These two volcanoes, located ~40km apart, share the same parent hot spot, but each has its own independent shallow magma feeder system, magma chamber and chemical signature. Eruption frequencies between the two volcanoes appear linked in an opposite sort of way: when one is active (Kilauea at present), the other is dormant, and vice versa. Investigating these reciprocal eruptive cycles, researchers from Rice University, the University of Hawaii, Carnegie Institute and the USGS modeled the magma storage and transport system beneath the volcanoes. The results show that these two sister volcanoes are indeed connected by a partially molten layer at about 50 km depth -- that is, a deep magma reservoir. This depth places this magma reservoir well into the earth’s mantle underlying Hawaii.
The chemical differences between the two volcanoes show that the volcanoes are drawing magma from different areas in this deep magma chamber, competing in a sisterly way for the same magma supply. This deep magmatic plumbing connection further suggests that Kilauea eruptions act as a pressure relief valve for Mauna Loa. Monitoring the bulging of the summit of Mauna Loa helps to predict when Kilauea may overflow, and vice versa.
Image: Thanks to the USGS who provides 637 public domain photos of Hawaii: