Volcaniclastic Sedimentation and Facies

(From Fisher, R. V. and Schmincke, H.-U., 1994. Volcaniclastic sediment transport and deposition. In K. Pye, Sediment transport and depositional processes. Blackwell Scientific Publications. p. 351-388.


The interaction between volcanism and sedimentation and development of concurrent facies are governed largely by two factors. These are that (1) active volcanism produces abundant sediment that is rapidly delivered to sites of deposition, and (2) lateral changes are the result of flow transformations. During eruptions, large volumes of pyroclastic and hydroclastic sediment are released far more rapidly than any process of production of epiclastic particles (Kuenzi, 1979; Walton, 1979; Vessel and Davies, 1981; Ballance, 1988; Houghton and Landis, 1989). The episodic nature of eruptions may profoundly disrupt sedimentary environments and processes resulting in rapid changes in depositional systems through time. Removal and transfer of these materials from active volcanoes occur through flow transformations as material is carried into contiguous basins of deposition. Sediment is carried from the volcano to the sea to be stored for a time in subaqueous borderland environments, and then remobilized and carried into deep marine basins (Fisher, 1984). During times of quiescent volcanism, smaller volumes of pyroclastic, hydroclastic and volcanic epiclastic sediment are remobilized by similar flow transformations (Walton, 1979).

Volcaniclastic facies depend ultimately upon magma composition, which governs eruptive rates, types of particles, manner of emplacement, total volume and therefore type of volcano. In subduction settings, andesite to dacite suite magmas construct high-standing stratovolcanoes with large volumes and great heights, and therefore large reservoirs of sediment (Hackett and Houghton, 1989). They erode rapidly, providing large volumes of reworked pyroclastic and hydroclastic particles together with epiclastic volcanic debris that are deposited into surrounding basins. Large calderas, commonly built in extensional back-arc regions, are as voluminous as stratovolcanoes, but they are low-standing volcanoes. Very large craters of calderas are initially closed sedimentary basins in which lacustrine sediments and slump blocks from crater walls are deposited. Differences between volcanoes require that different facies aspects be considered in order to reconstruct volcanic areas. These facies aspects are (1) distance-related facies, (2) the type of source volcano and (3) whether vents were single, multiple, central or flank.

The presence of vitric fragments (shards, pumice) within sedimentary sequences indicates a pyroclastic or hydroclastic origin. Moreover, glass is metastable and readily alters to clays and zeolites, and therefore does not appear as an epiclastic fragment.


Volcaniclastic facies are defined by distance from source, type of transporting agent, environment of deposition, and in some cases, by composition. First-order volcaniclastic facies are generally defined by position of the rock body relative to source within non-marine or marine environments, e.g., proximal, medial and distal facies. These designations are generalized and depend upon the size and volume of deposits. For example, at Mount St. Helens, the 18 May 1980 blast surge went no farther than 24 km from source, therefore all of the proximal, medial and distal facies occur within that limit (Fisher, 1990). However, at Aso caldera Japan, one pyroclastic flow deposit, which travelled at least 155 km from source, is considered to be proximal out to 45 km (Suzuki-Kamata, 1988).

The proximal facies may include the source volcano (Vessel and Davies, 1981), but where the source is not exposed, proximal facies rocks can be defined by type of transport such as lava flows (short travel distance), lahars, and fallout layers (most far-travelled) and, in the case of reworked pyroclastics or volcanic epiclastic materials, on their coarsest and thickest parts (Smith, 1988a,b). Pyroclastic facies may be divided into different subfacies, such as lahar or pyroclastic flow and pyroclastic surge subfacies (mechanisms of transport), lacustrine, submarine fan or alluvial sub-facies (environment of deposition), etc. These criteria are the foundation for defining larger-scale facies environments such as source volcanoes and their surroundings (Hackett and Houghton, 1989; Fisher and Schmincke, 1984).

Depositional Units and Multiple Beds

Individual transport events result in the deposition of single layers or several layers that change in aspect such as thickness, texture or composition away from source. Changes in textures and structures of layers result from changing physical behavior within a single transportational event such as a block-and-ash flow deposit laid down by a single nue ardente, or, in marine regions, turbidity currents that deposit Bouma sequences. Unlike nonvolcanic events, changes in grain size and transportation characteristics can also be ascribed to changing variables of a volcanic eruption at the source, as for example, the widening of a vent during a Plinian eruption leading to eruption column collapse resulting in development of a pyroclastic flow on top of earlier fallout deposits (Sparks, 1976; Sparks et al., 1978).

One example of a multilayered deposit from a single event comes from the 18 May 1980, 8:32 a.m. eruption of Mount Saint Helens, Washington (USA). Five different layered units were formed from a single blast erupted laterally from the north side of the volcano. The layers include a ground layer containing a poor mixture of material from the original ground surface with some juvenile lithics from the eruption, overlain by blast surge deposits, capped by an accretionary lapilli fallout layer (Fisher, 1990).

The concept of tephra event unit has been developed which includes proximal to distal facies, all produced in a geologically very brief time interval (Schmincke and Bogaard, 1991).

