Holocene lava flows are more alkalic than the Pleistocene lava flows and comprise several xenoliths of chromium-spinel lherzolite, spinel clinopyroxenite, and rare ferroan websterite and spinel wehrlite. Xenoliths do not exist in the older lava flows. However, chemical evidence indicates that every lava flow was produced in a similar way by low degrees of piecemeal melting. The melts originally came from the upper mantle which, over time, was progressively depleted by every following melting event.
