how do sand dunes form?
348428
As sand is blown across the landscape, it begins to pile up. However, the sand reaches an angle at which it can no longer hold itself up. At that point, it starts to expand in order to keep the angle lower. As this occurs continuously, the sand pile becomes bigger and bigger, making a dune.
1058011
I think that what you are asking about might be better called coastal deposition.\n\nOver the span of geologic time the sea level has changed dramatically. As the sea level has moved up and down over time the coastal erosion process has created continental shelves that are covered in deposits eroded from dry land around some continents. This process has also helped shape the coastlines of the continents. \n\nOne of the more spectacular examples of coastal deposition are river deltas. The sediment carried by the river is deposited as the flow of river water slows. This tends to sort the sediments according to the energy level of the water that is flowing into the open ocean. First larger grains like gravel and pebbles settle down, and as the river slows, sand, and finally clay size particles settle out of the water. Many river deltas have built up sediment deposits that are several miles thick and cover areas as large as some states. Most of southern Louisiana is a river delta deposit. \n\nAlong many coasts there is a predominant wind direction that helps to build coast long-shore currents. These currents move parallel to the coastline and will cause sediment accumulations (from rivers) such as sand bars and islands to slowly migrate in one direction. Much of the sand on beaches is actually slowly migrating along the shore. As heavier winter waves move the sand offshore, the sand also tends to move along the shore and sometimes temporarily moves back up onto the beach at a new location. \n\nAnother geologic process that is involved in coastal change is both uplift and subsidence. Uplift is taking place on some coasts due to tectonic plate motion. Other coasts are subsiding, or sinking, due to both faulting and slow compaction of underlying sediments. This sort of subsidence is taking place along the Gulf Coast of the US where there are large growth faults that parallel the coast that are sinking towards the Gulf. Sediment deposited thousands and millions of years ago is still settling in some areas and this leads to loss of coastal wetlands and rising local sea level. \n\nIn other areas coastlines are being changed by local erosion of uplifted sediments (e.g. California). Landslides carry hillsides down to the coastline. Another change in many coastlines is being caused by a geologic process called isostacy. As glaciers accumulated across North America during the last ice age, they weighed down that part of the earth's crust and caused it to sink into the asthenosphere. The glaciers melted long ago, but the areas that were covered by glaciers are still rising. This creates a sort of teeter-totter effect, where some coastlines like parts of the East Coast of the US and the Louisiana coast are sinking. \n\nInformation on isostatic rebound:\nhttp://www.homepage.montana.edu/~geol445/hyperglac/isostasy1/\n\nMississippi River Delta:\nhttp://www.lacoast.gov/geography/basins/mr/\n\nExplanation of long shore drift:\nhttp://www.cofc.edu/CGOInquiry/longshoredrift.htm\n\nGood explanation of barrier islands:\nhttp://www3.csc.noaa.gov/beachnourishment/html/geo/barrier.htm\n\nA very good illustration of differential erosion in a coastal setting:\nhttp://www.rgs.edu.sg/events/geotrip/stack.html\n\nUN Coastal Types:\nhttp://www.oceansatlas.com/servlet/CDSServlet?status=ND0yNzYwJmN0bl9pbmZvX3ZpZXdfc2l6ZT1jdG5faW5mb192aWV3X2Z1bGwmNj1lbiYzMz0qJjM3PWtvcw~~
256680
Sand and bare rock cover only about one-fifth of the Sahara Desert. More than half of the Sahara is made up of soil known as yermosols that are shallow deposits over gravels and pebbles. These soils are no older than 50 million years. As recently as 5000 years ago the Sahara was probably not a desert, and may have supported vegetation such as savanna woodlands now present in eastern Africa. Prior to that wetter period it was probably a desert, according to some recent work on dating paleodunes in Chad.\n\nIt is a large region so it is easy to over generalize when trying to describe the geology. Much of the sand has originated from weathering of exposed Mesozoic and Paleozoic basement rocks. Several mountain ranges cut through the Sahara. In late Eocene the Tethys Sea had regressed from much of this area, likely leaving behind large deposits of sand and Cretaceous age sandstone that had been laid down while the area was beneath the sea. This probably accounts for much of the sand. Sedimentary deposits of sandstone have been once again eroded, reworked, and recycled. Most of the aeolian sand in the Sahara is considered to be Pleistocene and Holocene in age. \n\nThe sand is considered superficial by geologists working on understanding the underlying geomorphology. In the studies of the Selima Sand Sheet in the eastern Sahara, the sand was measured as from 1 to 6 meters in depth. In other areas where dunes have accumulated there are likely to be depths of sand of several hundred meters, however typical dunes are generally about ten meters in height, with some reaching 100 meters.
39471
hills, mountains, valleys, sand dunes
572852
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
416771
You see there are different kinds of aeolians deposits.\nIf you look at the sites you will see different formations. You will see at the windward side erosion occurs, in where wind pick up sediments, like sand, and deposits it at the leeward side or simply at the back.
