basalt being less common and mostly occurring around its base. The rocks define a mildly alkaline suite rich in potassium, unlike the rocks elsewhere in the volcanic field. The rest of the volcanic field also features alkali basalts, basanite and mugearite. Phenocrysts include aegirine, amphibole, anorthoclase, augite, clinopyroxene, fayalite, hedenbergite, ilmenite, kaersutite, magnetite, olivine, plagioclase and sanidine. Gneiss, granulite, harzburgite, lherzolite and tholeiite xenoliths are found in the volcanic field and form the core of many lava bombs. Inclusions in xenoliths indicate that the gaseous components of the Mount Melbourne volcanic field magmas consist mainly of carbon dioxide. The rocks in the volcanic field have porphyritic to vitrophyric textures. The trachytes and mugearites formed through magmatic differentiation in a crustal magma chamber from alkali basalts, defining an alkali basalt-trachyte differentiation series. Basalts were mainly erupted early in the history of the volcano. During the last hundred thousand years the magma chamber became established; this allowed both the differentiation of trachytes and the occurrence of large eruptions. A gap in the rock spectrum ("Daly gap") with a scarcity of benmoreite and mugearite has been noted at Mount Melbourne and other volcanoes in the region. There is no agreement on which processes contributed to petrogenesis in the Mount Melbourne volcanic field but diverse mantle domains and assimilation and fractional crystallization processes appear to have played a role. The magmatic system that feeds Mount Melbourne appears to have a composition distinct from the one associated with the Mount Melbourne volcanic field. Hydrothermal alteration has affected parts of the summit area, leaving yellow and white deposits that contrast with the black volcanic rocks. Hydrothermal sinter deposits have formed in geothermal areas from past liquid water flow. Clay containing allophane, amorphous silica and feldspar are found in the summit area. Eruption history Mount Melbourne was active beginning 3.0–2.7million years ago. Activity has been subdivided into an older Pliocene Cape Washington stage, an early Pleistocene Random Hills stage, the Shield Nunatak stage that is 400,000 to 100,000 years old, and the recent Mount Melbourne stage. Volcanic activity migrated north from Cape Washington towards the Transantarctic Mountains and eventually became centralized at Mount Melbourne. During the last hundred thousand years Mount Melbourne has produced about of magma. The earliest records of the volcano noted its young appearance. Mount Melbourne volcanic field Ages obtained on the Mount Melbourne volcanic field include 2.96 ± 0.20million years, 740,000 ± 100,000