Agglomerate is a pyroclastic coarse accumulation of blocks of volcanic metarial that contains atl least 75% bombs, rounded clasts in a matriix of lava or ash. The clasts are fragments particles that may be derived from, country rock or pyroclastic rocks.

Name origin: The name derived from the Latin agglomerare meaning “to form into a ball

Color: Various

Grain size: coarse-grain rock

Group: Extrusive igneous rock

Content: Igneous rock fragments

Agglomerate Composition and Formation

Agglomerates are different kind of occur. They consist of volcanic minerals formed in the lava, but exhibiting indicate that have formed slowly under pressure considerable depths. They bear a resemblance to plutonic igneous rocks, but are more correctly to be regarded as agglomerations of crystals formed within the liquid lava as it slowly rose towards the surface, and at a subsequent period cast out by violent steam explosions. The sanidinites of the Eifel belong to this group. At Vesuvius, Ascension, St Vincent and many other volcanoes, they form a considerable part of the coarser ash-beds. Their commonest minerals are olivine, anorthite, hornblende, augite, biotite and leucite.

Agglomerate rounded clasts in a matrix of lava or ash. The clasts are fragments particles that may be derived from, country rock or pyroclastic rocks. The rounding of the clasts may have occurred either in the magma during eruption or by later sedimentary reworking. The rounded nature of these clasts is the key to designating the rock as an agglomerate rather than as a volcanic breccia. In a volcanic breccia, most of the clasts are angular. One type of agglomerate, vent agglomerate, is the rock that plugs either the main vent or a satellite vent of a volcano. The outcrop of this rock is of limited extent and appears circular on a geological map. Like other agglomerates, vent agglomerate contains a variety of clasts of different sizes, shapes, and compositions from the lava, other volcanic rocks, or country rocks. These clasts lie in a matrix of fine-grained igneous rock.

Where is it located

Agglomerates are typically found near volcanic vents and within volcanic conduits, where they may be associated with pyroclastic or intrusive volcanic breccias.


  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
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