This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary. Seafloor spreading, theory that oceanic crust forms along submarine mountain zones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. As the magma cools, it is pushed away from the flanks of the ridges.
The speed of spreading affects the shape of a ridge — slower spreading rates result in steep. In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading. Seafloor spreading helps explain continental drift in the theory of plate tectonics. Our Earth is a warm planet sailing through cold space.
The cold outer layer of our planet, which holds together as a rigid shell, is not made of one solid piece. Active plate margins are often the site of earthquake s and volcano es. 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 transition s from oceanic lithosphere to continental lithosphere. Passive margins are not sites of fault s or subduction zone s. 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.
Eventually, geologist s predict, seafloor spreading will completely separate the two continent s—and join the Red and Mediterranean Seas. Mid-ocean ridges and seafloor spreading can also influence sea level s. As oceanic crust moves away from the shallow mid-ocean 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 theorize d that the continents moved drifted through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity. 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 subduct s, 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. After the war, scientists pieced together the ocean depths to produce bathymetric maps, which reveal the features of the ocean floor as if the water were taken away. Even scientist were amazed that the seafloor was not completely flat.
What was discovered was a large chain of mountains along the deep seafloor, called mid-ocean ridges. Scientists also discovered deep sea trenches along the edges of continents or in the sea near chains of active volcanoes. Finally, large, flat areas called abyssal plains we found. When they first observed these bathymetric maps, scientists wondered what had formed these features.
Sometimes, for reasons unknown, the magnetic poles switch positions. North becomes south and south becomes north. During normal polarity, the north and south poles are aligned as they are now. With reversed polarity, the north and south poles are in the opposite position.
0コメント