Volcanoes get their name from the Greek word Vulcano. “To the ancient Greeks, Vulcano was Hiera Hephaistou…to the ancient Romans it was the home of the forges of Vulcan; to both Vulcano was the lair of the god of fire” (Bullard 1984). Since then the myths have all gone but the name Vulcano has remained as the symbol for all volcanoes. “A volcano is both the place and opening from which molten rock, solid rock or gas issues from a planetary interior” (Scarth 1994). It is also defined as the mountain or hill built up around the opening at the surface by an accumulation of the ejected materials.
Sources of molten rock below the surface of the earth is called a magma reservoir, magma being the term given to molten or semi-molten rock below ground level. The magma travels up to the surface from the magma reservoir through conduits and emerges at the surface through volcanic vents (Fig 1). If the eruption happens to be explosive the magma is ejected from the vent as a dense cloud of volcanic ash, bombs and other forms of pyroclastic rock. When the eruption is non-explosive, it is said to be effusive and the magma issues from the volcano as lava.
In the following paragraphs the different types of volcanoes that are present on Earth will be looked at and discussed. Such as the lava domes, calderas and shield volcanoes. Fig 2: Schematic representation of the internal structure of a typical volcanic One type of volcano is the lava dome. These are formed by relatively small, round masses of lava too thick to flow any great distance. As a result on extrusion, the lava piles over and around its vent. The dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides.
Although they are much less common than Cinder cones, it has been calculated with admirable precision that 217 domes have been formed in the past 10,000 years” (Scarth 1994). The shapes of most Lava domes are determined by the way they grow and solidify. They grow from below when viscous lava wells up the vent and then solidify from their outer layers inwards. As the dome formsthere is a conflict between the upward-surging, plastic mass and the solid, outer shell of brittle rock, which prevent expansion.
Great upsurges can overcome the strength of the outer shell and cause its crest to burst open and emit molten material, while at the same time gas is also released. The appearance of a dome ultimately depends on the interplay of upwelling, explosion and solidification. The Novarupta Dome, which formed during the 1912 eruption of the Katmai Volcano in Alaska, measured 800 feet across and 200 feet high. The internal structure of Novarupta indicated by the layering of lava fanning upward and outward from the center shows that it grew largely by expansion from within.
Figure 3: The Novarupta Dome formed during the 1912 eruption of the Katma Volcano Cinder Cones Figure 4: Schematic representation of the Internal structure of a typical cinder cone Cinder cones are among the most common volcanic landforms found in the world and are also 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 an oval cone. Cinder cones are mostly produced by Strombolian eruptions.
They commonly grow in groups and are often found on fissures or in swarms, in both oceanic environments and continental environments. Shield volcanoes are built up of almost entirely of fluid lava. Flow after flow pours out in all directions from a central summit vent, or group of vents. This builds 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 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. Some of the Earth’s grandest mountains are composite volcanoes, which are sometimes called strato-volcanoes. Typically they are steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs.
They rise to be as much as 8,000 feet above their bases. Figure 5: Shishaldin Volcano, A composite cone that towers 9,272 feet above sea level in the Aleutian Islands, Alaska The composite volcano is formed when magma rising upward through a conduit, erupts at the Earth’s surface to form a volcanic cone. The lava spreads over the surrounding area. As the volcanic activity continues, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys.
When the volcanic activity finally ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened “volcanic plug” in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or “volcanic neck,” a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau (Fig 5).
Figure 7: Aerial view of Crater Lake Caldera, Oregon “A caldera is a large volcanic depression, at least 1km in diameter and often much more, that is enclosed by nearly vertical walls facing into a central flattish floor” (Brown 1970). They are so beautiful that it is hard to imagine that they have been produced by some of the most catastrophic events on Earth. Many develop within the space of a few hours. Huge volumes of material are erupted and huge volumes of material sink into the crust.
At least ten caldera forming eruptions in the past 10,000 years have been ejected more than 50km of ash and pumice. Calderas fortunately are not formed very often and only two or three are created every century. Hydrovolcanic landforms develop when an external source of water interacts with magma approaching the Earth’s surface. The large areas covered by water, ice, snow, the frozen ground, abundant precipitation, subsurface qualifiers, all combine to ensure that some interaction between water and magma.