There are around 1510 'active' volcanoes in the world. We currently know of 80 or more which are under the oceans.
A volcano is a mountain that opens downward to a reservoir of molten rock below the surface of the earth, but unlike most mountains, which are pushed up from below, volcanoes are vents through which molten rock escapes to the earth’s surface. When pressure from gases within the molten rock becomes too great, an eruption occurs, which can be quiet or explosive. There may be lava flows, flattened landscapes, poisonous gases, and flying rock and ash that can sometimes travel hundreds of miles downwind
A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat (geothermal activity), explosive release of gases (including steam with the interaction of magma and surface of ground water), and the non-explosive extrusion or intrusion of magma. An event could be non-destructive without release of solids or magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or explosive activity.
Volcanic eruptions can be accompanied by other natural hazards, including earthquakes, mudflows and flash floods, rock falls and landslides, acid rain, fire, and (under special conditions) tsunamis.
Active volcanoes in the U.S. are found mainly in Hawaii, Alaska and the Pacific Northwest. The danger area around a volcano covers approximately a 20-mile radius however some danger may exist 100 miles or more from a volcano.
The first eruption of a volcano necessarily leaves a mound of scoriae and lava, while numerous eruptions at length raise mountains, which are frequently of an amazing extent and height. These mountains, which are generally called volcanoes, though in reality they are but an effect of volcanic action situated far beneath their base, are called extinct when for many centuries they have exhibited no signs of combustion; active, when, either perpetually or from time to time, eruptions or exhalation of lava, scoriae, or gases take place from their summits, or from vents in their sides.
The line of demarcation between active and extinct volcanoes is not easily drawn, as eruptions have sometimes taken place after such long intervals of repose as to warrant the belief that the vents from which they issued had long since been completely obliterated. Thus, though nearly six centuries have passed since the last eruption of Epomeo in the island of Ischia, we are not entitled to suppose it extinct, since nearly seventeen centuries elapsed between this last exposition and the one that which preceded.
Magma is liquid rock inside a volcano. Lava is liquid rock (magma) that flows out of a volcano. Fresh lava ranges from 1,300° to 2,200° F (700° to 1,200° C) in temperature and glows red hot to white hot as it flows.
Volcanologists have classified volcanoes into groups based on the shape of the volcano, the materials they are built of, and the way the volcano erupts. The groups are: Composite volcanoes (also called strato volcanoes), shield volcanoes (also called shields),cinder cones, spatter cones, and complex volcanoes (also called compound volcanoes).
Small active volcanoes at Papua-New-Guinea, deep beneath the sea, have thrown precious metals out of the earth. The sea-bottom there contains so much gold and other metals that commercial ventures were investigated.
Some benefits from volcanoes are that the earth's water and atmosphere, consequently life on earth, evolved from the gases produced by volcanic eruptions; rich volcanic soils form by alteration of volcanic products; volcanoes provide geothermal power; health spas and hot springs for recreation and health, for example, Beppu, Japan; and etc.