Lava flows

Lava flows are classified in terms of their surface characteristics. Flows showing smooth or hummocky, gently undulating surfaces and crusts locally wrinkled into ropelike forms are called pahoehoe. Flows showing very rough, irregular surfaces covered by jagged spinose fragments resembling furnace clinker are called aa (Fig. 5). The terms pahoehoe and aa are of Hawaiian origin. Flows in which the fragments that constitute the upper part of the flow are fairly smooth-sided polygons called block lava. Fluid basaltic and related mafic lavas characteristically form pahoehoe or aa flows, or flows intermediate in character between these two end members. In contrast, more viscous lavas such as andesites more commonly form block lava flows. All lava flows that have poured out on land contain various amounts of open cavities (vesicles), which mark the sites of gas bubbles formed as the rising magma reached regions of increasingly lower pressure before breaching the surface to erupt. Over geologic time, vesicles can be filled by minerals precipitated from circulating mineralizing fluids or ground water. Where pahoehoe flows enter bodies of water or wet ground, they may form heaps of irregular ellipsoids, in cross section somewhat resembling sacks of grain or pillows. Basaltic pillow lavas form an overwhelming bulk of the ocean floor. See also: Andesite

Fig. 5 An aa lava flow overriding an older pahoehoe flow during the 1990 eruption of the Kilauea Volcano, Hawaii, USA. (Credit: T. Mattox/USGS)

Pyroclastic materials

Magma, at depth and under great pressure, contains gas in solution, but, as it rises into regions of lower pressure near the surface of the Earth, the gas starts to exsolve and escape from the liquid. The gas generally escapes readily from fluid lavas, with little or no explosion; in more viscous liquids, however, the gas may acquire considerable pressure before it escapes and bursts forth in strong explosions. Thus, the emergence at the surface of highly gas-charged, viscous silicic lava may be attended by a sudden frothing as the contained gas rapidly exsolves and vesiculates. The sudden expansion of the gas may tear the froth into countless tiny shreds, each of which chills virtually instantaneously to form fragments of volcanic glass. Continued ebullition of gas results in a mass of small solid or quasisolid fragments, each surrounded by an envelope of still-expanding gas that pushes against all adjacent expanding envelopes. The net effect of this process is the isolation of the solid fragments from contact with one another, and the entire mass obtains an expansive quality that is enhanced further by the expansion of heated air trapped in the hot, moving mass. The result is a very mobile suspension of incandescent solid fragments in gas that may flow—hugging close to the ground—at great speed down slopes and spread out to great distances from the erupting vents, forming extensive deposits having nearly flat surfaces. The denser varieties of such flows are called pyroclastic flows (also ash flows), and their associated lighter-density components are called pyroclastic surges. When such flows come to rest, they commonly are still so hot that the fragments of glass stick together or even merge in the center of the deposit to form a layer of solid black obsidian. The resulting deposits are known as welded tuffs or ignimbrites; the degree of welding of ash flows is largely dependent on the temperature and thickness of the deposit. Rapidly moving incandescent ash flows have been called glowing avalanches or nuée ardentes; they can be highly destructive to life and property. For example, in 1902 a devastating nuée ardente produced during a violent eruption of Mont Pelée (island of Martinique in the Lesser Antilles) virtually destroyed the entire city of St. Pierre, killing some 30,000 persons. More recently, nuée ardentes generated during the March–April 1982 eruption of El Chichón Volcano (State of Chiapas, southeastern Mexico) wiped out all settlements within 5 mi (8 km) of the volcano, resulting in the loss of more than 2000 people (Fig. 6). See also: Ignimbrite; Obsidian; Tuff

Fig. 6 Remnant of a building in the village of Francisco Leon that was destroyed after being swept by nuées ardentes from the 1982 eruption of the El Chichón Volcano in southeastern Mexico. The reinforcement rods in the concrete wall are bent in the direction of the flow. (R. I. Tilling/USGS)

