John Seach, Volcano Live

 Pyroclastic Deposits - John Seach

Three types of pyroclastic deposits

Fall deposits
Drape the landscape. Well sorted. Deposited cool. Vertical trajectory.
Surge deposits
Do not evenly drape the landscape, but pinch and swell. Vary from poor to well sorted. Transient high temperatures such as burn victims or carbonised vegetation. Horizontal trajectory.
Flow deposits
Confined to valleys. Poorly sorted.
High temperation deposition indicated by carbonised vegetation, thermal oxidation colours, and eruption welding. Horizontal trajectory.

A pyroclastic surge is a turbulent cloud of gas and rock fragments that flows across the ground.
Surges are not constrained by topography but can move over obstacles such as ridges and hills.

There are 4 factors operating during deposition

  • Trajectories of particles from vertical to horizontal
  • Concentration of particles in the deposited material.
  • Fluctuation in deposition rate. Determines degree of sorting.
  • Degree of cohesion. More cohesive means longer lasting deposits.


Pyroclastic Deposits
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