Ultramafic rock but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO which can be considered "ultramafic". Ultrapotassic, ultramafic igneous rocks such as lamprophyre, lamproite and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved. Most of these rocks occur as dikes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lamproite occurrences occur as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown. Vents of Proterozoic lamproite (Argyle diamond mine), and Cenozoic lamproite (Gaussberg, Antarctica) are known, as are vents of Devonian lamprophyre (Scotland). Kimberlite
Black Rock Desert volcanic field Black Rock Desert volcanic field The Black Rock Desert volcanic field in Millard County, Utah, is a cluster of several volcanic features of the Great Basin such as Pavant Butte and Tabernacle Hill. The field's Ice Springs event was an explosive eruption followed by lava flows that were Utah's most recent volcanic activity (1140–1440 AD). which overlapped the older flows of Pavant Butte. The Pleistocene-Holocene field has been an active tectonic area for approximately 2.7 million years. Within the past 1.5 million years, local volcanism has produced both rhyolitic and basaltic cones from near twenty-four vents from which lava escapes