Pyroclastic materials are classified by the nature of the shreds and fragments: essential—the erupting molten lava; accessory—solidified lava of previous eruptions of the same volcano; and accidental—solid material of still older, not necessarily volcanic, rocks from the crust beneath the volcano. The classification of pyroclastic materials also considers size, shape, and consistency of constituent fragments and matrix. Blobs or drops of material still liquid enough to assume rounded or aerodynamically drawn-out forms during flight are known as lapilli (if 0.16–1.2 in. or 4–32 mm in average diameter) and bombs (if greater than 1.2 in. or 32 mm). Depending on their final shapes when they strike the ground, bombs are variously called cow-dung bombs, spindle or fusiform bombs, or ribbon bombs.

Irregular fragments of frothy lava of bomb or lapilli size are called scoria or cinder; if the fragments are sufficiently plastic to flatten or splash as they hit, they are called spatter. The still-molten fragments of spatter often adhere to each other to form welded spatter, or agglutinate. Angular fragments larger than 1.2 in. (32 mm) either solid or too viscous to assume rounded forms during flight are known as blocks; their accumulation forms a volcanic breccia. Ejecta smaller than 0.16 in. (4 mm) are called ash, and those smaller than 0.01 in. (0.25 mm) are called dust. Indurated (hardened) volcanic ash or dust is called tuff.

The term tephra had been applied liberally to all pyroclastic materials, but this term should be reserved, as originally defined, for those pyroclastic deposits of air-fall origin regardless of the size of the ejected material. The individual pyroclastic fragments—the building blocks of pyroclastic rocks—are called pyroclasts. See also: Pyroclast; Pyroclastic rocks

Volcanic gases

In general, water vapor is the most abundant constituent in volcanic gases; the water is mostly of meteoric (atmospheric) origin, but some volcanoes can have a significant magmatic or juvenile component. Excluding water vapor, the most abundant gases are the various species of carbon, sulfur, hydrogen, chlorine, and fluorine (such as CO₂, CO, SO₂, SO₃, H₂, H₂S, Cl₂, F₂, HCl). As discussed above, the related processes of rapid exsolution, expansion, and release of gas from the magma as it ascends near the Earth's crust provide the energy for driving and sustaining volcanic eruptions. Volcanic gases, as well as the erupted solid volcanic materials, can pose a hazard to humans. In August 1986, at Lake Nyos, a volcanic lake occupying the crater of a geologically young volcano in Cameroon (western Africa), the sudden overturn of deep lake waters caused the massive release of carbon dioxide (CO₂) gas of magmatic origin, asphyxiating more than 1700 people in low-lying areas.

Volcanic aerosols

Violent volcanic explosions may throw dust and aerosols high into the stratosphere, where it may drift across the surface of the globe for many thousands of miles. Fine ash and dust from eruptions of Icelandic volcanoes have fallen in the streets of Moscow. Studies have shown that the particles in the eruption cloud are mostly angular bits of lava, many of them glassy; but some glass spheroids also are present, as well as liquid droplets of hydrous solutions of sulfuric acid and various sulfates and chlorides. Most of the solid particles in the volcanic cloud settle out within a few days, and nearly all settle out within a few weeks, but the gaseous aerosols (principally sulfuric acid droplets) may remain suspended in the stratosphere for several years. Such stratospheric clouds of volcanic aerosols, if sufficiently voluminous and long-lived, can have an impact on global climate. For example, the volcanic cloud from the June 15, 1991, eruption of Mount Pinatubo (Luzon, Philippines)—the world's second largest in the twentieth century—resulted in the lowering of the average surface temperature in the northern hemisphere by about 1°C for nearly two years. See also: Aerosol; Atmospheric chemistry

