Abstract

An outcrop of andesite with an extraordinary striated surface stands facing the Inca ruins of Saqsaywamán, near Cusco, Peru. The andesite has an aplitic texture and is considerably altered. The striations, which range from microgrooves to channels a metre deep, are equally well developed regardless of the attitude of the surface on which they occur. The striated surface is deformed. It is cut by hairline fractures (microjoints), conjugate shears, and locally has been thrust over itself. The andesite with its striated surface has been deformed, both in a plastic and in a brittle fashion, due to collapse in response to loss of support from below.

The andesite reached the Earth's surface along an eruptive fissure. The first materials ejected were hot but solid blocks which built an elongated, steep-sided mound. Later, viscous lava was extruded through the crest of the mound and flowed down its flanks. The stretching of the lava during flowage caused its surface to become striated, analogous to the striations on the surface of pulled taffy. The hairline fractures, conjugate shears, and thrusts were produced by the changing conditions of stress during the different stages of the flow. Compaction of the pyroclastic mound during and after flowage of the viscous lava caused first plastic collapse and then brittle collapse of the andesite.

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