A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat (geothermal activity), explosive release of gases (including steam with the interaction of magma and surface of ground water), and the non-explosive extrusion or intrusion of magma. An event could be non-destructive without release of solids or magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or explosive activity. Destruction usually refers to the works of mankind (buildings, roads, agricultural land, etc.).
A volcanic event can include
(1) an eruptive pulse (essentially an explosion with an eruption plume, but also non-explosive surges of lava. A pulse may last a few seconds to minutes,
(2) an eruptive phase that may last a few hours to days and consist of numerous eruptive pulses that may alternate between explosions and lava surges, and
(3) a single eruption or eruptive episode, composed of several phases, that may last a few days, months or years (Fisher and Schmincke, 1984). Paricutin, Mexico was in eruption for nine years. Stromboli, Italy has been in eruption for over 2000 years.
Simkin et al. (1981) define eruptions in terms of inactive periods. An eruption that follows its predecessor by less than 3 months is considered to be a phase of the earlier eruption unless it is distinctly different (explosive versus effusive, different magma type).
Some volcanoes (e.g., domes and basaltic scoria cones) may form completely within a few weeks or months. Others, such as shield volcanoes and composite volcanoes may show high order discontinuities such as major chemical changes, volcano-tectonic events like caldera collapse, or long erosional intervals, and may last 10 m.y. or more before volcanism completely dies out.
During a single eruption, styles of activity and types of products may change within minutes or hours, depending upon changes in magma composition, volatiles, or other magma chamber and vent conditions.
Common eruptions types are Plinian, Hawaiian, Strombolian, and Vulcanian. Gas-only eruptions are not so common.
At the present time, carbon dioxide gas is seeping upward through Mammoth Mountain, a composite volcano on the edge of Long Valley Caldera. The carbon dioxide leaks are occurring at several places around the volcano. Long Valley and Mammoth Mountain are being watched by the U.S. Geological Survey. It is not known whether or not the CO2 leaks could be a precursor to a volcanic eruption.
(This article is still "in progress")
Sparks et al. (1978) concluded that a pyroclastic flow develops around the base of a collapsing eruption column, deflates, and then moves outward across the landscape under its own momentum. In their model, the momentum that a pyroclastic flow acquires is proportional to the height from which the eruption column collapses.
The conclusion that momentum is the main cause of transport of pyroclastic flows influenced the "energy line" concept of Sheridan (1979). Sheridan explained that the slope of the energy line as proposed by Hsu (1975) for avalanche runout, traces the potential flow head from the top of the gas-thrust region of an eruption column to the distal toe of a flow along the line of transport. However, only a tiny fraction of the total fragmental component reaches the top of the gas-thrust part of an eruption column. Most of the fragmental material that falls back is located between the top of the gas-thrust and the ground surface. The total momentum acquired cannot be a single mass number that attains a particular height, but a summation of all the fragmental mass and the different heights to which they attain. Thus, the calculated momentum value is exaggerated.
McEwen and Malin (1989) argued that the energy-line model predicts velocities that are too high, resulting in flow paths that are insufficiently responsive to topography. They suggest that velocity-dependent resistance factors such as Bingham or turbulent models are needed for accurate velocity predictions. Viscosity-dependent factors, however, are less effective resistance factors than internal processes that effect sediment gravity flow such as flow transformations, density stratification, decoupling and blocking, particularly in mountainous regions. These processes need to be considered as resistance factors that affect the forward progress of flows. Fisher (1990), for example, shows that mountainous terrain itself can be considered as a roughness element that significantly effects runout distance.
Pyroclastic flows can originate in several ways. It is generally accepted that the main origin is by gravitational collapse of a vertical eruption column (Sparks and Wilson, 1976; Sparks et al., 1978). The collapse of vertical eruption columns as a process in the origin of pyroclastic flows was first recognized by Kozu (1934) and discussed by Smith (1960a) and Williams (1942). Collapse of columns was postulated from sedimentological data by Hay (1959a).
Other ways by which pyroclastic flows originate is (1) the "boiling-over" of a highly gas-charged magma from a crater (Wolf, 1878; Anderson and Flett, 1903; Taylor, 1958), (2) inclined blasts from the base of an emerging spine or dome (Lacroix, 1904; Perret, 1937), (3) collapse of a growing dome (Escher, 1933; Schmincke and Johnston, 1977; Mellors et al., 1988), (4) low elevation fountaining (Hoblitt, 1986; Valentine and Wohletz, 1989), and (5) explosive disruption from the front of a lava flow as observed at Santiaguito Volcano, Guatemala in 1973 (Rose et al., 1977) (Fig. 8-40).
