How is continental crust formed?
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Basically one of three ways. These are extrusion, intrusion, and accretion. \n\nExtrusion is the process of volcanism. Volcanoes erupt molten rock that may have originated as subducted oceanic crust or deeper rock. This magma adds huge volumes of rock to the crust as it erupts and cools into solid rock. \n\nIntrusion is a similar process, but no eruption takes place. Large bodies of molten rock float upward over long time periods through the crust. When these cool and harden they are called plutons or batholiths (depending on size-batholiths are larger). These form large parts of the crust.\n\nAccretion is the other method. In some instances plate tectonics can force oceanic plates up and over continental plates. These often form what are called ophiolites where we can examine oceanic crust on dry land. Large parts of continents or islands are often forced against continental plates. This can force sediments and oceanic crust upward creating what are called 'accretionary terrane'. Much of California is an accretionary terrane, built of islands in a fore arc subduction zone that were forced up against a paleocoastline that once was near the eastern border of present day California. \n\nHere two other explanations similar to my own that illustrate these processes:\nhttp://www.physicalgeography.net/fundamentals/10j.html\nhttp://earth.leeds.ac.uk/assyntgeology/extra_info/ehistory.htm
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The Mid-oceanic ridges are the largest mountain chain on the earth, and in some places they go under continents. The Atlantic Ocean part of this ridge has one section, the Island of Iceland, where the ridge is above the surface of the ocean. The ridge is where the convection of the mantle is rifting the crust, making new oceanic crust in the process. Since the oceanic crust is denser than the continental crust (3.3 g/cc vs. 2.8 g/cc), the oceanic crust "sinks" in relation to the continental crust.  Since the surface of the Earth contains abundant amounts of liquid water, oceanic crust is submerged.
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The Himalayan Mountains were/are still being created by compressional stresses between two different continental plates "colliding" together. This is also known as a convergent boundary.\n\nThe Indian plate is moving northward into the Eurasian plate. When two continental plates "collide" neither one will be subducted because both are of about the same density. Because of this, the Indian plate continually pushes into the Eurasian plate causing deformation of both crustal plates. The plates buckle from the pressure and stress being applied over such long periods of time and essentially they have no where to go but "up".\n\nNot to be disrespectful...but the San Andreas Fault is a transform or strike-slip fault where two blocks of rock slide horizontally past each other in opposite directions. There is no subduction taking place at all. Also, continental crust cannot be subducted under continental crust because they are of equal density.
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Divergent Boundaries...\n\nContinental/Continental: rift valleys i.e. East African Rift Valley\n\nOceanic/Oceanic: new crust is being formed on the ocean floor at oceanic ridges. The Mid-Atlantic Ridge is an example. The U.S. moves about 5cm away from Europe each year b/c of this.\n\nConvergent Boundaries...\n\nContinental/Continental: Both are of the same density & both are less dense than the molten material beneath so neither will be subducted. They form folded mountain chains such as the Appalachians of the U.S. & the Alps of Europe.\n\nContinental/Oceanic: Ocean crust is much denser & will be subducted under the continental. Friction from plate movement underneath the crust will cause melt & the crust will thin causing some volcanic mountains to appear.\n\nOceanic/Oceanic: One of the oceanic crusts will be subducted & a trench will be created. Volcanic island arcs are associated w/ this type of boundary.
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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)
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The continental plates float on the LITHOSPHERE because the continents, including the mountain ranges, are composed of LIGHTER materials than the partial BASALT of which the seafloors are composed and of which the lithosphere is composed.  Even GRANITE weighs less.\n\nThe lithosphere is very hot in comparison to the crust of the earth.  The materials there are 'plastic' and flowing.  this enables crustal adjustments to granitational, earthquake and other stresses.\n\nIt is all somewhat similar to bread floating on the top of pea soup (representing the molten sea of cushiony basalt).  If you put two large pieces there (continents), ones that fill nearly the whole surface, and if you push one down, the other will rise.\n\nWith erosion onto the sea floor continental masses also become lighter.  This can make land masses rise.  The pressure of the ocean waters also keeps the continents up.\n\nThe heavier ocean floor plates dive under the continental plates where these two thrust into one another.  There are such areas around the Pacific rim.  Deep marine trenches are associated with these.\n\nLook on the internet for tectonic plates, deep marine trenches, basalt (also from Hawaiian volcanoes) and you will find lots of info.
