Lapilli are usually irregular or angular in shape due to violent magma fragmentation but can be spherical, teardrop, dumbbell, or button shaped droplets if more fluid. They are typically molten or partially molten
lava ejected from a volcanic eruption that fall to earth as solid or partially molten rock. Lapilli that forms from this process usually becomes pumice if felsic or scoria if mafic. These granules are the direct result of liquid rock cooling as it travels through the air. Country rock can also be accidentally blasted out during an eruption and can fall into lapilli size category. Lapilli
tuffs are a very common form of volcanic rock typical of
rhyolite,
andesite and
dacite pyroclastic eruptions, where thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and
welding of semi-molten lapilli into what is known as a
welded tuff. If an eruption is unusually rich in pumice, it can become a type of pumiceous lapilli tuff or if from a pyroclastic flow, an
Ignimbrite. The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out and then become welded. Welded tuff
textures are distinctive (termed
eutaxitic), with flattened lapilli,
fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs, which are unconsolidated and easily
eroded.
Accretionary lapilli Stac Fada Member of the
Torridonian, of probable impact origin Rounded balls of
tephra are called
accretionary lapilli if they consist of layered
volcanic ash particles. Accretionary lapilli are formed by a process of wet
ash aggregation due to moisture in volcanic clouds that sticks the particles together, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus. This texture can be confused with
spherulitic and
axiolitic texture.
Armoured (or cored) lapilli These lapilli are a variety of accretionary lapilli, though they contain lithic or crystal cores coated by rinds of coarse to fine ash. Armoured lapilli only form in hydroclastic eruptions, where significant moisture is present. The vapour column contains cohesive ash which sticks to particles within it. == See also ==