A rift zone is one of the most common volcanic features
constructed on both subaerial (e.g., Hawaii, Iceland, etc.) and submarine volcanoes
(e.g., Loihi and other seamounts, mid-ocean ridge segments). The morphological
characteristics of a rift zone are typically used to infer the internal
architecture of volcanoes. Both of these facts underscore the broad
applicability of studying the Puna Ridge, and the importance of fully
understanding the formation and evolution of a volcanic rift zone. Currently,
the controls on the injection and transport of magma along a rift zone, the
role of magma storage within the dike system, and the controls on the shapes,
sizes, and styles of lava deposits are not well known.
Kilauea’s subaerial rift zone system is one of the best studied in
the world (e.g. Tilling and Dvorak, 1993; and references therein). The volcano
is fed from a central magma chamber (or system of magma conduits) beneath the
summit (e.g., Ryan et al., 1981; Ryan, 1988). Lava is erupted at the summit
and/or one of the volcano's two rift zones, the South West Rift Zone (SWRZ) and
the East Rift Zone (ERZ). The onset of a rift zone eruption is marked by
seismicity that migrates from the summit region down one or the other rift zone
to the site of eruption, where the early phase of eruption is normally through
a fissure that may be several hundreds of meters long (e.g., Klein et al.,
1987; Wolfe et al., 1987). If fissure eruptions persist they normally become
confined to a single vent. Since 1983 eruptions have been continuously
occurring along the ERZ centered at either the Pu`u `O`o or Kupaianaha vents
(e.g., Wolfe et al., 1987; Mangan et al., 1995), and has produced more than 1
km3 of lava. Surface deformation associated with the seismic
activity (e.g. Pollard et al., 1983), the fissure eruptions, and the
observation of dikes within the eroded cores of Hawaiian volcanoes (e.g.,
Walker, 1987) indicate that rift-zone eruptions are dike-fed, and that the
seismic activity is associated with magma moving through the underlying magma
conduit system. The subaerial ERZ is 55 km in length and the zone of eruptive
fissures, and hence of active dike intrusion, ranges in width from 1.5 to 3 km
(Holcomb, 1987; Moore and Trusdell, 1991).
Since the 1950’s the average magma supply rate to Kilauea during
long term eruptions has been ~3 m3/s (Tilling and Dvorak, 1993). The
Mauna Ulu eruption is a good example of eruptive volumes and styles during a
long-lived eruption. Between 1972 and 1974 about 160 x 106 m3
of lava was erupted (Tilling et al., 1987). The cone of Mauna Ulu was built to
a height of ~120 m above pre-1969 topography. Channels and a tube system typically
transported lava 3-5 km from the summit of Mauna Ulu, and some lava flowed as
much as 10 km from the vent to the shoreline (Tilling et al., 1987; Peterson et
al., 1994). The surface area covered by lava during the eruption was about 45
km2. Much of the lava erupted during this time was in the form of
pahoehoe flows, formed during slow, steady eruption from the vent at 1-5 m3/s
(Peterson et al., 1994).
Puna Ridge, the submarine extension of Kilauea’s ERZ, runs ~75 km
from the shoreline to its distal end. Over its length, it is 55-130 km from the
summit magma reservoir, and plunges from sea level to a depth of 5400 m.
Multibeam bathymetry data has been collected over its entire length. These
data, along with deep-tow side-scan sonar images of the Puna Ridge at its
distal end (Lonsdale, 1989; Smith et al., 2002), photographic imagery (Moore
and Fiske, 1969; Clague et al., 1988; Lonsdale, 1989; Smith et al., 2002),
submersible dive observations (Fornari et al., 1978; Johnson et al., 2002), and
a recently completed study using deep-towed 120 kHz sidescan, ARGO II bottom
photography, seafloor magnetics, and rock sampling (Smith et al., 1998, 2002)
confirm that the Puna Ridge crest is a constructional volcanic feature and that
the crest is the location of dike intrusions and fissure eruptions. Existing
sea surface magnetic data show an elongate, normally-polarized magnetic anomaly
centered over the axis of the Puna Ridge, consistent with the presence of a
11-km wide, 70-km long, nearly-vertical magnetic source, presumably
representing the dike complex along the ridge (Malahoff and McCoy, 1967). This
characteristic magnetic anomaly high disappears at ~4500 m water depth.
Eruptions appear to be less frequent on the Puna Ridge than on the
subaerial ERZ. Holcomb (1987) estimated that 70% of the subaerial portion of
Kilauea is younger than ~500 years. Based on palagonite thicknesses, Clague et
al. (1995) estimated that dredged lavas from the Puna Ridge range from 700 to
24,000 years in age, and most are 2000 to 7000 years old. The most recent
submarine eruptions are thought to have occurred in 1790, 1884, and 1924,
although direct observation and collection of very fresh, glassy, nearly
unsedimented seafloor pillow lavas along the ridge by Shinkai 6500 submersible
and ARGO II suggest that many eruptions have occurred within the last 100
years. The 1884 eruption was witnessed just offshore at 20 m water depth. In
1790 and 1924, explosions at the summit of Kilauea are thought to have been
associated with magma withdrawal from the summit reservoir and it was inferred
that they fed submarine eruptions on the Puna Ridge (Stearns and Macdonald,
1946).
