This review presents some of the current knowledge of volcanoes in Hawai'i. It was originally written for a NASA-sponsored workshop about Hawaiian volcanism. We hope that with this review you can gain a better understanding of the processes and landforms that are associated with Hawaiian volcanoes. Many of these processes and features can also be found at other basaltic volcanoes on Earth. Additionally, Kilauea and Mauna Loa and have also become the primary volcanoes used by planetary geologists as analogs for volcanoes on Mars and Venus.
This review presents ideas derived by many volcanologists over the last few decades, the most prominent of whom are George Walker, Dave Clague, Jim Moore, and the late Gordon Macdonald. Scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory, the University of Hawai'i, and elsewhere have made further important contributions to the study of Hawaiian volcanism, and their willingness to share their knowledge is gratefully acknowledged. Hypertext references link to a bibliography that provides just a taste of the extensive literature available to those interested in studying Hawaiian volcanoes.
Basaltic shield volcanoes comprise a small percentage of Earth's
volcanoes (~8%; Simkin et al. 1981).
Hawaiian volcanoes are by far the best-studied examples of basalt
shields. This means we have really only studied a small sample
of a small percentage of Earth volcanoes. This review starts with
large-scale structures and works to smaller and smaller features.
Keep in mind how they all fit together to form a complex volcano.
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).
The old Hawaiians noticed that the Hawaiian islands showed an
obvious progression from old (Kaua'i) to young (Hawai'i). They
attributed this to the southeast-ward travels of Pele, the goddess
of volcanoes, in her search for a home. 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).
The exact nature of hotspots is poorly known, but it is known
that they are sources of heat and/or magma that is supplied to
the surface. Because they are stationary with respect to the moving
lithosphere (as well as with respect to each other), linear chains
of volcanoes form on the overlying plates and these volcanoes
get older as you look in the direction of plate motion.
the Pacific plate is moving at ~9 cm/year towards the northwest.
The Hawaiian volcanoes grow progressively older from the submarine
volcano Lo'ihi and the island of Hawai'i at the southeast end
of the chain through the main islands, through the leeward islands
(mostly atolls formed on old submerged volcanoes), and beyond
Kure atoll to the Emperor seamounts, the northernmost and oldest
of which (Meiji Seamount) is being subducted under Kamchatka.
The bend in the Emperor seamount chain reflects a change in plate
motion about 40 million years ago.
Uplift caused by the hotspot has bulged the Pacific plate upward
over a broad region approximately 400 kilometers wide called the
Hawaiian swell. This brings an excess of dense mantle material
to near the earth's surface, and this extra mass actually results
in gravity being slightly higher around the Hawaiian chain. At
the same time, the loading of the volcanoes onto the heat-weakened
swell has warped the center downward. This combination of uplift
and subsidence (see image) has formed a broad m-shaped profile; the resulting
structures are the Hawaiian trough adjacent to the islands, and
the Hawaiian arch outboard of that. Recently the arch has been
found to be the source of very high-volume lava flows (Holcomb et al., 1988;
Clague et al. 1990).
Subsidence is greatest directly over the hotspot, for it is here
that the lithosphere is the most thermally weakened and at the
same time the greatest amount of lava is being loaded. At Hilo,
tide gauge measurements during the past century have recorded
sea level going up relative to the land at a rate of 4 mm per
year. Worldwide, sea level has been rising ~1.5 mm/year during
this time, so the extra 2.5 mm/year change in Hilo must be due
to subsidence of the big island (Moore 1987).
The southern part of the big island, more directly over the hotspot,
must be sinking even faster.