For example, the East African Rift Valley (a divergent boundary), is pushing apart the African plate, separating Somalia from the continent. Divergent boundaries create rift valleys on land and eventually ridges on ocean floors, where hotter material wells up from the mantle, cools and form new crust. A divergent boundary is where two tectonic plates are actively being pushed apart. The banding furnishes a map in time and space of both spreading rate and polar reversals.Divergent boundaries occur at the boundary between plates. This was confirmed by measuring the ages of the rocks within each band. The magnetic banding directly corresponds with the Earth's polar reversals. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley–Vine–Matthews hypothesis. Scientists had been studying polar reversals and the link was made by Lawrence W. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. (from thermal contraction and subsidence). Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper. If one views the seafloor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear.
A seafloor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. Divergent boundaries can create massive fault zones in the oceanic ridge system. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface, and the kinetic energy is thought to be sufficient to break apart the lithosphere.ĭivergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.īridge across the Álfagjá rift valley in southwest Iceland, that is part of the boundary between the Eurasian and North American continental tectonic plates.Īt divergent boundaries, two plates move away from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other. This supplies the area with huge amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area, forming large flood basalt or lava flows. Ĭurrent research indicates that complex convection within the Earth's mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries within continents initially produce rifts, which eventually become rift valleys. In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Oceanic divergent boundary: mid-ocean ridge (cross-section/cut-away view)
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