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.
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. 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. 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. 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 California, Mt. Hood in Oregon, and Mt. St. Helens and Mt. Rainier in Washington (e. g. USGS/Cascades Volcano Observatory, Vancouver, Washington) (see figure 4). Cinder and scoria cones are the simplest type of volcanoes. They are built from particles and blobs of congealed lava ejected from a single vent.
When gas-charged lava is blown violently in the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world. Calderas are among the most spectacular and active volcanic features on Earth. When a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera (see figure 5).
Most calderas very active, but this does not always lead to an eruption. The possibility of violent explosive eruptions forces detailed scientific study and monitoring of some active calderas. The Five Types of Volcanic Eruptions The five types of volcanic eruptions are Pelean, Vulcanian, Strombolian, Hawaiian, and Icelandic. The Pelean type of eruption, which is the most explosive, is represented by many volcanoes in Central America and the West Indies. A Pelean eruption has violent and destructive activity that takes place during the opening stages when glowing avalanches of magma are produced.
Airfall material is much less widespread than those of most Vulcanian eruptions. The eruptive cycle generally lasts only a few years, but there are exceptional cases such as in Santiaguito, Guatemala, where the cycle lasts more than 50 years (McBirney 240). The second type of volcanic eruption is referred to as the Vulcanian type. In a Vulcanian volcano the crater crusts over solidly between eruptions. When gases accumulate beneath the crust, the upper part of the magma column becomes thoroughly gas-saturated.
Strong eruptions sometimes disrupt the cone, followed by a cauliflower-shaped cloud, dark in color because of high ash content, as the obstruction is blown out. As the pressure is suddenly reduced, the gas-charged magma is disrupted by the explosively expanding gases into pumice and ash (Bullard 185). Lava flows may flow from the crater or from fissures on the sides of the cone. Vulcano, in the Lipari Islands off the coast of Sicily, was the example used by Mercalli (1891), Lacroix (1908), and other scientists for the Vulcanian type of eruption.
However, many others consider Vesuvius to be an even better example (Bullard 185). The prime characteristics of a Strombolian eruption are the throwing out of incandescent fragments of lava accompanied by a white eruption cloud (Bullard 242). The Strombolian eruption is named after a volcano named Stromboli. Stromboli is one of the few volcanoes in the world, which is in a state of permanent moderate activity. In contrast with the Vulcanian eruption cloud, which is heavily charged with ash, the Strombolian cloud contains little ash.
The lava column crusts over lightly, and at frequent intervals mild explosions break the crust, hurling the pasty, incandescent fragments into the air (Bullard 242). The Mauna Loa and Kilauea volcanoes are examples of Hawaiian type eruptions, in which basaltic lava issues more or less quietly from a fissure that may extend for a number of miles. Earthquakes usually proceed these eruptions, as the fissure opens to allow the magma to reach the surface. The lava is quite fluid, and the gases escape readily without the disruption of the lava into ash or cinders which occurs in the Pelean type of eruption (Bullard 263).
Less than 0. 5 percent of the portion of the Hawaiian Islands exposed above sea level consists of fragmental material, thus attesting to the great predominance of the outpourings of lava at these vents (Bullard 263). The Icelandic type of eruption has many features in common with the Hawaiian type. The lava in both cases is primarily basalt, and the eruptions are quite similar (Bullard 99). During Hawaiian type eruptions the lava piles up in dome-shaped masses, while in the Icelandic type the lava forms plateaus with flat-laying layers. The volume of lava involved in individual plateau building and in dome making is similar.