Chapter 2.1 Where do tuffs fit into the framework of volcanoes?
Published:January 01, 2006
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R.V. Fisher, Grant Heiken, M. Mazzoni, 2006. "Chapter 2.1 Where do tuffs fit into the framework of volcanoes?", Tuffs - Their Properties, Uses, Hydrology, and Resources, Grant Heiken
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Tuffs are consolidated pyroclastic or volcaniclastic rocks. To understand the properties of tuffs, the heart of this handbook, the processes that create and deposit them need to be understood. This brief chapter is to acquaint the reader with the basic concepts and terms associated with explosive volcanism.
A Summary of the Origins of Pyroclastic Rocks
Explosive volcanic eruptions that deposit tuff are frequent and exhibit a wide range of activity and magnitude. To understand the explosive eruption process is difficult because we can approach only the smallest and least energetic of eruptions, the lava fountains of basaltic magmas. In contrast, the world's largest explosive volcanoes can erupt hundreds or thousands of cubic kilometers of ash and pumice in a few days, devastate thousands of square kilometers around a volcano, and produce ash clouds that circle the planet—these eruption processes are difficult to investigate. Even for the smallest of eruptions, we cannot see what is going on in the vent, so how do we know how magma is fragmented and erupted explosively to produce the deposits that eventually become tuffs?
On a few well-instrumented volcanoes, observations of long-period seismic tremor below and within volcanoes led seismologists to conclude that explosive eruptions are caused by pressure disturbances within the rising magma (e.g., Morrissey and Chouet, 1997). Morrissey and Chouet interpreted the long-period tremor, with periods of 0.2–2 s as resonances within magmafilled volcanic conduits or flow-induced nonlinear oscillations. Some of these events occur before explosive activity begins. This approach to studying explosive volcanic eruptions is still in its infancy, but may provide the observations needed to understand explosive eruption phenomena.
Theory and Numerical Modeling
An understanding of magma fragmentation began to develop in the 1930s, and major theories evolved in the 1950s and 1960s. These theories were based on rheological properties of a vesiculating magma. Magma “vesiculates” when dissolved gas comes out of solution to form bubbles because of decreasing pressure as magma nears Earth's surface. During the 1980s, papers on the theoretical behavior of vesiculating liquids and fragmenting magmas led to a new subdiscipline within volcanology—physics-based eruption-process modeling (e.g., Valentine and Wohletz, 1989). High-performance computing and computing codes originally developed for weapons research were applied to the understanding of explosive volcanism. These models demonstrated that, depending on magma composition, gas content, and magma flux, fragmentation can occur along a moving rarefaction wave, moving down the volcano conduit from the surface to depths of 2–3 km. The gas and fragmented products (ash and pumice) are subsequently accelerated out of the vent to form ash fallout and pyroclastic flows.
Wohletz and McQueen (1984) conducted a series of experiments in which a melt (thermite) was mixed with differing quantities of water. When mixed in the proper proportions (H2O:melt of 0.2 to ∼10), the two fluids mixed explosively and produced finely fragmented hydrovolcanic (or hydromagmatic) ash. These ashes are typical of what we find for volcanoes associated with shallow-water bodies or very productive aquifers. Numerical modeling of magma fragmentation caused by gas coming out of solution is a current topic of active research by groups in Germany and Italy.
Studies of Gases Released from Volcanoes
Recent studies at Galeras volcano, Colombia (e.g., Stix et al., 1997) and Popocatépetl volcano in Mexico have shown that decreases in the flux of gases released from the volcano may precede explosive eruptions of domes slowly growing within the crater. An understanding of the volume and composition of gases within a magma and how they are released to the atmosphere is crucial to understanding explosive volcanic processes.
Laboratory Studies of the Products of Explosive Volcanic Eruptions
Volcanic ash and pumice are essentially bits of fragmented foam formed as gases come out of solution when confining pressure drops as magma approaches Earth's surface. Early theories concluded that explosive activity occurred when all the vesicles (gas bubbles) coalesced. This is partly true, but if all the gas bubbles were to coalesce, there would be no pumice or scoria. By studying bubble walls (shards), pumice and scoria, it is possible to relate their shapes to the viscosity of the magma, the amount of gas within that magma, and estimate the degree of magma-water interaction (Heiken and Wohletz, 1985; Wohletz and Heiken, 1992).
