Because of complex interactions of the different plates, spreading ridges have different spreading rates, as do the plates themselves. In general, mid-oceanic ridges divide into those with high spreading rates, and those with low spreading rates. These differences in the rates of magma rise cause unique patterns of rock types. As all the new crust created at the mid-oceanic spreading ridges does not seem to expand the Earth, the older crust must somehow be returned back into the mantle.
This occurs at the plate edges along subduction zones. As oceanic crust cools down, it becomes denser so that on its boundary with adjacent continental crust, it sags below the continental crust under the push from the spreading ridges. Subduction zones also occur between oceanic plates, particularly oceanic plates of differing age. In some regions behind the oceanic plate, spreading starts when new magmas well up from the subduction of the down-going oceanic plate to give back-arc basins.
The back-arc basins themselves influence the down-going plate and deflect it towards the back-arc spreading. Down-going oceanic crust contains hydrous minerals and some trapped oceanic water , and these are released at depth.
This released water initiates more melting so that new magmas rise up through the plate and produce large volcanoes along an arc, forming island arcs.
Robert Hooke in recognised that earthquakes came from land movements and Mallet realised earthquake damage resulted from waves called seismic waves generated by the movement. A wave propagates strain through a material, which then tends to become restored to its original state Newton's Law of Equilibrium in the Universe. Earthquakes simply release strain built-up within the Earth, from plate tectonic movement or movements of molten rock. Although seismic waves spread out from disturbances of the ground, only earthquakes and nuclear explosions are large enough sources of waves to be detected around the world.
When seismic waves are generated, they spread out spherically but become distorted by regions of different density or elastic properties. The faster P-waves are the first to arrive at recording stations and the time delays between P- and S-wave arrivals help determine where the earthquake originated. It takes several such seismic stations to determine the location and size of an earthquake accurately. When two tectonic plates converge, huge frictional forces build up with time until at a critical point rupturing occurs, producing a fault and an earthquake.
Earthquakes within the ocean basins produce tsunamis but those on the continents also devastate by releasing energy in rebounding waves. The largest earthquakes are often along the collision zone of two continental plates. Most large earthquakes around the world occur along plate margins, particularly around the edges of the Pacific Plate. Many of the world's volcanoes occur along the plate margins e. Pacific Rim of Fire , but some occur wholly in isolated pockets within plates e.
The latter form from hotspots of upwelling magma that pierce the crust, forming voluminous volcanic outpourings. These stationary upwellings underlie the plate movements, so they form a line of volcanoes that migrate in the opposite direction to the plate movement. Perhaps the best known hotspot chain is the Hawaiian Islands, in which the older volcanoes finally disappear below sea level as subsided mounts.
Some of the hotspots seem to come from the Earth's core, others from less deep layers in the mantle. At present about 50 hot spots are known on Earth. Hotspots have erupted along the margin of eastern Australian and in the Tasman Sea. Because plate tectonics is a large-scale process that transfers heat, water and magmas, it underpins the formation of many mineral deposits. Since these deposits form in special plate tectonic settings, we can use our knowledge of present plate tectonic processes to search for deposits formed in the past in similar tectonic settings.
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It runs as a single structure for over km between the Puysegur Trench in the south and the Marlborough Fault System in the northeast. As the Pacific Plate is subducted below North Island, the part of the Australian Plate that makes up the central North Island is stretched and has, over many millions of years, become thinner than normal crust. Water released from the Pacific Plate deep under North Island combines with the hot rock of the Australian Plate at about km depth and causes a small amount of that rock to melt.
This molten rock rises to the surface through the thinned crust and is either erupted from volcanoes like Ruapehu, Tongariro and Ngaruhoe or sits within the crust and heats it, and the water it contains, up causing geothermal activity around Taupo and Rotorua.
The area of volcanic activity is referred to as the Taupo Volcanic Zone see map above. More information about volcanoes. The subduction zone in the north is linked to the subduction zone in the south by a series of very large faults that run through Marlborough Marlborough Fault System and down the west coast of South Island Alpine Fault.
The Marlborough Fault System is a series of subparallel strike-slip faults which run northeast-southwest. Not only does it try to pull one over on us with its pink-colored lakes and insistence that Vegemite is edible, the country—home to more than 23 million people—has actually moved. That might not sound like a major journey.
The North American plate, by contrast, travels roughly one inch per year. Thanks to various countries moving at different rates and in different directions, our models of the earth soon become out of date, if only by a few inches.
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