Pyroclastic Deposits I: Pyroclastic Fall Deposits

Transcription

1 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

2 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

3 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 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 don t mantle topography) 3

4 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) 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

5 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: V = 13.08T max b 2 Where T max 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

6 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 T max isopach Fragmentation Index, F Fraction of material finer than 1 mm at a distance from vent where thickness is 10% of T max i.e., at 0.1 T max isopach. Dispersal and Fragmentation Indicies 6

7 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. 7

8 Surtseyan Eruption Surtseyan Eruption 8

9 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

10 Surtseyan Eruption Phreatomagmatic Fall Deposit 10

11 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

12 Graded Bedding, Aira Volcano, Japan 12