Review Questions and Answers; Volcanoes

The eruptive cycle represented the ascent of a "new" batch of magma from depth. This eruption was the most powerful in the cycle. After some early, small-volume, ash eruptions, a small magma chamber high in the cone began inflating (filling with more magma). However, the chamber expanded horizontally rather than vertically, causing the upper portion of the cone to bulge outward and, in a sense, to overhang the lower flanks. When this bulged mass of rock broke away from the main cone and slid rapidly downhill, the magma chamber was suddenly "opened" to the atmosphere and decompressed, generating the powerful May 18 eruption (Box 4.1).

2. List three factors that determine the nature of a volcanic eruption? What role does each play?

An eruption, explosive or otherwise, depends on three, fundamental factors: the nature of the magma, the depth of the magma chamber below the surface, and the excess pressure built up in the magma chamber (compared to what the pressure would be if an unobstructed column of magma extended to the surface). Important magma characteristics are temperature, composition, viscosity, and dissolved gas (volatile) content. Cooler, highly viscous, volatile- and silica-rich magmas are very likely to erupt explosively; hotter, more fluid, silica- and volatile-poor, basalt magmas commonly erupt nonexplosively as lava fountains.

High ratios of excess pressure to depth increase the chances for explosive eruptions. High volatile content and high viscosity both contribute to the buildup of high internal pressures in a magma body, increasing the chances for an explosive eruption. Magmas with low volatile contents generate correspondingly low internal fluid pressures, and a low magma viscosity allows exsolved gas bubbles to expand, rise, and exit the magma chamber without much buildup in pressure. Intrusion of hot mafic magma into the deeper levels of a cooler, more silicic, and more volatile-rich felsic magma chamber is an important triggering mechanism for explosive, caldera-forming eruptions. The mafic magma heats the more felsic magma and displaces it higher toward the surface. Heating and upward movement both favor increased magma overpressures and thus increase the likelihood of an explosive eruption. Later-stage, ash-flow tuffs from smaller-volume, caldera-forming eruptions, such as Crater Lake, OR, typically show more mafic character than the earlier tuffs (Fig. 3.7), showing that prior to the eruption, the magma deeper in the chamber was hotter and more mafic than the shallower magma.

3. Why is a volcano fed by highly viscous magma likely to be a greater threat than a volcano supplied with very fluid magma?

The more fluid magma is typically hotter and has a lower volatile content than the more viscous magma. The most important difference is that the more viscous magma has much more mechanical strength to resist movement and expansion of gas bubbles, thus confining the volatiles, promoting the buildup of excess pressure in the magma chamber, and increasing the likelihood of an explosive event. In a fluid magma, the gas bubbles can freely expand, rise, and escape from the magma chamber, reducing the probability of an explosive eruption.

4. Describe pahoehoe and aa lava.

These terms describe basaltic lava flows with different surface and flow-front characteristics. Aa flows are relatively thick with high, steep, flow fronts; their surfaces are covered with angular, congealed, lava rubble. Pahoehoe flows are thinner, the flow fronts are more gently sloping, and the surface is smooth or rippled (ropy). As the pahoehoe flow advances, small lava prongs break out, forming rippled blobs that move a short distance beyond the main flow front. When pahoehoe lava congeals, the smooth, rippled surfaces of blobs are preserved.

5. List the main gases released during a volcanic eruption. Why are gases important in eruptions?

Water (H2O) is generally the dominant gas; carbon dioxide (CO2) is typically the secondmost abundant gas in Hawaiian eruptions, but can be dominant at specific volcanoes, such as Mt. Vesuvius. In other eruptions, such as El Chichon, Mexico, and Pinatubo, the Philippines, sulfur dioxide (SO2) was the dominant volatile. Nitrogen (N2), hydrogen (H2), argon (A), hydrogen chloride (HCl), and hydrogen fluoride (HF) may also be released to the atmosphere during eruptions and fumarolic activity. Dissolved gases are important in volcanism because the large volume expansion that accompanies their dissolution from the melt pushes magma upward toward the surface and generates explosive overpressures in silicic magma chambers.

6. How do volcanic bombs differ from blocks of pyroclastic debris?

Both are pebble sized or larger pyroclastic fragments. Bombs are cooled from ejected magma blobs. They typically have very fine-grained, chilled margins, are vesicular, exhibit surface patterns characteristic of solidified liquid, have rounded, twisted shapes produced in flight, and may be flattened and cracked on impact. Essentially all bombs are vesicular to a greater or lessor extent.

