when 2 plates of the earths crustpush against each other,what 2 things happen?
363137
I think Stillkelsie and GruHairy have the best answers.  \n\nIf one (or both) of the colliding plates are oceanic plates (thinner and a bit denser), one will slide underneath the other and be pushed down.  You'll typically get a subsea trench where the two plates meet, such as the Marianas Trench in the western Pacific Ocean.  After the sliding's gone on long enough, some of the "pushed-down" plate will melt, and the lighter melted rock will try to rise to the surface, where it can form volcanoes or cool underground as granites and such.\n\nIf two continental plates (thinner and less dense than oceanic plates) collide, they'll tend to mash each other and "pile up".  The Himalaya Mountains, earth's tallest, were formed when the Indian continental plate collided with Asia.\n\nYou get earthquakes associated with both types of collisions.
561068
An earthquake occurs where the stress and strain on the fault pane ruptures or slips. If we take the San Andreas fault for an example, it is at the edge of two tectonic plates. One of these plates is moving up the other down. Both plates are loading the fault plane with massive amounts of stored energy which when it reached a critial point 'gives way'. Think of it this way, you have two american football players (these can each be a side of the fault plane) facing each other and they get down to tackle (this cane be the stress and strain). There will come a point where one player gives way to the other player; this is the critial point where the strain and stress becomes too much to handle, the result an earthquake! However I gave the example of the S.A fault which is at a plate boundry, but bear in mind that eartquakes can occur where ever there are weakness in the rocks.
640247
The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. \n\nOnly lithosphere has the strength and the brittle behavior to fracture in an earthquake. \n\nThe map below locates earthquakes around the globe. They are not evenly distributed; the boundaries between the plates grind against each other, producing most earthquakes. So the lines of earthquakes help define the plates: \n\n\nIn cross section, the Earth releases its internal heat by convecting, or boiling much like a pot of pudding on the stove. Hot asthenospheric mantle rises to the surface and spreads laterally, transporting oceans and continents as on a slow conveyor belt. The speed of this motion is a few centimeters per year, about as fast as your fingernails grow. The new lithosphere, created at the ocean spreading centers, cools as it ages and eventually becomes dense enough to sink back into the mantle. The subducted crust releases water to form volcanic island chains above, and after a few hundred million years will be heated and recycled back to the spreading centers.
950622
the tectonic plates that shift happen  underground, if your lucky to see a plate shift at ground level your truly lucky, these plate shifts cause earthquakes so please be careful.
797755
It happens in the brittle part of the crust, which is not usually deeper than 20 km.\n\nHere's the jist:\n\nIt all comes down to stress and strain. It is not 'the plates rubbing together' as many people say; or at least that's a very simplistic view.\n\nPlate tectonics causes stress on the continents and oceans, all over the surface of the earth. In some places, the stress is very small (usually within the plates). Elsewhere, the stress is high, usually where the plates meet each other. Since each plate moves, when two come in contact, the stress each other. They can push on each other and cause compressional stress, they can pull with extensional stress, and they can slide past or shear each other with tensional stress.\n\nFaulting, causing earthquakes, comes from the fact that this stress is building up all the time, but rocks and continents are strong materials. Just like hitting a rock with a small hammer, you do put stress on it, but a small amount. It would take a sledgehammer to put enough strain (effects and accumulation of stress) to build up and cause breakage. In the earth, the area around an active fault builds up strain from the stress of plate tectonics. Most faults become locked, because of this strength, and thus can not release their strain. Away from the fault, the stress produces very small and slow movement of the rock masses as a whole. Eventually, the strain is too much and the rest of the plate has moved too far and the fault releases the strain build-up all at once in a big stress release called an earthquake. This is called the elastic rebound theory, and it explains most (but not all) movements.\n\nThis is why an earthquake's size is relative to the fault size. The bigger the fault, the bigger the strain build up, and the bigger the release in an earthquake. Subduction zones and collision zones, where large portions of plates actually can rub together as a whole, have the really big earthquakes, like the 2004 Sumatra earthquake. In other plate boundaries, the entire plate boundary does not act together, so the faults become spread out and many faults take up the strain from the tectonic stress, like with the San Andreas Fault in California (it only takes up ~3/4 of the stress between the plates).\n\nEarthquakes are really tricky things; there is still so much we need to learn. As of now, there is no way to predict them, but we can say where the danger is highest and about how long between events.
