Agglomerate

Agglomerate (from the Latin agglomerare meaning "to form into a ball") is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs.

These bombs were viscous at the moment of ejection and by rotation in the air acquired their shape.

They typically form proximally during Strombolian eruptions, and are common at strongly peralkaline volcanoes.

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.

At Vesuvius, Ascension, St Vincent and many other volcanoes, they form a considerable part of the coarser ash-beds.