Chapter 6
Volcanism at divergent plate boundaries is most often associated with which magma type? What causes rocks to melt in these settings?
At divergent plate boundaries, the volcanism is most often associated with basaltic rock compositions. The source of melting at these settings is related to separation of the plates leading to decompression of underlying mantle rock that allows the hot mantle rock to partially melt and produce magma reservoirs of basaltic composition.
How is a crater different from a caldera?
A crater is a bowl-shaped depression at the summit of a volcanic cone. Calderas are very large summit depressions with diameters greater than 1 kilometer.
Are volcanoes fed by highly viscous magma more or less likely to be a greater threat to life and property than volcanoes supplied with very fluid magma?
A volcano fed by highly viscous magma poses a greater threat to life and property because of the less predictable, highly explosive nature of a potential eruption.
How is magma generated along convergent plate boundaries?
Along convergent plate boundaries where subduction is occurring, the subducted lithosphere sinks into the mantle where higher temperatures drive volatiles out of the rock. Under the pressures at this depth, the volatiles rise and trigger partial melting of the overlying hot mantle rock. Once molten, this rock buoyantly rises upward through the crust.
Where do most shield volcanoes form—on the ocean floor or on the continents?
Although a few examples of large shield volcanoes on continents exist, most shield volcanoes form on the ocean floor where the majority of basaltic volcanism occurs.
What is meant by the term country rock?
As magma rises through the crust, it displaces the preexisting crustal rocks, which are referred to as the "country rock" or "host rock."
Over what time span does a typical cinder cone form?
Cinder cones usually form over short intervals of time. Most are formed by a single, short-lived eruptive event with 95 percent of cinder cones forming in less than a year.
What is emitted from a fumarole?
Fumaroles are vents on the flanks of a volcano, similar to parasitic cones, but they only emit volcanic gases.
What is thought to be the source of magma for most intraplate volcanism?
Intraplate volcanism is thought to originate from plumes of hotter-than-normal, solid, yet mobile, mantle rock that rises toward the surface. When the plume reaches a certain depth, typically the base of the lithosphere, the lower pressure can trigger partial decompression melting to produce the liquid magma.
What is scoria? How is scoria different from pumice?
Scoria is a red or black highly vesicular ejecta material formed from basaltic to andesitic lavas. Pumice is a light-colored highly vesicular rock formed from more felsic lavas and appears more glassy and threadlike in its texture compared to scoria. In addition, pumice has so many vesicles that samples will usually float, while scoria is rarely seen to float.
What type of volcanic structure is Shiprock, New Mexico, and how did it form?
Shiprock, New Mexico is the remnant of a volcanic conduit, the pipe or neck of an ancient volcano. It formed when magma solidified in the conduit of the inactive volcano and erosion removed the less resistant rocks forming the flank of the volcano, leaving the more resistant igneous rock of the volcanic neck.
Explain the process of decompression melting.
Since melting temperatures increase at depth due to the associated increase in confining pressures, reducing the confining pressure can lower a rock's melting temperature. Decreasing a rock's confining pressure sufficiently can trigger decompression melting to begin. This process can occur at divergent boundaries where convective upwelling brings hot rocks into shallower depths with lower pressure and partial melting can occur.
Briefly compare the 1980 eruption of Mount St. Helens to a typical eruption of Hawaii's Kilauea Volcano.
The 1980 eruption of Mt. St. Helens was a violent, explosive event that sent ash extremely high into the atmosphere, and the blast flattened trees over a 400-km2 area. The eruption also melted glaciers on the mountain that mixed with ash to form mudflows into nearby drainages. In contrast, a typical eruption of Kilauea Volcano generates quiet outpourings of fluid lava
Where are the best-known shield volcanoes in the United States? Name some examples in other parts of the world.
The best-known United States shield volcanoes would be the Hawaiian volcanoes, specifically Mauna Loa and Kilauea. Other well-known shield volcanoes include the Galapagos Islands, the Canary Islands, Iceland, and Easter Island. On the continental crust, Oregon's Newberry Crater is a well-known shield volcano.
_________________ magma is the most abundant type of magma erupted on Earth. a) Basaltic b) Granitic c) Andesitic d) Pegmatitic
a) Basaltic
_______________ tend to increase the explosive potential of a magma body beneath a volcano. a) High viscosity and high dissolved gas content b) High viscosity and low dissolved gas content c) Low silica content and low viscosity d) Low viscosity and low dissolved gas content
a) High viscosity and high dissolved gas content
Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.
Batholiths are massive, discordant plutonic bodies. They are the largest plutons with a surface exposure greater than 100 km2. Stocks are similar to batholiths but with a much smaller surface area, less than 100 km2. Laccoliths are also considered massive plutons, but they are concordant rather than discordant, meaning they do not cut across the sedimentary layers they are intruding. Laccoliths form by arching sedimentary layers upward to allow more intrusive material to accumulate without becoming discordant.
