Tuff
Image courtesy Ministerio de Obras Públicas Republica de El Salvador (fair use policy)
The tierra blanca tuff shown in this roadcut belongs to the San Salvador Formation, which underlies the metropolitan region of El Salvador's capital, San Salvador. Tuff is an igneous rock formed by the accumulation of volcanic ash.
Volcanic ash and pumice or scoria are the principal ingredients in tuff. Tuff tends to form when erupting lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting it escape. After the ash falls, it may be reworked by rainfall and streams. That accounts for the crossbedding near the top of the lower part of the roadcut.
If tuff beds are thick enough, they can consolidate into a fairly strong rock. In parts of San Salvador, the tierra blanca is thicker than 50 meters. Presumably this roadcut is in such a place. In other places, the tuff must be carefully compacted before buildings can be constructed on it. Salvadoreans have learned this through centuries of rueful experience with major earthquakes. Residential and suburban buildings that short-change this step remain prone to landslides and washouts, whether from heavy rainfall or from the inevitable quakes, like that which struck the area 13 January 2001.
See more closeup pictures of tuff, plus opther related rocks, in the gallery of extrusive igneous rocks.
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