Pyroclastic Deposits I: Pyroclastic Fall Deposits
EAS 458 Volcanology
Introduction . We have seen that physics is useful in understanding volcanic processes, but physical models must be constrained by and tested against observation. . We have 1925 years of historic observations of Vesuvius (79 AD to present) . Far less for most other volcanoes . In all, a very, very small fraction of eruptions . Most descriptions are of limited use . Observations about volcanic processes must depend primarily on geologic observations . The geologic record of volcanic eruptions consists primarily of the deposits produced by them.
1 Pyroclastic Deposits
. Three types of pyroclastic deposits . Fall Deposits . Fallout from an eruptive column . Flow Deposits . Produced by pyroclastic flows . Surge Deposits . Often associated with flow deposits . Associated with explosive events, such as phreatomagmatic explosions
Pyroclastic Deposits
. Characteristics . Fall Deposits . Mantle topography . Parallel bedding . Well sorted . Often graded . Flow Deposits . Topographically constrained . Poorly sorted . Often graded . Surge Deposits . Partially topographically constrained . Cross bedding characteristic . Intermediate sorting . Often graded
2 Pyroclastic Fall Deposits . General term: tephra . Types . Scoria (mafic , larger size) . Pumice (silicic, larger size) . Ash (fine grained)
Fall Deposits: Bedding . Except very near vent, “fall” particles settle vertically. . Therefore, extensive deposits (such as those of Plinian eruptions) will be equally thick at any given distance and direction from the vent. . Hence, they mantle topography . (Scoria cones produced by Hawaiian and Strobolian eruptions obviously don’t mantle topography)
3 Fall Deposits: Sorting
. The distance a particle will travel from the vent depends on: . Ejection velocity . Particle size . For conditions at any particular time and place, particles of a small range of sizes will fall out. . Hence, fall deposits tend to be very well sorted at any given horizon. . Grading reflects changes in eruption and/or wind conditions
Ash Dispersal
. Scoria deposits (associated with Hawaiian & Strombolian erupitons) tend to be concentrated near the vent. . Pumice deposits, though sometimes concentrated near the vent, tend to be more widely dispersed. . Ash deposits tend to be widely dispersed and thin with distance from the vent. . Grain size also decreases with distance.
4 Isopleths and Isopachs
. Mapping out ash dispersal can provide key information about past eruptions . Isopleths . Lines of constant grain size . Isopachs . Lines of constant deposit thickness.
Estimating Deposit Volume . In the simplest case, circular, thickness decreases exponentially with distance from the vent. . In such cases, deposit volume can be estimated as follows:
2 V = 13.08Tmaxb
. Where Tmax is maximum thickness and b is the distance over which thickness halves. . Only in rare cases will the deposit form a circular pattern. . In most cases, wind produces an elliptical pattern.
5 Dispersal and Fragmentation Indicies . Walker defined two key parameters than can be determined by mapping out fall deposits. . Dispersal Index, D . Area covered by deposit that is at least 1% as thick as maximum observed thickness
. i.e., area inside 0.01 Tmax isopach . Fragmentation Index, F . Fraction of material finer than 1 mm at a distance
from vent where thickness is 10% of Tmax
. i.e., at 0.1 Tmax isopach.
Dispersal and Fragmentation Indicies
6 Relationship to Eruption Dynamics
Relationship to Eruption Dynamics
. Dispersal Index is related to the height of the eruption cloud . Fragmentation Index is related to the explosiveness of the eruption . Phreatomagmatic eruptions anomalous.
Surtseyan Eruption
8 Surtsey
Phreatomagmatic Deposits
. Phreatomagmatic eruption are more explosive than ‘dry’ eruptions of comparable volume due to extra energy from vaporization & expansion of additional water . Phreatomagmatic deposits also tend to be more poorly sorted than ‘dry’ eruptions . This is due to ‘clumping’ of ash in the eruption column . Recondensed water vapor effectively makes the ash “sticky” with large and small particles sticking together.
9 Surtseyan Eruption
Phreatomagmatic Fall Deposit
10 Accretionary Lapilli
Graded Bedding in Fall Deposits
. Fall Deposits Often graded. . Can reflect: . Change in exit velocity . Vent radius . Gas content of magma . Clogging of vent by lithic fragments . Change in wind speed or intensity
11 Graded Bedding, Aira Volcano,Volcano, Japan
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