695195
Convergent boundaries occur where two plates are being pushed together and collide. This is sometimes called a "destructive plate margin". One thing to keep in mind is that when subduction occurs, the denser plate (usually the older one) generally is considered to be the one that is pushed into the hot asthenosphere. Its rocks are melted and the resulting hot magma migrate upward along cracks and weakened areas in the overriding plate forming volcanoes. Of course, you always get earthquakes along with this process. The regions where the earthquakes occur are called the Wadati-Benioff zones. However, what actually happens depend on the composition of the crustal plates involved.\n\nOceanic colliding with continental crustal plates: An example of this is the subduction of the denser oceanic Nazca Plate under the South American Plate, pushing limestone strata upward to form the towering Andes and generating destructive earthquakes. Another example is the Cascadian subduction zone off the northwest coast of the U.S. you can find volcanoes along the subduction zones. Deep trenches are formed as a result of subduction.
616929
1) when the plates collide - eg himalayas\n2) when the plates pull apart - eg mid oceanic ridge, it is underwater but still a mountain chain\n3) when one subsides under another, when that plate melts, it can cause a chain of volcanoes, which can also make mountains - pacific rim
742010
Earthquakes: \nIt all comes down to stress and strain from the interaction of different plates, chiefly at the boundaries. It is not 'the plates rubbing together' as many people say; or at least that's a very simplistic view.\n\nPlate tectonics causes stress on the continents and oceans, all over the surface of the earth. In some places, the stress is very small (usually within the plates). Elsewhere, the stress is high, usually where the plates meet each other. Since each plate moves, when two come in contact, the stress each other. They can push on each other and cause compressional stress, they can pull with extensional stress, and they can slide past or shear each other with tensional stress.\n\nFaulting, causing earthquakes, comes from the fact that this stress is building up all the time, but rocks and continents are strong materials. Just like hitting a rock with a small hammer, you do put stress on it, but a small amount. It would take a sledgehammer to put enough strain (effects and accumulation of stress) to build up and cause breakage. In the earth, the area around an active fault builds up strain from the stress of plate tectonics. Most faults become locked, because of this strength, and thus can not release their strain. Away from the fault, the stress produces very small and slow movement of the rock masses as a whole. Eventually, the strain is too much and the rest of the plate has moved too far and the fault releases the strain build-up all at once in a big stress release called an earthquake. This is called the elastic rebound theory, and it explains most (but not all) movements.\n\nThis is why an earthquake's size is relative to the fault size. The bigger the fault, the bigger the strain build up, and the bigger the release in an earthquake. Subduction zones and collision zones, where large portions of plates actually can rub together as a whole, have the really big earthquakes, like the 2004 Sumatra earthquake. In other plate boundaries, the entire plate boundary does not act together, so the faults become spread out and many faults take up the strain from the tectonic stress, like with the San Andreas Fault in California (it only takes up ~3/4 of the stress between the plates).\n\nEarthquakes are really tricky things; there is still so much we need to learn. As of now, there is no way to predict them, but we can say where the danger is highest and about how long between events.
