Earth's surface is composed of two kinds of crust , continental and oceanic. Most continental crust is over 3 billion years old, while virtually all oceanic crust is less than 180 million years old. Oceanic crust is young because it is continually destroyed in some places and created in others. Subduction is the process that destroys oceanic crust, and sea-floor spreading is the process that creates oceanic crust.
Sea-floor spreading is driven by crust formation along the mid-ocean ridges , meandering undersea mountain ranges that span Earth like the seams of a baseball. Oceanic crust is continually produced by magma welling up along the centerlines of the mid-ocean ridges. This new crust flows away from each ridgeline in two symmetric sheets, one on each side. The rate of sea-floor spreading resulting from this process is from 0.5 to 8 inches per year (1–20 cm/yr), depending on the particular mid-ocean ridge.
The Mid-Atlantic Ridge offers a particularly clear case of sea-floor spreading. About 165 million years ago, the Americas were matched to Africa and Europe like the pieces of a puzzle. Then, magma upwelling at the Mid-Atlantic Ridge began to produce oceanic crust, parting the continents to form the Atlantic Ocean. Today the Mid-Atlantic Ridge snakes down the center of the Atlantic all the way from Iceland to the Antarctic Plate and remains an active site of sea-floor spreading.
A dramatic proof of sea-floor spreading was discovered in the mid 1960s when data revealed alternating stripes of magnetic orientation on the sea floor, parallel to the mid-ocean ridges and symmetric across them—that is, a thick or thin stripe on one side of the ridge is always matched by a similar stripe at a similar distance on the other side. This mirror-image magnetic orientation pattern is created by steady sea-floor spreading combined with recurrent reversals of Earth's magnetic field . Iron atoms in liquid rock welling up along a mid-ocean ridge align with Earth's magnetic field. When this magma solidifies into crust, its iron atoms lock into position. This solid crust flows away from the mid-ocean ridge in both directions, carrying its original magnetic orientation with it. Eventually Earth's magnetic field reverses. Previously solidified crust retains its original field state, but crust just forming along the ridge is locked into the new orientation. As crust feeds steadily and symmetrically away from the ridgeline and Earth's magnetic field reverses over and over again, a symmetric striped pattern of magnetism is created.
See also Geographic and magnetic poles; Lithospheric plates; Magnetic field; Mantle plumes; Mapping techniques; Ocean trenches; Paleomagnetics; Plate tectonics