how are barrier islands formed?
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A bar is a linear shoaling landform feature within a body of water. Bars tend to be long and narrow (linear) and develop where a current (or waves) promote deposition of particles, resulting in localized shallowing (shoaling) of the water. Bars can appear in the sea, in a lake, or in a river. They are typically composed of sand, although could be of any particulate matter that the moving water has access to and is capable of shifting around (for example, soil, silt, gravel, cobble, shingle, or even boulders). The size of the particles comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important. A vessel run aground on a bar is arguably worse off than one run aground on stationary rocks, because of the destructive capacity of the shifting action.\n\nThe term bar can apply to landform features over a considerable range in size, from just a few meters in a small stream to marine depositions stretching for hundreds of kilometres along a coastline, often called barrier islands.
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due to earthquakes formed under sea...
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This is due to the seawater having a greater heat capacity than the land, thus they heat and cool off at different rates, causing pressure differentials in the air above them. See the source for details.
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Dikes (not dykes) are artificial earthern walls made to hold back waters from sea and rivers.  As most are made from soils nearby, their compositions are different in regard to where they are - gradation depends on how high the waters are expected to get and the force of the water - best as I can explain it off the top of my head.
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Water.\n\nWater is one of the key parts of the formation of magma in subduction zones. The water that is in the subducting plate lowers the melting point of the rock by a few hundred degrees C. The water in the subduction plate will boil and migrate upwards into the overlying wedge of mantle rock where it will reduce the melting point of the overlying mantle. This magma will migrate upwards and form a volcano. \n\nAnother part of this process is that within rock there are many different minerals, all with different melting points, so the ones that melt at the lowest temperatures tend to form magma first, so the resulting magma comes from a partial melt of the source rock.\n\nHere's another diagram (I'd draw you my own but I can't post anything but text here):\nhttp://earth.leeds.ac.uk/dynamicearth/subduction/cordillera_islandarc.htm
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There are three major ways that rocks are formed. The first of these is from heat ie- igneous rocks. They are the volcanic rocks, among which are obsidian, and granite. Both obsidian and granite contain the same minerals but their different appearances are caused by differences in the speed of their cooling. Obsidian cooled very rapidly and has a glassy texture.  Granite cooled very slowly and the separate minerals crystallized into larger crystal. The three major minerals of granite and obsidian are quartz, feldspar, and mica, each cool at a different rate.\n\n\nGranite is an Intrusive rock=\nIgneous rock that cools and solidifies beneath the Earth’s surface\n\nObsidian is an Extrusive rock=\nIgneous rock that cools and solidifies at or very near earths surface eg- in a lava flow
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If you are asking how rocks are formed, there are three main processes: 1) Igneous rocks are formed by the solidification molten magma from deep in the earth; 2) Sedimentary rocks are formed by pressure acting on sand or other small fragments under a body of water or under other rock layers; 3) Metamorphic rocks start as another type, but are changed into a different, often more complex structure or crystalline matrix by the action of pressure, heat, friction, and/or water.
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Plates collide in areas called subduction zones.  At these zones, oceanic crust is subducted underneath continental crust due to its higher density.  Subduction results in great amounts of heat and pressure building up in the zone.  When these factors become strong enough, they are forced to the Earth's surface in the form of volcanoes.  That is why volcanoes typically occur along plate boundaries (e.g., the ring of fire).  However, volcanoes are not exclusive to plate boundaries.  They can also form over hot spots within the interior of the Earth.  The islands of Hawaii are an example.  The islands are formed on the Pacific Plate.  The plate is moving in a northwest direction, while the hot spot underneath the crust is stationary.  As the plate moves over the hot spot, volcanic eruptions occur.  The result is a new island being created.  The islands are actually the products of a series of shield volcanoes that grow in age as one moves to the northwest.
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Galveston Island helps to form the southernmost part of Galveston Bay, but is on the edge of the Gulf of Mexico.  It is primarily bordered by West Bay (on its northwest side) and the Gulf (on its southeast).  Galveston Bay probably has its name because in order to reach the bay by ship, you must first pass by Galveston.\n\nThe attached map should be helpful.
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I think you mean rock fragments. If yes,\n\nRock fragments are further eroded by forces of nature: wind or water. depending on the level of disintegration, it forms various kinds of soil or sand.\n\nSometimes if fragments get buried under several layers of soil, fragments clump together to form sedimentary rocks\n\nHope that helps\n\nR
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The rocks of the Giant's Causeway crystalized from molten rock or melted rock called magma.  This process makes igneous rocks. As the magma and rock cools, it loses heat and water, and as a result shrinks.   The shrinkage causes the polygon pattern which makes the Giant's Causeway famous, see photos at this web site: http://www.webvider.com/wwm/images/gb_u162.jpg\n\nGeologists call this pattern, when seen in igneous rocks, columnar joints.  Joints are the cracks which form during cooling.   Columnar joints are often seen in volcanic and shallow formed igneous rock such here in Northern Ireland, Iceland, and in North America,........all over the world where igneous rocks are found.