Blocks are lithic clasts broken from preexisting rock. They are typically angular and show none of the morphological features associated with impacts, in-flight movements, and solidification of liquid or partly-liquid magma masses. Blocks may or may not be vesicular. If present, the vesicles show no particular relationship to edges or interior portions of the block; some vesicles may be amygdaloidal. Blocks may be heterolithologic since they represent random chunks torn out from the walls of an erupting vent. Bombs almost always consist of the major magma type being erupted from the vent.

7. Compare a volcanic crater to a caldera.

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.

8. Compare and contrast the main types of volcanoes (size, shape, eruptive style, and so forth).

Volcanoes are constructed of erupted volcanic material. With the lone exception of basaltic cinder cones, volcanoes are products of many eruptions and generally have long (a million years or so) eruptive histories. Cinder cones are small, fairly steep-sided cones comprised mainly, or entirely, of basaltic ash and cinders; they develop during a single, short-lived, eruptive cycle. Internal layering in the pyroclastic strata is parallel to external slopes. Shield volcanoes are very large, gently sloping, dome-shaped mounds built of successive outpourings of basaltic lavas. Composite volcanoes (stratovolcanoes) are massive, steep-sided, volcanic cones built from repeated outpourings of lava and pyroclastic material. Composite volcanoes may erupt some basalt but are more likely to erupt andesite and other magmas richer in silica such as rhyolite. Internal layering of lavas and pyroclastic beds is roughly parallel to the external slopes of both kinds of volcanoes.

9. Name a prominent volcano for each of the three types.

cinder cone - Sunset Crater (Fig. 4.11) near Flagstaff, AZ, is a very young, well-preserved, basaltic cinder cone. It was formed about 900 years ago. Sunset Crater, numerous nearby cinder cones, and associated basaltic lava flows have been set aside as a national monument.

composite volcanoes - The great volcanoes of the world such as Vesuvius near Naples, Italy; Pinatubo in the Philippines; and the Cascade Range volcanoes in Oregon, Washington, and northern California, are good examples.

shield volcanoes - The very large basaltic volcanoes of Hawaii (Mauna Loa and Kilauea) are good examples.

10. Briefly compare the eruptions of Kilauea and Parícutin.

Parícutin is a small, basaltic cinder cone built in a corn field in southern Mexico during a few years of eruptive activity in the 1940s. During the cone-forming phase, mainly pyroclastic materials (bombs, cinders, and ash) were erupted; later in the eruptive cycle, lava flows broke out from the base of the cinder cone and spread over the surrounding countryside. After a few years of continuing activity, the eruptive episode ended as abruptly as it had started.

Kilauea is the most active volcano on Hawaii, the largest of the Hawaiian Islands, and is part of a massive, basaltic, shield volcano complex that forms the island. Eruptions are mainly fluid, basaltic lava flows and minor pyroclastic activity. The volcanic activity began millions of years ago when submarine lava flows were erupted on the ocean floor. With continued activity, a massive, mound-shaped seamount was constructed; eventually it grew above sea level, forming the present-day island of Hawaii. Kilauea is the youngest, southeasternmost, subaerial volcano on the island but has yet to reach the elevation and size of the much larger shield volcanoes Mauna Loa and Mauna Kea. A newer, submarine, eruptive center (Loihi Seamount, Box 4.3) is currently forming southeast of Kilauea in keeping with the west-northwest migration of the Pacific plate over a hot spot deep in the mantle.

11. Contrast the destruction of Pompeii with the destruction of St. Pierre.

A nuée ardente generated by the 1902 explosive eruption of Mt. Pelée devastated the city of St. Pierre. The nuée ardente was evolved from a massive, pyroclastic flow that sped to the sea along a stream valley outside the city. However, at a fairly sharp curve in the valley, the nuée ardente portion of the flow jumped a low ridge and bore on straight toward the city. It was all over in a few minutes. The hot, violently turbulent, dust-and-ash cloud, moving at hurricane speeds, flattened buildings and suffocated all living beings in its path. Only a few centimeters of hot, very-fine size ash were deposited over the ruined city.