"Bulk subsidence" (column collapse) was suggested as the cause for development of pyroclastic flows by Fisher (1966b). The term bulk subsidence was used for the processes that led to the base surge development from an underwater atom bomb explosion at Bikini Atoll (Brinkley et al., 1950). Bulk subsidence of an eruption column was described from a series of photographs showing the development of a base surge at Capelinhos (Azores) (Waters and Fisher, 1971).
The connection between column collapse and the origin of pyroclastic flow and surge deposits was firmly established by Sparks and Wilson (1976) and Sparks et al. (1978), but it must be pointed out that bulk subsidence and column collapse are visualized as two different processes. Development of nues ardentes by column collapse was recognized during the 1969 eruption of Mayon volcano (Phillipines) (Moore and Melson, 1969), during the 1974 eruption of Ngauruhoe Volcano (New Zealand) (Nairn et al., 1976; Nairn and Self, 1978). Devastation on all sides of El Chichon volcano (Mexico) occurred during one of the eruptive episodes of its 1982 eruption (Sigurdsson et al., 1984, 1986), and on all sides of Mt. Pele (Martinique) during its 30 August 30 1902 eruption. Observations of base surge and nuee ardente development, coupled with well reasoned theoretical arguments on the criteria for column collapse (Wilson, 1976; Sparks and Wilson, 1976; Sparks et al., 1978; Wilson et al., 1980; Wilson and Walker, 1987) and numerical modeling (Valentine and Wohletz, 1989) has established the validity of the process. Some pyroclastic flows and surges, however, have originated without development of high vertical eruption columns, such as the 1951 eruption of Mount Lamington (Papua) (Taylor, 1958). Several times during its eruption, convoluted clouds filled the crater but had little tendency to rise; instead, the heavier parts flowed through low gaps in the crater wall whereas the lighter parts poured over the crater rim. Wolf (1878) reported an eruption at Cotopaxi (Ecuador) which "glowing lava" "boiled over" from the crater and flowed with furious velocity in all directions down the slopes, a description similar to that given by Taylor (1958) to some of the flows at Mount Lamington. Activity similar to the "massive disgorgements" at Mount Lamington also occurred during at least one eruptive episode on July 9, 1902 at Mt. Pele (Anderson and Flett, 1903, p. 492-493). During the 18 May eruption of Mount St. Helens, following the the blast phase, most of the pumiceous pyroclastic flows formed when "bulbous masses" of inflated ash, lapilli and blocks erupted to a few hundred meters like a fountain above the inner crater and then spilled out through the open crater to the north (Rowley et al., 1981). These upwellings took place before or during the development of the gas thrust of the Plinian column that occurred without visible column collapse. Pyroclastic flows and surge development also preceded the vertical eruption column during the 22 July and 7 August 1980 eruptions of Mount St. Helens (Hoblitt, 1986). Each eruptive pulse began with a fountain of gases and pyroclasts around the vent that generated a pyroclastic density current. The change from fountaining to vertical column activity is interpreted to be caused by an increase in the gas content of the eruption jet or else a decrease in vent radius with time. Sparks et al. (1978) postulated that pyroclastic flows originate following the fall-back of a turbulent, collapsing eruption column and then move outward as a non-turbulent flow. Their calculations, using flow velocities ranging from 10 to 200 m/s, a drag coefficient of 0.01, and terminal velocity measurements of pyroclastic particles by Walker (1971), showed that grains >1 mm could not be carried in suspension. The runout length of pyroclastic flows and their ability to surmount topographic barriers are topics of continuing research germane to the distribution of ignimbrite sheets. Pumice-rich pyroclastic flows are known to have crossed topographic barriers of considerable height (Yokoyama, 1974; Miller and Smith, 1977; Koch and McLean, 1975; Rose et al., 1979). The 22,000 yr B.P. Ito pyroclastic flow (Japan) traveled 70 km over topographic barriers as high as 600 m (Aramaki and Ui, 1966; Yokoyama, 1974). The 18,000 B.P. Taupo Ignimbrite, only ~30 km3 in volume, is spread out over a ~20,000 km2 area and mantles mountains as high as 1500 m above the inferred vent as far as 45 km from the source (Wilson, 1985; Wilson and Walker, 1985). Pyroclastic flows from Aniakchak and Fisher calderas in the Aleutian Islands traveled as far as 50 km over mountainous barriers between 250 and 500 m high (Miller and Smith, 1977). Currently, there are two general models that describe the way that pyroclastic currents move across the landscape -- (1) as expanded flows (EFs) thicker than the height of the mountains they traverse, or (2) as dense flows (DFs) moving as a nonturbulent ground-hugging sheet across the landscape (Sparks, 1976). The purpose of the present paper is to test these ideas by analyzing the stratigraphy and flow directions, as determined