Welded Tuff

Welded tuffs are formed during violent volcanic eruptions; gas-charged material is expelled and deposited while still partially molten, so that the droplets become attached, or "welded" to one another. Typically, the weight of overlying tuff flattens pumice fragments and produces aligned grains and lithic fragments. Tuffaceous textures are commonly best viewed in plane polarized light, particularly if the matrix is glassy, so that grain outlines can be seen.

The photos above show welded tuffs with irregularly shaped, light gray glass shards and mashed pumice clasts in a glassy matrix.

The pictures below show (on the left) a plane polarized light view of a highly welded tuff with numerous glass shards and broken plagioclase grains, and on the right a crossed polarizers view showing extinct glass matrix, lithic clasts, and broken plagioclase grains.

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