Volcanic landforms are common features of the New Zealand environment. They originate by two major processes: first, by the eruption of various magmas onto the Earth's surface creating a wide array of landforms; and second, by largely physical, and to a lesser extent chemical, weathering, which progressively erodes and greatly modifies the original landform. In the first process the landforms are constructional in origin. They vary principally according to the chemistry and gas content of the magmas that control the explosivity of an eruption and the viscos ity of lavas. Magmas are subdivided into four groups principally on the basis of the silica content of the eruptive materials (Table 3.1) . Many other properties vary with the silica content, including everything from the viscosity of lavas to their mineral content. For example, increasing the water content of magma decreases its viscosity, so that any resulting lava will flow more readily. However, the gas content of the magma is of prime importance. If it is low, relatively quiescent extrusion of lavas will form steep-sided domes (in high-viscosity rhyolites) or extensive lava fields (in low-viscosity basalts). If it is high, vesicles (bubbles) will form just prior to or during eruption and a violent explosion results. The principal landforms associated with these variables are identified in Table 3.1. In the second process the constructional volcanic landforms become modified by erosional agents, particularly running water, glaciers, the sea and the wind. These are therefore destructional or erosional in origin. In this chapter, volcanic landforms are described according to their composition and subdivided as to whether they are constructional or erosional in origin.