench" - 36,198 ft)

• the subducted crust is remelted in the mantle, & then migrates back up through the crust via fissures & cracks to form volcanic mountains (e.g., Andes Mountains)

Subduction

F. The Earth's Plates (Plate Tectonics)

• the crust is split-up into 14 plates

(2) convergent boundaries - where areas of continental and/or oceanic crust collide - e.g., subduction zone off western S.A., between Nazca Plate & S.A. Plate - produces mountain building & volcanism

(3) transform boundaries - plates move laterally past one another - typically no associated volcanic eruptions

• (defn: "fault" - an area where fracturing & displacement occur between two portions of the Earth's crust)

• lateral motion - no new crust formed or destroyed

Transform Fault

** Earthquake & volcanic activity occur primarily at the plate boundaries **

    Summary of 3 Types of Plate Boundaries

G. Hot Spot Volcanism: "hot spots" - points of upwelling mantle material (from the asthenosphere) - not necessarily associated with sea-floor spreading centers - they remain fixed relative to the moving plates - e.g., the Hawaiian Islands