Fragmentation processes that form pyroclasts can be evaluated by studying older volcanoes that have been eviscerated by erosion or by drilling into the conduits of relatively young volcanoes. Heiken et al. (1988) studied cores from drill holes adjacent to young rhyolite domes along the Inyo Dome chain in California. They found that fractures at depths of 300 m and 380 m, which formed during the eruption, were filled by poorly vesiculated shards and fragmented host rock. These shards were similar in composition to much more vesicular pumices erupted at the surface, which suggested that much of the actual vesiculation (bubble growth) had occurred within the uppermost 400 m of the conduit.
GLOSSARY OF TERMS
The term pyroclastic refers to volcanic materials ejected from a volcanic vent. But there are several other ways to make volcanic particles. Volcaniclastic refers to explosively fragmented volcanic particles and their deposits, regardless of origin (Fisher, 1961). Volcanic particles generated by weathering and erosion are termed epiclastic.
Generic Types of Volcaniclastic Particles
Pyroclasts are pyroclastic particles formed by vesiculation of magma and subsequent fragmentation by brittle fracturing caused by shock waves moving down the conduit from the surface. They are ejected from a volcanic vent either in air or beneath water.
Hydroclasts form by steam explosions during magma-water interactions. In many explosive eruptions, fragments are formed both by magma expansion and fracturing as described above and by explosive magma-water interactions; there is no special term for these “hybrid” clasts.
Autoclasts form by mechanical friction during movement of lava and breakage of cool brittle outer margins, or gravity crumbling of spines and domes.
Alloclasts form by disruption of pre-existing volcanic rocks by igneous processes beneath Earth's surface.
Epiclasts are lithic clasts and minerals released by ordinary weathering processes from pre-existing consolidated rocks. Volcanic epiclasts are clasts of volcanic composition derived from erosion of volcanic rocks.
The terms pyroclastic, hydroclastic, and epiclastic refer to the initial process of fragmentation. A pyroclast, for example, cannot transform into an epiclast by reworking with water, wind, or glacial action.
Varieties of Pyroclastic Ejecta According to Origin
Essential (or juvenile). Essential pyroclasts are derived directly from erupting magma and consist of dense or inflated particles of chilled melt, or crystals (phenocrysts), in the magma prior to eruption.
Cognate (or accessory). Cognate particles are fragmented comagmatic volcanic rocks from previous eruptions of the same volcano.
Accidental. Accidental fragments (or lithoclasts) are derived from the subvolcanic basement rocks and therefore may be of any composition.
Names of Pyroclasts and Deposits According to Grain Size
Originally, terms used to describe volcaniclastic materials were developed based on the grain size of the fragmental material. The classification followed, more or less, the size classification used by sedimentologists to describe clastic rocks.
Ash: particles <2 mm in diameter. Volcanic ash is composed of vitric, crystal, or lithic particles (rock fragments of juvenile, cognate, or accidental origin). Tuff is the consolidated equivalent of ash. Classification may be made according to environment of deposition (lacustrine tuff, submarine tuff, sub-aerial tuff) or manner of transport (fallout tuff, ash-flow tuff). Reworked ash (or tuff) is named according to the transport agent (fluvial tuff, aeolian tuff).
Lapilli: fragments 2 mm to 64 mm in diameter. Lapilli-size particles may be juvenile, cognate, or accidental. Lithified accumulations with >75% lapilli is lapillistone. Lapillituff is a lithified mixture of ash and lapilli, with ash-sized particles making up 25%–75% of the pyroclastic mixture. Lapilli are angular to subrounded. Subrounded forms are commonly of juvenile origin. Accretionary lapilli are lapilli-size particles that form as moist aggregates of ash in eruption clouds, by rain that falls through dry eruption clouds, or by electrostatic attraction (Schumacher and Schmincke, 1991). Armored lapilli form when wet ash becomes plastered around a solid nucleus, such as crystal, pumice, or lithic fragments, during a hydrovolcanic eruption.
