The continual process of seafloor spreading separated the stripes in an orderly pattern. Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling. 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. 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. The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. The newest, thinnest crust on Earth is located near the center of mid-ocean ridges-the actual site of seafloor spreading. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop! It spreads about 6-16 centimeters (3-6 inches) every year. The East Pacific Rise, on the other hand, is a fast 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 Mid-Atlantic Ridge, for instance, is a slow spreading center. Rapidly spreading ridges have a much more gentle slopes. Slowly spreading ridges are the sites of tall, narrow underwater cliffs and mountains. Seafloor spreading is not consistent at all mid-ocean ridges. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic 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 Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate. Seafloor spreading occurs along mid-ocean ridges-large mountain ranges rising from the ocean floor. This rock ( basalt) becomes a new part of Earth’s crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock. Hot magma fueled by mantle convection bubbles up to fill these fractures and spills onto the crust. The less-dense material rises, often forming a mountain or elevated area of the seafloor. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. Seafloor spreading occurs at divergent plate boundaries. Convection currents also “recycle” lithospheric materials back to the mantle. Convection currents carry heat from the lower mantle and core to the lithosphere. Mantle convection is the slow, churning motion of Earth’s mantle. Seafloor spreading and other tectonic activity processes are the result of mantle convection. Seafloor spreading is a geologic process in which tectonic plates-large slabs of Earth's lithosphere-split apart from each other.
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