L200 - Volcanic breccia

L200 hand specimen. Note very poor sedimentary sorting and rounding.

L200 thin section, plane-polarised light. Note isolated igneous crystals in volcanic ash matrix.

L200 thin section, cross-polarised light. Note sedimentary, metamorphic and igneous source lithologies in clasts.

Age

Carboniferous

Location

Melrose, Scotland

Hand Specimen

Sedimentary textures, very poorly sorted.
Sub-round to angular grains up to 20mm across.
Wide variety of clasts: dark purple, cream, rusty orange, grey, several types of lava.
Some black, shiny igneous crystals.
Brown-grey matrix of volcanic ash.

Thin section

Wide range of clasts including
- Basalt
- Slate or phyllite
- Schist
- Chert
- Sandstone
- Carbonate
- Quartz

Matrix of fine volcanic ash.

Rock History

This is an example of the coarser type of material deposited by explosive volcanic action.

NB, this is not a typical volcanic breccia, as one usually sees a greater abundance of igneous clasts; other clasts could be wall-rock. Another name for this rock could be “lithic wacke”.

Rock Name

volcanic breccia
lithic wacke