Contents<br />1 Introduction<br />2 Plate tectonics and hotspots<br />3 Volcanic features<br />4 Erupted material<br />5 Volcanic activity<br />6 Notable volcanoes<br />7 Effects of volcanoes<br />8 Volcanoes on other planetary bodies<br />9 Panoramas<br />
INTRODUCTION<br />A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano island off Sicily which in turn, was named after Vulcan, the Roman god of fire.<br /> Volcanoes are generally found where tectonic plates are diverging or converging. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"). Volcanoes can be caused by mantle plumes.<br /> The study of volcanoes is called volcanology, sometimes spelled vulcanology.<br />
Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station, May 2006<br />
Plate tectonics and hotspots<br />Divergent plate boundaries <br /><ul><li> At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. for example, Iceland.</li></li></ul><li>Convergent plate boundaries<br /><ul><li>Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide.Oceanic plat submerges under the continental plate forming a deep ocean trench just offshore.Examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.</li></li></ul><li>Pinatubo ash plume reaching a height of 19 km, 3 days before the climactic eruption of 15 June 1991<br />
Hotspots<br /><ul><li>Hotspots are not usually located on the ridges of tectonic plates, but above mantle plumes, where the convection of the Earth's mantle creates a column of hot material that rises until it reaches the crust, which tends to be thinner than in other areas of the Earth.TheHawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot.</li></li></ul><li>Yellowstone Caldera<br />
Volcanic features<br /><ul><li>The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit.Somevolcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material and gases can be located anywhere on the landform.
Fissure vents - Flat, linear cracks through which lava emerges.
Shield volcanoes - broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent.</li></li></ul><li><ul><li>Lavadomes -slow eruptions of highly viscous lavas, violent, explosive eruptions, do not flow far from the originating vent.
Cryptodomes -viscous lava forces its way up and causes a bulge, under great pressure.
Volcanic cones (cinder cones) -erupt mostly small pieces of scoria and pyroclastics.
Stratovolcanoes (composite volcanoes) -made of cinders, ash and lava.
Supervolcanoes-a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. </li></li></ul><li><ul><li>Submarine volcanoes-common features on the ocean floor, Hydrothermal vents are common near these volcanoes.
Subglacialvolcanoes-develop underneath icecaps, flat lava which flows at the top of extensive pillow lavas and palagonite.
Mud volcanoes-geo-excreted liquids and gases, although there are several different processes which may cause such activity. </li></li></ul><li>lava dome,ChaiténVolcano, southern Chile<br />Lakagigar fissure vent in Iceland<br />Skjaldbreiður, a shield volcano <br />Mayon,stratovolcano Philippines.<br />Holocene cinder cone volcano,Utah.<br />Pillow lava (NOAA)<br />
Erupted material<br />Lava composition<br /> Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions :-<br />If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.<br />If the erupted magma contains 52–63% silica, the lava is of intermediate composition.<br />If the erupted magma contains <52% and >45% silica, the lava is called mafic or basaltic.<br />Some erupted magmas contain <=45% silica and produce ultramafic lava.<br />
Lava texture<br /> Two types of lava are named according to the surface texture: ʻAʻa (pronounced [ˈʔaʔa]) and pāhoehoe ([paːˈho.eˈho.e]).ʻAʻais characterized by a rough, clinkery surface and is the typical texture of viscous lava flow.Pāhoehoeis characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows.<br />
Volcanicactivity<br />Scientific classification of volcanoes<br />Philippine Institute of Volcanology and Seismology provides a scientific classification system for volcanoes.<br /><ul><li>Active- Eruption in historic times - Historical record - 500 years - C14 dating - 10,000 years - Local seismic activity - Oral / folkloric history.
Potentially Active - Solfataras / Fumaroles - Geologically young (possibly erupted < 10,000 years and for calderas and large systems - possibly < 25,000 years). - Young-looking geomorphology (thin soil cover/sparse vegetation; low degree of erosion and dissection.
Inactive-No record of eruption and its form is beginning to change by the agents of weathering and erosion via formation of deep and long gullies.</li></li></ul><li>Popualarclassification of volcanoes<br />Active -erupt regularly, erupted in holocene times.<br />Extinct - unlikely to erupt again, because the volcano no longer has a lava supply.<br />Dormant - may remain dormant for a long period of time, and it is not uncommon for a so-called "extinct" volcano to erupt again. <br />Notable volcanoes<br /><ul><li>Avachinsky-Koryaksky, Kamchatka, Russia
create new islands </li></ul>Volcanoes on other planetary bodies<br />The Earth's Moon has no large volcanoes and no current volcanic activity. he planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. Jupiter's moon Io is the most volcanically active object in the solar system<br />
Panoramas<br />Mount Bromo, East Java, Indonesia.<br />