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Submarine Volcaniclastic Rocks Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Submarine volcaniclastic rocks R. V. Fisher SUMMARY: The type, relative abundance and stratigraphical relationships of volcanic rocks that comprise island volcanoes are a function of (i) depth of extrusion beneath water, (ii) magma composition, and (iii) lava-water interactions. The water depth at which explosions can occur is called the pressure compensation level (PCL) and is variable. Explosive eruptions that occur above the PCL and below sealevel can give rise to abundant hydroclastic and pyroclastic debris. Below the PCL, clastic material cannot form explosively; it forms from lava by thermal shock. The volcaniclastic products are widely dispersed in basins adjacent to extrusion sources by three principal kinds of marine transport processes. These are slides, sediment gravity flows and suspension fallout. Volcaniclastic debris can be derived in subaqueous and subaerial-to-subaqueous environments (i) directly from eruptions, (ii) from remobilization of juvenile volcaniclastics, or (iii) from epiclastic material which initially develops above sealevel. Sediment gravity flows (fluids driven by sediment motion) exhibit the phenomenon of flow transformation. This term is used here for the process by which (i) sediment gravity flow behaviour changes from turbulent to laminar, or vice versa, within the body of a flow, (ii) flows separate into laminar and turbulent parts by gravity, and (iii) flows separate by turbulent mixing with ambient fluid into turbulent and laminar parts. Dominant kinds of subaqueous volcaniclastic sediment gravity flows are debris flows, hot or cold pyroclastic flows and turbidites. Fine grained material can be thrown into suspension locally during flow transformations or underwater eruptions, but thin, regionally distributed subaqueous fallout tephra is mostly derived from siliceous Plinian eruptions. Volcaniclastic rocks in marine sequences occur marine volcaniclastic deposits are most abundant in many kinds of sedimentary environments and near island arcs and ocean islands because large tectonic settings (Table 1). Some are the result volumes of volcaniclastic material are trans- of eruptions on land which deliver fallout ash, ported from land into the sea during eruptions lava and pyroclastic flows into water. Others and during subsequent erosion. Volcanogenic are derived from underwater eruptions which sedimentation and tectonics, an area of consid- extrude lava flows and volcanic fragments of erable research, is reviewed by Mitchell & various kinds, or are pyroclastic, hydroclastic or Reading (1978). epiclastic materials reworked from land or remobilized under water. This review is mostly concerned with subaqueous sediment gravity Underwater eruptions flow deposits, because of their abundance in Whether underwater eruptions are effusive or marine environments adjacent to volcanic explosive is determined by (i) the depth (pres- regions. sure) of the water column (Fig. 1), (ii) the com- The site of most voluminous volcanism is at position of the magma, especially amounts of divergent plate boundaries where basaltic sheet volatiles, and (iii) the extent of interaction be- flows, pillow basalts and smaller amounts of pil- tween magma and water. Subsequent transport low breccias and hyaloclastites are formed. An depends upon slope, which in turn partly estimated 4-6 km 3 of such material is added depends upon the growth rate of the volcano each year to the Earth's crust at mid-ocean and the initial lava-to-clastic ratio of extruded ridges (Nakamura 1974). The second important products. The manner of transport also depends site of volcanism is at convergent plate bound- upon whether or not eruption columns can aries. Basaltic and andesitic island arcs develop develop. When vapour pressure in the magma at converging oceanic plates; converging exceeds water pressure, vesiculation com- oceanic and continental plates give rise to mences. As depth decreases, vesicles become dominantly andesitic to rhyolitic volcanic chains more abundant (Moore 1965, 1970; Moore & on the edges of continents. Perhaps the most Schilling 1973) and, at a shallow level, explo- varied and complex environments are the dif- sions caused by exsolution of magmatic vol- ferent kinds of back-arc settings. A third import- atiles can occur. This depth, which is variable, ant site of oceanic volcanism is represented by is here called the pressure compensation level intra-plate seamounts and ocean islands. Sub- (PCL) and 'volatile fragmentation depth' by Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 TABLE 1. Simplified partitioning of environment, kind of extrusion, transport and emplacement processes and