volcanoes

Molten material, steam, and gases rise to the surface at volcanoes. The activity of volcanoes depends upon their source of heat, which is dependent upon their location and is ultimately derived from Earth's radioactivity, and upon the nature of their lava.

(left - USGS schematic of a composite volcano - click to see larger image in vulcanism item on Terms) [gallery of lava flows]

Volcanoes are submarine or terrestrial and are located at:
● divergent plate boundaries – spreading centers where ocean is spreading, like the volcanoes of Iceland atop the mid-Atlantic Ridge, or where continents are spreading as in Africa's volcanic Rift Valley
● convergent plate boundaries – subduction zone island arc volcanoes and explosive stratovolcanoes around the Ring of Fire
● hotspots or mantle plumes – intraplate volcanoes like the Hawaiian island shield volcanoes

Volcanoes release/extrude
● gases – carbon dioxide, sulfur and chlorine compounds, minor amounts of carbon monoxide, fluorine and boron compounds, and ammonia
● steam
● ash and pyroclastic flows (ignimbrites), bomb, lapilli, pumice, reticulite, obsidian, scoria
● molten lava – felsic dacite and rhyolite, intermediate andesite, mafic basalt, and ultramafic magma (molten rock)
● unusual materials like the warm black lava carbonates (natrocarbonatite) of the intracontinental volcano Oldoinyo Lengai in Tanzania, which are white when cooled and solidified (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)

Volcanoes are classified by lava composition, and/or by shape, or by type of eruption:
shape
_ ● composite or stratovolcanoes
_ ● supervolcanoes and calderas
_ ● cinder cones
_ ● shield volcanoes
_ ● lava domes or volcanic domes
___ ● maar
other
_ ● calderas within shield volcanoes
_ ● fissures
_ ● lava lake
_ ● lava tube and lava cave
_ ● hornito – a small rootless spatter cone that forms on the surface of a basaltic lava flow (usually pahoehoe).[Mauna Ulu hornito, Petunia Skylight hornito, another hornito above lava tube, and view through a puka]
_ ● spatter cone – spatter, spatter and cinder cone, spatter cone and rampart, spatter photo
_ ● mud volcano
_ ● submarine volcanoes [diagram, black smoker, hydrothermal vent, limu, pillow lava, Nikko submarine volcano]
_ ● subglacial volcanoes
_ ● vents

◙ subduction zone magmas ◙

type of eruption VEI
Table  Volcanoes  Flood Basalts and Stratigraphic Boundaries
● low gases, low silica – shield volcanoes like Kilauea
● low gases, high silica – lava domes such as have formed in the crater Mt. St. Helens since the explosive eruption
● high gases, low silica – cinder cone like Paricutin, which explodes with fountains of fluid lava
● high gases, high silica – composite or stratovolcanoes, like Mt. St. Helens before the explosive eruption

_ ● Strombolian eruptions produce intermittent explosion or fountaining of volcanic gases and clumps of basaltic lava, which becomes rounded in flight, from a single vent or crater. Volcanoes that often exhibit strombolian activity also include Etna (Italy), Pacaya (Guatemala), and Erebus (Antarctica). [Stromboli erupting, Stromboli volcano, Stromboli gallery]
_ ● Plinian eruptions produce tall columns of ash and tephra, eject large amounts of pumice, and are often characterized by powerful, continuous gas-blast eruptions. [eruption cloud]
_ ● Vesuvian
_ ● Peléan or Nuée Ardente eruptions produce pyroclastic flows, which are extremely hot, ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. [Mayon Volcano, 2, Philippines]
_ ● Hawaiian or Effusive eruption
_ ● Vulcanian eruptions are explosive eruptions in which ejected lava fragments are too viscous to take on a rounded shape during their flight through the air (unlike strombolian fragments). This type of eruption is named for Vulcano, one of the Aeolian Islands north of Sicily. Other volcanoes that have recently shown vulcanian behavior include Sakurajima in Japan (ongoing) and Irazu in Costa Rica (1965). [Tavurvur Volcano in Rabaul Caldera, Papua New Guinea.]
_ ● Phreatic (ultravulcanian) eruptions occur when rising magma comes into contact with ground or surface water, causing near-instantaneous evaporation of the water and resulting in an explosion of steam, water, ash, rock, and volcanic bombs. [Mt. St. Helens]

◙ subduction zone magmas ◙

'Normal plumes look like smooth-domed mushroom clouds. But this one, photographed in Ecuador in 2002, has unusual scalloped edges. Volcanic clouds can collapse back to Earth in searing, high-speed events called pyroclastic flows. Unlike with normal plumes, the ash from the scalloped cloud must have been unusually heavy, causing it to collapse more rapidly.' [NG news]

Video The Ring of Fire.

NG interactive : gallery of images : Alaska's Augustine Volcano : Time Lapse Movies :
> Volcanoes > Bromo Merapi Etna Kilauea, Hawaii Lokon Piton de la Fournaise (La Réunion) Semeru Stromboli Welirang (East-java) Lava flows Strombolian eruptions Lava lakes lava entering sea, close-up of lava entering sea