Magma migration and resupply during the 1974 summit eruptions of Kilauea Volcano, Hawai`i

Lockwood, John; Tilling, Robert; Holcomb, Robin; Klein, Fred; Okamura, Arnold; Peterson, Donald

Three brief eruptionsthe longest lasting 3 days, the shortest only 6 hourstook place in the summit region of Kilauea Volcano during the last half of 1974: July 19-22, September 19, and December 31. These eruptions followed an inferred constriction of the upper East Rift Zone (ERZ) supply conduits to the long-live Mauna Ulu eruption in early 1974, causing magma and excess pressure to accumulate beneath Kilauea's summit. The 1974 eruptions each shifted farther westward across Kilauea's summit, reflecting the apparent westward migration of magmatic storage, from the upper ERZ (Mauna Ulu), across Kilauea's summit caldera, and eventually down the volcano's southwest flank. Subsurface migration of magma storage centers across Kilauea is documented not only by the loci of eruptive activity, but also by seismic and geodetic observations, which clearly trace the westward propagation of magma-generated seismic activity and deformation centers. These eruptions occurred during a period of increasing unrest at neighboring Mauna Loa Volcano, a factor possibly related to tectonic processes that also influenced the 1974 Kilauea summit eruptions. The July eruption was fed by separate, subparallel fissures that bridged the zone between the upper ERZ and the southeastern margin of the modern Kilauea Caldera. These lavas, characterized by bimodal compositions, erupted from separate vent systems and in part represented the eruption of stored, 'old' magma that was forced to the surface by an intrusion of more primitive magma from depth. The highly differentiated lavas from the southern vent system are characterized by megascopic plagioclase phenocrysts, a rarity at Kilauea's summit. Lavas erupted in September are much more homogeneous than those erupted in July, but only slightly more mafic than the mean composition of the two groups of July lavas combined. The December lavas, however, include some of the most mafic compositions erupted at Kilauea's summit since 1959 (12.2-15.6 percent MgO) and may include compositions little modified from a batch of 'new' magma that is postulated to have entered the Kilauea summit storage area before the July eruption. The September eruption began within Halema`uma`u Crater, in the same area of the Kilauea eruption in September, 1971. In contrast to 1971, however, 1974 caldera-floor 'curtain-of-fire' fountains abruptly halted their advance when they reached the southwest wall of the caldera. In 1971, eruptive fissures extended beyond Kilauea Caldera and down a strand of the Southwest Rift Zone (SWRZ) during earliest phases of that eruption. The SWRZ blockage of September 1974 is discussed. The December eruption took place southwest of Kilauea Caldera in a historically noneruptive area between the Koa`e Fault System (KFS) and the SWRZ. This eruption apparently continued a geologically recent pattern of magmatic injection into the KFS, a new mode of activity first suspected in 1965 and which later included the first recorded KFS surface eruption (1973). This activity may be a new element in the tectonic evolution of Kilauea and suggests that the Koa`e Fault System may become an increasingly important locus of magmatic injection and intrusion in the future. The volcanic conditions that existed on Hawaiii in the mid-1970s bears some similarities to the present situation (late 1990s). The long-lived ERZ eruption of Mauna Ulu was apparently beginning to wane in mid-1973, following several brief eruptive outbreaks and intrusions uprift, and Mauna Loa was beginning to inflate. Twenty-five years later, another long ERZ eruption has at times waned in intensity, eruptive and intrusive activity has taken place uprift of the main vent, and Mauna Loa is again slowly inflating. These parallels in eruptive history may well be coincidental, but they would merit further study should the Pu`u O`o eruption cease, eruptions return to Kilauea's summit, and then activity resume at Mauna Loa.

Magma migration and resupply during the 1974 summit eruptions of Kilauea Volcano, Hawai`i

Series