Pompeii and its sister city of Herculaneum were buried over a three to four day, cataclysmic phase of the 79 A. D. eruption of Mount Vesuvius. Pompeii was buried by 20 to 30 feet of airfall pumice and ash. Written accounts and archeological excavations suggest that many people escaped during the early phase of the eruption, and others managed to survive a day or two before succumbing to thirst and suffocation. Herculaneum was evidently buried suddenly by mudflows or pyroclastic flows unleashed simultaneously with, or shortly following, the climactic phase of the eruption that buried Pompeii. Additional, detailed, historical accounts and geological interpretations of the Vesuvius eruption can be found in "Volcanoes of the Earth" by Fred Bullard (1976).

12. Describe the formation of Crater Lake. Compare it to the caldera formed during the eruption of Kilauea.

Crater Lake (Oregon) caldera (Figs. 4.16 & 4.17) is about six miles in diameter. It formed following a major eruption of ash and pyroclastic flows about 7000 years ago. Glacial valleys cutting through the caldera rim and other geologic evidence prove that a complex, composite volcano once existed above the site of the present-day caldera. Indian legends and geological evidence suggest that the former mountain, Mount Mazama, had subsided within a few days following the end of the eruption. To have been completely buried and lost from view, the mountain must have subsided at least 5000 feet.

In contrast, the summit caldera block of Kilauea is about three miles in diameter and acts somewhat like a floating cork, rising when magma is accumulating and sinking after an eruption. The rising and sinking movements are gradual as contrasted with the catastrophic collapse that follows large-volume pyroclastic flow eruptions.

13. What is Shiprock, New Mexico, and how did it form?

Shiprock (Fig. 4.21), 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.

14. How do the eruptions that created the Columbia Plateau differ from eruptions that create volcanic peaks?

Large, voluminous, volcanic edifices such as Mts. Rainier, WA, and Shasta, CA (Fig. 4.13), 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 such as Shastina, the lower, steeper cone on the flank of Mt. Shasta (Fig. 4.13). 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 (Box 4.2). Higher viscosity magmas (andesite through rhyolite) erupt explosively or form thick, stubby, lava flows that, unless the lava is unusually hot, move only short distances from the vent.

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. At first only low areas are buried; eventually, the lava stack thickens and higher parts of the former land surface are buried. Later flows rest exclusively on earlier ones, and the lava pile attains a relatively flat upper surface. Thicknesses may exceed a few kilometers and the flows spread over vast areas (Fig. 4.18). Relatively low viscosities allow the hot lavas to move long distances before solidifying.

15. Where are fissure eruptions most common?

Fissure eruptions are generally associated with basaltic volcanism. Flood basalt provinces on the continents and mid-ocean ridges (Iceland; centers of seafloor spreading in the ocean basins) are common sites for fissure eruptions. Most flank eruptions on large, basaltic shield volcanoes, such as Kilauea, are fissure eruptions. However, most of the magma is extruded from one, major, lava fountain that remains active after magma injected along most of the length of the fissure has chilled and solidified.

16. Extensive pyroclastic flow deposits are most often associated with which volcanic structures?

Voluminous, pyroclastic-flow deposits are always accompanied by collapse of the rock above the evacuated part of the magma chamber, forming a caldera. The large magma volumes and high extrusion rates make collapse inevitable. Caldera collapse is often simultaneous with pyroclastic-flow emission, as shown by the tremendous thicknesses of ash-flow tuff deposited in large Tertiary calderas in Nevada, Utah, and the San Juan Mountain region in southwestern Colorado.

17. Describe each of the four intrusive features discussed in the text.

These are dikes, sills, laccoliths, and batholiths. Dikes are tabular, sheetlike igneous rock bodies emplaced into fractures and fissures cutting through the wall rock. Most dikes are steeply dipping to vertical, but some low-angle dikes are recognized. Dikes are generally discordant in that they usually cross cut bedding and other structures in the wall rocks. Extensive swarms of dikes occur in some areas. In some swarms, the dikes are crudely parallel to one another; in others, the dikes have a radial geometry and appear to diverge outward from a central point.

Sills are tabular, sheetlike igneous rock bodies emplaced parallel to bedding in enclosing strata or intruded as subhorizontal sheets into older igneous and metamorphic basement rocks. The Palisades Sill, a Triassic, mafic rock body exposed along the west side of the Hudson River valley near New York City, is a well-studied example. Older petrological studies emphasized sinking of early-formed crystals to explain the stratalike concentrations of olivine in the sill. More recent studies have suggested that these zones crystallized from later, olivine-rich, basaltic magma sheets concordantly injected when the sill was still partially liquid.