Though both the subaerial ERZ and the Puna Ridge are constructed in
the same way, by lavas erupted from a rift zone, there are clear morphological
differences between them, which must reflect the differences between subaerial
and submarine volcanology. For example, the longitudinal slope of the subaerial
portion of the ERZ is fairly constant at ~23 m/km (Lonsdale, 1989), while that of
the upper part of the Puna Ridge is much steeper, at ~51 m/km. Fialko and Rubin
(1998) suggested that longitudinal slopes of rift zones may be an important
factor in driving dike intrusion along the length of the rift. Their model
would predict that the ratio of longitudinal slopes in the subaerial and
submarine environments should be approximately as/apr = (rL - rw)/(rL) where as and apr are subaerial and
submarine slope angles, respectively; and rL and rw are lava and water
density, respectively. The observed ratio for the initial change in slope
immediately offshore is as/apr = 0.45, while that
predicted by their relationship is about 0.6. Below 2700 m the longitudinal
slope of the Puna Ridge steepens further to ~95 m/km, but the cause of this
second steepening is not understood.
The styles of volcanic features on the lateral slopes of the rift
zone change significantly as the crest of the rift dips below sea level. Lavas
erupted from the subaerial ERZ form smooth, low angle slopes, except where
interrupted by faults. These slopes are gently dipping low-relief lava flow
surfaces, and where they reach beyond the shoreline they are believed to be
submarine debris flows formed as the lava breaks upon flowing into the water
(e.g., Moore et al., 1973). Large edifices are not commonly constructed. By
contrast, the lateral slopes of the Puna Ridge are both steeper (~200 m/km)
than the subaerial slopes of the ERZ (~50 m/km), and also topographically
irregular on a scale of 1-2 km. Lava flow features on the flanks of the Puna
Ridge include large semi-circular flat-topped features that have diameters of 1
km or more and sides several hundreds of meters high. These flat-topped
features often appear to form staircases of features one on top of the next.
Many of them have pit craters in their tops that can be resolved by multibeam
bathymetry. Scattered along the crest of the Puna Ridge are volcanic cones
(Lonsdale, 1989) that presumably represent primary eruptive vents. The large
flat-topped vent located on the rift axis ~10 km from the shoreline is unique
along the length of the Puna Ridge. It is about 200 m high, similar to heights
that the subaerial Pu`u `O`o cone has reached, although the submarine cone has
a much larger volume because of its flat top.
On the Puna Ridge the lateral slopes of lava deposition range
between 160-240 m/km. These slopes do not change significantly with distance
from the shoreline until a water depth of about 4500 m (below which dikes may
not propagate), suggesting that the slopes have remained the same throughout
the construction of the ridge. This in turn indicates that lavas are added
uniformly to the flanks averaged over time, thus maintaining the lateral
slopes. These slopes must thus reflect the volcanic processes that take place
during the construction of the submarine ridge. The slopes of the Puna Ridge
extend, where the ridge is close to sea level at its upper end, for more than
15 km down to the deep ocean floor. The morphology of the flanks of the ridge
must therefore represent a characteristic of submarine basaltic flows.
Some fundamental scientific questions to be addressed are:
1) What is the cause of elevated Chlorine and Al2O3
concentrations in some distal Puna Ridge lavas? Results from previous work on Puna Ridge (e.g., Johnson et al.,
2002) show that volcanic glass from some rock samples from the south flank and
the deep, distal end of the ridge contain elevated Cl and Al2O3
concentrations. Johnson et al.
(2002) speculated that the elevated Al2O3 was caused by either high pressure
crystallization or high water contents in the melts, causing suppression of
plagioclase crystallization. The
high Cl points to contamination of the melts by seawater or altered basaltic
crust. We collected samples from
the distal portion of the Puna Ridge on Shinkai Dive 688 to further investigate
this question of high pressure versus high water content in the melts.
2) How are dikes able to propagate 55-130 km from Kilauea’s summit to
feed the Puna Ridge? The lateral extent
of dikes is most likely controlled by the size of the summit reservoir and its
resupply rate, the recent history of magma intrusion into the dike system,
and/or the stress conditions along the ridge. To provide constraints on these
controls, data on small-scale tectonic and volcanic morphology, high-resolution
magnetic structure, and geochemistry are important to map out eruption volume,
rate, style, lava composition and age, and the distributions of faults,
fissures, and graben as a function of distance along the Puna Ridge. These data
can be compared to the subaerial ERZ.
3) How are the deep terraces formed? An intriguing aspect of the Puna Ridge is the presence of a
series of large terraces at ~3000 - 5500 m water depth, the deepest portion of
the Ridge. This deep zone represents a change in volcanic morphology from a
preponderance of cratered, individual smaller benches above to the construction
of large lava terraces or benches toward the distal end. The point where this
volcanic morphology changes also marks a break in along-axis slope from 51 m/km
along the shallower portion to 95 m/km in the deeper end of the Puna Ridge.
Possible explanations for the change in slope include changes in magma supply
and effusion rates, and significant changes in lava properties, such as those
related to the increase in ambient pressure. Understanding the processes
important in constructing slopes, and the overall shape of a rift zone, are
important to understanding the construction of any basaltic volcano.
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