Illustrated Volcano Glossary

An adjective describing the chemical composition of magmas (or lavas) having relatively high content of silica (SiO2). It has little to do with the use of the term in chemistry where it is used to describe low pH values found in acids.

In petrology and volcanology, magmatic rocks (or the magmas they derived from) are often classified by their bulk silica (SiO2) content, which can typically range from around 40-75 weight %, an results in the 4 main types of magma by composition. These are, in order of increasing silica content: basaltic (45-53%), andesitic (53-62%), dacitic (62-69%), and rhyolitic (69-77%).
Rocks with high silica content (above around 66% SiO2) are called acidic, whereas magmas or rocks with less silica content are called "intermediate" (52-66%) or "basic" (less than 52%) in composition.
The most well-known and common examples of acidic magmatic rocks are granite (intrusive) and rhyolite (extrusive). They usually contain the minerals quartz, feldspar and biotite.
In today's scientific literature, the term "acidic" is considered outdated and often replaced by the newer term "felsic". However, it remains widely in use at many levels of geoscience.