Continental Drift
Paleomagnetism
Seafloor Spreading

Continental Drift

As early as the fifteenth century, European scholars studying new maps of the world produced as a result of the early explorations of the new world and Africa noticed the complementary fit of the continents on opposite sides of the Atlantic Ocean. By the mid-nineteenth through early twentieth century a small number of people had proposed some form of continental motion either in terms of continental separation to explain this complementary fit or in terms of collisions to explain mountain belts. The most coherent hypothesis was that of American geography and geologist F.B. Taylor (1910). But even Taylor's proposal had little supporting evidence and no real mechanism.

Alfred Wegener (1880-1930) was a German meteorologist and geophysicist. He was also an avid balloonist and a member of three expeditions to Greenland. He died on the third expedition in 1930 possibly due to a heart attack.

Wegener's primary career interest was in meteorology. But he became interested in geology when he became aware of the very good correspondence of coastlines on opposite sides of the Atlantic. He then read of a postulated land bridge connecting Brazil and Africa to account for similarity of fossil species on the two continents. Land bridges were an ad hoc favorite of paleontologists (those who study fossils). Land bridges were postulated under the geosynclinal theory of Suess. But Wegener argued that rising and falling land bridges in the oceans were not likely considering the observation that the ocean crust was made of denser (basaltic) rock than the continents. He argued on isostatic grounds that this denser oceanic crust could not rise up above sea level. Likewise, if the land bridge was less dense (granitic) continental rock it would be too light to sink into the denser rock below.

Wegener recognized the similarity of rocks on opposite sides of the Atlantic, especially matching mountain belts. One way to account for this similarity, as for the similarity in the shapes of the coastlines, is if the continents had once been together and somehow had drifted apart.

Wegener knew of many Late Paleozoic and Mesozoic age fossil species found on widely dispersed southern continents (Africa, South America, India, Australia, and Antarctica). Fossils of a shallow water reptile, Mesosaurus, were found in both Africa and South America even though they could not swim across the Atlantic Ocean. Fossils of a family of seed ferns, Glossopteris, were known from Africa, South America, and India. When the Scott expedition to the south pole perished in a blizzard in 1912 on their return from the pole they were hauling rock samples they had picked up in the Transantarctic Mountains. The relief team that found the expedition's remains sent the samples to the British Museum. These Antarctic rock samples contained fossils of Glossopteris thus helping to relate Antarctica to the other continents. How could apparently identical species have evolved on such widespread continents with no geographic connection unless they had at one time been together and then drifted apart.

Wegener studied paleoclimate indicators in sedimentary strata. In upper Paleozoic (Carboniferous and Permian) strata he found tropical coals in northwestern Europe and glacial tillites in equatorial Africa. These anomalies could be best explained if the continents had moved (Europe from near the equator, Africa from the polar region into the equatorial region).

Wegener used all of his compiled evidence to assemble a supercontinent he named Pangea, meaning all land. Pangea existed in the late Paleozoic and began to break up during the Mesozoic. The continents have drifted apart since that time.

Wegener first presented his ideas in 1912 and they were elaborated with successive editions of his book, The Origin of Continents and Oceans, through 1929. His hypothesis that came to be known as continental drift was read with interest but also with much skepticism because there was no plausible mechanism to account for continental motions. Basic intuition wonders how a continent can be slid across the ocean against tremendous frictional forces. Wegener believed in the isostacy concept (continents afloat in the mantle) and that the problem was more akin to pushing an iceberg across the ocean. Nevertheless there were no known forces of sufficient magnitude to account for continental motions.

The British geologist Arthur Holmes worked on theories of the Earth's interior in the late 1920's. The radioactive heat that was generation in the interior was thought to be greater than heat liberated from all of the worlds volcanoes. Therefore the Earth's interior would continue to heat up unless there was some other mechanism to remove the heat. Holmes proposed that hot mantle, behaving as a very viscous fluid, would rise by convection toward the surface where it would cool and contract (become denser) and then descend back deep into the Earth. There it would heat up and expand (become less dense) and then rise again. These proposed convection currents could provide the driving force for continental drift.

Alex DuToit (1878 - 1948) was a South African geologist who added many detailed observations from the southern hemisphere. The Parana flood basalts of South America and the Etendeka flood basalts of Africa were both extruded around 125 million years ago during the Cretaceous period. They lie on conjugate margins of these continents and both are cut off at the coast as if they were once two halves of one large flood basalt province. Connecting South America with Africa in a good jigsaw puzzle fit across the Atlantic reunites these two geologic terranes into one. DuToit published his influential book, Our Wandering Continents, in 1937. In it he made a more detailed supercontinent reconstruction but he preferred to think in terms of two separate supercontinents Gondwanaland (Africa, South America, India, Australia, and Antarctica) and Laurasia (North America, Europe, and Asia).

