Radiating away from the summits of Hawaiian volcanoes are (usually two) linear rift zones. The rift zones conspicuously do not point towards adjacent volcanoes, but instead parallel the volcano-volcano boundaries. Rift zones mark preferred directions of sub-horizontal magma excursions from the magma chamber. At the surface they are characterized by numerous vents, fissures, earth cracks, cinder cones, graben, pit craters, and the sources of lava flows. All of these are indications that magma preferentially intrudes into the rift zones and is also often stored there for periods of time up to a few years.

There has been much discussion about the formation and persistence of Hawaiian rift zones (e.g. Fiske & Jackson 1972; Deterich 1988). The general idea is that because Hawaiian volcanoes are close to one another relative to their size, a younger volcano is growing through the flank of an older one. The gravitational stress field caused by the pre-existing volcano tends to yield downslope-directed directions of least compressive stresses. Because dikes orient themselves so that their direction of widening is parallel to this least compressive stress, the dikes end up propagating parallel to the volcano-volcano boundary. Once a preferred direction of dike propagation is established, it is self-perpetuating as long as there is a mechanism for the flanks of a volcano to move outward to accommodate the repeated dike injections. The most popular mechanism for this outward movement is sliding along the volcano-ocean floor interface which consists of easily-deformable sediments (e.g. Nakamura 1982). The focal mechanism for the 1975 M7.2 Kalapana earthquake indicated a slip plane that was nearly horizontal with a slight dip towards at a depth consistent with the base of the volcano (e.g. Lipman et al. 1985). Such an orientation would be expected due to the downward warping of the oceanic lithosphere under the load of the island.

Rift zones probably become preferred directions of dike propagation due to stress orientations, and they evolve thermally to perpetuate themselves. This means that eruptions are rare elsewhere on the flanks of the shields. Except at the summit, the vents of Kilauea are found exclusively along the rift zones. On Mauna Loa, however, there is a class of vents called "radial vents " (Lockwood & Lipman 1987) that are found on the northern and western flanks. This is the sector on the obtuse side of the angle formed by the two rift zones, and circumferential tension caused by a bending moment set up by the rift zones and the westward push of neighboring may be leading to the formation of these vents (Walker 1990).

Probably the most studied rift zone is the east rift of Kilauea. The northern flank of this rift is stable, probably because it abuts Mauna Loa. The south flank, however, is notably mobile. It has been shown to move seaward during both earthquakes and intrusive events. There is nothing in this direction to buttress the flank so the continued pressure caused by numerous dike intrusions produces this seaward displacement (Swanson et al. 1976; Lipman et al. 1985). This relative displacement between the non-mobile north flank and mobile south flank has caused a wide graben to form along the crest of the rift. Thus even though the rift axis is the locus of most eruptive activity it is in places topographically subdued. Some of the faults bounding this graben are visible near Napau crater.

Pit Craters:

Continued transport of magma down the rift zone results in the establishment of a thermally efficient conduit probably 2-3 km below the surface. Some evidence for this was provided by the first 10 km of propagation of the dike marking the onset of the Pu'u 'O'o/Kupa'ianaha eruption being aseismic (Klein et al. 1987). This indicates that there was a pre-existing conduit could be utilized by the migrating magma. This distance corresponds rather closely with the distribution of pit craters along the east rift Kilauea. Beyond the first 10 km, earthquakes marked the propagation of the dike.

Pit craters are not explosion craters or vents, but rather they are locations of localized collapse into a void. The above-mentioned conduit is the best candidate for such a void. A pit crater forms from the bottom up by stoping of a cavity. Evidence for this is provided by a pit crater called the "Devil's Throat." When first noticed by Westerners, Devils Throat was a hole in the ground a few 10's of m across. A very brave man was lowered through this opening on a winch, and he soon found himself in a huge cavity, much wider than the hole he'd come through. It was evident that he was in a bell-shaped void and that the top layers of lava had not yet collapsed into it. Since then the last layers have fallen in, leaving Devil's Throat with the more typical cylindrical form of a pit crater. Eruptive fissures occasionally cut right across pit craters apparently without noticing the difference in topography. An eruptive fissure can extend from the floor of a pit crater, up the wall, and continue on beyond the rim.
In summary, vent, graben, and pit crater distributions yield insight to the preferred directions of magma travel within a Hawaiian volcano. These in turn can yield information about the stress directions within the edifice (e.g. McGuire & Pullen 1989; Rubin 1990). There have been some attempts to tie these stress directions to the stress field within the Pacific plate but it appears that the local stress field caused by neighboring volcanoes is much more important in determining the eventual direction of rift zone formation.

VolcanoWorld