They are extrusive igneous rocks with fragmental texture.
They form in the air, when volcanic ash, debris and other volcanic material is thrown into the air.
They can also consist of pieces of other rocks, glass and mineral crystals, or be a mixture of them all.
The fragments have first solidified, and then, later cemented or packed together. One very good example is tuff.
Tuffs of a volcano. Poster by AllPosters. Click on thumbnail to buy
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