Some volcanoes have very large circular depressions, called ____________, which ave diameters that are greater than 1 kilometer (0.6 mile) and that in rare cases exceed 50 kilometers (30 miles).
Calderas
How do cinder cones compare in size and steepness of their flanks with shield volcanoes?
Cinder cones are smaller than shield volcanoes because they usually erupt for only a small interval of time. In addition, they are composed of pyroclastic scoria fragments that fall and accumulate near the vent providing a steep slope due to the high angle of repose of the loose, fragmented material.
Describe the composition of a cinder cone.
Cinder cones tend to be basaltic in composition. However, because they are composed mostly of scoria fragments, they could have compositions that range from basaltic to slightly more andesitic than basaltic.
Describe the materials that compose composite volcanoes.
Composite volcanoes contain layers formed from fluid lavas that are low enough in viscosity to flow away from the vent and build gentle-sloped flanks. These layers alternate with layers formed from more viscous, silica-rich lavas that produce pyroclastic material that accumulates near the summit, giving the volcano some height.
Describe dikes and sills, using the appropriate terms from the following list: massive, discordant, tabular, and concordant.
Dikes are tabular, discordant intrusive rock bodies that cut across the structures of the country rock. Sills are tabular, concordant intrusive rock bodies that are squeezed into the weaknesses between sedimentary beds or horizontal structures of the country rock.
What is a lahar?
Lahars are destructive mudflows produce when volcanic ash and debris mix with water and move down steep volcanic slopes. The water source can be snowmelt or glacial meltwater, formed by magmatic heat as magma nears the surface or heavy rainfall that saturates weathered volcanic material.
How do lava tubes form?
Lava tubes form as conduits through which lava will move to feed the leading edge of a flow. As such, they are usually associated with fluid basaltic lavas and form in the interior parts of the flow where temperatures remain higher and the lava remains more fluid.
List these magmas in order, from the highest to lowest silica content: basaltic (mafic) magma, granitic/rhyolitic (felsic) magma, andesitic (intermediate) magma.
Most silica rich to least silica rich: felsic (rhyolitic), intermediate (andesitic), mafic (basaltic)
Describe the formation of Crater Lake. Compare it to the calderas found on shield volcanoes such as Kilauea.
Mt. Mazama,a composite volcano in Oregon, erupted explosively about 7000 years ago. After the large eruption, about 1500 meters of the summit collapsed into the space once occupied by the erupted material producing a 10-kilometer wide caldera that later filled with water to form Crater Lake. The caldera on a shield volcano such as Kilauea, by comparison, forms when the liquid lava in the underlying magma chamber is slowly lost during eruption and the summit slowly collapses without the support.
List at least three volcanic hazards besides pyroclastic flows and lahars.
Other volcanic hazards include volcano-related tsunamis, volcanic ash erupted into flight paths of aircraft, and volcanic aerosols erupted to great heights that can alter levels of incoming solar radiation.
What is the term for the pulverized rock and fragments of lava and glass ejected from the vent when volcanoes erupt energetically?
Pyroclastic Materials
Describe pyroclastic flows and explain why they are capable of traveling great distances.
Pyroclastic flows are fiery flows of incandescent ash and lava fragments that race down volcanic slopes propelled by the force of gravity. They can reach great speeds and travel far from the volcano because the expanding gas helps separate the pyroclastic particles, creating a near frictionless environment.
Other than composite volcanoes, what volcanic landform can generate a pyroclastic flow?
Pyroclastic flows can be associated with lava domes. When a lava dome grows and becomes over steepened, it can collapse, releasing gases from the interior magma and triggering a pyroclastic flow.
Are pyroclastic materials a significant component of shield volcanoes?
Pyroclastic materials are not a significant component of shield volcanoes because of their low-viscosity lava. However, as a shield volcano matures, its lavas increase in viscosity, which can cause cinder cones to form at the summit.
How do the eruptions that created the Columbia Plateau differ from the eruptions that create large composite volcanoes?
The eruptions that produced the Columbia Plateau emitted fluid basaltic lava from elongate features in the Earth's crust called fissures rather than from central vents as in composite volcanoes. Additionally, due to the low viscosity of the lava, fissure eruptions form broad, flat fields of basalt (flood basalts) rather than the cone-shaped composite volcanoes associated with more silica-rich eruptions.
What is the geothermal gradient? Describe how the geothermal gradient compares with the melting temperatures of the mantle rock peridotite at various depths.
The geothermal gradient is the rate at which temperatures increase with depth. At various depths below Earth's surface, the melting temperature of mantle rock (peridotite) increases due to increasing pressure, and the geothermal gradient predicts that Earth's temperature will always stay below the temperature needed to begin melting the rock. Therefore, based solely on the geothermal gradient, mantle rock should never begin melting.
What name is given to the region having the greatest concentration of composite volcanoes?
