what is a under water volcano?
84803
The islands of Hawaii all started as underwater volcanoes before they got tall enough to be above water. Another famous example is Surtsey (see reference).
459846
Not much water in the form of free liquid water entrapped in pores of the rocks in subducted lithosphere that makes it into the mantle. However, the subducted plate *does* contain a substantial amount of water in the form of hydrous minerals  (e.g., clays, serpentine, amphiboles, chlorite, epidote, micas, etc.) that form by hydrothermal alteration of the oceanic crust, and are present in the overlying sediments (some of which are subducted).\n\nMuch of the water in these hydrous minerals is released when the subducted plate heats up as it is warmed by the hot mantle surrounding the plate, and the hydrous minerals dehydrate (really just a special type of prograde metamorphism).  The water that is released is thought to leave the plate itself and rise (because it is less dense than the rock, and hence bouyant) into the overlying mantle.  Water reduces the temperature at which  silicate rocks begin to melt, and because the mantle overlying the subducted slab is actually hotter than the slab itself, this water can trigger melting in the mantle.  This is why there is almost always a volcanic front above a subduction zone (e.g., the "Pacific ring of fire").  The magma associated with these subduction-zone volcanoes is usually volatile-rich (water and CO2 coming from hydrous minerals and carbonates in the subducted slab), and gives rise to explosive eruptions like Mt. St. Helens, or in a more extreme example, Krakatoa.
1066571
stratovolcano is a tall, conical mountain (volcano) composed of both hardened lava and volcanic ash. The shape of these volcanos is characteristically steep in profile because the lava flows that formed these volcanos were highly viscous, and so cooled and hardened before spreading very far. Such lava are classed as acidic to be high in silica. At the other end of the spectrum are shield volcanoes (such as Mauna Loa in Hawaii), which are formed from less viscous, basic lavas (high in metals such as magnesium and iron), giving them a wide base and more gently sloping profile. Many stratovolcanos exceed a height of 2500 m. They are often created by subduction of tectonic plates.\n\nAlthough stratovolcanoes are sometimes called composite volcanoes, volcanologists prefer to use the term stratovolcano to distinguish among volcanoes because all volcanoes of any size have a composite (layered) structure — that is, are built up from sequential outpourings of eruptive materials.
93724
...a composite volcano.\nThese types of volcanoes are built up of layers of lava, ashfall, tephra, cinder, etc.   One name is stratovolcano (strato- refers to the layers, and the other is composite, which refers to the fact that the volcano is made up of many types of material.\nFor more information on stratovolcanoes:\nhttp://vulcan.wr.usgs.gov/Glossary/StratoVolcano/description_composite_volcano.html
71124
Actually there is land surface more than the water one, but its under water.
510967
It is the area of land that is underwater for half the day and above water the other half. This is caused by the rise an fall of sea levels due to high and low tides.
56322
Volcanoes occur in hotspots where the earths crust is thin, most often around subduction zones, or areas where the tectonic plates are colliding and one plate is sliding under the other. There are other places where volcanos or hotspots occur, such as Hawaii, but in that case the chain of islands known as Hawaii are being pushed north by the movement of the tectonic plate there. In fact right now there is another island being built beneath the ocean just south of the main island. The earth is slowly cooling though events such as these. Beneath the crust the magma is under intense pressure and it has to escape somewhere, so it does escape at these weak places in the earths crust which we call volcanos.\n\nOf course I could have just plagarized my answer like  mikah_nine, but instead I opted to answer from my own knowledge and even include a source.
463663
It's a stratovolcano:\nhttp://volcano.und.nodak.edu/vwdocs/volc_images/img_mt_pelee.html
924357
volcanoes are built when tectonic plates push together, the way mountains are formed. except volcanoes have magma inside of them. the gas and magma inside are under extreme pressure, and this pressure pushes its way through small cracks in the rock, eventually escaping in an eruption of gas that brings lava with it. a lot of people have been killed by eruptions, but i cant tell you how many. the lava takes weeks or months to cool down unless it hit the water, which will cool it in a matter of days.
