Seafloor spreading

Seafloor spreading is the mechanism of continental drift. Convection currents above magmatic plumes in Earth's mantle pull the solid crust apart, allowing new material to well up and solidify. Driven by magmatic convection, the sea floor spreads until it encounters a continental plate and is forced into the mantle. The motion of seafloor material pushing sections of continental plates together leads to the formation of exotic terranes along the western coast of North America and elsewhere, characterized by seismic faults and volcanism.

Origins of the Theory of Seafloor Spreading

The idea of seafloor spreading was preceded by German geophysicist Alfred Wegener's 1910 theory of continental displacement, or continental drift, in which Wegener posited that Earth's continents had once been conjoined into a single supercontinent, which he called Pangaea. In time, the Pangaea segments drifted apart, eventually taking up their current positions. Wegener's theory was met with disdain and ridicule until well after his death. The primary failing of the theory was that it contained neither a notion of a mechanism whereby such a process could take place nor a notion of a driving force for such displacement.

Not until the 1950s was the first evidence to support Wegener's theory observed. US Navy and civilian field studies observed the paleomagnetic signature embedded in seafloor bedrock formations. The Navy's mission, which crisscrossed sections of the Pacific Ocean to map the topography of the seabed, was accompanied by a civilian crew who mapped the Pacific seabed's paleomagnetic signature. This mapping revealed numerous geomagnetic bands with opposite magnetic polarity. While this was not a surprise in itself because magnetic pole reversal in rock formations had been observed before, it was surprising that the bands could be matched precisely across seismic fault lines, indicating that the bands had shifted in time from the demarcation point of that fault line.

Oceanic topography mapping in the Atlantic Ocean revealed the presence of the Mid-Atlantic Ridge, a line of volcanically active sea floor that essentially bisected the Atlantic Ocean. Magnetic anomaly mapping across this feature revealed a symmetrical pattern of geomagnetic bands cataloging some 171 magnetic pole reversals in 76 million years. In 1960, American geologists Harry Hammond Hess and Robert S. Dietz, working independently, concluded from these geomagnetic observations that new seafloor material was being generated by the continuous upwelling of magma from Earth's mantle at mid-oceanic ridges. Coupled with a theory put forward by British geologist Arthur Holmes in 1945, which suggested magmatic convection currents as the means by which the new crustal material was carried away from ridges and subducted back into the mantle at deep ocean trenches, the overall mechanism was termed “seafloor spreading.” This discovery provided the final vindication of Wegener's theory of continental displacement by revealing the mechanism missing in Wegener's hypothesis.

The Structure of the Earth and the Mechanism of Action

The theories of continental drift and seafloor spreading are intimately bound into plate tectonics and concepts of the Earth's interior structure. The Earth's inner core is believed to be a sphere of solid nickel-iron at a temperature well above its melting point but kept in the solid state by gravity and the pressure of the surrounding material. An outer core of liquid nickel-iron surrounds the inner core. Seismic investigations show that the outer core may not be spherical but instead covered with radial prominences. The remaining material, extending practically to the planet's surface, is the mantle, composed of molten rock in a plastic state. The final outer layer of the planet's structure is the crust, a mere 70 to 80 kilometers (40-50 miles) thick across the sea floor and nearly twice that size across the continental plates. The thickness of the crust is only about 1 percent of the planet's radius, roughly the same proportion as the shell of an egg.

Hot spots are formed in the mantle above the various prominences that dot the outer core, increasing the plasticity of the magma in those locations and giving rise to volcanic activity. The Hawaiian Islands, the South Pacific Island chains, and the infamous volcano Krakatoa in Indonesia are formed atop such hot spots. Elsewhere, as at the Mid-Atlantic Ridge and the mid-Pacific rise, the convective movement of magma within the mantle drives the material upward with sufficient force to break the crustal surface. At those locations, magma continually and slowly oozes against the pressure of thousands of meters of seawater, forming new crustal material as it freezes. The convective motion of the underlying magma drives the new seafloor material from the ridge structure where it was formed and eventually into the continental plates on either side.

