Is Andesite a Felsic or Mafic Rock?

Andesite is an intermediate volcanic rock that has characteristics of both felsic and mafic rocks. Determining whether andesite is more accurately categorized as a felsic or mafic rock has been the subject of much debate amongst geologists.

Felsic rocks like granite and rhyolite typically contain high levels of silica (>63%), potassium, and sodium. They also contain lower levels of iron, magnesium, and calcium compared to mafic rocks. Mafic rocks like basalt and gabbro have <52% silica and higher amounts of iron, magnesium, and calcium.

Andesite lies between the 52-63% silica range, classifying it as an intermediate rock. But its exact mineral composition can vary between different locations and volcanic systems. This variability makes definitively classifying andesite as felsic or mafic difficult.

Several key characteristics suggest andesite is more similar to mafic than felsic rock types:

  • Andesite commonly contains phenocrysts (large mineral crystals) of plagioclase feldspar, pyroxene, and amphibole – minerals more typical of mafic rocks. Felsic rocks usually contain quartz and potassium feldspar phenocrysts.
  • The calcium content of andesite is generally higher than that found in felsic rocks. Calcium is an important constituent of mafic minerals.
  • Andesite magma commonly forms via mixing of felsic rhyolitic and mafic basaltic magmas. The end result of this mixing skews closer to the mafic end of the compositional spectrum.

However, certain andesite properties align more closely with felsic rocks:

  • The intermediate silica content of andesite at 57-63% is greater than that of basaltic rocks. It approaches the >63% level defining felsic rocks.
  • Andesite lava tends to be more viscous than mafic lava due to its higher silica content. Increased viscosity is a property of felsic magmas.
  • Trace element analysis shows andesite has higher large ion lithophile elements (LILEs) like barium and rubidium. LILE enrichment is characteristic of felsic magmas.

In summary, while andesite displays a mix of both mafic and felsic geochemical and mineralogical properties, current scientific consensus categorizes andesite as a mafic rock. It typically forms via mixing of mafic and felsic magmas, with the mafic component being dominant. The mineral assemblage, calcium content, and lower viscosity of andesite lavas also aligns it more closely with mafic rock types like basalt.