some typical pyroclastic deposits Directly from eruptions Environment Eruptions Transport Emplacement Deposits Subaqueous Effusive Lava flows Congealing flows Massive flows; pillow lavas; hyaloclastites. (marine, ~ (shallow lacustrine, or deep) sub-ice) Explosive Dilute suspension Suspension fallout Thin, well sorted, normally graded beds. May (shallow) in water be reworked by bottom currents. Local occurrence near source. Pyroclastic flows Suspension fallout Thin, fine grained well sorted, normally ~ (dilute suspensions graded beds. May be reworked by bottom from tops of flows). currents. Rest on turbidites. Turbulent flows from tops of mass flows. Thin sequence of fairly well sorted beds, may be cross-bedded. Tops may be reworked by Laminar mass flows. bottom currents. Rest on pyroclastic flows. (Flows also develop from slump and flow of Thick, poorly sorted, poorly bedded, water-logged pyroclastic debris on subaqueous non-welded. Mixing with water can result in slopes of volcanoes to give turbidites and lahars.) lahars. Tops may be reworked by bottom currents. May contain rip-ups. Bases erosive to non-erosive. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Subaerial Effusive Lava flows from Congealing flows, Massive flows. Broken pillow complexes and eruptions with land into water. pillows, broken hyaloclastites. Explosive disruption may subaqueous ~ pillows. Explosive produce littoral cones. deposition disruption. Explosive Turbulent Fallout on water to Thin, well sorted, normally graded beds. suspensions in air. bottom. Sharp bases, bioturbated tops. May be in \ deep sea 100s of km from source. Pyroclastic flows Dilute suspension Thin, well sorted, normally graded. Partly from land into fallout from tops of derived from air fall and surge from land water (may be flows. which does not enter water. Rest on destroyed by turbidites. May be reworked by bottom explosive Turbulent flows currents. disruption upon from tops of mass entering water). flows. Thin sequence of fairly well sorted beds, may be cross-bedded. Rest on pyroclastic flows. Laminar mass flows. Tops may be reworked by bottom currents. Thick, poorly sorted, poorly bedded. May be welded to base. Subaerial Effusive Flows Congealing flows Massive forms, sometimes mostly rubble. No pillows. r~ Explosive Ballistic; Fallout from air ~ turbulent Thin to thick, well sorted; may show normally suspension in air. graded bedding. Pyroclastic flows. Fallout. Derived from Thin, well sorted beds. tops of flows. Turbulent suspensions Thin sequences, fairly well sorted, commonly (pyroclastic surge.) well bedded, may be cross-bedded. Laminar mass flows. Thick to thin, poorly sorted, massive to poorly bedded, welded to non-welded. "--.I Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 8 R.V. Fisher Volcanic Products Sea Level EXPLOSIVE DEBRIS CAN BE DOMINANT OVER PILLOWED AND MASSIVE FLOWS. EXPLOSIVE HYDROCLASTIC DEBRIS DOMINANT OVER PYROCLASTIC DEBRIS. POWERFUL ERUPTION COLUMNS CAN BREAK THE WATER SURFACE AND DISTRIBUTE VOLCANICLASTICS SUBAERIALLY. LARGE DRAG FORCES ON SUBAQUEOUS ERUPTION 500 - COLUMNS CAN CAUSE COLLAPSE OF COLUMN TO PRODUCE UNDERWATER SEDIMENT GRAVITY FLOWS. ! Pc _ ! LAVA AND PYROCLASTIC FLOWS FROM LAND INTO WATER ! CAN PRODUCE PILLOW LAVAS, HYDROCLASTICS, HOT - I ? PYROCLASTIC FLOWS AND OTHER SEDIMENT GRAVITY FLOWS. .E I000 NON- EXPLOSIVE HYDROCLASTIC PRODUCTS, * PCL = PRESSURE PILLOWED AND MASSIVE FLOWS. FLOWS INCREASE RELATIVE TO HYDROCLASTICS WITH DEPTH: MAXIMUM 1500 - COM PENSATION RATIO OF FLOWS TO HYDROCLASTICS ~ 5 TDI, FLOW-TO- LEVEL CLASTIC RATIO DECREASES WITH INCREASING SLOPE OWING TO SLUMPS, SLIDES AND OTHER GRAVITY PRO- CESSES, SUCH AS ON GROWING SUBSEA VOLCANOES. 20001Mafic Silicic COMPOSITION FIG. 1. Submarine volcanic products related to water depth and composition. Pressure compensa- tion levels (PCL) are not specifically known and depend upon magmatic gas pressures, volumes and expansion rates relative to pressure exerted by the water column. (Fisher & Schmincke 1984). It might exceed 1972), kinetic processes of falling, slumping 1000 m for silicic and volatile-rich mafic alkalic and breaking of detached pillows on steep magmas, but apparently is less than 500 m for slopes, rupturing of growing pillows (Rittmann most mafic basaltic magmas (McBirney 1963), 1962) and implosions of evacuated pillows well within the range of marine shelves that where water pressure exceeds internal pillow fringe continents and many islands, and of the pressures (Moore 1975). tops of some submerged islands. Geological Explosive hydroclastic processes occur when evidence suggests that the PCL for explosive (i) pore water in rocks is rapidly vaporized by alkali basaltic volcanism may be 500-1000 m an underlying
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