Bombs or blocks: fragments >64 mm. Bombs are thrown from vents in a partly molten condition and solidify during flight or shortly after they land. Bombs are exclusively juvenile. Molten clots are shaped by drag forces during flight and are modified by impact if still plastic when they hit the ground. Bombs are named according to shape—ribbon bombs, spindle bombs (with twisted ends), cow-dung bombs, spheroidal bombs, and so on. Bread-crust bombs are so-called because of bread-crust patterns on bomb surfaces resulting from stretching of their outer solidified shell by internal gas expansion. They are commonly from magma of intermediate and silicic compositions. Basaltic bombs usually show little surface cracking, although some may have fine cracks caused by stretching of a thin, glassy surface over a still-plastic interior upon impact. Cauliflower bombs have coarsely cracked surfaces and dense interiors caused by rapid quenching in aqueous environments. They develop during hydrovolcanic eruptions.
Blocks are angular to subangular fragments of juvenile, cognate, and accidental origin derived from explosive extrusion or crumbling of domes.
Pyroclastic breccia is a consolidated aggregate of blocks containing <25% lapilli and ash.
Volcanic breccia applies to all volcaniclastic rocks composed predominantly of angular volcanic particles >2 mm in size.
Agglomerate is a nonwelded aggregate consisting predominantly of bombs. It contains <25% by volume lapilli and ash.
Pumice, scoria, and cinders are named without reference to size, but are usually lapilli or larger sizes. Their degree of vesicularity differs.
Pumice is a highly vesicular glass foam with a density of <1 g/cm3; bubble walls are composed of translucent glass.
Scoria (also called cinders) are mafic particles that are less inflated than pumice. They are generally composed of tachylite (basaltic glass with quench crystals).
Spatter applies to bombs, usually basaltic, that form from lava blebs that readily weld (agglutinates) upon impact and contrasts with scoria, which do not stick together. Scoria (or cinder) cones, for example, are composed largely of loose particles; spatter cones are composed mainly of agglutinated blebs or larger isolated lava tongues.
Volcanoes and Facies
A volcano is a mound, hill, or mountain constructed during the eruption of lava or volcaniclastic material—usually both. Volcanoes are grouped into a few families according to shape, size, and types of extruded volcanic material. The composition of the original magma or the effects of mixing with surface water are the principal reasons for similarities and differences. Composite volcanoes, domes, shield volcanoes, cinder cones, calderas, and their associated facies are described here.
Lava flows and volcaniclastic deposits are the two main facies of volcanoes. Volcaniclastic deposits are the most varied and are composed of pyroclasts, reworked pyroclasts, autoclastic pyroclasts, and epiclastic particles. Pyroclasts may be deposited as fallout deposits, pyroclastic flow deposits, and pyroclastic surge deposits. Reworked pyroclastic deposits are deposited by streams, wind, and glaciers. Epiclasts are generated by weathering that releases particles from any hard rock.
Calderas are closed volcanic depressions that form during the subsidence of large blocks in a roof zone above a partially evacuated magma chamber. These collapse craters are mostly filled with slump blocks from crater walls, ash and pumice from the eruption, and lacustrine sediments. Most calderas are larger than composite volcanoes, but are less evident because of their broad low-standing profiles.
Description. Andesitic to dacitic magmas construct high-standing composite volcanoes with large volumes and great heights, and therefore are important sources of sediment (Hackett and Houghton, 1989). Composite volcanoes are often associated with subduction zones.
Composite volcanoes are constructed of interbedded lava flows, pyroclastic and autoclastic materials, and reworked volcanic debris. They may be the products of several volcanic episodes. They erode rapidly and are sources of epiclastic volcanic debris. Between eruptive episodes, the volcano can be severely eroded, then rebuilt, sometimes with compositionally different products. Their slopes are made of innumerable layers of rubble derived from the breakup of brittle lava and dome rocks, pyroclastic layers, a few lava flows, and reworked volcanic debris. Composite volcanoes may have satellite domes on their flanks or in craters.
Andesitic eruptions provide abundant fragmental debris to construct the composite cones, but not all of the magma rises to the summit of a volcano. Some penetrates the fragmental layers of a volcano as dikes or as sills, which parallel the boundaries of rock layers. Multiple intrusive events therefore build a rigid framework that supports the accumulation of volcanic rubble and lava flows to heights greater than cones that do not have such a framework.
Some of the highest composite volcanoes in the world include Klyuchevskoy volcano, Kamchatka, Russia (4725 m above sea level [asl]) and Ojos del Salado (6870 m asl) and Llullaillaco (6723 m asl) on the border of Argentina and Chile. Depending upon latitude, large volcanoes create their own microclimate where seasonal winter snow and rain lead to widespread erosion of loose debris from their slopes. Debris flows that originate on the steep slopes may travel more than 100 km down valleys.
