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(volcanic) breccia

(volcanic) breccia

SRI Petrography ID: 36

Names

[de] (vulkanische) Breccie
[en] (volcanic) breccia

Description

hard or soft rock, composed of angular, gravel-size fragments in a finer sediment matrix. Breccia is the consolidated equivalent of rubble.
The gravel size of the fragments refers to a mean particle diameter above 32 mm (more than 50% of the particles have to be larger than 32 mm), below this grain size: lapillistone (4 – 32 mm), tuff (< 4 mm).
Volcanic (igneous) breccias are mainly of pyroclastic origin (formed by an explosive escape of gas from viscous lava that is solidifying) but can also be formed as intrusive breccias by intrusion of magma.

Color

variable. Matrix mostly grey

Grain Size

Matrix fine grained, often glassy, fragments from 32 mm to m

Texture

brecciated

Minerals

matrix fine grained lava or glass

Occurences

In all volcanic areas, Southern Italy, Iceland

Comments

Volcanic breccias can be divided from agglomerates, which accumulate as lava bombs solidified during flight. These are commonly rounded.

Classification

Class

IGNEOUS ROCKS

Broader Terms

pyroclastic rocks