The mechanism is slightly different at the mid-Pacific rise because of the more intense hot spot beneath the Hawaiian Islands and the much more extensive, relatively thin crust that makes up the Pacific Ocean floor. The rate of spreading may be limited to some extent in the Atlantic Ocean by the relative nearness of the American and African continents to either side. This effect is not present in the Pacific Ocean, and the rate of seafloor spreading is as much as ten times greater there. This rate indicates that considerably more magmatic material is being extruded from the mid-Pacific rise; also, the lack of restraining pressure allows the magma over the Hawaiian hot spot to roll out almost continuously, building up volcanic shield seamounts in the process. The effect of seafloor spreading in this area is evidenced by the chains of volcanic islands in different regions of the Pacific Ocean.

The Hawaiian Islands comprise a series of seamount islands that have formed over the stationary hot spot that currently provides the energy for the activity of Kilauea, Maunaloa, and other active volcanic sites in Hawaii. The island chain began to form from the movement of the active vents away from the hot spot as the sea floor moved westward from the mid-Pacific rise; the slow turning movement of the Pacific plate caused the curve of the chain. Other Pacific Island chains resulting from the combination of these movements include the Javanese island system, home to Krakatoa, and the East Pacific chain, which consists of the volcano Pinatubo in the Philippines.

When the seafloor material approaches the continental plates, it meets the resistance of the continental mass's more massive, though less dense, material. The natural tendency is for the lighter material to float atop the denser material in isostatic equilibrium. The now-downward convective movement of the mantle magma augments this. As a result, the spreading sea floor is subducted beneath the continental plate to become part of the magmatic mantle once again. Subduction zones are characterized by deep oceanic trenches where the ocean-side sea floor trends downward into the mantle, and the continent-side sea floor is pushed upward by the material sliding beneath it.

Terranes and Fault Zones

The movement of seafloor material from mid-oceanic source ridges and toward subduction zones has had some significant effects on the structures of the continents. It can be assumed that the breakup of Pangaea and its daughter segments, Laurasia and Gondwana, was accompanied by several small subcontinental fragments that separated from the main mass. Inevitably, these relatively small fragments were carried toward the major continental plates by the seafloor spreading mechanism to locations far removed from their points of origin.

The fragments' greater size at the subduction zone brought them up against the continental plate instead of into subduction. The force of the moving sea floor inexorably drove the smaller fragments against the continental mass, as it still does, essentially welding them and driving up high mountain ranges. These regions, called terranes, are notable for their essential geological difference from the main continental mass, sometimes composed of entirely incongruous structural features. The junctures of these smaller fragments and continental plates are characterized by active seismic faults and volcanic activity fueled by the displacement of material in the subduction zone. The fault lines that permeate terranes tend to run parallel to the motion of seafloor spreading, with the San Andreas fault in California being a notable example.

Plate tectonics, however, does not only involve seafloor spreading. Regarding the Earth's entire crust, several tectonic plates other than the continental plates exist and are in motion, driven by the same magmatic convection forces that drive seafloor spreading. The largest of these plates, the Pacific, is also affected by the planet's rotation and, correspondingly, has a rotational motion of its own. This additional motion of tectonic plates creates transform faults because of the shearing force between moving plates. At these locations, new seafloor material is neither produced nor removed.

Fault boundaries are classified as either convergent or divergent. The Mid-Atlantic Ridge and mid-Pacific rise are divergent boundaries, where solid plate material, such as the sea floor, is driven to separate, allowing new material to form from upwelling magma. The deep trenches of subduction zones are convergent boundaries where solid plate material is forced down into the mantle; here, it is melted again and then circulated into the overall motion of the magma. Convergent boundaries destroy crustal material, and divergent boundaries create crustal material or sea floor.

Subduction Zone

The difference in the direction of movement of tectonic plates and the sea floor as it spreads gives rise to a different form of seismic activity: the earthquake. Plate edges and shapes are irregular and do not slip smoothly past each other. The lateral relative movement of plates is interrupted as plate edges bind, generating lateral torsional stress. Earthquakes and related seismic activity in fault zones are generated when the torsional stress exceeds the resistance of the binding sites, and the plates surge forward in their normal movement. This type of plate movement generates continuous minor earthquakes, with few being sufficiently powerful to cause damage.

