Historical Development of the Plate Tectonic Paradigm

The plate tectonic paradigm was developed from a number of

different models, ideas, and observations that were advanced

over the prior century by a number of scientists on different

continents. Between 1912 and 1925, Alfred Wegener, a meteorologist,

published a series of papers and books outlining

his ideas for the evolution of continents and oceans. Wegener

was an early proponent of continental drift. He looked for a

driving mechanism to move continents through the mantle,

and invoked an imaginary force (which he called Pohlfluicht)

that he proposed caused the plates to drift toward the equator

because of the rotation of the Earth. Geophysicists were

able to show that this force was unrealistic, and since Wegener’s

idea of continental drift lacked a driving mechanism, it

was largely disregarded.

In 1929, Arthur Holmes proposed that the Earth produces

heat by radioactive decay and that there are not

enough volcanoes to remove all this heat. He proposed that a

combination of volcanic heat loss and mantle convection can

lose the heat, and that the mantle convection drives continen-

tal drift. Holmes wrote a widely used textbook on this subject,

which became widely respected. Holmes proposed that

the upwelling convection cells were in the ocean basins, and

that downwelling areas could be found under Andean-type

volcano chains.

Alex du Toit was a South African geologist who worked

on Gondwana stratigraphy and published a series of important

papers between 1920 and 1940. Du Toit compared

stratigraphic sections on the various landmasses that he

thought were once connected to form the supercontinent of

Gondwana (Africa, South America, Australia, India, Antarctica,

Arabia). He showed that the stratigraphic columns of

these places were very similar for the periods he proposed the

continents were linked, supporting his ideas of an older large

linked landmass. Du Toit also was able to show that the floral

distributions had belts that matched when the continents

were reconstructed but appeared disjointed in the continents’

present distribution.

In the 1950s paleomagnetism began developing as a science.

The Earth has a dipolar magnetic field, with magnetic

field lines plunging into and out of the Earth at the north and

south magnetic poles. Field or flux lines are parallel or

inclined to the surface at intermediate locations, and the magnetic

field can be defined by the inclination of the field lines

and their deviation from true north (declination) at any location.

When igneous rocks solidify, they pass through a temperature

at which any magnetic minerals will preserve the

ambient magnetic field at that time. In this way, some rocks

acquire a magnetism when they solidify. The best rocks for

preserving the ambient magnetic field are basalts, which contain

1–2 percent magnetite; they acquire a remnant inclination

and declination as they crystallize. Other rocks,

including sedimentary “red beds” with iron oxide and

hematite cements, shales, limestones, and plutonic rocks also

may preserve the magnetic field, but they are plagued with

their own sets of problems for interpretation.

In the late 1950s Stanley K. Runcorn and Earl Irving

first worked out the paleomagnetism of European rocks and

discovered a phenomenon they called apparent polar wandering

(APW). The Tertiary rocks showed very little deviation

from the present poles, but rocks older than Tertiary showed

a progressive deviation from the expected results. They initially

interpreted this to mean that the magnetic poles were

wandering around the planet, and the paleomagnetic rock

record was reflecting this wandering. Runcorn and Irving

made an APW path for Europe by plotting apparent position

of the poles, while holding Europe stationary. However, they

found that they could also interpret their results to mean that

the poles were stationary and the continents were drifting

around the globe. Additionally, they found that their results

agreed with some previously hard to interpret paleolatitude

indicators from the stratigraphy.

Next, Runcorn and Irving determined the APW curve for

North America. They found it similar to Europe from the late

Paleozoic to the Cretaceous, implying that the two continents

were connected for that time period, and moved together, and

later (in the Cretaceous) separated, as the APW curves

diverged. This remarkable data set converted Runcorn from a

strong disbeliever of continental drift into a drifter.

In 1954 Hugo Benioff, a seismologist, studied worldwide,

deep-focus earthquakes, to about 435-mile (700-km)

depth. He plotted earthquakes on cross sections of island arcs

and found earthquake foci were concentrated in a narrow

zone to about 700 km depth. He noted that volcanoes of the

island arc systems were located about 62 miles (100 km)

above this zone. He also noted compression in island arc

geology and proposed that island arcs are overthrusting

oceanic crust. Geologists now recognize that this narrow

zone of seismicity is the plate boundary between the subducting

oceanic crust and the overriding island arc and have

named this area the Benioff Zone.

Development of technologies associated with World War

II led to remarkable advancements in understanding some

basic properties of the ocean basins. In the 1950s the ocean

basin bathymetry, gravity, and magnetic fields were mapped

for the U.S. Navy submarine fleet. After this, research vessels

from oceanographic institutes such as Scripps, Woods Hole,

and Lamont-Doherty Geological Observatory studied the

immense sets of oceanographic data. In the 1950s raw data

was acquired, and the extent of the mid-ocean ridge system

was recognized and documented by geologists including

Bruce Heezen, Maurice Ewing, and Harry Hess. They also

documented the thickness of the sedimentary cover overlying

igneous basement and showed that the sedimentary veneer is

thin along the ridge system and thickens away from the

ridges. Walter Pitman happened to cross the oceanic ridge in

the South Pacific perpendicular to the ridge and noticed the

symmetry of the magnetic anomalies on either side of the

ridge. In 1962 Harry Hess from Princeton proposed that the

mid-ocean ridges were the site of seafloor spreading and the

creation of new oceanic crust, and that Benioff Zones were

sites where oceanic crust was returned to the mantle. In 1963

Frederick John Vine and D. H. Matthews combined Hess’s

idea and magnetic anomaly symmetry with the concept of

geomagnetic reversals. They suggested that the symmetry of

the magnetic field on either side of the ridge could be

explained by conveyor-belt style formation of oceanic crust

forming and crystallizing in an alternating magnetic field,

such that the basalts of similar ages on either side of the ridge

would preserve the same magnetic field properties. Their

model was based on earlier discoveries by a Japanese scientist,

Matuyama, who in 1910 discovered recent basalts in

Japan that were magnetized in a reversed field and proposed

that the magnetic field of the Earth experiences reversals.

Allan Cox (Stanford University) had constructed a geomagnetic

reversal timescale in 1962, so it was possible to correlate

the reversals with specific time periods and deduce the

rate of seafloor spreading.

With additional mapping of the seafloor and the midocean

ridge system, the abundance of fracture zones on the

seafloor became apparent with mapping of magnetic anomalies.

In 1965 J. Tuzo Wilson wrote a classic paper, “A New

Class of Faults and Their Bearing on Continental Drift,”

published in Nature. This paper connected previous ideas,

noted the real sense of offset of transform faults, and represented

the final piece in the first basic understanding of the

kinematics or motions of the plates. Wilson’s model was

proved about one year later by Lynn Sykes and other seismologists,

who used earthquake studies of the mid-ocean ridges.

They noted that the ridge system divided the Earth into areas

of few earthquakes, and that 95 percent of the earthquakes

occur in narrow belts. They interpreted these belts of earthquakes

to define the edges of the plates. They showed that

about 12 major plates are all in relative motion to each other.

Sykes and others confirmed Wilson’s model, and showed that

transform faults are a necessary consequence of spreading

and subduction on a sphere.

See also CONVERGENT PLATE MARGIN PROCESSES; DIVERGENT

OR EXTENSIONAL BOUNDARIES; TRANSFORM PLATE MARGIN

PROCESSES.

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