Illustrated Volcano Glossary

An adjective describing the chemical composition of magmas (or lavas) having relatively low content of silica (SiO2), as opposed to "acidic", and a comparably high content of iron and magnesium.

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 %, and 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 low silica content (below around 52% SiO2) are called acidic, whereas magmas or rocks with higher silica content are called "intermediate" (52-66%) or "acidic" (more than than 66%) in composition.
The most well-known and common examples of basic magmatic rocks are gabbro (intrusive) and basalt (extrusive). They often contain the minerals olivine, pyroxene, and plagioclase.