New crust is formed at divergent boundaries on the ocean floor where the lithosphere is thin. Magma from the upper mantle presses against the plate, pushing it upward, then flows off in opposite directions at the plate. The plate, constructed of brittle lithosphere rock, is stretched by the movement of the convection and soon cracks. Magma fills the crack, cools and hardens, forming new crust. As convection continues under the plate, the rock of the new cooling crust becomes brittle and eventually cracks again, reforming the rift and pushing new crust to either side.
As new crust is formed, other plates are pushed by the spreading ocean floor. When convection pushes against land, the thicker rock layer doesn't split as easily as thin ocean plates. Convection pushes the thick plate upward, stretching and fracturing it, forming a rift. Faults develop on either side of the rift. The rift between the faults begins to sink as the gap continues to widen. Effects that are found at this type of plate boundary include: a rift valley sometimes occupied by long linear lakes or a shallow arm of the ocean; numerous normal faults bounding a central rift valley; shallow earthquake activity along the normal faults.
Volcanic activity sometimes occurs within the rift. Contributor: Hobart King Publisher, Geology. What is the San Andreas Fault? How did the Hawaiian Islands Form? Find Other Topics on Geology. Maps Volcanoes World Maps. Teaching Plate Tectonics.
Earth's Internal Structure. Divergent Boundary. Convergent Boundary. Transform Boundary. Tectonic Features Map. Divergent Plate Boundary - Oceanic. Divergent Plate Boundary - Continental. The rate of spreading along the Mid-Atlantic Ridge averages about 2. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past to million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. The map also shows Reykjavik, the capital of Iceland, the Thingvellir area, and the locations of some of Iceland's active volcanoes red triangles , including Krafla.
The consequences of plate movement are easy to see around Krafla Volcano, in the northeastern part of Iceland. Here, existing ground cracks have widened and new ones appear every few months. From to , numerous episodes of rifting surface cracking took place along the Krafla fissure zone.
Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise m before abruptly dropping, signalling an impending eruption. Between and , the displacements caused by rifting totalled about 7 m. When the continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes.
The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone. East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about million years ago. Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa the Horn of Africa a large island.
The size of the Earth has not changed significantly during the past million years, and very likely not since shortly after its formation 4. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction recycling of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks is subducted under another.
The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates. If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor.
Trenches are the deepest parts of the ocean floor and are created by subduction. Also called lithospheric plate. Also called a transform fault. Also called a conservative plate boundary.
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