THE LANGUAGE OF THE
EARTH – PART IV
PLATE TECTONIC
THEORY:
THE EVIDENCE
Rock Cycle
Alfred Wegener
1880-1930
1915 – The Origin of Continents and
Oceans (3rd edition, 1922)
Meteorologist by training
Introduced the hypothesis of
Continental Drift
Postulated the existence of the
supercontinent Pangea
Evidence:
Fit of the continents
Fossil record
Similar geologic structures
Similar paleoclimates
Fit of the Continents
~250 Ma
Wegener’s 1915 Reconstructions
Fossil Evidence
Similar Geologic Structures
Wegener’s Sketch (1931)
The Appalachian-Caledonian
Mountains
~250 Ma
Paleoclimate Evidence
~300 Ma
General Dismissal of Wegner’s Theory
• an amateur (not even a geologist!)
• inadequate driving mechanism for horizontal motion of
continents (tidal energy, earth’s rotation)
• uncertainty about the seafloor and earth’s mantle
• some supporters: DuToit (South Africa), Holmes
(England), but many more detractors especially in the US
Continental Collapse
Continental Drift (Wegner)
from J. Holden
Continental Drift and
Seafloor Spreading
Paleomagnetism:
New Evidence for Continental Drift in the 1950’s
Declination – azimuth
direction to the
magnetic pole
Inclination – angle that
magnetic field is tilted
into the earth
Apparent Polar Wandering
Investigations of the Seafloor
The Final Pieces of the Puzzle
Earthquakes, Volcanoes,
Ocean Ridges, and Ocean Trenches
Ocean Ridges
Earthquakes
Volcanoes
Ocean Trenches
Benioff Zones
Sites of Ocean Floor Subduction and Melting
Seafloor Spreading
An “essay in geopoetry” by Harry Hess (1961)
Magnetic Striping of the Seafloor
Confirmation of the Seafloor Spreading Hypothesis
Vine and Matthews (1963)
Further Confirmation
Youth of the Ocean Crust
Continental Drift + Seafloor Spreading =
Theory of Plate Tectonics (late 1960’s)