by anisotropy of magnetic susceptibility (AMS) measurements (see below), of the Campanian Tuff. The two models require fundamentally different hydrodynamic behaviors. For EFs, the pyroclastic current must remain turbulent to maintain its expansion to a thickness greater than the topography that it overtops. Also, an expanded current can travel across water because expansion reduces its bulk density so that only its basal part interacts with the water (Sigurdsson et al., 1991). Dense pyroclastic flows probably cannot easily travel above water. Moreover, should they enter and travel beneath water, viscous boundary effects, mixing with water, and other conditions would inhibit flow, and it is unlikely that they could re-emerge. One critical observation applicable to the problem discussed herein is that pyroclastic currents such as nues ardentes are known to separate gravitationally into a lower part containing most of the solid fragmental mass. This natural density stratification in initially turbulent pyroclastic currents (Valentine, 1987) and other sediment gravity flows (Fisher, 1983, 1984) commonly results in flow transformations from turbulent to nonturbulent behavior in basal zones where concentration values become high. Having different densities and turbulent behaviors, the different parts of the current can decouple and travel different paths, depositing material independently (Fisher, 1990). In mountainous terrain, flow transformations, decoupling and divergent flow directions are especially amplified. Study of these effects can contribute to a better understanding of pyroclastic flow emplacement processes. Valentine (1987) concluded that pyroclastic flows may become density stratified and do not necessarily completely collapse to a non-turbulent condition of flow. According to his model, density stratification does not necessarily form a surface above which is mostly gas and below which is a dense flow, but rather there is a continuous gradation from one to the other. At flow velocities of r100 m/s and r300 m/s, particles as large as 1 cm and 10 cm repectively can be turbulently supported -- considerably larger than sizes calculated by Sparks et al. (1978). The differences in supportable clast sizes stem from the choices of substrate roughness and boundary layer thickness. Valentine (1987) proposed a rougher terrain than Sparks et al. (1978), with roughness elements (such as tree stumps) up to 1 m. Sparks et al. (1978) assumed a flat terrain with a roughness of 1 cm and considered the whole flow as a boundary layer. The EF model contends that a pyroclastic current may initially be of medium- to low-density, but unlike the DF model, it remains expanded as it travels over the landscape leaving behind a depositional carpet deposited from its basal part, a model proposed by Fisher (1966) and extended by Branney and Kokelaar (1992). Flow of DFs is based upon a plug flow model whereby deposition is thought to occur by en masse freezing of the debris, similar to nonvolcanic debris flows, rather than layer by layer accretion (Sparks, 1976). Pyroclastic eruptions commonly produce eruption columns that transport volcaniclastic particles from beneath the ground into the atmosphere. The eruption column is a gas-solid dispersion that is columnar-shaped and extends into the atmosphere from the surface vent. The physical properties and dynamic processes within eruption columns affect many physical attributes of pyroclastic deposits. Moreover, the different properties of eruption columns define the diverse styles of classically defined pyroclastic eruptions. Sustained explosive volcanic eruptions into the atmosphere commonly produce volcanic plumes. Many features of their origins, shapes, and dynamic behavior have become quantitatively known only in the past decade (Wilson, 1976; Blackburn et al., 1976; Sparks and Wilson, 1976; Settle, 1978; Wilson et al., 1978; Sparks, 1986; Valentine and Wohletz, 1989). Dispersal of fragments from them are becoming clarified (Carey and Sparks, 1986; Wilson and Walker, 1987). The height of eruptions columns (up to 50 km) and wind vectors determine particle distributions from volcanic eruptions (Chap. 6) Eruption columns are divided into three main components: the lower or gas thrust part, a central convective thrust part (Wilson, 1976; Blackburn et al., 1976; Sparks and Wilson, 1976; Wilson et al., 1978), and an upper part known as the umbrella region (Sparks et al., 1986; Sparks, 1986). Expansion of juvenile volcanic gas, and, in Vulcanian eruptions, pressure from expanding steam are the driving forces for the gas thrust part. Collapse criteria have been based upon the effects of exit velocity, gas content, vent radius (Sparks et al., 1978; Wilson et al., 1980; Wilson and Walker, 1987), but an important parameter is also shown to be the effect of exit pressure (Valentine and Wohletz, 1989) based upon numerical modelling. The numerical modelling by Valentine and Wohletz (1989) also suggests that column behavior is much more sensitive to the exit pressure ratio than to the density ratio between the column and the atmosphere. Widely dispersed sheets of pumice and ash are derived from high eruption columns that result from high-velocity voluminous gas-rich eruptions, commonly lasting for several hours to about four days (Fig. 4-4). These are called Plinian because Pliny the Younger described the famous 3-day eruptions of Vesuvius in 79 AD during which the towns of Pompei and Herculaneum were buried by several meters of fallout pumice, followed by pyroclastic surges and flows (Sigurdsson et al., 1985) that are an integral part of many Plinian deposits (Figs. 8-21, 8-31). Plinian fallout is commonly associated with voluminous pyroclastic flow deposits from calderas (Chaps. 