The San Borja (or El Rosarito) volcanic field, is located in central Baja California NE of the Vizcaino Peninsula. Holocene lava flows occur at Rosarito and Morro San Domingo at the western end of the field along the coast north of the Vizcaino Peninsula. Some lava flows are less vegetated than flows of the San Quintín volcanic field to the north that overlie 5,000-6,000 year-old midden deposits (Rogers et al., 1985). The Holocene lava flows in the western part of this alkalic volcanic field are of basaltic-andesite and andesitic composition.