Rock Sequences and Volcanoes

Rock sequences deposited within marine or nonmarine basins that are derived from depositional events originating from many eruptions can be divided into large-order facies groups that reflect the history and dynamics of volcanism through sedimentary analysis (Busby-Spera, 1988b). Such basinal sequences occur adjacent to volcanic fields, including magmatic arcs with large stratovolcanoes close to marine basins and island volcanoes.

The growth rate of andesitic stratovolcanoes, with consequent influence upon depositional environments, is geologically extremely rapid --- on the order of a few hundred to a few thousand years. These large constructional landforms are composed of great volumes of easily remobilized fragmental material. Their growth is therefore reflected almost instantly in the sedimentary record of the surrounding region (Kuenzi et al., 1979; Vessel and Davies, 1981) by direct deposition from airborne tephra, by deposition of ground-hugging pyroclastic flows, or from eruption-related debris avalanches, lahars and fluvial materials. Rapid construction of a volcano results in an increase in rate of erosion as slopes steepen and local climates are altered. Large volcanoes create climatic barriers, where rainfall and consequent erosion can be dramatically high on the windward side.

Andesitic volcanism produces three conditions necessary to lahar development -- steep slopes, relatively high rainfall and abundant loose fragmental material. With high enough elevations glaciers may also form, including their consequent abundant outwash. Rapid growth of volcanoes can result in oversteepened unstable slopes leading to collapse of sections of the mountain and development of debris avalanches (Glicken, 1986; Siebert et al., 1987).

Rapid growth rates of volcanoes profoundly influence the progressive facies changes associated with an entire volcano system. For example, later products from a stratovolcano at its maximum height and volume can be carried farther than its earlier products when it was smaller. In subaqueous environments, as a volcano grows from deep through shallow water to subaerial environments, explosivity increases which leads to greater production of particles and their more efficient dispersal (Staudigel and Schmincke, 1984; Schmincke, 1982)). Thus, coarsening upward, progradational sequences in adjacent marine basins, as demonstrated by Busby-Spera (1988a), result from actively growing stratovolcanoes. Incision occurs during inactive periods with reworked primary pyroclastic and epiclastic volcanic debris being carried away by fluvial systems leaving little or no record of sedimentation near the source (Smith, 1987a; Smith et al., 1988b). Fining upward sequences develop in sedimentary basins as a volcano lowers by erosion, with products being dominantly of epiclastic and reworked pyroclastic origin as shown by the Great Valley sequence of California (USA) (Ingersoll, 1978).

Flow transformations and facies lineage

All of the volcaniclastic sediments discussed in other sections of this web site can be accomodated within a stratovolcano facies framework that is linked by flow transformations (Fisher, 1983). A flow transformation, which occurs within single-event sediment gravity flows, can be defined as the change from laminar to turbulent behavior (or vice versa) involving (1) separations caused by gravity (gravity transformations), (2) a change without much variation in water or gas content content (body transformation) as when slope changes, and (3) separations caused by turbulent mixing with ambient fluid above a flow surface (surface transformation)). Freundt and Schmincke (1986) show that pyroclastic flows may transform from surge on the higher slopes of a volcano to plug flow on the plains via a hydraulic jump.

Scott (1988) extends the concept of flow (laminar-turbulent) transformation to include changes in transport agents whereby lahars are transformed to lahar-runout flows (hyperconcentrated flows), or hot pyroclastic flows are transformed to lahars. Weirich (1989) demonstrates that subaqueous debris flows transform to turbidity currents by hydraulic jumps. Thus, the concept of transformations links the general volcaniclastic facies (multiple event facies) to flow processes (single event facies) within a space-time framework from source (the volcano) to final deposition in marine or non-marine basins (proximal to distal facies). Because of erosion, however, the lateral facies changes are commonly truncated. For example, in volcanic arc environments, the proximal source is commonly missing, with only distal facies (turbidite, submarine fan) and intermediate facies (fluvial, lahar to delta and shelf with or without submarine lahars) being present (Smith, 1988b).

As shown by the 1980 Mount St. Helens eruptions, one facies lineage, linked by flow transformations, is as follows (Scott, 1988): eruption of pyroclastic surge or flow > lahar > hyperconcentrated flood flow > normal fluvial transport (in the Columbia River). Another lineage is fallout ash from vertical eruption plumes > initial large-scale debris avalanches > stop-gap storage of sediment on submarine shelves or slopes > submarine landslides > subaqueous lahars > turbidity currents.


Ruapehu volcano, New Zealand, is divided into two parts: a composite cone of volume 110 km3 surrounded by a ring plain (Hackett and Houghton, 1989). Complementary parts of the volcano history are preserved in these two environments. Cone-forming sequences are dominated by sheet- and autobrecciated-lava flows, that seldom reach the ring plain. The ring plain is built from the products of explosive volcanism including distal primary pyroclastic deposits and reworked material eroded from the cone. Much of the material of the ring plain is deposited as lahars directly resulting from eruption processes or triggered by high intensity rain storms on volcano flanks. Deposits of the ring plain are further reworked and carried farther into alluvial systems and depositional basins immediately following eruptions or more gradually in the longer intervals between eruptions.

