Spreading centers

Volcanism occurs along spreading centers where the Earth's crust is rifting apart. This volcanism has formed the mid-ocean ridge system, the largest mountain chain on the Earth. It also contributes to the dense oceanic crust, forming ocean basins. Associated hydrothermal activity produces valuable deposits of copper, zinc, and other metals.

Mid-ocean Ridges

Plate tectonics explains that the Earth's crust is composed of relatively thin, rigid plates that float on top of the denser, less rigid mantle. Where the individual plates of crust meet, three types of motion are possible: Plates may collide, forming a convergent boundary; they may slide past each other, forming a lateral, offset boundary; or they may move away from each other, forming a divergent boundary. Along divergent boundaries, cracks or rifts in the crust are formed between the plates. Lavas made of basalt seep up and fill these cracks from below, forming new crust. Where the new basalt erupts, great ridges form.

As spreading continues, the newly formed volcanic crust is slowly forced away from the ridge, as if carried on a great conveyor belt. This motion seems to be driven by large-scale convection currents within the mantle, fueled by the release of heat from the Earth's interior. As basaltic crust moves away from the spreading center, it gradually subsides. The basaltic crust produced by volcanism at spreading centers along divergent boundaries is heavier than the granitic continental crust. Because it is denser, it sits lower on the Earth's surface and forms the great topographic depressions of the ocean basins.

This vast mountain chain extends for more than 60,000 kilometers and spans the globe. The mid-ocean ridge system covers approximately 33 percent of the ocean floor, a surface equal to that of all the Earth's continents. The segment of the mid-ocean ridge system in the Pacific basin is called the East Pacific Rise. The Atlantic segment of the ridge is known as the Mid-Atlantic Ridge and bisects almost the entire Atlantic Ocean from the Arctic to the Antarctic Circle. Most of the ridges are submerged, though a few of the highest peaks (notably Iceland and the Azores) extend above sea level as islands.

The typical mid-ocean ridge stands about 2 to 3 kilometers above the surrounding sea floor. The ridges are broader than most mountain ranges. Individual ridges are typically 1,000 to 4,000 kilometers wide at their base. Unlike most continental mountains, the ridges typically have a central rift a few kilometers wide and about 1 kilometer deep. Volcanic activity associated with spreading occurs within this central rift.

Undersea Eruptions

The basaltic lava that erupts along the spreading centers is exceptionally hot (1,000 to 12,000 degrees Celsius) and, therefore, very fluid compared with other types of volcanic flows. Consequently, these basaltic eruptions are less violent and less explosive than other types of volcanic events, and the lava is spread over large areas, similar to eruptions recorded on land in Iceland and, to a lesser extent, those of Hawaii.

At depths of more than 2,000 meters below the sea, volcanic eruptions are considerably less explosive than on land. At these depths, the water pressure is so high that the gases released during volcanic eruptions do not explode but immediately go into solution with the seawater. Consequently, below 2,000 meters, there are no explosions, only effusive outpourings of lavas. This concept is supported by the low percentage of vesicles, the spherical voids formed around gas bubbles found in deep mid-ocean ridge basalts.

At depths of less than 2,000 meters, steam, ash, and sulfuric gases may be released. However, unless volcanism occurs at very shallow depths, the seawater will absorb steam, ash, and gases before reaching the surface. Many of these eruptions go completely unnoticed by humans unless an ocean traveler notes a yellow tinge to the sea, the characteristic evidence of volcanism at depth. If the eruption occurs at shallow water depths (less than a few hundred meters), the eruption may be more explosive than if it had occurred on land because of the secondary explosions caused by the violent vaporization of the water. The eruptions of steam propel the volcanic projectiles higher and faster through the air than they would normally travel. Spectacular ash plumes known as “cock's tail” explosions may arise.

