Welded tuff, or
ignimbrite
Many volcanoes erupt more ash than lava. If the magma is
very rich in gas, the gas expands as the pressure is
released, and the eruption is explosive, blasting pieces of
molten lava of all sizes out of the volcano. This rock is
made of the accumulated fragments of lava. Because they were
still very hot when they fell to earth, the larger lumps
have become flattened and all the lava and ash particles
have become welded together into a hard rock The rock is
light coloured because the composition of the magma was
acid, rather than basic. The brown staining is due to iron
oxides deposited from percolating groundwater after the rock
was formed.