Welded Tuff
Welded tuff is a pyroclastic rock, of any origin, that was sufficiently hot at the time of deposition to weld together. Strictly speaking, if the rock contains scattered pea-sized fragments or fiamme in it, it is called a welded lapilli-tuff. Welded tuffs (and welded lapilli-tuffs) can be of fallout origin, or deposited from pyroclastic density currents, as in the case of ignimbrites. During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact together, resulting in a 'eutaxitic fabric' (see image and contrast with the ash shapes in unwelded tuff).
Welded ignimbrites can be highly voluminous, such as the Lava Creek Tuff erupted from Yellowstone Caldera in Wyoming 640,000 years ago. Lava Creek Tuff is known to be at least 1000 times as large as the deposits of the May 18, 1980 eruption of Mount St. Helens, and it had a Volcanic Explosivity Index (VEI) of 8—greater than any eruption known in the last 10,000 years. The intensity of welding may decrease towards the upper margin of a deposit, towards areas in which the deposit is thinner and with distance from source. Welded tuff is commonly rhyolitic in composition, but examples of all compositions are known.
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Famous quotes containing the word welded:
“How marvellous it all is! Built not by saints and angels, but the work of men’s hands; cemented with men’s honest blood and with a world of tears, welded by the best brains of centuries past; not without the taint and reproach incidental to all human work, but constructed on the whole with pure and splendid purpose. Human, and yet not wholly human—for the most heedless and the most cynical must see the finger of the Divine.”
—Archibald Philip Primrose, 5th Earl Rosebery (1847–1929)