DEFINITION OF CONTINENTAL DRIFT

A theory that may be thought of as the

precursor to plate tectonics. It was proposed most clearly by

Alfred Wegener in 1912 and states that the continents are relatively

light objects that are floating and moving freely across

a substratum of oceanic crust. The theory was largely discredited

because it lacked a driving mechanism, and it seemed

implausible if not physically impossible to most geologists

and geophysicists at the time. However, many of the ideas of

continental drift were later incorporated into the paradigm of

plate tectonics.

Early geologists recognized many of the major tectonic

features of the continents and oceans. Cratons are very old

and stable portions of the continents that have been inactive

since the Precambrian and typically have subdued topography

including gentle arches and basins. Orogenic belts are

long, narrow belts of structurally disrupted and metamorphosed

rocks, typified (when active) by volcanoes, earthquakes,

and folding of strata. Abyssal plains are stable, flat

parts of the deep oceanic floor, whereas oceanic ridges are

mountain ranges beneath the sea with active volcanoes,

earthquakes, and high heat flow. In order to explain the

large-scale tectonic features of the Earth, early geologists proposed

many hypotheses, including popular ideas that the

Earth was either expanding or shrinking, forming ocean

basins and mountain ranges. In 1910–25, Alfred Wegener

published a series of works including his 1912 treatise on

The Origin of Continents and Oceans. He proposed that the

continents were drifting about the surface of the planet, and

that they once fit together to form one great supercontinent,

known as Pangea. To fit the coastlines of the different continental

masses together to form his reconstruction of Pangea,

Wegener defined the continent/ocean transition as the outer

edge of the continental shelves. The continental reconstruction

proposed by Wegener showed remarkably good fits

between coastlines on opposing sides of ocean basins, such as

the Brazilian Highlands of South America fitting into the

Niger delta region of Africa. Wegener was a meteorologist,

and since he was not formally trained as a geologist, few scientists

at the time believed him, although we now know that

he was largely correct.

Most continental areas lie approximately 985 feet (300

m) above sea level, and if we extrapolate present erosion

rates back in time, we find that continents would be eroded

to sea level in 10–15 million years. This observation led to

the application of the principle of isostasy to explain the elevation

of the continents. Isostasy, which is essentially

Archimedes’ Principle, states that continents and high topography

are buoyed up by thick continental roots floating in a

denser mantle, much like icebergs floating in water. The principle

of isostasy states that the elevation of any large segment

of crust is directly proportional to the thickness of the crust.

Significantly, geologists working in Scandinavia noticed that

areas that had recently been glaciated were rising quickly relative

to sea level, and they equated this observation with the

principle of isostatic rebound. Isostatic rebound is accommodated

by the flow of mantle material within the zone of low

viscosity beneath the continental crust, to compensate the rising

topography. These observations revealed that mantle

material can flow at rates of several centimeters per year.

In The Origin of Continents and Oceans, Wegener was

able to take all the continents and fit them back together to

form a Permian supercontinent, known as Pangea (or all

land). Wegener also used indicators of paleoclimate, such as

locations of ancient deserts and glacial ice sheets, and distributions

of certain plant and animal species, to support his

ideas. Wegener’s ideas were supported by a famous South

African geologist, Alexander L. Du Toit, who, in 1921,

matched the stratigraphy and structure across the Pangea

landmass. Du Toit found the same plants, such as the Glossopterous

fauna, across Africa and South America. He also

documented similar reptiles and even earthworms across narrow

belts of Wegener’s Pangea, supporting the concept of

continental drift.

Even with evidence such as the matching of geological

belts across Pangea, most geologists and geophysicists doubted

the idea, since it lacked a driving mechanism and it seemed

mechanically impossible for relatively soft continental crust

to plow through the much stronger oceans. Early attempts at

finding a mechanism were implausible and included ideas

such as tides pushing the continents. Because of the lack of

credible driving mechanisms, continental drift encountered

stiff resistance from the geologic community, as few could

understand how continents could plow through the mantle.

In 1928 Arthur Holmes suggested a driving mechanism

for moving the continents. He proposed that heat produced

by radioactive decay caused thermal convection in the mantle,

and that the laterally flowing mantle dragged the continents

with the convection cells. He reasoned that if the

mantle can flow to allow isostatic rebound following glaciation,

then maybe it can flow laterally as well. The acceptance

of thermal convection as a driving mechanism for continental

drift represented the foundation of modern plate tectonics. In

the 1950s and 1960s, the paleomagnetic data was collected

from many continents and argued strongly that the continents

had indeed been shifting, both with respect to the magnetic

pole and also with respect to each other. When seafloor

spreading and subduction of oceanic crust beneath island

arcs was recognized in the 1960s, the model of continental

drift was modified to become the new plate tectonic

paradigm that revolutionized and unified many previously

diverse fields of the Earth Sciences.

See also PLATE TECTONICS.

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