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True. Magma can be generated at subducted plate boudaries. \n\nI am still uncertain about the question. Does your question read as "Can magma be formed at subducted plate boundaries" then the answer would be yes
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4 ways:\n\nDivergent plate boundaries\nAt the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling down and solidifying. In these places, the crust is very thin and eruptions occur frequently because of the pull by the tectonic plates. The main part of the mid-oceanic ridges are at the bottom of the ocean, and most volcanic activity is submarine. Black smokers are a typical example of this kind of volcanic activity. Where the mid-oceanic ridge comes above sea-level, volcanoes like the Hekla on Iceland are formed. Divergent plate boundaries create new seafloor and volcanic islands, such as Cjertce and Hawaii.\n\nConvergent plate boundaries\nIn places where one tectonic plate submerges beneath another, the crust melts and becomes magma. This surplus amount of magma generated in one location causes the formation of the volcano. Typical examples for this kind of volcano are the volcanoes in the Pacific Ring of Fire, and also Mount Etna and Mount Vesuvius.\n\nHotspots\nHotspots are not located on the ridges of tectonic plates, but on top of mantle plumes, where the convection of Earth's mantle creates a column of hot material that rises until it reaches the crust. The temperature of the plume causes the crust to melt and form pipes, which can vent magma. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes extinct after a while and a new volcano is then being formed as the plate shifts over the hotspot. The Hawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the current part of the North American plate over the hotspot.\n\nPetitspots\nIn July 2006, volcanoes were discovered that did not fit in any of the above-mentioned categories, since they are located far from the plate boundary, but are too small to be the result of a mantle plume. A new theory suggests that submergence of tectonic plates causes stress all over the plate, which causes the plate to crack in some places. However, other scientists believe the mantle plume theory to be incorrect, and consider this discovery a confirmation of their ideas.
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Evaporites (IPA: /iˈvapərʌɪt/) are water-soluble, mineral sediments that result from the evaporation of bodies of surficial water.\n\nEvaporites are formed by evaporation of restricted bodies of water at the Earth's surface.\n\nAlthough all water bodies on the surface and in aquifers contain dissolved salts, in order to form minerals from these salts, the water must evaporate into the atmosphere in order to precipitate the minerals.\n\nIn order for this to happen the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small basin fed by a limited input of water.\n\n--> http://en.wikipedia.org/wiki/Evaporite#Evaporitic_formations\n\nSedimentary rocks are laid down in layers called beds or strata. Each new layer is laid down horizontally over older ones. There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.\n\n--> http://en.wikipedia.org/wiki/Sedimentary_rocks
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B.Types of Islands: Low Islands\nLike the high islands, low islands are also further classified into two subgroups: eroded volcanic islands and atolls. The eroded volcanic islands are much like the oceanic, or volcanic, high islands, only they have been eroded to such a point that they are barely above sea level. Examples of these islands include the smaller, lower islands of Hawaii.\n\nAtolls are a series of islands that form a rough ring enclosing a central body of water called a lagoon. The various islets of the ring are called motus and are separated by sea channels that lead into the lagoon. An atoll is continually built upward from an underlying coral reef, itself formed from the skeleton of a tiny, lime-secreting animal called a polyp. Typically, some parts of an atoll are above sea level while other parts remain below. Many scientists believe that atolls were once the fringing reef around volcanic islands that have subsided below the ocean surface. In a subset of atolls, known as raised atolls, geologic action has elevated the coralline limestone above the surrounding sea. Nauru and Niue are examples of raised atolls. Examples of other atolls include most of the Marshalls, Kiribati, Tuvalu, Tokelau, and the Cook Islands. Although most atolls are tiny, some—probably those that once surrounded large volcanic peaks—are quite large. Kwajalein, in the Marshall Islands, is the world’s largest atoll in terms of its diameter around the lagoon, with a length of 130 km (80 mi) and a width of 32 km (20 mi). Kwajalein’s land area, however, is only 16 sq km (6 sq mi). Kiritimati, in Kiribati, is the largest atoll in terms of land area, with a total area of about 390 sq km (150 sq mi).
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Shield volcanoes are composed of basaltic, low viscosity lavas. which have low height/area aspect ratio (think about Hawaii).\nComposite volcanoes have a high height/are aspect ratio and are formed formed by andesitic to dacitic lavas, and pyroclasts. They are also called strato-volcanoes.\nThe domes, are dome shaped eruptions, lavas or volcanoes formed by dacitic to rhyolitic lavas, with some pyroclasts in the rims. They can also form cake like shapes or even very impressive protruding spine-shaped extrusion of very viscous magma.
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Erosion forms sand\nSand settles as sediment\nSediment hardens over time\nVoila, Sandstone