Laccoliths are relatively small-volume, intrusive igneous rock bodies. The typical laccolith is emplaced into subhorizontal sedimentary strata as a sill that simultaneously spreads laterally and inflates vertically, producing a magma body with dome-shaped upper contact and more-or-less horizontal, planar lower contact. Wall rock strata above the laccolith bend upward and stretch to conform with the upper contact of the magma body. Laccoliths are intruded at shallow depths; they represent intrusion in a subvolcanic environment.

Batholiths, generally of granitic rock, are the largest plutons. They are massive and discordant; they occur as extensive, linear arrays of separate plutons, many of which are large enough (> 100 square miles in cross-sectional area) to individually qualify as batholiths. Smaller plutons, not large enough to qualify as batholiths, are called stocks. Groups of contiguous, large, dominantly-granitic plutons, such as those Sierra Nevada, CA, region, are commonly designated as comprising a "regional batholith". The Sierra Nevada batholith is one of many mid-Mesozoic to early Cenozoic, regional batholiths intruded along the then-convergent, western margin of North America (Fig. 4.27).

18. Why might a laccolith be detected at Earth's surface before being exposed by erosion?

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.

19. What is the largest of all intrusive igneous bodies? Is it tabular or massive? Concordant or discordant?

The largest of all felsic, intrusive, igneous rock bodies are batholiths. They are massive, possibly tear-drop shaped, and discordant. Thicknesses may easily exceed 10,000 feet.

20. Spreading center volcanism is associated with which rock type? What causes rocks to melt in regions of spreading center volcanism?

Spreading centers are divergent plate boundaries. They lie above slowly rising, largely solid, mantle peridotite plumes that turn laterally as they near the surface, carrying the diverging plates in opposite directions. Melting temperatures of rock-forming minerals increase with higher pressure and decrease with lower pressure. As the plume rises, pressures and melting temperatures are lowered but the plume loses very little of its heat; thus rock temperatures stay constant. Eventually, temperatures exceed the melting range and partial melting occurs. More melting ensues as the plume rises closer to the surface. Basalt magma is the most common partial melt formed in a rising, mantle-rock plume.

21. What is the Ring of Fire?

This term refers to the strings and arcs of large, composite volcanoes that surround much of the Pacific Ocean. These volcanoes lie above subduction zones, where plates that comprise the Pacific Ocean floor are sinking beneath other oceanic plates or beneath plates carrying continents.

22. Are volcanic eruptions in the Ring of Fire generally described as quiescent or violent? Name a volcano that would support your answer.

Very large, composite volcanoes (stratovolcanoes), like those around the Pacific margin, typically erupt explosively. The 1991 eruption of Pinatubo in the Philippines was the secondmost 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.

23. Describe the situation that generates magma in subduction zone volcanism.

Partial melting associated with a subducting slab seems to begin at depths of about 100 km. Fluids released from the slab promote melting of hot peridotite in the overlying lithosphere. Also, materials at the top of the slab, such as sediments, hydrated volcanic rocks, and continental-rock slivers are in contact with hot, non-slab peridotite and may undergo partial melting. As the slab tip penetrates to deeper levels, upward, counter flows of hot peridotite are set in motion, resulting in decompression melting and production of basaltic magma. If the slab is cool and dense enough at the start of subduction, it may sink, unmelted, to depths as great as 700 km, as indicated by the deepest-known earthquake foci.

24. The Hawaiian Islands and Yellowstone are associated with which of the three zones of volcanism?

These volcanic areas both lie above fixed "hot spots", sites of long-lived, partial melting deep in the mantle below the base of the lithosphere. The hot spot track on the North Pacific plate is marked by the Hawaiian Island-Emperor Seamount chain; the track of a different hot spot beneath the North American plate is marked by the Columbia Plateau flood basalts, the Snake River Plain volcanic rocks, and the volcanic activity of the past few million years in Yellowstone National Park. The hot spots are thought to mark the tops of slowly rising mantle plumes; partial melting results from decompression accompanied by insignificant heat loss. Thus as it rises, the plume rock remains at essentially constant temperature while its melting temperatures decrease with the lower pressures. When rocks temperatures exceed the lower limit of the melting temperature range, partial melting begin and continues as the partly-melted plume rises to even shallower depths.