Pyroclastic Flow (Tuff-breccia), Mt. Garibaldi, British Columbia

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Pyroclastic flow (tuff-breccia), Mt. Garibaldi Provincial Park, B. C., Canada. Composite XPL. Imaged area 1.3 mm x 3.1 mm. Specimen collected by the late Tom Zelinski. Photo by Dan Snyder.

Above: Weakly-metamorphosed (perhaps deuteric) pyroclastic flow, containing gray feldspar crystals, brightly-colored mafic crystals,  and light brown devitrified volcanic glass.

According to the generally-accepted classification of pyroclastic materials, any particle smaller than 2 millimeters in diameter is classed as "ash". By this definition, the images in this post show only ash particles. Even the comparatively large feldspar fragments in the image above are ash-sized. And in the image below, the mafic crystals range in diameter from about 20 micrometers to 70 micrometers, whereas the maximum size for ash is 2000 micrometers (2 mm). Particles with diameters from 2 mm to 64 mm are termed lapilli.

Mafic crystals entrained in volcanic glass (now devitrified). Pyroclastic flow (tuff-breccia), Mt. Garibaldi, B. C., XPL. Imaged area 1.3 mm by 2 mm. Photo by Dan Snyder

Crystals are abraded and rounded by colliding with other solid particles in the fast-moving but viscous pyroclastic flow. Abrasion of crystal edges exposes minerals to alteration by circulating fluids after the flow settles.

Abraded and corroded olivine (left) and augite (right) phenocrysts erupted in pyroclastic flow from Mt. Garibaldi, B. C., XPL. Each image covers 0.42 mm by 0.56 mm. Photos by Dan Snyder

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Abraded and embayed feldspar crystals with feldspar fragments altering to clay minerals or zeolites. Pyroclastic flow (tuff-breccia), Mt. Garibaldi, B. C., XPL.Imaged area 1.3 mm by 2 mm. Photo by Dan Snyder

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Feldspar fragments. Pyroclastic flow (tuff-breccia), Mt. Garibaldi, B. C., XPL.Imaged area 0.5 mm by 0.8 mm. Photo by Dan Snyder

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Feldspar fragments in cloudy silica gel, altering to clay minerals or zeolites. Pyroclastic flow (tuff-breccia), Mt. Garibaldi, B.C., XPL. Imaged area 0.09 mm by 0.14 mm, 40X objective. Photo by Dan Snyder

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Feldspar fragments and devitrified volcanic glass. See next image for enlargement of area in rectangle. Pyroclastic flow (tuff-breccia), Mt. Garibaldi, B. C., XPL.Imaged area 0.5 mm by 0.8 mm. Photo by Dan Snyder

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Digitally-enlarged (6X) view of area outlined blue rectangle at upper right of previous image, showing fibrous fringe of neoforming mineral, likely a zeolite.

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Devitrified volcanic glass in tuff-breccia, Mt. Garibaldi, B.C., XPL. Imaged area 0.09 mm by 0.13 mm, 40X objective. Photo by Dan Snyder.

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Neoformed mineral mass, likely an iron-rich phyllosilicate, possibly nontronite, surrounding a relict mafic mineral crystal. Pyroclastic flow (tuff-breccia), Mt. Garbaldi, B. C. .XPL. Imaged area 0.5 mm by 0.8 mm. Photo by Dan Snyder.

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As above, ordinary light. Note opaque iron-oxide grains in former core of a mafic mineral in the center of  the brown-stained mineral mass.

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