6, 8). Here we briefly summarize some features of the controls of eruptive processes in Plinian and related eruptions (Walker et al., 1971; Walker, 1973; Wilson, 1976, 1980; Sparks and Wilson, 1976; Sparks et al., 1978, Wilson et al., 1978, 1980). The energy and characteristics of a Plinian eruption depends on many factors, among which gas content of the magma, exit pressure, viscosity, vent radius and shape, and volume of magma erupted are especially important. Most Plinian eruptions result from explosions of highly evolved rhyolitic to dacitic, trachytic and phonolitic magmas with liquidus temperatures from about 750x to 1000xC. Thus, a mean temperature of 850xC is assumed in the following discussion (Wilson et al., 1980). The eruption velocity, Uv, is nearly proportional to the square root of temperature. This enables adjustments for different temperatures. Density is assumed to be 2.3x103 kg m-3, and volatile content about 5 weight percent, dominantly water, as discussed in the previous section. The viscosity is about 104 to 107 Pas (rhyolite). Wilson et al. (1978) have shown that maximum column height, H, is proportional to the fourth root of the mass eruption rate. If 70 percent of the heat released by the erupted material is used to drive convection, then H = 236.6 m1/4. Wilson et al. (1980) have discussed three combinations of vent radius and gas content in monitoring exit velocities in column height (Fig. 4-7). They show that while velocity drastically decreases with decreasing gas content, column height is mainly dependent on vent radius. Column collapse, at conditions of constant vent radius equal to 200 m, only occurs when water contents drop below 2.4 percent. Widening vent radius may also lead to reverse grading commonly reported in Plinian deposits. An initial Plinian phase will be followed by pyroclastic flows when either gas content decreases or the vent widens (Fig. 4-8). It should be noted, however, that numerical modeling by Valentine and Wohletz (1989) suggests that the formation of a Plinian column does not require entrainment and heating of atmospheric air, and the pressure effects that they present do not support the assumption that column behavior is determined entirely by the efficiency of air entrainment. Salient features of Plinian type eruptions and their products are summarized in Table 4-2. Carey and Sigurdsson (1986) have developed a model of pyroclastic dispersal that discriminates between eruption column height and transport by local winds based upon the geometry of particle isopleth maps constructed from field measurements. They used the model to calculate eruption column height and from that, eruption intensity based on the behavior of convective plumes under a variety of atmospheric conditions. Eruption intensity is defined as the volume-rate at which magma is discharged. They (Carey and Sigurdsson, 1989) further show that peak eruption intensities (i.e., magma discharge rate) is positively correlated with the magnitude (total erupted mass; all erupted products). Initial Plinian fall phases with intensities > 2.0 x 108 typically precede the onset of a major pyroclastic flow (chap. 8) and caldera subsidence. During eruptions of large magnitude, the transition to pyroclastic flows is likely to be the result of high intensity, whereas the generation of pyroclastic flows in small magnitude eruption s may occur more often by reduction of magmatic volatile content or other transient changes in magma properties. As shown in figure 4-8, transitions from a convecting column to a collapsing column can occur by two different trajectories shown by arrows: (1) large increases in intensity or vent size or (2) decrease in volatile content or exit velocity of magma. Carey and Sigurdsson (1989) suggest that caldera-forming events which generate large-volume pyroclastic flows follow path (1) and small-volume pyroclastic flow may occur along path (2).
Strombolian eruptions, named after Stromboli Volcano, Italy, are discrete explosions separated by periods of less than a second to several hours. They give rise to ash columns and abundant ballistic debris. Ejecta consist of bombs, scoriaceous lapilli and ash. Stromboli, and other Italian volcanoes are described in Boris Behncke's page on Stromboli.
The 1980 eruptive episode of Mount St. Helens included more than one type of eruptive behavior and more than one kind of volcanic hazard. It is not uncommon for volcanoes to exhibit a range of eruptive types during an eruption.