752486
These are all related to the crusts. \n\nA divergent boundary occurs when two Earth tectonic plates move away from each other, causing a depression or a range. \n\nSubduction occurs underwater when seawater tectonic plates converge into continental plates. Since seawater tectonic plates are more dense than continental plates, it 'subduct's and goes under. \n\nThere's more than 3 types of magma, depending on its temperature, density, and composition.\n\nAs you know, the plate tectonics theory says that the Earth's crust is moving constantly. By applying the observations of divergent boundaries through time (ex, Great Rift Valley created over centuries), that will -support- and -clarify- the plate tectonics theory. So, divergent boundaries being observed serves as evidence and support for the theory.\n\nSubduction occurs in sea-floor spreading, which explains why crusts closer to the mid-atlantic ridge are "warmer" and "younger", while the ones further away, and most likely closer to a subduction zone is colder. This shows that the Earth's crust is once again moving, once again indicating that the theory is true. \n\nMagma is formed when divergent boundaries move away from each other, and is replaced by new crusts formed by magma. The cycle of sea-floor spreading undergoing subduction, sinking under the continental crust, melting into magma, then re-forming  at the Mid-Atlantic ridge.
840305
The plate which is being subducted under the other plate carries down volatiles with it (carbon dioxide, water etc from in the clay and sediments and rocks and stuff) - and these volatiles allow melting to occur in the mantle at a lower temperature than it would ordinarily do. Some of this melt (magma) may then rise up through the overlying crust (so on the overriding plate, not the one which is being pushed under) and erupt out at the surface as lava... so forming volcanoes.
1089394
earthquakes occur when the tectonic plates that cover the earth noticably shift. These movements can also form volcanoes and trenches
305725
Yes, the seismic activity that causes one can also cause the other.  When the Earth's techtonic plates shift but experience some resistance, they shake, jump, rattle the area around that resistance.  Sometimes a crack in the Earth's crust is created or widened which creates the eventual environment for a volcano.  Othertimes volcanoes are created over a hot spot (thin crust) in the Earth's crust, like the Hawaiian islands.  This is somewhat different, although some tremors can be felt.
958787
Mantle convection moves the plates\n\nAt plate boundaries, there are many forces at work: ridge push, slab pull, trench suction, slab roll back and basal shear resistance to name a few.\n\nThe links go into more detail about the forces and where they working.
123973
Giant rock plates pushing against each other suddenly slip causing the earth to move, sending out ripples like in a pond.\n\nChildren can recreate the action of a fault slipping with their hands.  Place both hands together, palm-to-palm.  Push the palms together.  While pushing the palms together, slide one hand forward away from the body while sliding the other hand back toward the body.  At first, nothing will happen.  As force is added to the forward-backward movement, the hands will suddenly slip, much as the earth does during an earthquake.
809527
I think its called 'subduction', where one tectonic plate goes under another. It occurs at 'subduction zones', such as on the west coast of the United States. The reason it occurs is that one plate is expanding and the edges need to go somewhere (under another plate). For example, the Atlantic plate is expanding from the center because there is a north-to-south line of underwater volcanoes, spewing out magma and forcing the 2 halves of the plate apart to the west and east.
89723
Some are moving away from each other (i.e. their separation is increasing), such as Africa and South America. Some are moving closer together, like North America and Asia. Some places continents or pieces of continents are moving past each other, like in the Pacific Northwest, or in the Afar Triangle in Somalia. Some places are expanding, like Iceland. \n\nThere are lots of ways to measure this, but the first clue that something was going on came from looking at the shapes of coastlines. Look at the coastlines of western Africa and eastern South America. They look like they could fit together, like a puzzle. The rocks match each other on both continents too, as if the two continents were once fit together. \n\nHowever, the big clue that something was going on came from magnetic stripes on the sea floor. As new basalt forms in the middle of the Atlantic, and some other oceans, it pushes out from the central mid-ocean ridge, and cools. When it cools, it sets, taking on a magnetization aligned with the magnetic field of the earth at that moment. However, the earth's magnetic field flips every few thousand years (just like the Sun's does, every 11.5 years or so). This creates a record of magnetic stripes on the sea floor that alternate polarity. When the scientists Vine and Mathews saw this, they knew they had proved that plate tectonics existed, and that the continents were moving (sometimes away from each other, and sometimes towards each other or past each other)
149677
Subduction zones occur where oceanic crust collides with continental crust.  Since oceanic crust has more density, it is always subducted underneath continental crust.