Composition Variations
Pyroclastic falls exhibit lateral and commonly vertical variations in the nature and size of fragments. This is commonly known as an inversion of the magma chamber.
The 79 AD eruption of Mount Vesuvius produced the Pompeii Pumice which is an example of lateral and vertical variations. The deposit is well sorted with density and size of pumice, and the content and size of the lithic fragments increasing upwards. The bottom layer of the pumice is white felsic rich pumice with a darker grey mafic pumice overlying it. These changes represent the increasing vigour of the eruption. The mafic upper part of the deposit reflects the increasing depth of the origin or compositionally zoned magma chamber (mafic lava is denser and settles to the bottom of the chamber as well as crystals which settle out, e.g., olivine). This unit represents an inversion of the magma chamber as progressively deeper materials from the chamber were tapped as the eruption progressed.
Read more about this topic: Pyroclastic Fall
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