4.4 Developing the Continental Drift Theory

Developing the Theory

• Alfred Wegener described continental drift in 1912 and expanded the theory in his 1915 book The Origin of Continents and Oceans.
• The continental drift scientific debate ended 50 years later when plate tectonics theory was accepted.
• Continents were once a single land mass called Pangea.
• These continents drifted apart; continents were released from the Earth's mantle.
• Continents are like low-density granite "icebergs" floating on a sea of denser basalt.
• Evidence supporting continental drift includes the dovetailing outlines of South America and Africa.
• Matching rock formations along the edges of South America and Africa support continental drift.
• Fossil plants Glossopteris and Gangamopteris further support continental drift.
• Lystrosaurus, a mammal-like reptile whose fossils were discovered in South America, Africa, Antarctica, India, and Australia, supports continental drift.
• Alex du Toit compiled more evidence in his 1937 publication Our Wandering Continents.
• Alex du Toit recognized links between the Gondwana fragments.
• Scientists such as Harold Jeffreys and Charles Schuchert were outspoken critics of continental drift.
• Debate ensued between "drifters"/"mobilists" who supported continental drift and "fixists" who opposed it.
• Convection currents might have driven plate movement, and spreading may have occurred below the sea in the oceanic crust during the 1920s-1940s.
• Geophysicists and geologists such as Vening-Meinesz, Holmes, and Umbgrove proposed concepts close to current plate tectonics.
• Paleomagnetism was one of the first geophysical pieces of evidence to support lithospheric plate movement.
• Rocks of different ages displayed variable magnetic field direction based on studies since the mid-19th century.
• Magnetic north and south poles reverse over time.
• The relative position of the magnetic north pole varies through time.
• "Polar wander" was introduced to explain the phenomenon which assumed that the north pole location had been shifting through time during the 20th century.
• Continents moved, shifting and rotating relative to the north pole to create their own "polar wander path".
• Data showed this in Keith Runcorn's 1956 paper and by Warren Carey in a symposium held in March 1956, to validate continental drift.
• Data from deep ocean floors, oceanic crust nature, and magnetic properties supported continental drift in the late 1950s and early 60s.
• Marine geology saw the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published by Heezen, Dietz, Hess, Mason, Vine, Matthews, and Morley between 1959-1963.
• Early seismic imaging techniques in Wadati-Benioff zones showed how oceanic crust could disappear into the mantle.
• Oceanic crust disappearing into the mantle provided a mechanism to balance ocean basin extension with shortening along its margins.
• Continental drift was determined feasible around 1965, and the theory of plate tectonics was formed between 1965-1967.
• Plate tectonics revolutionized earth sciences and explained geological phenomena.
• Plate tectonics has implications in paleogeography and paleobiology.

Continental Drift

• Geologists the late 19th and early 20th centuries thought Earth's features were fixed and geologic features were caused by vertical crustal movement.
• Geosynclinal theory was placed in the context of a contracting planet Earth due to heat loss.
• The opposite coasts of the Atlantic Ocean or the edges of the continental shelves had similar shapes and appeared to have once fitted together.
• A solid Earth assumption made the theories to explain observed phenomena difficult to accept.
• Radioactivity and heating properties spurred a re-examination of Earth's age in 1895.
• Earth was estimated to have been cooling for tens of millions of years.
• The Earth's core was still sufficiently hot and liquid, with the knowledge of a new heat source.
• Alfred Wegener argued for continental drift in the first edition of The Origin of Continents and Oceans, after having published a first article in 1912.
• The east coast of South America and the west coast of Africa looked like they were once attached.
• Evidence including fossil, paleo-topographical, and climatological evidence to support continental drift, was collected by Wegener.
• Alex du Toit supported Wegener in his findings.
• Similar rock strata margins on separate continents suggests that these rocks formed the same way, which means they were joined initially.
• Parts of Scotland and Ireland share rocks with Newfoundland and New Brunswick.
• Caledonian Mountains found in Europe and parts of the Appalachian Mountains of North America share similar structure and lithology.
• Geologists did not take Wegener's ideas seriously because of the lack of mechanism.
• There was no explanation as to how continental rock could plow through much denser oceanic crust and what force would drive continental drift.
• Wegener's vindication came after he died in 1930.

