129 Andesites of Mount Mazama and Scott – Andesites of the Hillman Vent

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography


Andesites of Mount Mazama and Scott

     Andesites of the Hillman Vent

The conduit of the Hillman cine is mainly occupied by vesicular, pale-gray, pilotaxitic pyroxene andesite. Not only is this andesite more coarsely crystalline than most of the surface flows, but it is characterized further by paucity of hypersthene. In many places, cavities are lined with cristobalite; elsewhere cracks in the andesite are lined with specular hematite, deposited by fumarolic vapors rising through the conduit.

Along the western, lower edge of the Hillman vent, there are finer-grained and more fluidal andesites characterized by an unusual abundance of hornblende. No lava in the park contains this mineral in greater proportion. Many of the crystals are 2 mm. long. Though some retain a green color, and an extinction angle, Z to c, of 10°, most of them have the yellow to deep-brown pleochroism and small extinction angle of oxyhornblende, and many are partly or wholly replaced by magnetite. In an average sample, the content of hornblende is 5 per cent. Next in importance among the dark phenocrysts is augite (4 per cent). Porphyritic hypersthene is again subordinate (1 per cent). These minerals lie in a pilotaxitic base.


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