Volcaniclastic breccia is a sedimentary rock, made up of gravel sized (2mm to 4096mm) angular pieces (clasts) of other rocks with a finer grained groundmass. Breccias are similar to conglomerates, but the roundness of the clasts differentiates the two. Volcaniclastic breccia are formed from explosive volcanic eruptions, breaking rocks up into small pieces. These clasts are then combined with fine sediment of broken up rocks and ash to form a cohesive rock.
Often in volcaniclastic breccia, clasts will be a wide variety of sizes, shapes and colours. This is because they were all formed separately, under different conditions and with different components. Explosive volcanic eruptions aren’t too good at sorting rocks by size (unlike rivers), so these rocks look very jumbled and messy. The clasts in a volcaniclastic breccia are also pretty sharp and jagged, this is due to the explosive nature of their break down and the short distance they travel to deposition. In a river, rocks are gradually smoothed out as they travel downstream, from bumping into other rocks and scraping along the riverbed. This doesn’t happen in breccias, leaving the rocks pointy.
These rocks have been supplied by:
Lyttelton Port Company