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Well believe it or not it is the composition of the two. \ncontinental crust is "typically" silicic, meaning it is composed largely of silica rich minerals, oceanic crust is more commonly "mafic" meaning it is composed of minerals with abundant Iron and Magnesium.  This oceanic crust is heavier.\nBecause of this the continental crusts are "floating" higher than the oceanic crusts.  When the two converge (i.e. along the west coast of south america for example) typically the lower oceanic crust is subducted beneath the continental crust..\nCool huh
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The reponse is related to plate tectonics. The earth crust is composed of a larger number of plates that float, very slowly of course, over the liquid interior. Continents therefore move relative to each other. When two plates meet, generally one moves under the other and it is here that we find mountains and volcanoes.\n\nWhat relation with oceans? Well, between some of the plates there is an upwelling of new magma, new crust. This is for instance the case in the middle of the Atlantic Ocean. The new crust needs room and pushes the plates on the left and right away: the Americas are moving to the west, while Europe and Africa are moving to the east. \n\nOceans (ah, at last!) are watermasses in which such upwelling of fresh crust takes place. An ocean therefore is partly in two or more different plates. On the other hand, seas are bodies of water that are on one continental plate only, or on two plates were no new crust is formed.\n\nHere is a pictures of the different continental plates:\n http://en.wikipedia.org/wiki/Image:Plates_tect2_en.svg
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When plates are diverging they are pulling apart and magma can come up from under the crust to fill the gap. This can create mid-ocean ridges, like the Mid-Atlantic Ridge, which is an underwater mountain range that runs about the whole length in the center of the Atlantic ocean.
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In a subduction zone, one plate is being pulled under another. When continental crust & oceanic crust are involved, the oceanic crust is subducted b/c it is denser.\n\nThis type of stress is called compressional stress.\n\nIn the case of ocean ridges, like that found in the middle of the Atlantic Ocean, they are being pulled apart. That is a tensional stress. New crust is constantly being created there.\n\nIn the case of the San Andreas Fault, which is in the news a lot lately, it is two plates that are moving laterally. They are sliding past each other. This is a shearing stress.
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I will answer first the easy one. The oldest rocks are found in the pacific plate, the ones near the marianes trench, but their age is not 4.6 bya. Because old rocks return inside the earth through subduction zones. \nRefer to rock cycle.\nNew rocks are produce at the MOR (mid-oceanic ridges) and old ones are consumed at the subduction zones.\nThere are no very old rocks in continental crusts because they were weathered and eroded and brought to seas through rivers.\nNow about fossils. Most fossils are found in sedimentary rocks. Sedimentary rocks forms when sediments accumulates and cemented. Like lakes, and seas. Usually these place occur life. For example a leaf fall from a tree and left at the bottom of the river. Sediments pile over the leaf and thus preserved.\nIf lava piled over a life (like a leaf) the life or existence will burn and thus not preserved.\nFor fossil to occur it must be preserved. Its very difficult to preserved something if heat is involved.
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I'm not going to write this in 6 sentences for you but...\n\nIgneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes -- an increase in temperature, a decrease in pressure, or a change in composition.
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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.
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Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from either the Earth's mantle or pre-existing rocks made molten by extreme temperature and pressure changes. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. The word "igneous" is derived from the Latin ignis, meaning "fire".\n\nIgneous rock are classified according to mode of occurrence, texture, chemical composition, and the geometry of the igneous body.\n\nIgneous rocks make up 95% of the Earth's upper crust.
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Subduction zones occur where oceanic crust collides with continental crust.  Since oceanic crust has more density, it is always subducted underneath continental crust.
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Like most of the interior of the earth, the movements and dynamics of magma are poorly understood. However, it is known that an eruption usually follows movement of magma upwards into the solid layer (the earth's crust) beneath a volcano and occupying a magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet surface as lava, or the rising magma can heat water in the surrounding landform and cause explosive discharges of steam; either this or escaping gases from the magma can produce forceful ejections of rocks, cinders, volcanic glass, and/or volcanic ash also known as tephra. While always displaying powerful forces, eruptions can vary from effusive to extremely explosive.\n\nMost volcanoes on the land are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust. As the oceanic crust 'subducts', it is forced into a higher-temperature region called the asthenosphere, and melts. Friction between these moving plates may also be a cause of melting. The reduced density of the melting rock forces the newly formed magma to rise. As the magma rises through weak areas in the continental crust it may eventually erupt as one or more volcanoes. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate. The oceanic crust is normally a highly-'mafic' (iron and magnesium rich) rock, but as it rises through granitic (more silicic) continental plates, it may cause 'partial melting' of both mantle rocks and the plate rocks, producing magmas of varying silicity. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silicity (high-viscosity) stratovolcanoes. Examples include the South American Andes and the Cascade range.\n \nA volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame. The truth is that a volcano seldom emits either smoke or flame, although various combinations of hydrogen, carbon, oxygen, and sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust (called volcanic ash), mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.\n\nPerhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular shape, formed by a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.\n\nVolcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones or fractures. When the cone is eroded these rifts or lava filled fractures remain as radial near vertical dikes of volcanic rock. For example the radiating dikes at Shiprock in NW New Mexico.
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The crust but more specifically the lithospheric plates.\n\nThe lithosphere is divided into 7 major plates and about 7 minor ones. The major ones contain the continents.\n\nThese plates move in response to the convection of molten material in the mantle. As the molten rock circulates, it pulls lithospheric plates along like they were on a conveyor belt.\n\nThe crust floats atop the molten rock below b/c it is less dense.