Pillow Lava and Pahoehoe

The eruptions along the mid-ocean ridge system are episodic and frequently emanate from conical vents of basalt along the central rift. These vents have been measured up to 20 meters high. In some areas, there is evidence of multiple lava flows erupting in quick succession. Many of these flows form thick layers of basaltic “pillows,” which are structures that form only underwater. Pillow lavas are cemented masses of bulbous, slightly squashed ovoids of rock. They form as the molten lava contacts the cold water (which, at the depths of the mid-ocean ridges, is barely above freezing, at 2 degrees Celsius). As the lava flows, its surface is chilled by the cold water and quickly cools into a bulge of dark, glassy rock. The still-molten interior of the mass may burst under pressure and stretch or may open the pillows, allowing small buds or toothpaste-like bulges to protrude.

Along the axis of the central rift, flows of pillow basalts have been recorded up to 200 meters high and 500 to 1,000 meters wide, particularly along the Mid-Atlantic Ridge. These flows form steep-sided, flat-topped ridges similar to the table mountains or möberg mountains formed by eruptions under glaciers.

Smooth or ropy flows of lava, known as pahoehoe, also occur within the central rift. These flows form when the slopes are too gentle for pillows to take shape. Underwater, a flow's outer 10 to 30 centimeters quickly solidifies, allowing the lava beneath to advance under this hardened shell. These flows form sheets or lava plains. Single flows emanating from spreading centers have been measured up to 20 kilometers from their source.

Sometimes, a single eruption may create a rapid outpouring of basalt that a lava pond or lake may form. Some of these lakes have been measured up to hundreds of meters long and 5 meters deep. Lava lakes may display several interesting features. In places, the surface may have caved in, forming a collapse pit. Often, there are residual hollow basalt pillars, which upheld the roof of the lava lake as it drained and collapsed. Horizontal ribs, like the rings around a bathtub, occur repeatedly on the pillars and the lake's edges. These ribs record changes in the lava level.

In addition to the flows, large, circular volcanic seamounts sometimes form near the ridge crests, particularly along the Mid-Atlantic Ridge. These may have been formed from lava with a slightly lower temperature (and therefore lower fluidity), which allowed the basalt to pile up. Some seamounts occur as pairs equally spaced from the central rift, indicating that they were probably once part of a single volcano that split and was separated.

Eruption Frequency and Volume

Along spreading centers, the amount of basalt generated and the time between eruptions may vary considerably. The East Pacific Rise is spreading relatively fast (averaging 6 centimeters per year). The frequency of eruptions at a given location is probably about once every few thousand years, and the volume of material per eruption may reach up to 200 cubic kilometers. The spreading rate along the Mid-Atlantic Ridge is approximately three times slower. Along parts of this ridge, eruptions are expected to occur with an average frequency of once every 14,000 years. Estimates of the amount of material erupted vary considerably. At one submarine location, the rate of lava being generated has been speculated to be 8,700 cubic meters per kilometer per year. Another study near Iceland suggested that ten times that amount of fresh material was being released. To put this amount into perspective, that would be enough material to cover the entire landmass of Great Britain with a meter of new basalt every year.

Although volcanism has not been directly observed on the ocean floor, scientists have seen heated fluids (over 350 degrees Celsius) spewing from chimneylike vents along the rifts in the mid-ocean ridges. These chimneys rise from layers of basaltic lava and stand up to 30 meters tall. In some areas, they coincide with the source of the most recent lava flows. Sometimes, the fluids emanating from the chimneys are black because of large quantities of sulfide precipitates. The chimneys are called “black smokers” or “white smokers,” depending on the clarity of their fluids. They are produced as seawater circulates through cracks in the basalt to the molten magma beneath. As the water heats up, it dissolves elements such as zinc, iron, copper, lead, silver, and cadmium. The hot, mineral-rich water rises and is recycled back into the ocean. Upon contact with the cold water, sulfide minerals precipitate, forming chimneys around the rising jets.