Thus, on present-day stratovolcanoes, major volcaniclastic facies associations can be divided on the basis of distance and geographic location -- cone-forming sequences surrounded by voluminous ring plains corresponding to proximal and medial facies as presented above. Distal facies are far-travelled ash blankets that may be physiographically separated from the other deposits of the source volcano (Fisher and Schmincke, 1984).


Calderas, as well as stratovolcanoes, can produce enormous amounts of volcaniclastic debris. Unlike stratovolcanoes, calderas have large-diameter craters generally without high-standing edifices, with correspondingly lower rates of erosional reworking of deposits. Commonly they form in backarc and other extensional tectonic regions such as rifts and grabens, therefore the chance for preservation of caldera fills, rim sequences and marginal caldera faults is greater than high-standing stratovolcanoes that can be rapidly worn down (Francis, 1983). Other volcanoes commonly associated with each of the large volcano forms -- stratovolcanoes and calderas -- are domes, scoria cones and maar volcanoes, with chances of survival within sedimentary basin sequences dependent upon whether or not they occur as satellites on the slopes of the larger volcanoes, on highlands or in basins.

Volcaniclastic sedimentation and plate margins

Volcaniclastic sedimentation is characteristic of convergent plate margins in marine forearc sequences (Dickinson, 1976; Davies et al., 1978; Kuenzi et al., 1979; Ingersoll, 1978; Vessel and Davies, 1981; Miller, 1989), in marine to nonmarine intra-arc grabens (Busby-Spera, 1986, 1988b), and in marine and nonmarine environments of backarc or interarc areas (Van Houten, 1976; Mathisen and Vondra, 1983; Smith, 1987a,b,1988a,b; Busby-Spera, 1988a; Turberville et al., 1989). The forearc region between the volcanic arc and the down-going subducting crustal slab, above which lies the trench, can be up to 300 km wide and can form a large forearc basin. Sedimentary environments along the shoreline include beach-shelf-slope-rise with fan-deltas, deltas, submarine canyons and submarine fans. The volcaniclastic component within the sedimentary fill depends upon intensity of volcanic activity and the volume of debris that enters this environment. Within the forearc basin itself, sediments are largely turbidite-dominated, and clastics are epiclastic volcanic and reworked pyroclastites and hydroclastites. Pyroclastic and hydroclastic materials dominate during episodes of volcanism, whereas epiclastics dominate between volcanic episodes. Thin primary fallout tephra deposits may be interbedded depending upon prevailing winds during volcanic eruptions.

Facies analysis of volcaniclastic aprons surrounding oceanic islands has led to the definition of several overlapping stages in the evolution of oceanic islands: 1) deep water stage, (2) shallow water--shield stage, (3) mature island stage and (4) regressive erosional stage, each with different clastic processes giving rise to characteristic clast types and mixtures (Schmincke, 1987, 1988).

The above described sedimentary environments also occur in marine backarc regions, and therefore are difficult to separate only on the basis of volcaniclastic lithology or type of transporting agent. Tectonic associations (extensional structures, grabens), chemical affinities (more alkaline in back arc) and rock associations (interbedding of cratonic sediments) may also be necessary to determine tectonic environment. In the North and South American Cordilleran environment, fallout tephra is much more common in backarc regions than in forarc environments, whereas, in the Lesser Antilles, fallout tephra is far more common in the forearc region (Sigurdsson and Carey, 1981; Sigurdsson et al., 1980).

There are several types of extensional environments, generally in back arc regions, both marine and nonmarine, and in intra-arc regions (Busby-Spera, 1988a,b; Smith et al., 1987). Chemical affinities of extensional volcanics are commonly alkaline. Although stratovolcanoes may grow within extensional environments to provide volcaniclastic debris, felsic ignimbrite deposits are characteristic products associated with caldera formation. Basaltic scoria cones (and maar volcanoes in water-rich environments) may be abundant. Extensional environments generally include graben structures that act as sediment traps. In lowland graben environments, basaltic volcanism is likely to give rise to abundant hydroclastics, and volcanic land forms can be partly covered and preserved within the sedimentary fill, which is likely to be epiclastic volcanics depending upon the intensity of volcanism. In addition to chemical evidence and rock type mentioned above, preservation of the volcano edifice signifies an extensional tectonic environment different from that of the convergent island arc stratovolcano that stands high and erodes away. Features of and evidence for arc graben depressions are briefly reviewed by Busby-Spera (1988b) for the early Mesozoic of the southwest Cordilleran United States. Important evidence is the great thickening of the depositional bodies, and the interbedding of quartz sandstone from the craton trapped in the graben and interbedded with pyroclastic and epiclastic materials, including ignimbrite. A modern arc graben analogue occurs in Central America (Burkart and Self, 1985). Smith et al. (1988) describe a late Miocene graben from the central Oregon High Cascades filled with volcaniclastics of the Cascade arc.

Volcaniclastic rocks of the arc are characteristically calc-alkaline andesite or basaltic andesite. Rhyolitic to dacitic ignimbrites also occur in arcs but are more abundant and widespread in intra-arc grabens, backarc regions and continental extensional tectonic zones.

Cited References

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