The Hawaiian shield volcanoes are the largest volcanoes on earth (e.g. Peterson & Moore 1987) rising some 9 km above the ocean floor (see image), with volumes of 42,500 and 24,800 cubic kilometers (not counting subsidence) for Mauna Loa and Mauna Kea, respectively. Kilauea is a relatively small bump on the flank of Mauna Loa with a volume of 19,400 cubic kilometers. This can be contrasted to an average of ~100 cubic kilometers for strato volcanoes such as Mount Saint Helens (Wood & Keinle 1990). In the other direction, Olympus Mons on Mars rises 24 km above its base and has a volume of almost 4,000,000 cubic kilometers.
Hawaiian volcanoes reach these huge volumes in relatively short periods of time. Mauna Loa is thought to have begun forming on the sea floor some 500,000 years ago, although this is poorly constrained. For Mauna Loa, these numbers yield an average eruption rate of 0.085 cubic kilometers/year or 2.7 cubic meters/second. Interestingly, this is almost exactly the same eruption rate that is seen during low effusion-rate eruptions, and from observation of such eruptions this has been proposed to be the supply rate from the mantle (e.g. Swanson 1972, Dzurisin et al. 1984).
Plate tectonics provides a modern explanation for the presence of the Hawaiian volcanoes and their age progression from young in the southeast to old in the northwest. The lithosphere consists of the crust and uppermost mantle, both of which are rigid, and together can be divided into sections called plates. Beneath the lithosphere is the asthenosphere, a hot plastic layer on which the lithospheric plates can slide. Somewhere beneath the asthenosphere, and possibly as deep as the core-mantle boundary, is a hotspot, and the Hawaiian volcanoes are formed because of it. There are approximately 42 hotspots on earth (Duncan & Richards 1991).