1. Magma and lava both refer to molten rock from which igneous rocks form. However, magma refers to molten rock below Earth's surface, and lava refers to molten rock at the surface.
2. The nature of a volcanic eruption is determined by 1) the composition of the magma, 2) the temperature of the magma, and 3) the quantity of dissolved gases contained in the magma. The composition and temperature of the magma influence the viscosity of this material. The viscosity, in turn, helps determine whether the eruption will be violent or quiet. The viscosity is least for very hot magmas with a relatively low silica content. The gases dissolved in the molten material provide the force to propel the liquid rock from the volcano. The quantity of gases present and the ease with which they can escape (dependent upon viscosity) determines the nature of the eruption.
3. When magma moves to a near-surface environment, the gases dissolved at great depth begin to migrate upward and expand. The viscosity of the lava determines the ease with which these gases can escape. Highly viscous magma inhibits the escape of gas, which may then accumulate to the point where the lava is violently ejected from the volcano.
4. Pahoehoe lava forms from fluid basaltic lava and has a smooth or ropy appearance. Aa lava, with its sharp and jagged surface, results when more viscous magma cools.
5. The main gases released during a volcanic eruption are water vapor (70%), carbon dioxide (15%), nitrogen (5%), sulfur compounds (5%), and smaller amounts of chlorine, hydrogen, and argon.
6. water vapor
7. Fragments of volcanic rock and partly solidified lava blown from a volcano are termed pyroclastics. Dust is the finest in size, followed by ash. Welded tuff is a rock produced when the hot glassy shards of ash stick together. Larger pyroclastic debris includes blocks and bombs. Bombs were apparently partly molten when ejected and became streamlined as they sailed through the air. (any three of the above)
8. A volcanic crater is a relatively small
depression marking the vent or exit site of erupting lava or pyroclastic
material. A crater is excavated by the boring or drilling action of the
erupting magma and gases. A caldera is a much larger volcanic depression that
forms during or following a large outpouring of lava or pyroclastic debris.
Extremely rapid emission of huge quantities of magma, such as occurs during a
powerful explosive eruption, evacuates upper portions of the former magma
chamber. Thus, the rocks above the chamber fail and a large, circular to
elliptical volcanic depression is formed by collapse and subsidence.
9. Shield volcanoes are among the largest on Earth. These gently sloping domes are associated with relatively quiet eruptions of fluid basaltic lava. They contain very little pyroclastic material. Cinder cones are composed almost exclusively of pyroclastics, are steep-sided, and are the smallest of the volcanoes. Composite cones, as the name suggests, are composed of alternating layers of lava (usually andesitic or rhyolitic in composition) and pyroclastic debris. Their slopes are steeper than those of a shield volcano but more gentle than a cinder cone. Composite cones are associated with violent volcanic activity.
10. Mauna Loa, one of five volcanoes comprising the island of Hawaii, serves as an excellent example of a shield volcano. Paricutin, as well as many small cones on the Colorado Plateau north of Flagstaff, Arizona, are good examples of cinder cones. Mt. Fuji in Japan and Mt. Shasta, in California, as well as the many volcanoes of the Cascade Range are examples of composite cones.
11. Mauna Loa, a shield volcano, was built up from the ocean floor 5000 meters below sea level over nearly one million years by many eruptive cycles. Paricutin, on the other hand, is a cinder cone that was created over a period of a few years. It is, by comparison to the volcanoes of Hawaii, tiny.
12. Crater Lake is a caldera believed to have formed when the summit of Mt. Mazama collapsed into a partially empty magma chamber following a violent volcanic eruption. In contrast, Kilauea's caldera formed by collapse as magma was slowly drained from the summit magma chamber by flank eruptions during relatively quiet activity.
13. The largest volcanic structures on Earth are the Yellowstone-type calderas that occur in continental regions. They are not associated with a composite volcano, such as Crater Lake in Oregon. Instead, they occur as very large (tens of miles in diameter) depressions in volcanic terrains dominated by explosive rhyolitic and andesitic magmas. Good examples include Yellowstone National Park in Wyoming, Long Valley Caldera in California, and the Valles Caldera in New Mexico.
14. Shiprock, a well-known landmark in northwestern New Mexico, marks the subsurface “plumbing” system of a former volcano. The igneous rock is much harder than surrounding sedimentary strata. As erosion gradually cut into the bedrock, spires and sharp ridges of igneous rock were left towering above the more easily eroded sedimentary rocks. Shiprock itself is the central magma pipe that once fed magma upward to the volcano. The sharp ridges extending outward from the central spire are dikes representing radial cracks filled with magma injected outward from the central pipe.
15. Large, voluminous, volcanic edifices such as Mts. Rainier, WA, and Shasta, CA, are composite cones (stratovolcanoes). They are built by repeated, central-vent eruptions over time spans ranging up to a million years or more, interspersed with eruptions from flank fissures and satellite centers. Pyroclastic activity and lava flows add to the volume of the volcano; mudflows and mass wasting redistribute debris
to the lower flanks of the volcano and contribute to preserving the distinctive, steepening-upward, conical shape.
The Columbia Plateau is an eroded, uplifted flood basalt province of mid-Tertiary age. Elsewhere, flood basalts comprise the most voluminous, volcanic accumulations on Earth (Deccan basalts, India, and the Siberian traps, for example). Over a million years or more, basaltic lava flows are erupted repeatedly from fissure vents. The lavas collect as pools in topographically low areas and solidify to sheets of basalt.
16. Dikes and sills are tabular masses (thin in one dimension). They differ because sills are concordant, whereas dikes are discordant. Massive intrusions include laccoliths, which are concordant, and batholiths, which are discordant. Batholiths are by far the largest of all intrusive features.
17. Laccoliths are known to be emplaced at shallow depths. Domed strata above a laccolith may be exposed at the surface before erosion cuts down far enough to expose the igneous rock. Thus the domed strata may suggest that the top of a laccolith lies a short distance below the surface.
18. Batholiths are the largest of all intrusive bodies. They are massive, discordant igneous features.
19. During spreading center volcanism, a reduction in pressure lowers the temperature of the mantle rocks and partial melting generates large quantities of basaltic magma, which become the rock basalt as it fills the newly formed cracks and cools.
20. The volcanoes on the “Ring of Fire” lie above subduction zones, where plates that comprise the Pacific Ocean floor are sinking beneath other oceanic plates or beneath plates carrying continents.
21. Very large, composite volcanoes (stratovolcanoes), like those on the “Ring of Fire,” typically erupt explosively. The 1991 eruption of Pinatubo in the Philippines was the second most powerful eruption of the twentieth century, being surpassed only by the 1902 eruption of Santa Maria in Guatemala. The 1980 eruption of Mount St. Helens is another good example.
22. Hot spot volcanism refers to the volcanic activity produced at localized areas not related to tectonic plate boundaries. Hot spots are thought to be the result of large heat plumes in the mantle that induce melting in the overriding lithospheric plate. Yellowstone National Park in Wyoming and the island of Hawaii are good examples of hot spot volcanism.
23. Volcanic islands in the deep ocean are composed primarily of the igneous rock basalt.