Floating Continents, Paleomagnetism, and Seismicity Zones

• Continental granite and seafloor basalt led to the 20th-century concept of "sial" continental crust and "sima" oceanic crust.
• A static shell of strata existed under the continents.
• A basalt "sial" layer underlies continental rocks.
• Pierre Bouguer deduced that less-dense mountains project downward into the denser layer underneath based on plumb line deflection by the Andes in Peru.
• The concept that mountains had "roots" later confirmed by George B. Airy, during the study of Himalayan gravitation.
• Seismic studies detected corresponding density variations of mountains "roots".
• By the mid-1950s, it was undetermined if mountain roots were clenched in surrounding basalt or floating like an iceberg.
• 20th-century improvements enabled scientists to learn through seismographs that earthquakes tend to happen in specific areas.
• Earthquake locations are notably along oceanic trenches and spreading ridges.
• Seismologists began to identify several prominent earthquake zones parallel to trenches at a 40–60° inclination and extending hundreds of kilometers into Earth by the late 1920s.
• These zones became known as Wadati-Benioff zones named after seismologists Kiyoo Wadati and Hugo Benioff.
• The study of global seismicity advanced with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor compliance of the 1963 Nuclear Test Ban Treaty.
• Data from WWSSN instruments allowed seismologists to map the zones of earthquake concentration around the world.
• Debates developed around the phenomenon of polar wander.
• Evidence was used to suggest that polar drift had occurred since continents seemed to move across different climatic zones.
• Paleomagnetic data showed that the magnetic pole shifted over time.
• Reasoning oppositely, the continents shifted and rotated.
• Evidence of magnetic polar wander initially used to support the movements of continents in Keith Runcorn's 1956 paper and successive papers by his students, Ted Irving and Ken Creer.
• The evidence was used in the expansion theory of global crust during a symposium in Tasmania in March 1956.
• Shifting continents were explained by an increase in Earth's size.
• This expansion was unsatisfactory because there was no mechanism that could credibly expand the Earth.
• There is no evidence the moon has expanded in the past 3 billion years.
• Work showed evidence supporting continental drift on a globe with a stable radius.
• Modern plate tectonics theory was formed by Vening-Meinesz, Holmes, and Umbgrove in the thirties up to the late fifties.
• Arthur Holmes proposed in 1920 that plate junctions lie beneath the sea.
• Arthur Holmes proposed convection currents within the mantle as a driving force in 1928.
• These contributions are forgotten because continental drift was not accepted, ideas were discussed in the context of a deforming globe and abandoned fixistic ideas.
• Publications occurred during extreme political and economic instability that hampered scientific communication.
• Research was published by European scientists and not initially mentioned by sea floor spreading published by American researchers in the 1960s.

Mid-Oceanic Ridge Spreading and Convection

• A team of scientists led by Maurice Ewing confirmed the existence of a rise in the central Atlantic Ocean.
• The team found the floor was basalt-based, not granite-based.
• The ocean crust was thinner than the continental crust.
• New data showed mid-oceanic ridges, leading to the "Great Global Rift'' concept.
• Bruce Heezen's crucial paper described the "Great Global Rift," a real conclusion in thinking.
• Seafloor spreading means new crust continues being created along oceanic ridges.
• Heezen advocated for the theory of "expanding Earth”.
• Arthur Holmes, Vening-Meinesz, and Coates realized the disappearance of crust in excess along oceanic trenches.
• Multiple members of the scientific community began to reason regarding data at their disposal of the ocean floor, and the pieces quickly fell into place.
• Harry Hammond Hess and Robert S. Dietz coined the term seafloor spreading.
• Dietz and Hess understood sea floor spreading implication which would agree with continental drift and mobilistic models made by previous workers like Holmes.
• Robert R. Coats described island arc subduction in the Aleutian Islands.
• Work by European scientists on island arcs and mountain belts were performed and published during the 1930s up until the 1950s to be applied and appreciated in the United States.
• Hess and Dietz reasoned that new oceanic crust spreads away in a conveyor belt-like motion.
• Oceanic crust descends along continental margins where oceanic trenches form.
• Convection currents were the driving force like Holmes correctly concluded.
• The thinning of the ocean crust was performed using Heezen's mechanism of spreading along the ridges.

Magnetic Striping

• Victor Vacquier used magnetometers to recognize odd magnetic variations across the ocean floor in the 1950s.
• Basalt contains a strongly magnetic mineral (magnetite) and can locally distort compass readings.
• Presence of magnetite gives basalt measurable magnetic properties leading to magnetic variations.
• Newly formed rocks record the Earth's magnetic field at the time.
• Ocean floor showed a zebra-like pattern of magnetic variations turning out not to be random.
• This pattern consists of alternating bands of normally and reversely polarized rock, now known as magnetic striping.
• Ron G. Mason and co-workers published magnetic striping in 1961 sans explanation.
• Heezen, Hess, and Dietz theorized that mid-ocean ridges mark zones where the ocean floor was ripped in two lengthwise in the early 1960s.
• New magma rises and erupts through these zones to create new oceanic crust in a process called seafloor spreading.
• The "conveyor belt hypothesis" and seafloor spreading operate for years to form new ocean floor across the 50,000 km-long system of mid-ocean ridges.
• The connection between sea floor spreading and magnetic stripes was correctly placed independently by Lawrence Morley, and Fred Vine and Drummond Matthews in 1963.
• These scientists created the Vine-Matthews-Morley hypothesis which linked geomagnetic reversals and was supported by evidence including symmetrical stripes around mid-ocean ridges, where rocks are youngest and the youngest rocks always have present-day polarity.
• Zebra-like magnetic striping and the construction of the mid-ocean ridge system led to the seafloor spreading hypothesis (SFS).
• Oceanic crust became a natural "tape recording" of geomagnetic field reversals (GMFR).
• Studies exist dedicated to calibrating the normal-reversal patterns in the oceanic crust to estimate spreading rates and plate reconstructions.

Definition and Refining of the Theory

• Plate Tectonics (or "New Global Tectonics") became accepted.
• Tuzo Wilson added transformative fault concepts to the model to make plate mobility work in 1965.
• A symposium on continental drift was held at the Royal Society of London in 1965.
• Edward Bullard and co-workers showed a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean ("Bullard's Fit").
• Wilson published a paper that referred to previous plate tectonic reconstructions, introducing the "Wilson Cycle" in 1966.
• W. Jason Morgan proposed Earth's surface consists of 12 rigid plates during the 1967 American Geophysical Union's meeting.
• Xavier Le Pichon published a model based on 6 major plates with relative motions.
• McKenzie and Parker presented a model similar to Morgan's using translations and rotations on a sphere to define plate motions.