The Santo Tomas-Black Mountain basalts were erupted during the Quaternary from four centers. Six lava flows are present at Black Mountain, three at Santo Tomas, and one each at Little Black Mountain and San Miguel. The basalts are grouped into three major types of phenocryst mineralogy: (l) plagioclase abundant, (2) olivine abundant, and (3) both olivineand plagioclase abundant.
All three types are alkali-olivine basalts, showing high alkali-silica ratios and total alkali content increasing with silica.
Seven periods of basaltic extrusion among the centers have been established on the basis offieldevidence, phenocryst mineralogy, and pyroxene-olivineratios. K-Ar dates show the basalts to be less than 0.3 × 106 m.y. old.
The basalts are thought to have originated from a single small, shallow magma chamber which was under the influence of a high thermal gradient during differentiation.