Composite volcanoes can grow to 3000 or 4000 m in height, although, commonly, the instability of the accumulated deposits results in sector collapse and debris-flow avalanches similar to that which occurred during the 1980 eruption of Mount St. Helens, USA. Debris avalanches occur when a large sector of an edifice collapses, sending material down the mountain at a high rate of speed. The cause of the collapse may be that the core of the volcano is highly altered by hydrothermal activity (Crandell, 1971; Vallance and Scott, 1997), or oversteepened by intrusion of magma into the edifice (Siebert et al., 1987). The rocks of the edifice shatter during transport, but commonly remain in approximately the same stratigraphic relation with each other as blocks within a moving shattered-rock matrix (Voight et al., 1981). As the slope decreases, the avalanche slows and deposits the blocks as hummocks of shattered rock, while the matrix between the hummocks continues on, in many cases mixing with water to form mudflows (lahars) or high-concentration flows downstream.
Facies. Composite volcanoes have complex facies relationships that can be grouped into near-source, intermediate-source, and distal facies (Fisher and Schmincke, 1984).
Near-source facies. The near-source facies of composite volcanoes consists of lava flows and volcaniclastic debris. The near-source facies includes all rock types that comprise the edifice of the volcano, including abundant poorly bedded or massive autoclastic and pyroclastic breccias. These are interbedded with coarse-grained fallout tuffs and reworked pyroclastic material. In areas where erosion has removed all but the volcanic roots, the near-source facies consists mainly of a complex of stocks, sills, and dikes and some intrusive or extrusive breccia and tuff. Widespread fumarolic alteration is common.
Fragmentation of material in the near-source facies occurs during collapse of brittle lavas, domes, and spines, and from explosive eruptions. Volcaniclastic accumulations are typically thick, wedge-shaped, and discontinuous. Deposition occurs by accumulation in alluvial fans, by slump and flow within valleys, or by fallout mantling ridges and valleys. Unconformities and deposition on irregular surfaces make establishing lateral continuity of the deposits difficult. Interbedded breccias and lava flows originate from different places at different times from the volcano. Breccias may interfinger with tuff from other volcanoes within the region. Stratigraphic continuity of contemporaneous but isolated sections can sometimes be established by identification of distinctive fallout volcanic ash layers. More commonly, it can only be done by the piecing together several interfingering units that link the local sections to known stratigraphy, or, in many cases, it cannot be done at all (Smedes and Prostka, 1972).
Intermediate-source facies. The intermediate-source facies is commonly found in the lowlands immediately surrounding the volcanic center. Rocks include those deposited from pyroclastic flows, lava flows, fallout processes, and their reworked and erosional products, such as materials deposited by debris flows, floods, and common stream deposits. The outer edges of this facies have increasing amounts of reworked pyroclastic debris and volcaniclastic sediment.
Distal facies. The distal facies consist of fallout tephra (volcanic ash) and fine-grained epiclastic volcanic sediments in areas far from the source, much farther than lava or pyroclastic flows can travel. Its transition with the intermediate-source facies is generally lost by erosion. The distant-source facies rocks are thin, well sorted, and are commonly interbedded with nonvolcanic sediments. If deposition of ash occurs slowly over thousands of years, ash sequences may become very thick, as is the case for the Miocene John Day Formation of central Oregon (Fisher and Rensberger, 1973).
Description . Volcanic domes may occur as individual volcanoes or as dome-like protrusions of lava in craters or on the flanks of composite cones. Lava domes result from the slow extrusion of highly viscous, usually silica-rich magma. Domes are generally small, but some are volcanoes that stand alone and exceed 25 km3 in volume. Many dome-producing eruptions begin explosively and form explosion craters rimmed with pyroclastic debris. Explosive activity wanes as gas content and pressure within the magma decreases. With lowered gas pressures, the magma extrudes slowly as viscous rhyolite lava that forms thick stubby flows or bulbous domes (Fink, 1987). As a lava dome grows, steep margins become covered with sharp glassy blocks of rubble. Domes on steep slopes may collapse as a mass of hot rubble that fragments into pyroclastic flows.