The greatest concentration of composite volcanoes (stratovolcanoes) on Earth occurs along the "Ring of Fire" that rims the Pacific Ocean.
Which type of plate boundary generates the greatest quantity of magma?
The greatest volume of magma is produced at divergent plate boundaries (60 percent of Earth's total yearly output).
How does the composition and viscosity of lava flows differ between composite volcanoes and shield volcanoes?
The lava flow portion of a composite volcano is silica-rich lava of andesitic to rhyolitic composition compared to the almost exclusively silica-poor basaltic flows of shield volcanoes.
List the main gases released during a volcanic eruption.
The main gases released during an eruption are water vapor, carbon dioxide, and sulfur dioxide.
Contrast pahoehoe and aa lava flows
The surface of aa is covered with rubbly, blocky lava and the flows have a steep front. Aa flows are made from fluid basaltic flows that are slightly cooler that the pahoehoe flows. Because pahoehoe lava is hotter, it is more fluid and the resulting surface is smooth and ropey with a more gently sloping front.
List the two primary factors that determine the manner in which magma erupts.
The two primary factors that determine how magma erupts are its viscosity and gas content.
Describe the composition and viscosity of the lava associated with shield volcanoes.
The viscosity of lava that produces shield volcanoes needs to be relatively low so the lava can be fluid enough to flow great distances and build the broad, low-sloped features of the shield volcano. Therefore, the lava associated with shield volcanoes is low-viscosity basaltic in composition.
Distinguish among a conduit, a vent, and a crater.
The volcanic conduit is a pipe-like, circular path that magma takes to the surface. The vent is where the conduit reaches the surface and the lava and other material are released. The crate is the bowl-shaped depression that surrounds the vent.
Define viscosity.
Viscosity is a fluid's resistance to flow. Higher viscosity means higher resistance to flow, or more "thick and gooey." Lower viscosity means less resistance to flow, or more "thin and runny." The three factors that affect a magma's viscosity are: its temperature (higher temperature results in lower viscosity, just like pancake syrup), its composition in terms of silica content (higher silica content results in higher viscosity due to more silicate structures linking together), and its dissolved gas content (more dissolved gases result in lower viscosity due to the gases reducing the formation of the long silicate chains).
What is a parasitic cone, and where does it form?
Volcanic activity from an eruption on the flank of a volcano can build small cones referred to as "parasitic cones."
How do volcanic bombs differ from blocks of pyroclastic debris?
Volcanic bombs are pyroclastic particles larger than 64 millimeters in diameter and are ejected as incandescent, semi-molten lava; therefore, they take on a streamlined shape as they fly through the air. Pyroclastic material called "blocks" differ from bombs in that they are ejected as angular, hardened rock fragments.
Are volcanoes in the Ring of Fire generally described as effusive or explosive? Provide an example that supports your answer.
Volcanoes in the Ring of Fire are generally considered explosive because they are often the result of silica-rich, viscous magma that erupts explosively. A classic example of this is Mount St. Helens' 1980 eruption that devastated a 400-km2 area in Washington State.
What roles do water and other volatiles play in the formation of magma?
Water and other volatiles, under pressure, can lower the melting temperature of rocks. Therefore, at locations where water-rich ocean lithosphere is subducted, magmas can be generated.
In the movie "Volcano" basaltic lava flows are shown running down flat streets at speeds faster than humans could run. From your knowledge of magma viscosity, is this realistic? a) No, the viscosity of basalt magma is much higher than water, and thus would flow much slower than water b) Yes, the viscosity of basalt magma is similar to water so it would flow at comparable rates c) No, the viscosity of basaltic magma is much higher than water, and thus, would flow much faster than water d) There is no way of knowing because basaltic magmas can have viscosities similar to water or higher depending on temperature and fluids, and thus there isn't enough information
a) No, the viscosity of basalt magma is much higher than water, and thus would flow much slower than water
Which type of basaltic lava flow has a fairly smooth, unfragmented, ropy surface? a) Pahoehoe b) AA c) Pegmatitic d) Scoria
a) Pahoehoe
Which one of the following shows the correct order (left to right) of decreasing magma viscosity in order of composition? a) Rhyolite, andesite, basalt b) Andesite, rhyolite, basalt c) Basalt, rhyolite, andesite d) Basalt, andesite, rhyolite
a) Rhyolite, andesite, basalt
________________ are usually the most abundant gases emitted during basaltic volcanism. a) Water and carbon dioxide b) Chlorine and sodium c) Neon and ammonia d) Oxygen and nitrogen
a) Water and carbon dioxide
Which is not one of the factors in determining how magma erupts? a) Width of crater b) Viscosity c) Gas content d) They all are factors
a) Width of crater
What type of volcanic material is indicative of eruption under water? a) Pahoehoe flows b) Aa flows c) Pillow lavas d) Pyroclastic flows
c) Pillow lavas
