THE 1983-86 PU'U'O'O ERUPTION OF KILAUEA VOLCANO, HAWAII - A STUDY OFDIKE GEOMETRY AND ERUPTION MECHANISMS FOR A LONG-LIVED ERUPTION

The 1983-1986 Pu'u 'O'o eruption of Kilauea volcano had a very distinc tive episodic character which later changed to steady-state activity. In this paper we present a physical model of episodic activity which e xplains the process largely in terms of non-uniform magma cooling and yield strength development resulting from variable dike width. The mod el is applied to the Pu'u 'O'o data set and shows that, to explain the pressure variations within the summit magma reservoir and the pattern s of summit deflation, a yield strength of 80 to 200 Pa needs to be de veloped. This corresponds to a temperature decrease in the magma of ap proximately 10-15-degrees-C (consistent with temperature variations me asured in the erupted lavas). The model simulates the variation in vol ume flux as a function of time through the eruption and produces resul ts which agree well with observed patterns of magma movement. Gradual evolution of the dike system through the eruptive series is explained by progressive widening of its narrowest sections. The loss of the epi sodic character and the transition to continuous activity is explained by a slight change in the thermal efficiency of the system. The chang e in pattern of gas release (from lava fountaining to gas pistoning an d strombolian activity) which accompanied the change from episodic act ivity to steady-state eruption results from the reduction in magma flo w rate at shallow levels in the dike system. However, we demonstrate t hat continuous eruptions do not inherently lack lava fountains - highe r magma supply rates and thus higher flow velocities would allow fount aining to occur during continuous eruption. We demonstrate that episod ic eruptions result from a fine balance between the effects of cooling of magma within the dike and the generation of pressure within the ma gma reservoir and thus that, thermally, episodic eruptions reflect a m iddle ground between short-lived fissure eruptions in which magma cool ing dominates and continuous eruption in which cooling is minimised, t he initial few months of an eruption being critical to establishing th e thermal viability of the dike system.