 

The Earth's Magnetic Field and Paleomagnetism

It has been known since the time of Gilbert that the Earth has a magnetic field. The Earth's magnetic field is a dipole field like a common bar magnet. A bar magnet has two poles, a "north pole" and a "south pole." You may have seen an experiment where a sheet of paper is placed over a bar magnet, and iron filings are sprinkled on the paper. The iron filings will be arranged into curving arcs connecting the two poles following the lines of magnetic force. The lines of force of the bar magnet and the Earth originate at the south pole and curve around to reenter at the north pole. The lines of force point straight out of the magnet or Earth at the south pole. They are perpendicular to the Earth's surface at the south pole. As you move from the south toward the north pole the lines of force curve around so they become less and less steeply inclined until they are horizontal at the equator. As you continue toward the north pole the lines of force continue curving so that they point more and more steeply into the Earth until they point straight into the Earth at the north pole.

We can use these properties of dipole magnetic fields to define a geographic reference frame. The lines of force everywhere point toward the north pole. The lines of force are vertical at the poles, horizontal at the equator, inclined upward in the southern hemisphere, and inclined downward in the northern hemisphere. The angle of inclination is proportional to latitude (the higher the latitude, the steeper the inclination). From measurements of the Earth's magnetic field we could therefore determine the direction to the pole and the latitude of the measurement site.

Actually, the Earth's magnetic field is not a perfect dipole. The magnetic north pole today lies over the Canadian Arctic. But it has been shown that the magnetic field wobbles around the true pole so that over a period of 10,000 to 20,000 years the magnetic field averages out to be equal to the geographic north pole.

When rocks are formed they typically contain small amounts of magnetic minerals, like the iron oxides magnetite and hematite. The magnetism of these magnetic minerals becomes aligned with the Earth's magnetic field as the rock forms. Rocks retain, in many cases, a permanent record of this field direction. This record of the Earth's magnetic field frozen in the rock can tell us the direction to the pole and the latitude at which the rock formed. The latitude also tells us the distance to the pole.

Apparent Polar Wander Paths
During the 1950's and into the early 1960's a group of scientists studied the magnetism in rocks of various ages. Chief among them was Keith Runcorn and his former students Ted Irving and Ken Creer. The magnetism recorded in rocks formed during the past few million years was consistent with the present magnetic field. However, the magnetism in old rocks was generally found to be inconsistent with the present field. The magnetism in these older rocks pointed in directions other than towards the pole and the recorded latitudes often did not coincide with the rocks' present latitudes. The magnetic direction and inclination (or latitude) recorded in the rocks allows an apparent pole to be calculate for rocks of that age. The apparent magnetic poles for old rocks are often far removed from the geographic pole. Moreover, Runcorn showed that these apparent poles were closest to the geographic pole for younger rocks and farther from the geographic pole for older rocks. Was this because the poles had moved or was it because the continents carrying the rocks had moved with respect to the geographic pole? Runcorn compared the apparent polar wander paths of Europe and North America and found that they were different. If apparent polar wander was caused by motion of the pole while the continents remained stationary, then the apparent polar wander paths of all the continents should be the same. The best explanation for the observation that all the continents had unique apparent polar wander paths was that the continents were in motion with respect to the pole and one another.

This was direct evidence for continental drift. In fact, the paleomagnetic results from upper Paleozoic rocks fit very well with the Pangea and Gondwanaland reconstruction of Wegener and DuToit. These reconstructions bring the apparent paleomagnetic poles for late Paleozoic rocks back into agreement with the geographic pole. But these observations did not suggest a mechanism that could account for pushing the continents across oceans. Continental drift continued to be viewed with skepticism.

Reversals of The Earth's Magnetic Field
Also during the 1950's paleomagnetists, especially Hospers, discovered that the magnetization in some layers of volcanic rock pointed toward the north pole and other layers were magnetized toward the south pole. It was quickly established that these were true reversals of the polarity of the Earth's magnetic field and not some behavior of the magnetic minerals recording the field. They had discovered that the Earth's magnetic field reverses occasionally. Rocks magnetized in periods when the magnetic lines of force pointed toward the north pole are said to have normal polarity. Those magnetized when the lines of force point toward the south pole are said to have reverse polarity. Allan Cox determined the pattern of reversals during the past few million years from paleomagnetic studies of lava flows.

