BH250-133
Title
Subject
Description
Alteration: olivine to iddingsite
BH250-133 is an andesite sample collected from the Tieton Andesite near the Naches Valley, Washington. This rock is part of the extensive volcanic geology of the Cascade Range and reflects complex magmatic processes associated with the subduction of the Juan de Fuca Plate beneath the North American Plate.
Geochemically, the Tieton Andesite is close to a true andesite—an intermediate volcanic rock with moderate silica content that plots within the andesite field on the Total Alkali-Silica (TAS) diagram.
Thin sections of BH250-133 reveal that phenocrysts are clustered together, forming aggregates known as glomerocrysts. This type of texture is referred to as glomeroporphyritic.
Texture:
Porphyritic, with well-formed phenocrysts set in a fine-grained to glassy groundmass
Indicates a two-stage cooling history typical of volcanic arc settings
Major Minerals:
Zoned plagioclase feldspar (dominant phenocrysts)
Amphibole (hornblende)
Pyroxene (augite or hypersthene)
Opaque minerals (magnetite or ilmenite)
Groundmass:
Fine-grained to glassy, reflecting rapid cooling of lava flows or pyroclastic deposits near or at the surface
This sample provides excellent opportunities for teaching volcanic rock textures, mineral zoning, and magmatic evolution in a convergent plate boundary setting.
Coverage
Nearby Geographic Feature: Mt. Rainier
GPS Coordinates: 46°37'4.39"N, 121°20'49.76"W
Creator
Source
Sample 133. Housed at Carleton College in Minnesota.