Volcanoes

Volcanoes Introduction 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.

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Such as the lava domes, calderas and shield volcanoes. Types of Volcanoes Lava Domes 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 forms there 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 Volcano 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. Composite volcanoes 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). Calderas 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 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. Science.

Volcanoes

Volcanoes Volcanoes JACK KNOFF WR 327 Technical Report Spring ’99 Introduction In this report I plan to discuss the geological event of volcanic eruptions and the disasters they cause. To me, this is a fascinating topic and timely seeing how the 19th anniversary of the eruption of Mt. St. Helens is upon us. I hope to inform people of the mass destruction that is caused by the eruption of a volcano. The scope of my report will be limited to: 1) describing what comes out of a volcano, 2) explaining the seven different types of volcanoes, 3) explaining the five types of volcanic eruptions, and 4) explaining the disasters they can cause people.

The procedure for completing this report first started by watching educational television programs that featured volcanoes and the upcoming anniversary of the eruption of Mt. St. Helens. From there, I decided that the topic of volcanoes would be a good subject for my analytical report. Then I began my research, first looking online for websites that contained information and pictures of volcanoes. After this, I looked for publications about volcanoes in the library, finding many books that pertained to my topic.

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Having an abundance of data, I began to sort through all of it and found what I thought to be the most informative. I then prepared an outline of the subjects I wanted to write about and arranged the data and visuals to fit my outline. And of course from there, I began to write this report using the technical writing methods taught during lecture and described in the book. Collected Data If we look through volcanoes we can view the interior of the earth. More than just lava flows are spewed out of volcanoes when they erupt.

The three main components that erupt out of a volcano are: lava, ash, and bombs. When all three of these components lump together, the solid fragments are called pyroclastics. Pyroclastic rocks can be erupted in two different ways: they can be airfall deposits or pyroclastic flows. There are seven different types of volcanoes: Submarine volcanoes; Ridges and vents; Shield volcanoes; Lava plateaus and Flood basalts; Lave domes; Composite volcanoes; Cinder and Scoria cones; and Calderas. Each of these volcanoes is found in different geographical locations and have different eruptions. Along with different types of volcanoes, there are also different types of eruptions.

The five eruption types are: Pelean, Vulcanian, Strombolian, Hawaiian, and Icelandic. These volcanoes have different levels of explosiveness, and their eruptions occur due to their geographic location. A volcanic hazard is destructive natural process that has a probability of reoccurring. Losses from volcanoes include: people’s lives, property, livestock, and the productive capacity of the area. The factors of predicting volcanic activity are: the longer a volcano is inactive, the greater the chances are for it to become active; eruptive behavior may change with time; and some hazards are indirectly related to an eruption, making it difficult to forecast. Being informed of volcanoes in your area and knowing cautionary steps can save your life. What Comes out of Volcanoes? Volcanoes are dark windows to the interior of the Earth (Decker 104). Volcanic products are our only direct samples of the Earth’s composition from deeper levels.

Most people think that lava flows are the only products spewed from volcanoes, but actually volcanic ash and larger solid fragments, called cinders and blocks, form the major products of observed volcanic eruptions (Decker 104). The three components that erupt from a volcano are: lava ash, and bombs. The volcanic debris that lumps together all the sizes of solid fragments is called pyroclastics (See Figure 1). Pyroclastics come from three sources: magma that is cooled and broken into fragments by expanding gases at the moment of eruption; fragments of old crater walls which are ripped loose in explosive eruptions; and clots of liquid lava thrown into the air which cool during their flight. Pyroclastic rocks are set apart by the general size of fragments. Volcanic dust is fine; volcanic ash is gritty, with particles up to the size of rice; cinders include pieces as big as Ping-Pong balls; and blocks cover all other fragments up to the size of a house.

Volcanic bombs are block-sized clots of liquid lava thrown from erupting vents. Pyroclastic rocks can be erupted in two different ways: they can be airfall deposits or pyroclastic flows. The contrast in explosive debris tells something of the nature of its previous eruptions and thus helps to predict the nature of possible future eruptions (Decker 105). The Seven Different Types of Volcanoes The seven different types of volcanoes are: Submarine volcanoes; Ridges and vents; Shield volcanoes; Lava plateaus and Flood basalts; Lava domes; Composite volcanoes; Cinder and scoria cones; and Calderas. Submarine volcanoes and volcanic vents are found on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea (e.g.

USGS/Cascades Volcano Observatory, Vancover, Washington). Many others lie so deep within the ocean floor that the tremendous weight of the water results in such confining pressure, that it prevents the formation and explosive release of steam and gases. Shield volcanoes are built almost entirely of fluid lava flows (e.g. USGS/Cascades Volcano Observatory, Vancouver, Washington). They are formed when flow after flow pours out from a central summit or vent, or group of vents which builds a broad, or gently sloping cone of a flat domical shape (See figure 2).

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 (e.g. USGS/Cascades Volcano Observatory, Vancouver, Washington). Flood basalts and plateau basalts occur at vast composite accumulations of horizontal or sub-horizontal flows and which, erupted in rapid succession over great areas, have at times flooded sectors of the earth’s surface on a regional scale (e.g. USGS/Cascades Volcano Observatory, Vancouver, Washington). The Columbia Plateau of the northwestern United States and the Deccan Plateau of southeastern India are classic examples of plateau basalt provinces.

Lava domes are masses of solid rock that are formed when viscous lava is erupted slowly from a vent (e.g. USGS/Cascades Volcano Observatory, Vancouver, Washington). If lava is viscous enough, it will pile up above the vent to form a dome rather than move away as a lava flow. Most domes are composed of silica-rich lavas that have a lower gas content than do the lavas erupted earlier in the same eruptive sequence; nevertheless, some lava domes still contain enough gas to cause explosions (see figure 3). Composite volcanoes and stratovolcanoes are typically steep sided, symmetrical cones of large dimensions built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs which may rise as much as 8,000 feet above their bases (e.g.

USGS/Cascades Volcano Observatory, Vancouver, Washington). Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mt. Fiji in Japan, Mt. Shasta in C …

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