This website would like to remind you: Your browser (Apple Safari 4) is out of date. Update your browser for more security, comfort and the best experience on this site. Encyclopedic Entry seafloor spreading For the complete encyclopedic entry with media resources, visit: http://education.nationalgeographic.com/encyclopedia/seafloor-spreading/ Seafloor spreading is a geologic process in which tectonic plates large slabs of Earth's lithosphere split apart from each other. Seafloor spreading and other tectonic activity processes are the result of mantle convection. Mantle convection is the slow, churning motion of Earth s mantle. Convection currents carry heat from the lower mantle and core to the lithosphere. Convection currents also recycle lithospheric materials back to the mantle. Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fill these fractures and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock. This rock (basalt) becomes a new part of Earth s crust. Mid-Ocean Ridges Seafloor spreading occurs along mid-ocean ridges large mountain ranges rising from the ocean floor. The Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate. The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate. Seafloor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliffs and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center. It spreads 2-5 centimeters (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon. The East Pacific Rise, on the other hand, is a fast spreading center. It spreads about 6-16 centimeters (3-6 inches) every year. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop! The newest, thinnest crust on Earth is located near the center of mid-ocean ridge the actual site of seafloor spreading. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. Geomagnetic Reversals The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century. Basalt, the once-molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s. What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching stripes on either side of the ridge. The specific magnetism of basalt rock is determined by the Earth s magnetic field when the magma is cooling. 1 of 5
Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern. Geographic Features Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust. In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquakes and volcanoes. Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire, the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin. In other cases, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from oceanic lithosphere to continental lithosphere. Passive margins are not sites of faults or subduction zones. Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate (on the east coast of North America) or the Eurasian plate (on the west coast of Europe). New geographic features can be created through seafloor spreading. The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other. Today, only the Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will completely separate the two continents and join the Red and Mediterranean Seas. Mid-ocean ridges and seafloor spreading can also influence sea levels. As oceanic crust moves away from the shallow midocean ridges, it cools and sinks as it becomes more dense. This increases the volume of the ocean basin and decreases the sea level. For instance, a mid-ocean ridge system in Panthalassa an ancient ocean that surrounded the supercontinent Pangaea contributed to shallower oceans and higher sea levels in the Paleozoic era. Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years. Seafloor spreading disproves an early part of the theory of continental drift. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity. Keeping Earth in Shape Seafloor spreading is just one part of plate tectonics. Subduction is another. Subduction happens where tectonic plates crash into each other instead of spreading apart. At subduction zones, the edge of the denser plate subducts, or slides, beneath the less-dense one. The denser lithospheric material then melts back into the Earth's mantle. Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant. VOCABULARY Term Part of Speech Definition active plate margin convergent tectonic plate boundary where an oceanic plate is crashing into a continental plate. basalt type of dark volcanic rock. churn verb to mix vigorously or violently. cliff steep wall of rock, earth, or ice. consistent adjective maintaining a steady, reliable quality. 2 of 5
continent one of the seven main land masses on Earth. continental crust continental drift convection current thick layer of Earth that sits beneath continents. the movement of continents resulting from the motion of tectonic plates. movement of a fluid from a cool area to a warm area. core the extremely hot center of Earth, another planet, or a star. crust rocky outermost layer of Earth or other planet. dense adjective having parts or molecules that are packed closely together. diameter width of a circle. disprove verb to prove wrong. divergent boundary area where two or more tectonic plates are moving away from each other. Also called an extensional boundary. earthquake the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity. fast spreading center mid-ocean ridge where seafloor spreading is occuring at more than 100 millimeters (4 inches) a year. fault a crack in the Earth's crust where there has been movement. fracture verb to break. frigid adjective very cold. geologic adjective having to do with the physical formations of the Earth. geologist person who studies the physical formations of the Earth. igneous rock rock formed by the cooling of magma or lava. lithosphere outer, solid portion of the Earth. Also called the geosphere. magma molten, or partially melted, rock beneath the Earth's surface. magnetic field area around and affected by a magnet or charged particle. magnetism force by which objects attract or repel one another. magnetometer scientific instrument used to measure the presence, strength, and direction of Earth's magnetic field. mantle middle layer of the Earth, made of mostly solid rock. mantle convection Mid-Atlantic Ridge mid-ocean ridge slow movement of Earth's solid mantle caused by convection currents transferring heat from the interior of the Earth to the surface. underwater mountain range that runs from Iceland to Antarctica. underwater mountain range. molten adjective solid material turned to liquid by heat. 3 of 5
mountain range series or chain of mountains that are close together. ocean basin depression in the Earth's surface located entirely beneath the ocean. oceanic crust thin layer of the Earth that sits beneath ocean basins. ocean trench a long, deep depression in the ocean floor. Paleozoic Era about 542251 million years ago. Pangaea supercontinent of all the Earth's landmass that existed about 250 million years ago. passive plate margin lithospheric region where oceanic crust transitions to continental crust without faults or subduction zones. plastic chemical material that can be easily shaped when heated to a high temperature. plate tectonics movement and interaction of the Earth's plates. predict verb to know the outcome of a situation in advance. rift valley depression in the ground caused by the Earth's crust spreading apart. Ring of Fire horseshoe-shaped string of volcanoes and earthquake sites around edges of the Pacific Ocean. seafloor spreading rift in underwater mountain range where new oceanic crust is formed. sea level base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle. seawater salty water from an ocean or sea. sediment solid material transported and deposited by water, ice, and wind. slow spreading center mid-ocean ridge where seafloor spreading is occuring at less than 55 millimeters (2 inches) a year. subduct verb to pull downward or beneath something. subduction zone area where one tectonic plate slides under another. supercontinent ancient, giant landmass that split apart to form all the continents we know today. symmetrical adjective having the same arrangment of parts on either side. tectonic activity movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes. tectonic plate large, moveable segment of the Earth's crust. theorize verb to formulate and propose a group of ideas to explain a scientific question. transition movement from one position to another. transitional crust lithospheric region where oceanic crust transitions into continental crust. volcano an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. volume space an object occupies. 4 of 5
For Further Exploration Articles & Profiles National Geographic News: Oldest Rocks on Earth Discovered? National Geographic News: Seafloor Still About 90 Percent Unknown Audio & Video National Park Service: Plate Tectonics Animations 1996 2015 National Geographic Society. All rights reserved. 5 of 5