Origin

Agglomerates occur in different types. They consist of volcanic minerals formed in the lava, but exhibite sings that indicate that they 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.  

References

• Schmidt, R. (1981). "Descriptive nomenclature and classification of pyroclastic deposits and fragments: recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks". Geology. 9: 41–43. doi:10.1007/BF01822152. S2CID 128375559.
• https://geologyscience.com/rocks/agglomerate/
• https://www.britannica.com/science/agglomerate