Sands of volcanic origin are of two dominant types: those that are eroded from volcanic terrain, and those that are produced by an explosive volcanic eruption (pyroclastic sediments or tephra).
The first group of volcanic sands are similar to most other types of sands in the way they are created. However, they generally have very different composition from the so-called normal continental quartz rich sands. Such volcanic sands are typically immature because most of its components are susceptible to weathering. They are often composed mostly of rock fragments rather than individual mineral crystals.
The second group consists ash flow and ash fall deposits which are collectively called tephra. This material is composed of volcanic glass, rock fragments, and crystals in all possible proportions. Lithified tephra is tuff. Lithified deposit of a pyroclastic flow is ignimbrite.
Volcanic sands have some characteristic minerals and mineral associations which are not present in every volcanic sand but if they are, they suggest that the particular sand could be of volcanic origin. Such minerals are olivine, pyroxene, and magnetite.
Most volcanic beach sands are dark-colored. Hence, they are often named black sand. Black color is given to these sands by minerals augite (pyroxene), magnetite, and hornblende. These minerals contain lots of iron in their crystal structure which often gives rust colored appearance to volcanic rocks and sands.
Volcanic sand composed of basaltic rock fragments. Kahena Beach, Hawaii. The width of the view is 10 mm.
Volcanic beach sand from Lehoula Beach, Maui. Sand is composed of partly weathered volcanic rocks, olivine crystals (green) and biogenic grains (white). The width of the view is 10 mm.
Mostly red volcanic glass fragments. California, Medoc County. The width of the view is 10 mm.
Fine-grained volcanic beach sand from Martinique. Green prismatic mineral is augite. Black is magnetite. The width of the view is 5 mm.
Olivine sand from Hawaii, Papakolea Beach. The width of the view is 10 mm.
Volcanic beach sand from Hawaii, Punalu'u Beach. Sand is composed of vesicular volcanic glass. The width of the view is 10 mm.
The components of a volcanic sand from the Azores archipelago.
