An alkali feldspar-rich volcanic rock with minor dark minerals such as biotite, amphibole, or pyroxene is called what?

An alkali feldspar-rich volcanic rock with minor dark minerals such as biotite, amphibole, or pyroxene is called what?

• A. Trachyte
• B. Andesite
• C. Rhyolite
• D. Granite

Answer: (A)

Starting with how these rocks are named: volcanic rocks are classified by their feldspar type and silica content. If a volcanic rock is dominated by alkali feldspar and contains only small amounts of dark mafic minerals, it fits the trachytic family. Trachyte is defined as an alkali feldspar–rich volcanic rock with relatively little quartz and typically minor dark minerals such as biotite, amphibole, or pyroxene. The other options don’t match this mineral balance: andesite is richer in plagioclase and intermediate in silica; rhyolite is high-silica and quartz-bearing; granite is a coarse, plutonic rock with numerous minerals including quartz. So the description best identifies trachyte.

Starting with how these rocks are named: volcanic rocks are classified by their feldspar type and silica content. If a volcanic rock is dominated by alkali feldspar and contains only small amounts of dark mafic minerals, it fits the trachytic family. Trachyte is defined as an alkali feldspar–rich volcanic rock with relatively little quartz and typically minor dark minerals such as biotite, amphibole, or pyroxene. The other options don’t match this mineral balance: andesite is richer in plagioclase and intermediate in silica; rhyolite is high-silica and quartz-bearing; granite is a coarse, plutonic rock with numerous minerals including quartz. So the description best identifies trachyte.