168408
Sand is only one form of silica and is mostly quartz, a hard to dissolve or wear away particle.  That is why sand fills an entire beach after rocks are broken down and through erosion.  However, if the sand is hot enough (near a volcano?) it may fuse into igneous rock (where igneous means fire as in the ignition that fires up a car).  Igneous rock is hard like iron but brittle rather than ductile.  Or the sand can be heated even higher to fuse into glass which is very hard but brittle and easier to crack.  On the other hand, sand at the bottom of a body of water (lake or ocean) can become cemented together perhaps with lime to become sandstone which is a sedimentary rock (made from sediment!).  I'd guess that if you filled a pot of molten lead with iron fillings you could make "sedimentary iron" or if you just fused the iron filings you could make "igneous iron" too.  Iron easily combines in nature with oxygen (rust) to form iron ores otherwise it might behave just like sand.  Hope that helps a little.
606522
If you mean how did they form...\n\nOpals form from hydrothermal solutions containing silicon dioxide. That simply means heated water containing dissolved quartz.\n\nThese solutions are deposited in sedimentary rocks where they come out of solution. Sedimentary rock is where you find opals.\n\nOpals contain small amounts of water in its structure. That is what gives it its "play of colors".\n\nIt is often referred to as hydrated silica & is called cryptocrystalline quartz because it is essentially quartz without a crystal structure.\n\nSome of the world's most beautiful opals come from Australia & Mexico. The ones found in Mexico often have a reddish "play of colors" & is called fire opal.
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Stream beds change due to sediment load that it carries & amount of water. They can meander or cut deeper into their bed. Oxbow lakes can form when meanders are cut off from the rest of the stream.\n\nRocks weather by wind & water. Self explanatory. Rock is removed by the wind & water and redeposited somewhere else as sediment.\n\nShorelines can either build up or erode depending on where they are located. Hurricanes can wash away shorelines or build them up too. Depending on how intense the storm is & amount of storm surge/ high tide or low tide, etc. Barrier islands are constantly changing. They move further inland during transgressions & move back out again during regressions. Hurricane can also close or open new inlets along barrier islands. Many of NC's barrier inlets have been closed & reopened by major hurricanes over the past couple hundred years.\n\nKarst topography. Areas w/ limestone bedrock will form sinkholes where the limestone is dissolved by naturally forming acids. This occurs especially where too much water is pumped out from these limestone aquifers. You can get sinkholes & sinkhole lakes/ponds. Lots of these in Florida.
1078413
the "winds aloft" at certain heights.  The higher the winds aloft decides what makes clouds flat on top or on bottom.  The wind shirrs them into that shape.  (Actually, they look flatter from the ground than they really are.)
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The most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide), usually in the form of quartz which because of its chemical inertness and considerable hardness is quite resistant to weathering. However, the composition of sand varies according to local rock sources and conditions.
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alluvial soils - soil deposited by water: sediment deposited by flowing water, especially soil formed in river valleys and deltas from material washed down by the river.\n \n\nwadi - (1) mainly dry water course: a steep-sided watercourse in dry regions of North Africa and southern Asia through which water flows only after heavy rainfalls  (2) an oasis, especially in North Africa\n\n\noasis - (1) fertile land in desert: fertile ground in a desert where the level of underground water rises to or near ground level, and where plants grow and travelers can replenish water supplies.\n\n\ndelta - deposit of clastic** sediment where a stream enters a lake, river, or ocean.\n\n** clastic sediment is sediment consisting of fragments of minerals, rocks, and other material.)
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Deltas and Estuaries are created at the mouths of rivers.\nAnother landform that is created close to the mouth of a river is an ox bow lake.
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Technically they do have soil. Just different kinds.\n\nSoil type is based mostly on texture & composition.\n\nSome deserts have loose sandy soil. Others have desert "pavement" which is essentially hard compacted soil.\n\nThere are 3 major types of soils under which all other subsoils fall. Pedocals are those soils found in the desert & other arid places. It is high in calcite b/c of lack of water filtering through the soil.\n\nThe other two are laterite which is found in humid tropical places that is high in iron oxide & pedalfers which are found in temperate climates. Like I said there are more but these are the main three types.
100603
Jetties and sea walls cause sand and silt to be deposited on their down-current side.  Example: The long jetty near the airport.\n\nWetlands are drained for development thus removing the buffer that wetlands provide against storm surges and flooding.  Example: Burnt Bridge Creek area - Lacamas Creek Enterprises Inc. filled approximately 1.2 acres of wetlands
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Iron and water.  Just wash the sand off the iron.
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Deserts can be either HOT or COLD. Antartica is also a desert.\n\nIn geography, a desert is a landscape form or region that receives little precipitation. Generally deserts are defined as areas that receive an average annual precipitation of less than 250 mm (10 inches).\n\nDeserts have a reputation for supporting very little life. Compared to wetter regions this may be true, although deserts often have high biodiversity, including animals that remain hidden (especially during the daylight) to preserve moisture. About one-fifth of Earth's land surface is desert.\n\nDesert landscapes have certain features. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony or hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation. Bottom lands may be salt-covered flats. Eolian (wind-driven) processes are major factors in shaping desert landscapes. Cold deserts have similar features but the main form of precipation is snow rather than rain. The largest cold desert is Antarctica (composed of about 98 percent thick continental ice sheet and 2 percent barren rock). The largest hot desert is the Sahara.