296278
Mauna Loa is a shield volcano, and as such it has comparatively gentle eruptions (as compared to stratovolcanoes, such as Mt. St. Helens.)  Shield volcanoes also produce high temperature low-viscosity basaltic lava, which contributes to the eruptions being gentle (except in the case of gas in the lava, which can cause fire fountains).\nMore information on shield volcanoes can be found here:\nhttp://en.wikipedia.org/wiki/Shield_volcano
216932
http://en.wikipedia.org/wiki/Volcanos
824523
I found a different website that talks about all the different types of volcanos.  There  are Shield volcanoes, Strato, Rhyolite caldera, Monogenetic fields, Flood basalts and Mid-ocean ridges.  There was also some interesting volcano facts that you might be able to use at the 2nd website below.
24742
There are volcanoes because of the interaction between multiple plate boundaries which causes the magma in the upper mantle (from beneath the crustal lithosphere) to rise, erupt and form lava at the surface.  \n\nVolcanoes tend to occur at either subduction zones, rifts or at hot spots.  Subduction zones are located at convergent oceanic/oceanic or oceanic/continental plate boundaries, where the denser oceanic crust sinks into the mantle beneath the other plate.  The minerals in the subducted slab dehydrates through metamorphic processes, which releases fluids to the mantle wedge to produce the melt (magma), which erupts to form volcanoes.  \n\nFor rifts, during the early stages of continental rifting (where continents are pulled apart because of extensional forces), volcanic activity can occur because crustal extension can allow magma to rise and intrude further up the crust.  Therefore, volcanic activity can occur in such situations.  eg: East African Rift. \n\nIn mid oceanic ridges, the divergent motion of oceanic plates allows mantle material to rise from below.  This tends to form a submarine ridge rather than a volcano.\n\nHowever, volcanoes can occur within the plate.  These are usually called hotspots, where a plume (a plume of anomalously hot magma) rise from deep within the mantle (probably as deep as the core-mantle boundary) to the surface.  These are stationary and therefore, it can leave a trail of volcanoes as the lithospheric plates move over the mantle.  Eg: Hawaii-Emperor seamount chain.
597381
Geologists generally group volcanoes into four main kinds--cinder cones, composite volcanoes, shield volcanoes, and lava domes. \n\nIn regard to your question, Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.\n\nShield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.
871525
A stratovolcano is a tall, conical volcano composed of many layers of hardened lava, tephra, and volcanic ash. These volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far. The source magma of this rock is classified as acidic, or high in silica to intermediate (rhyolite, dacite, or andesite. or basalt). This is in contrast to less viscous basic magma that forms shield volcanoes (such as Mauna Loa in Hawaii), which have a wide base and more gently sloping profile. Many stratovolcanoes exceed a height of 2500 m.\n\nStratovolcanoes are a by-product of the subduction of tectonic plates. The source melts of these volcanoes arise as a result of the dewatering of oceanic crust at specific pressure/temperature conditions as the plate subducts to lower depths. The water freed from the subducting slab allows partial melting of the overlying mantle rock, which rises due to its lower density relative to the surrounding mantle rock, and pools at the base of the crust. The melt is initially basic in composition. As it rises through the crust, it incorporates more acidic adjacent crustal rock, leading to a final intermediate composition. The water remains in the melt and upon eruption contributes to the explosive power of these volcanoes as superheated steam.\n\nAlthough stratovolcanoes are sometimes called composite volcanoes, volcanologists prefer to use the term stratovolcano to distinguish among volcanoes because all volcanoes of any size have a composite (layered) structure — that is, are built up from sequential outpourings of eruptive materials.
889442
Islands are pretty much the peaks of underwater mountains.  They stay above water because they've got more land underwater sitting on the ocean floor.
450623
go to\nhttp://en.wikipedia.org/wiki/Volcanoe