Volcanic mudflows

Mudflows are common on steep-side volcanoes where poorly indurated or nonwelded pyroclastic material is abundant. They may form by eruptions involving the water of a crater lake; after breaching its confining walls, the water sweeps down the mountainside, incorporating and mixing with loose volcanic debris to form a slurry. Such slurries can be quite dense and commonly have a consistency similar to wet concrete. Mudflows may also form by hot or cold volcanic debris avalanches descending into streams or onto snow or ice. Probably by far the most common cause, however, is simply heavy rain saturating a thick cover of loose unstable pyroclastic material on the steep slope of the volcano, transforming the material into a mobile, water-saturated “mud,” which can rush downslope at a speed as great as 50–55 mi (80–90 km) per hour. Such a dense, fast-moving mass can be highly destructive, sweeping up everything loose in its path. Volcanic mudflows can be hot or cold; they are sometimes called lahars, a term from Indonesia, where they have taken a heavy toll in property and human lives (Fig. 7). Fast-moving mudflows triggered by a small eruption on November 13, 1985, at Nevado del Ruiz, Colombia, killed about 25,000 people. The Ruiz catastrophe was the worst volcanic disaster in the twentieth century since the devastation associated with the eruption of Mont Pelée, Martinique in 1902.

Fig. 7 Houses in the Cibanjaran River buried by lahars (volcanic mudflows) generated during the 1982–1983 eruption of Galunggung Volcano, Province of West Java, Indonesia. (Credit: R.T.Holcomb/USGS)

Somewhat related to mudflows are the great floods of water, known in Iceland as Jökulhlaup, that result from rapid melting of ice by volcanic eruption beneath a glacier. A subglacial eruption in October 1996 at Grímsvötn Volcano beneath the Vatnajökull ice sheet, Iceland, produced abundant hyaloclastite. In November 1996, the catastrophic release of impounded melt water from the eruption produced a destructive glacial outburst flood that affected more than 270 mi² (750 km²) and destroyed or severely damaged several bridges. See also: Glaciology

Major edifices

The gross form of a volcano is largely determined by the viscosity and mode of eruption of the volcanic products, even though other factors may be operative locally on a smaller scale.

Not all volcanoes show a graceful, symmetrical cone shape, such as that exemplified by Mount Fuji, Japan, or Mayon Volcano, Philippines (Fig. 8). In reality, most volcanoes, especially those near tectonic plate boundaries, are more irregular, although of grossly conical shape. Such volcanoes, called stratovolcanoes or composite volcanoes, typically erupt explosively and are composed dominantly of andesitic, relatively viscous and short lava flows, interlayered with beds of ash and cinder that thin away from the principal vents. Volcanoes constructed primarily of fluid basaltic lava flows, which may spread great distances from the vents, typically are gentle-sloped, broadly upward convex structures that resemble a Germanic warrior's shield in form. Such shield volcanoes, classic examples of which are Mauna Loa and Mauna Kea volcanoes, Hawaii (Fig. 9), tend to form in oceanic intraplate regions and are associated with hot-spot volcanism. The shape and size of a volcano can vary widely between the simple forms of composite and shield volcanoes, depending on magma viscosity, eruptive style (explosive versus nonexplosive), migration of vent locations, duration and complexity of eruptive history, and posteruption modifications (Fig. 10a). Large shield volcanoes are much larger than the largest composite volcanoes (Fig. 10b). See also: Hot spots (geology)

Fig. 8 Mount Fuji, a symmetrical composite volcano on Honshu Island, Japan.

Fig. 9 Mauna Kea Volcano, 13,800 ft (4208 m) high, viewed from the Mauna Loa Solar Observatory, Hawaii, USA. Mauna Kea shows the classic gently sloping and broadly convex form of a shield volcano whose surface has been peppered with numerous smaller and steeper-sided cinder cones (see Fig. 10). (Credit: E. Endo/USGS)

Fig. 10 Profiles of volcanoes. (a) Some common types of volcanoes; the relative sizes shown are only approximate, and dimensions vary greatly within each group. For the major edifices, the vertical exaggeration is about 2:1, and for the minor edifices, about 4:1 (After T. Simkin and L. Siebert, Volcanoes of the World, 2d ed., Geoscience Press, 1994). (b) Hawaiian shield volcanoes (Mauna Loa and Kilauea) compared with Mount Rainier (Washington), one of the larger composite volcanoes of the Cascade Range, drawn at the same scale (no vertical exaggeration) (From R. I. Tilling et al., Eruptions of Hawaiian Volcanoes: Past, Present, and Future, USGS, 1987).

