Ngauruhoe is an active basaltic andesite to andesite composite cone volcano at the southern end of the Tongariro volcanic complex, and most recently erupted in 1954-55 and 1974-75. These eruptions constructed the inner crater of Ngauruhoe, largely composed of 1954-55 deposits, which are the basis of this study. The inner crater stratigraphy, exposed on the southern wall, is divided into seven lithostratigraphic units (A to G), while the northern stratigraphy is obscured by the inward collapse of the crater rim. The units are, from oldest to youngest: Unit A, (17.5 m thick), a densely agglutinated spatter deposit with sharp clast outlines; Unit B, (11.2 m) a thick scoria lapilli deposit with local agglutination and scattered spatter bombs up to 1 m in length; Unit C, (6.4 m thick) a clastogenic lava deposit with lateral variations in agglutination; and Unit D, (10 m thick) a scoria lapilli with varying local agglutination. The overlying Unit E (15 cm thick) is a fine ash fallout bed that represents the final vulcanian phase of the 1954-55 eruption. Unit F is a series of six lapilli and ash beds that represent the early vulcanian episode of the 1974-75 eruption. The uppermost Unit G (averaging 10 m thick) is a densely agglutinated spatter deposit that represents the later strombolian phase of the 1974-75 eruption. Units A-D juvenile clasts are porphyritic, with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, minor olivine, within a microlitic glassy groundmass. Quartzose and greywacke xenoliths are common in most units, and are derived from the underlying basement. The 1954-55 and 1974-75 eruptions are a product of a short-lived, continental arc medium-K calc-alkaline magma. The magma originated from the mantle, then filtered through the crust, undergoing assimilation and fractionation, and evolving to basaltic andesite and andesite compositions. The magma body stagnated in shallow reservoirs where it underwent further crustal assimilation and fractionation of plagioclase and olivine, and homogenisation through magma mixing. Prior to the 1954-55 eruption a more primitive magma body was incorporated into the melt. The melt homogenised and fed both the 1954-55 and 1974-75 eruptions, with a residence time of at least 20 years. The 1954-55 eruption produced alternating basaltic andesite and andesite strombolian activity and more intense fire fountaining, erupting scoria and spatter that built up the bulk of the inner crater. A period of relative quiescence allowed the formation of a cooled, solid cap rock that resulted in the accumulation of pressure due to volatile exsolution and bubble coalescence. The fracturing of the cap rock then resulted in a vulcanian eruption, depositing a thin layer of fine ash and ballistic blocks. The 1974-75 eruption commenced with the rupturing of the near-solid cap rock from the 1954-55 eruption in an explosive vulcanian blast, the result of decompressional volatile exsolution and bubble coalescence, and possible magma-water interaction. The eruption later changed to strombolian style, producing a clastogenic lava that partially flowed back into the crater.