BH250-119
Title
BH250-119
Subject
Basaltic Andesite/Basaltic Trachyandesite
Description
Major Minerals; plagioclase; orthoclase; clinopyroxene
BH250-119 (collected by Mike Smith, Class of 2929) is classified as either a basaltic andesite or a basaltic trachyandesite, as it plots directly on the boundary line between these two rock types on the Total Alkali-Silica (TAS) diagram. Both rock types are part of the intermediate volcanic spectrum and are prominently exposed in the region around Flagstaff, Arizona, particularly within the San Francisco Volcanic Field—a large and geologically active area in northern Arizona. Basaltic andesite and basaltic trachyandesite are compositionally transitional between basalt and andesite, typically containing 53–57 wt% silica (SiO₂). Mineralogically, they are dominated by plagioclase (often labradorite), with common mafic minerals including clinopyroxene (augite), orthopyroxene (hypersthene), and olivine. Accessory minerals such as magnetite or other oxides may also be present. These rocks commonly exhibit a porphyritic texture, often featuring prominent plagioclase phenocrysts set within a fine-grained groundmass.
BH250-119 (collected by Mike Smith, Class of 2929) is classified as either a basaltic andesite or a basaltic trachyandesite, as it plots directly on the boundary line between these two rock types on the Total Alkali-Silica (TAS) diagram. Both rock types are part of the intermediate volcanic spectrum and are prominently exposed in the region around Flagstaff, Arizona, particularly within the San Francisco Volcanic Field—a large and geologically active area in northern Arizona. Basaltic andesite and basaltic trachyandesite are compositionally transitional between basalt and andesite, typically containing 53–57 wt% silica (SiO₂). Mineralogically, they are dominated by plagioclase (often labradorite), with common mafic minerals including clinopyroxene (augite), orthopyroxene (hypersthene), and olivine. Accessory minerals such as magnetite or other oxides may also be present. These rocks commonly exhibit a porphyritic texture, often featuring prominent plagioclase phenocrysts set within a fine-grained groundmass.
Coverage
Flagstaff, AZ, USA, 35.341295, -111.576939
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab.
Sample 119. Housed at Carleton College in Minnesota.
Sample 119. Housed at Carleton College in Minnesota.
Contributor
Mike Smith
Type
Thin section, hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-119,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/141.