Said of an igneous rock having a silica content lower than that of a basic rock.
Percentage limitations are arbitrary; the upper limit was originally set at
44%. The term is frequently used interchangeably with ultramafic. Although most
ultrabasic rocks are also ultramafic, there are some exceptions; e.g.,
monomineralic rocks composed of pyroxenes are ultramafic but are not ultrabasic
because of their high silica content. A monomineralic rock composed of
anorthite would be considered ultrabasic (SiO2 = 43.2%) but not
ultramafic. Ultrabasic is one subdivision of a widely used system for
classifying igneous rocks on the basis of silica content; the other
subdivisions are acidic, basic, and intermediate. CF: ultramafic;
silicic. AGI
Source:
Dictionary of Mining, Mineral, and Related Terms