Seafloor spreading generates seismic activity in an entirely different manner because of the movement of the sea floor toward the mantle in a subduction zone. The regular motion of the sea floor is orthogonal to the edge of a plate rather than with the parallelism of lateral slippage. Accordingly, when binding occurs, there is a buildup of pressure between the advancing sea floor and the leading edge of the continental plate. This type of binding is not relieved by lateral slippage but by the sudden release of pressure as the sea floor suddenly breaks free and plunges to resume its regular motion. At the same time, the edge of the continental plate lurches vertically to ride up on top of the seafloor plate, then downward and toward a new isostatic balance. The energy released by the movement is often orders of magnitude greater than that released by horizontal slippage earthquakes.

Complicating the effect is the mass of water lying above the active fault. The sudden movement of the subducting plate is accompanied by a corresponding displacement of the water mass, generating a series of waves of mass equal to the mass of water that was displaced. This wave and its echoes come to shore as a tsunami that can result in a great deal of damage. To maintain the wave's energy, its height increases as the water depth below it decreases. Depending on the topography of the wave's path, a tsunami can attain a height of several meters (more than 6 feet), with the geological record of some locations indicating that tsunamis more than 300 meters (1,000 feet) in height have been produced.

The movement of seafloor spreading in the Atlantic Ocean threatens the collapse of one of the Canary Islands along an existing fault line. If that collapse occurs, it is expected to generate a tsunami of 200 meters (more than 650 feet) or more in height that will strike the eastern seaboard of North America, obliterating many major cities. Although seafloor spreading has slowed significantly over the past 19 million years, the process remains integral to contemporary issues, such as the rising sea level and the carbon cycle. Faster seafloor spreading rates induce volcanic activity, which, in turn, releases greenhouse gases into the atmosphere. Understanding these phenomena gives insight into past atmospheric conditions and also plays a vital role in the mitigation of global climate change. 

Principal Terms

asthenosphere: a thin boundary layer between the earth's solid crust material and the molten magma of the Earth's mantle

craton: a region of unaltered original magmatic crustal material, typically as continental shield structures

hot spot: the crustal region above a large rising plume of magmatic material in the mantle, presumed to be caused by an intense prominence of the outer core

magmatic: having its origin in the melted rock, or magma, of the planet's mantle layer

paleomagnetism: the planet's magnetic polarity signature embedded in molten rock as it cools and solidifies

plutonic: having its origin and focus on the planet's mantle in the asthenosphere below the level of the crust

seamount: a mountain formed under the sea through volcanic processes

subduction: the movement of dense crustal material back into the mantle as it is directed downward against the less dense material of the continental plates

terrane: a coastal subsection of a continent, formed by the accretion of a small continental mass or plate to a larger one under the influence of seafloor subduction movement; generally, a fault-ridden rock mass relocated from its point of origin and unrelated to adjacent rock structures

volcanic: having its origin within the crust, above the level of the asthenosphere

Bibliography

American Geophysical Union. “Seafloor Spreading Has Been Slowing Down.” Phys.org, 14 Apr. 2022, phys.org/news/2022-04-seafloor.html. Accessed 21 July 2024.

Chester, Roy. Furnace of Creation, Cradle of Destruction: A Journey to the Birthplace of Earthquakes, Volcanoes, and Tsunamis. New York: Amacom Books, 2008.

Doubek, Joshua. “Seafloor Spreading.” National Geographic Society, 29 Nov. 2023, education.nationalgeographic.org/resource/seafloor-spreading. Accessed 21 July 2024.

Karner, G. D., G. Manatschal, and L. M. Pinheiro. Imaging, Mapping, and Modelling Continental Lithosphere Extension and Breakup. London: Geological Society of London, 2007.

Kearay, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3rd ed., Hoboken, N.J.: John Wiley & Sons, 2009.

Merrill, Ronald T. Our Magnetic Earth: The Science of Geomagnetism. Chicago: University of Chicago Press, 2010.

Monroe, James S., Reed Wicander, and Richard Hazlett. Physical Geology: Exploring the Earth. 6th ed., Belmont, Calif.: Thomson Higher Education, 2007.

Pinet, Paul R. Invitation to Oceanography. 5th ed., Sudbury, Mass.: Jones and Bartlett, 2009.