I is for Ignimbrite
"I’ve seen fire and I’ve seen rain"
New Zealand geologist Patrick Marshall (1869–1950) coined the term ignimbrite in 1932, from Latin igni-, fire, and imbri-, rain (Notes on some volcanic rocks of the North Island of New Zealand: New Zealand J. of Science and Technology, 13: p. 198-200). The word is synonymous with “welded tuff,” which refers to a volcanic ash fall that has solidified (into tuff) at a time when it was still warm enough to anneal itself into a hard solid rock (welded).
The processes that result in these rocks can be straightforward ash-rich volcanic eruptions, but the heat to do the welding is often inferred to come from hot pyroclastic (Greek “fire, broken”) flows and nuées ardentes, French for “glowing clouds,” fast-moving clouds of hot, sometimes incandescent, gas and ash. When the flows settle and cool, they form welded tuffs, also known as ignimbrites.
About 40 to 25 million years ago, vast areas of western United States were covered by such volcanic deposits, centered especially in the basin and range of eastern Nevada and western Utah, and in the San Juan volcanic field of southwestern Colorado. The Mid-Tertiary Ignimbrite Flare-Up was a result of the low angle of subduction in western North America that generated breaks, roll-backs, and windows in the subducting plate, allowing heat from the mantle to rise and produce the extensive volcanism.
In the San Juan Mountains, at least 15 distinct caldera complexes exploded. A caldera is a wide zone of collapse, much larger than a typical volcanic crater, formed when the magma chamber below empties. The collapse releases pressure and can result in gigantic eruptions. The caldera eruptions at Yellowstone are perhaps the best known, but there have been many around the world over geologic time; some of Marshall’s original descriptions of ignimbrites are from the caldera of the Taupō Volcano in the North Island of New Zealand. The photo at top is near the headwaters of the Rio Grande River, in the San Juans of Colorado USA.
The ignimbrites of the North American Mid-Tertiary flare-up are largely rhyolitic (the fine-grained equivalent of granite) in composition, in part because they involve partial melting of the continental crust and also because the magmas differentiated into the rhyolitic composition, but there are some intermediate igneous rocks as well.
On the geologic map of Nevada above, most of the yellow, pink, and orange colors represent volcanic tuffs, mostly ignimbrites from the mid-Tertiary event. The ash is so tightly welded that it is like grainy glass in places, and where it is beneath a good seal and where it is fractured, it can even serve as a reservoir for oil.
The specimen above is a piece of ignimbrite from central Nevada. It’s mostly glassy ash and some broken crystal fragments, all welded together. In traditional volcanic rock terms, this is probably a crystal-vitric welded tuff, where “vitric” means glass. In the inset you can see a classic three-point Y-shaped glass shard that is the glass that occupied the space between gas bubbles in the almost-molten ash. My cartoon attempts to show how those Y-shaped glass shards originate.
The jar of oil above was pumped from Trap Spring Oil Field, in the fractured ignimbrite equivalent to the specimen above, in Railroad Valley, Nevada. I acquired it from the wellhead in 1979.
inyeresting and educational! Thanks
Here in UK, Lake District and Snowdonia, beautiful ignimbrites with the distinctive flattened pumice lumps in them. (Squashed by the overlying tuff while still semi molten). I gather that in Snowdonia the geologists puzzled out that the pyroclastic flows could keep right on down under the sea.