Cliffs of white rhyolite tuff at Kashe-Katuwe Tent Rocks National Monument
And why did they spray all that white concrete all over the rocks way out here?
Wait! The “High Desert Field Guide” I just bought says that’s a pumice deposit, and geologists call it tephra.
Most people instinctively know that each of these words probably has something to do with volcanoes, somehow, and of course they’re right. Tephra refers to any of the fragmental material a volcano ejects, regardless of the size of the fragments. This includes primary material like ‘bombs’, ‘cinders’, and ‘ash” as well as blocks of rock torn from the volcano’s cone or carried up from crust. As long as this fragmental stuff lies loosely in cones or drifts or layers on the ground, it is still called tephra.
Once this fragmental material consolidates into a more or less firm rock, it is called tuff. Some air-fall tephras are turned into rock – ‘lithified’ is the term geologists prefer, which means ‘turned into rock’ – by compaction and by ground water cementing the particles together. But most tuffs are formed almost immediately upon cooling, since the hot glassy fragments that make up the bulk of volcanic ash in pyroclastic flows are soft and hot enough to fuse together. This process is called welding. Depending upon the heat and size of the eruption and the proximity to the vent, a tuff can be welded weakly, or it can be fused together so firmly that you can scarcely distinguish it from lava.
Much of the Bandelier Tuff that visitors see on the way to Bandelier National Monument is only weakly welded, and once the weather-hardened rind is broken through, you can gouge out the tuff with simple tools. As you drive from Bandelier on into the Valles Caldera National Preserve, you might notice the tuff getting firmer and darker, breaking into hard plates of unmistakable rock.
A spectacular example of a welded tuff can be found within San Diego Canyon in the Jemez Mountains at Battleship Rock.
Firmly welded tuffs deposited from hot pyroclastic flows of ash are sometimes called ignimbrites. You really can’t help loving this word: “glowing cloud stone”.
Heat generated from volcanic activity in the Jemez Mountains has set up a naturally circulating hydrothermal system that dissolves calcium carbonate from beds of limestone below the mountains, and then redeposits it at the surface where the hot water leaks out. The springs fizz with bubbles of carbon dioxide as the water depressurizes – hence the name Soda Dam – and this abrupt change in chemistry precipitates out the lime. This process is probably assisted by the algae that thrives in the hot water, accounting for the crusty, porous, almost baklava-like texture of the rock:
So there you have it.