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The recent series on Red Mountain in the Ask A Ranger column prompts me to contribute my observations to the discussion.

Particular features of Red Mountain suggest its classification as a tuff cone, a common volcano worldwide characterized by interaction of lava and water at the start of the eruption. Diagnostic features at Red Mountain include:

1. The lowest layers in the amphitheater are composed of scattered gray cinders embedded in a matrix of small orange pieces of solidified lava; these pieces are not ragged cinders, but blocky fragments bound by flat surfaces. Geologists interpret fragments of this type as forming when lava encounters water, is quenched to glass and shocked, and shattered by steam explosions. The steam drives an eruption of hot water and glass fragments, which are deposited around the vent as steam-inflated slurries. After deposition, the hot water alters the glass to orange palagonite, at which time calcium and sodium are released from the glass and deposited in a cement of new minerals to form tuff.

2. Evenly depressed bedding below volcanic bombs (large projectiles of solidified lava) indicates that water gave the glassy fragments cohesion; impacts of bombs would disrupt loose, dry fragments.

3. Layers of gray cinders occur in the orange tuff, and layers of orange tuff occur in gray cinders. This association indicates alternation of steam explosions and cinder explosions from the vent(s).

Eventually, the lava conduit was sealed from groundwater, and the eruption transitioned to regular cinder explosions and lava effusion. Evidence for hot water and alteration early in the eruption does not disprove the late-stage amphitheater explosion, but makes it less likely.

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RICHARD HOLM

Flagstaff

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