Photograph by Rick Moscati, 1990. The lava within the lava lake is alkalic in composition. Specifically, it is called anorthoclase phonolite. Anorthoclase is a feldspar mineral. Phonolite refers to rocks made of the minerals alkali feldspar and nepheline.
Alkalic lava is common in rift volcanoes.
Mount Erebus from across Ross Island. Photograph by Rick Moscati, 1990.