  

Marine Magnetic Anomalies and Seafloor Spreading

Harry Hess was a U.S. Naval officer during World War II on a destroyer escorting ship convoys to England. His ship towed a sensitive magnetometer in an attempt to detect the steel hulls of Nazi submarines that preyed on Allied shipping. He noticed that as the ship sailed over the mid-Atlantic Ridge the magnetometer recorded small fluctuations in magnetic field intensity. After the war Hess went to Princeton and studied this phenomenon. He suggested that these fluctuations were due to varying magnetizations of the ocean crust. The magnetometer recorded primarily the direction and intensity of the Earth's magnetic field but also could detect changes in the magnetization of the ocean crust. Apparently the ship sailed over some sections of ocean crust that were magnetized such that they complemented the Earth's magnetic field therefore making the recorded intensity stronger. Other sections must be magnetized in such a way as to subtract from the Earth's magnetic field strength. The sections with complementary magnetization must be sections of the crust magnetized with normal polarity like the present field of the Earth. The sections of the ocean crust whose magnetism subtracts from the main field of the Earth must have been magnetized and formed during periods of reverse polarity.

Vine and Matthews (1963) mapped the varying magnetic intensity on one side of the midocean ridge. They reported linear stripes of alternately higher and lower magnetic field intensity, marine magnetic anomalies, parallel to the midocean ridge. They were the first to completely state the hypothesis of seafloor spreading. They believed that ocean crust was continuously created at the midocean ridges by igneous intrusion and volcanic activity; the newly-formed crust then breaks in two and spreads away from the ridge. The newly forming strips of crust become magnetized alternately in normal or reverse polarity as the Earth's magnetic field reverses.

Pitman and Heirtzler (1966) mapped the magnetic anomalies across a section of the Pacific-Antarctic Ridge and the Reykjanes Ridge south of Iceland. They showed that the magnetic anomalies were symmetric about the ridge; the same pattern of changing intensity was found on both sides of the midocean ridge. This was the conclusive evidence for the seafloor spreading hypothesis.

T.J. Wilson (1965) proposed and Lynn Sykes (1967) confirmed transform faults offsetting midocean ridge segments. Seismic evidence gathered from earthquakes by Sykes showed strike-slip (side-by-side) motion on the transform faults, no earthquakes on the fracture zones, normal fault (stretching) earthquakes on midocean ridges, and thrust fault (compression) earthquakes near deep ocean trenches. Other seismologists showed that there was a descending plane of earthquakes (Benioff Zone) descending from the trenches. Volcanic arcs like the Andes and Cascades mountains and volcanic island arcs like the Mariannas and Aleutians lie over Benioff Zones, set back from deep ocean trenches. The deep ocean trenches were then locations where the ocean crust was being subducted, or returned back into the Earth's interior.

Wilson (1963) showed that islands generally got older the farther they were from the midocean ridges. He set the stage for the understanding of hot spots, chains of volcanic islands in the middle of plates, that get progressively older away from the midocean ridge, suggesting that they form as their plate moves slowly over plumes of hot rising mantle material. The Hawaiian islands are one of the best examples of a hotspot track.

The theory of plate tectonics offers a mechanism, acceptable to the physics community, that can account for the continental motions described by Wegener, DuToit, and the paleomagnetists. It accounts for all of the major features of the ocean basin and the surface of the Earth in general. Beginning in 1966-1967 it has become overwhelmingly accepted by the scientific community. It is the primary predictive tool for understanding such ongoing processes as volcanoes and earthquakes.

Central to plate tectonics is the understanding of the linkage between seafloor spreading and currents of upwelling hot mantle rock that yield the molten rock that solidifies as new crust at the midocean ridges. The midocean ridge system is the main avenue for the release of heat from the interior of the Earth. In the most general sense, Hutton was correct in his idea that the upheaval of mountains was a result of the release of heat from the Earth's interior. But upheaval of mountains is the result of continental collisions, such as the formation of the Appalachians when Pangea formed by the collision of Gondwana and Laurasia. Continental collisions are in turn the result of tectonic plate motions which result from seafloor spreading which releases heat from the Earth's interior.

 

References

S. Berthon and A. Robinson, The Shape of the World, Rand McNally, New York, 1991.

R. H. Dott, Jr., and D.R. Prothero, Evolution of the Earth, fifth ed., McGraw-Hill, New York, 1994.

A. Hallam, Great Geological Controversies, Oxford University Press, New York, 1983.

P. Kearey and F.J. Vine, Global Tectonics, Blackwell Science, Cambridge, Mass., 1996.

E.K. Peters, No Stone Unturned, Freeman, New York, 1996.