This hydrothermal activity supports an incredible concentration of previously unknown animal life, including foot-long clams, fish, crabs, and giant tube worms up to a meter and a half long. These creatures thrive on bacteria that digest the sulfide provided by the vents. This is the only known ecosystem not directly or indirectly based on solar energy.

In the twenty-first century, scientists continue to study spreading centers, noting that within the past 19 million years, there has been a global slowdown in seafloor spreading. This showdown could have implications for volcanic activity and, in turn, the release of greenhouse gases, so the continued study of active and extinct spreading centers remains vital to climatologists studying global climate change. Recent research on spreading centers has also focused on their relationship to hydrothermal activity. 

Principal Terms

basalt: a fine-grained, dark, heavy rock of volcanic origin, primarily composed of calcic feldspars and pyroxene

convection currents: the transfer of material caused by differences in density, usually brought about by heating

crust: the thin, rigid outer layer of the Earth, extending generally 35 kilometers under the continents and 10 kilometers under the oceans

fissure: a fracture or crack in rock along which there is a distinct separation

hydrothermal: related to hot water, particularly involving the production or dissolution of minerals

lava: the fluid rock issued from a volcano or fissure and the solidified rock it forms when it cools

magma: molten rock below the Earth's surface that has not erupted and, therefore, retains its gaseous components

mantle: the layer of the Earth's interior between the crust and core, either semisolid (lower mantle) or plastic and nonbrittle (upper mantle)

seamount: a submarine mountain 1,000 meters or higher, often a volcanic cone

Bibliography

Ballard, Robert B. Exploring Our Living Planet. Rev. ed. Washington, D.C.: National Geographic Society, 1993.

Chernicoff, Stanley, and Donna Whitney. Geology: An Introduction to Physical Geology. Upper Saddle River, N.J.: Prentice Hall, 2006.

Chester, Roy. Furnace of Creation, Cradle of Destruction. New York: AMACOM Books, 2008.

Decker, Robert, and Barbara Decker. Volcanoes. 4th ed., New York: W. H. Freeman, 2005.

Dott, Robert H., Jr., and Donald R. Prothero. Evolution of the Earth. 8th ed., New York: McGraw-Hill, 2009.

Fletcher, Charles. Physical Geology: The Science of Earth. Hoboken, N.J.: John Wiley and Sons, 2011.

Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed., New York: Cambridge University Press, 2004.

Frisch, Wolfgang, Meschede, Martin, and Blakey, Ronald C. Plate Tectonics: Continental Drift and Mountain Building. New York: Springer, 2010.

Garrison, Tom S. Oceanography: An Invitation to Marine Science. Belmont, Calif.: Brooks/Cole, Cengage Learning, 2010.

Haymon, Rachel M., et al. “Manifestations of Hydrothermal Discharge from Young Abyssal Hills on the Fast-Spreading East Pacific Rise Flank.” Geology, vol. 33, 2005, pp. 153-156.

MacLeod, Sarah J., et al. "A Global Review and Digital Database of Large-Scale Extinct Spreading Centers." Geosphere, vol. 13, no. 3, 2017, pp. 911-949, doi:10.1130/GES01379.1. Accessed 27 July 2024.

Rona, Peter A., K. Boström, L. Laubier, and K. L. Smith, Jr., editors. Hydrothermal Processes at Seafloor Spreading Centers. New York: Plenum Press, 1983.

Rosbacher, L. A. Recent Revolutions in Geology. New York: Franklin Watts, 1986.

“Seafloor Spreading Has Been Slowing Down.” AGU Newsroom, 14 Apr. 2022, news.agu.org/press-release/seafloor-spreading-has-been-slowing-down. Accessed 27 July 2024.

Singh, Satish C., et al. “Discovery of a Magma Chamber and Faults Beneath a Mid-Atlantic Ridge Hydrothermal Field.” Nature, vol. 442, 2006, pp. 1029-1032.