Volcanic Mass Flow Processes and Deposits

Transcription

1 Volcanic Mass Flow Processes and Deposits Smith and Lowe, 1991 Lahars=hyperconcentrated (flood) flow (HFF) and debris flow Note ideal HFF deposit has normal grading, parallel bedding, better sorting Ideal debris flow deposit is poorly sorted, reverse graded base Debris flows are laminar flows, HFF s have significant fluid turbulence Cohesion typically provides little support in most volcanogenic debris flows (as not much clay-sized material) Debris flow (>80% sediment by weight) HFF (40-80% sediment by weight) Streamflow (<40% sediment by weight) Flow transformation is common (by bulking and dilution (below))

2 Lahar Deposit Lateral Variation Debris Avalanche Lateral Variation

3 Notes Lahars erode their banks and bulk-up (mostly) by undercutting banks and to a much lower extent by picking up clasts from surface. Large-volume lahars that spillover the river valleys can incorporate large amount of vegetation (with attached debris) Bulking-up may cause an original stream flow to change downstreaminto a HFF or debris flow (ie lahar) Dilution can occur due to sedimentation and by incorporation of water Syn-eruptive lahars tend to be more heterolithic than post-eruptive lahars Lahar depositional processes Deposition from ideal debris flows is mostly by en-masse freezing of a plug Deposition from ideal HFF s is mostly by gradual accretion from a density-stratified flow Deposition from laharic flows is some combination (Note the similarity with ignimbrite depositional processes) It is often not easy to be certain about debris flow vs HFF identification of a particular deposit Debris flow deposits Poorly sorted, massive, matrix-supported, no traction structures, boulders common May have reverse-graded base (or be entirely reverse-graded) Dewatering structures (eg dish structures, pipes) may be present May have vesicles (mostly cold air) (often overlooked) May show some clast imbrication Clasts are typically angular to sub-angular May contain a wide variety of clasts (heterolithic) HFF Hyperconcentrated Flood Flow HFF s are more dilute than debris flows They are laminar to turbulent flows. Lower clast concentrations allows for better bedding, sorting and grading than debris flow deposits HFF deposits are distinguished from debris flow deposits on basis of parallel bedding, better sorting, normal grading (above a possible reverse graded base) and better clast imbrication. Boulders not as common in HFF No basal traction (as in streamflows) Debris flows traveling down water-filled stream/river valleys initially push the water ahead of them but eventually may incorporate some water, and dilute to HFF s or even normal streamflows (with traction current deposits) Dewatering structures may be present Vesicles may be present Vertical Variation figure Lahar deposits are commonly interbedded with alluvium and talus Bulking-up or dilution may be recorded in vertical sections In vent-proximal areas lahars may be interbedded with PDC or DAD deposits Lateral Variation figure

4 Evidence for bulking-up and dilution can be preserved in lateral sections Note the more dilute flows travel further and note overbank facies from HFF and debris flow phases How do you distinguish lahar deposits from pyroclastic flow deposits, glacial tills, and dry mass flow deposits Lahar deposits can be easily confused with unwelded pyroclastic flow deposits glacial tills, and dry mass flow deposits like debris avalanche deposits (DAD s) Lahar deposits tend to be COLD (mag minerals; non-oxidized, non-welded but non-glacial) Lahar deposits are typically more heterolithic than pyroclastic flow deposits Facies associations can help Debris avalanche deposits commonly include in-situ shattered blocks and megablocks of intact stratigraphy Debris avalanche deposits commonly include in-situ shattered blocks and megablocks of intact stratigraphy Magnetic minerals in lahars would not be oriented with magnetic field (ie emplaced below their Curie T) Lahars don t typically display thermal oxidation color (eg reds, oranges, pinks) of some PF deposits

5 Debris Avalanches Volcanic debris avalanches: dry gravity-driven particulate flows of poorly sorted (ie debris ) clasts syn-eruptive or post-eruptive volcanic or non-volcanic clasts (including ice) large-volume deposits associated with flank collapse (of volcanic edifices) Syn-eruptive examples may include hot clasts and perhaps (?) some clast support from steam DADs Matrix facies: facies with low % of blocks (>1m) Block facies: facies with high % of blocks Hummocks: geomorphic feature associated with megablocks (or piles of blocks) in DAD s. Not restricted to DAD s Not recognized as significant volcanic product until Mt St Helens eruption in 1980 DAD s now recognised around hundreds of volcanoes. VERY COMMON. All volcanoes are piles of rubble on the brink Major volcanic hazard Submarine DAD s now recognised around many oceanic volcanic islands (eg Hawaii, Reunion, Canary Islands etc) Characteristic associated topography: amphitheatre-like source areas and hummocky topography Transport mechanisms of DAD s still not fully understood Triggering Flank failure (and large-volume DAD) from intrusion, earthquakes, explosion (phreatic, magmatic etc) Weakening of edifice by hydrothermal alteration is a common precursor Three types of DA proposed based on triggering mechanisms: Bezymianny: associated with magmatic eruptions Bandai: triggered by phreatic eruptions Unzen: triggered by seismicity DADs vertical variation DAD s typically display little vertical variation May have reverse graded base, but grading, sorting and bedding are poorly developed or absent Base may also be finer-grained (no blocks)

6 Like lahars and PDC deposits they are commonly reworked by fluvial processes (overlying (and lateral) lahar and streamflow facies are common) Syn-eruptive DAD s are associated with PDC and fallout deposits DADs clasts DAD s typically include a very high % (or exclusively) non-juvenile clasts Clasts typically occur in domains with poor (or no mixing) across domains Clasts are often hydrothermally-altered to varying degrees Lithic clasts may display jigsaw-like cracking (cold process developed during shearing) Megablocks to km across are common in proximal to distal areas Megablocks (and even matrix) may preserve flank stratigraphy (but sheared) Syn-eruptive DAD s may include breadcrust (and other) juvenile clasts DADs matrix and block facies Block facies display intact (but sheared, jointed ) blocks (usually >1m) of former stratigraphy and form hummocky topography Some blocks may be tilted/inverted, but paleomagnetic evidence suggests most only rotate on axis perpendicular to ground surface Matrix facies derived from shearing and abrasion of blocks and from poorly consolidated (or unconsolidated) parts of source area Matrix facies is topographically flat No jigsaw-like cracking in matrix Paleomag of matrix clasts suggests random orientation Juvenile clasts (if present) would mostly be in the matrix facies Exotic (not derived from edifice) clasts would be in matrix facies mostly Distinguishing DADs from similar deposits Easy to confuse with lahars, tills Hummocky topography alone is not diagnostic (also in lahars, tills (drumlins)) Low to absent juvenile content Matrix and block facies Intense internal shearing of large blocks is often best indicator No fluid escape structures No vesicles DAD blocks are often so large that they may be difficult to recognize as clasts