Facies. Facies around a dome mimic those around composite cones, but at a highly reduced scale. A near-vent facies consists of massive lava and the volcaniclastic rubble that surrounds the lava mound. Most of the volcaniclastic rubble that buries domes is formed of autoclastic debris. Such debris also forms from the collapse of the fronts of advancing silicic lava flows and the collapse of spines that tumble down the surface of steep-sided composite volcanoes. Intermediate facies are similar to those of a stratovolcano in those cases in which domes are explosive or have lava flows distinct from the source volcano. The autoclastic debris is interbedded with volcaniclastic deposits of nearly all origins that can occur on the sides of composite volcanoes. Where domes are isolated, the debris is monolithologic and massive.
Description . Shield volcanoes are broad, have low slopes, and are composed almost exclusively of solidified layers of basaltic lava flows. The fluid lavas can flow long distances, and therefore construct gentle slopes and broad summit areas, unlike the steeper-sided composite volcanoes or cinder cones. Some hydroclastic materials are formed by interaction with magma and water in the vent, as occurred at Kilauea in 1924. Scoria (cinder) cones, composed of basaltic tephra, occur on the surface of some shield volcanoes.
Facies. Shield volcanoes have a simple and distinct facies pattern, namely, layer after layer of thin basalt flows with the geometry of interlaced tongue-shaped bodies, some of which may have flowed 30 km. Unconformities marked by soil zones are common. Pyroclastic deposits consist of Pele's hair and tears, and hydrovolcanic tephra are rare.
Scoria (Cinder) Cones
Description. Scoria (cinder) cones are relatively small volcanoes composed of basaltic fragments. They are one of the most common volcanic landforms on Earth. Scoria clasts are small, nut- to fist-sized or larger pieces of lava containing abundant vesicle cavities that cooled during or soon after Strombolian eruptions (explosive bursts of molten and semi-molten clasts). Scoria cones are usually constructed completely of fragmental cinders and form steep-sided mounds with a small crater at the top. Scoria pyroclasts are deposited ballistically until the angle of repose is reached, when the loose material avalanches to the base of the cone. Most cinder cones are composed of interbeds of ballistic fallout and avalanche deposits. Very close to the vent, the cinders are often welded because they were deposited while still hot and soft. Lava flows commonly break through the lower slopes or overflow from lava lakes in the summit crater. Scoria cones may occur alone but commonly are in clusters, sometimes associated with more than 100 neighboring cinder cones. They also occur on the slopes of shield volcanoes.
Facies. Scoria cones are local features composed of lenticular and wedge-shaped red or black beds of bombs, lapilli-sized scoria, and ash. Hawaiian cinder cones commonly consist of bombs, lapilli consisting of Pele's tears, and lesser amounts of ash. In fields where there are many cinder cones, lava and basaltic tephra are interbedded.
Description. A caldera is a very large collapse crater formed during collapse of the ground surface during an eruption. Following the extrusion of such large volumes of ash, pumice, and rock emptied from the magma chamber, the ground above the magma chamber collapses into the resulting void. The dimensions of calderas are quite variable, from a few kilometers to as large as 60 km in diameter. The large ones are easily visible from space, but on the ground they may be difficult to recognize because their configuration is not visible from a single viewing place. Calderaforming eruptions usually produce widespread pyroclastic flow deposits and abundant ash fallout deposits. Calderas can also form during withdrawal of basaltic magma that is erupted along the flank of a shield volcano. The collapse occurs without any eruption at the volcano summit; an excellent example of this is Halemaumau caldera, Kilauea volcano, Hawaii.
Facies . Calderas, as well as composite volcanoes, can produce enormous amounts of volcaniclastic debris. Unlike composite volcanoes, calderas have large-diameter craters generally without high-standing edifices, with correspondingly lower rates of erosional reworking of deposits. Caldera collapse (Smith and Bailey, 1968) produces thick deposits that accumulate within the crater as an intra-caldera facies. The intra-caldera facies within the subsided area includes ignimbrite deposits hundreds of meters thick. With resurgence, the resulting moat is filled by pyroclastic rocks, lava flows, lake sediments, epiclastic volcanic sediments, and by landslide or talus breccias from the caldera wall. The caldera-outflow facies consist of ignimbrite sheets extending many kilometers from the caldera.