Some of the largest volcanic edifices are not shaped like the composite or shield volcanoes. In certain regions of the world, voluminous extrusions of very fluid basaltic lava from dispersed fissure swarms have built broad, nearly flat-topped accumulations, some covering hundreds of thousands of square miles with volumes of several tens of thousands of cubic miles. These voluminous outpourings of lava are known as flood basalts or plateau basalts. Two examples in the United States are the Columbia River Plateau (Fig. 11) and the Snake River Plain volcanic fields; similar large features exist in India (Deccan Plateau) and South America (Paraná Basin).

Fig. 11 Flood basalt lava flow in stacked layers viewed eastward across the Columbia River, Rowena Crest Viewpoint, Oregon making up part of the Columbia River Plateau in the northwestern region of the United States. (Credit: USGS)

The only volcanic products other than basalt that are sufficiently fluid and voluminous to form extensive volcanic plateaus or plains are ash flows. Some of the larger examples of these ash-flow or ignimbrite plains, such as the central part of North Island of New Zealand, the Jemez Mountains region of New Mexico, and the Yellowstone Plateau of Montana-Wyoming, involve repeated eruptions of ash flows covering many thousands of square miles.

Minor structures

In general, minor volcanic structures originate or develop on or near the major edifice during a single eruptive event or a short span of activity, whereas the major edifices represent the result of repeated activity spanning many thousands or even millions of years.

Lava too viscous to flow readily may accrete in or near the vent to form a steep-sided heap known as a lava or volcanic dome; such domes commonly develop following vigorous explosive activity at composite volcanoes (Fig. 12). Slender spires that thrust through apertures in such a dome are termed spines. The famous spine of Mont Pelée (Martinique, Lesser Antilles), formed during the eruption of 1902, reached a height of more than 1280 ft (390 m), but, like most such spines, was very short-lived. Other near-vent forms that may develop on the volcano include volcanic shield, cinder cone, spatter cone (pipe vent), spatter rampart (fissure vent), ash cone, tuff cone, and tuff ring.

Fig. 12 Ash and gas plume rises from the volcanic dome growing inside the large, amphitheater-like crater formed by the May 1980 eruption of Mount St. Helens. In this picture (taken May 1983), the dome measured about 2800 ft (850 m) long, 2600 ft (800 m) wide, and 750 ft (230 m) high; the dome continued to grow until October 1986. (Credit: L. Topinka/USGS)

Major edifices are commonly modified by negative or depressional landforms. Small bowl-shaped depressions that are formed by explosion, or by failure of pyroclastic ejecta to accumulate directly above a vent, are known as craters. Most of them are found at the summit or on the flanks of volcanic cones, but some are well away from any cones.

Larger depressions at the summit of volcanoes are formed by collapse of the summit region as the support beneath it is removed by the rapid withdrawal of magma, usually by surface eruption but sometimes by subsurface migration of magma within the volcano. A depression formed by collapse is called a pit or collapse crater, or if larger than about 1.2 mi (2 km) in diameter, a caldera. Perhaps the best-known examples of calderas in the United States are Crater Lake, Oregon; Kilauea Caldera, Hawaii; Yellowstone National Park (Wyoming); and Valles Caldera, Jemez Mountains, New Mexico. Larger, although less regular and obvious, features of similar origin are known as volcanic-tectonic depressions. Like many calderas, their formation commonly, if not always, is associated with the explosive eruption of great volumes of ash flows. See also: Caldera; Igneous rocks; Petrology