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Tracings of the seismic reflection records obtained .on two north-s outh lines during cruise MOL CAN II. See Figur e 1 for line positions,
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The north insular shelf of Molokai is a smooth plain, gently dipping seaward, with three slight steps, one occurring between the 30- and 60-foot isobaths, one between the 150- and 180-foot isobaths, and one near the 300-foot isobath. The shelf break occurs near the 500-foot isobath. Off East Molokai Volcano the shelf is cut by eleven submarine cany...
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Context 1
... the data presented in this study, from data obtained on two north-south profiles of the north slope of Molokai (Fig. 9), and from the study by Shepard and Dill ( 1966) , a structural profile of the north slope of Molokai was con- structed (Fig. 10). The suggestion that the submarine canyons were subaerially formed is supported by the long pr ofile of the canyon axes. The flatter gradient in the waterline sec-Or--------------------, tion of the profile ...
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... The northern coast of East Molokai is marked by a high sea cli¡, the steepest slope in Hawaii (Mark and Moore, 1987), that is incised by a series of deep canyons (the largest, from west to east, are Waialeia, Waikolu, Pelekunu, and Wailau; Fig. 2) that mainly cut into a sequence of ponded lavas interpreted to represent a caldera (Smith et al., 2002;Moore and Clague, 2002), Simrad EM300 bathymetry (MBARI Mapping Team, 2000;Dartnell and Gardner, 1999), and nearshore survey north of Molokai from Mathewson (1970). The general regions of blocks comprising the Nuuanu and Wailau landslides are labeled. ...
... Below the scarp the bottom becomes less steep and smoother before encountering the irregular topography of the landslide blocks. A series of 11 deep canyons extend from above sea level, across this scarp, and down to at least 2000 m depth (Shepard and Dill, 1966;Mathewson, 1970). These £at-£oored canyons are 1^2 km wide, up to about 200 m deep, and commonly are in line with major canyons above sea level on East Molokai Volcano. ...
... These major canyons, apparently cut into volcanic bedrock, are similar in morphology to Hawaiian canyons on land. The upper parts, shallower than 31300 m, are regarded as having been carved subaerially when the volcano stood at least 1300 m higher than at present (Shepard and Dill, 1966;Mathewson, 1970;Andrews and Bainbridge, 1972;Coulbourn et al., 1974). The deeper parts, extending down to about 32000 m, are regarded as having been carved subaqueously. ...
The main break-in-slope on the northern submarine flank of Molokai at −1500 to −1250 m is a shoreline feature that has been only modestly modified by the Wailau landslide. Submarine canyons above the break-in-slope, including one meandering stream, were subaerially carved. Where such canyons cross the break-in-slope, plunge pools may form by erosion from bedload sediment carried down the canyons. West Molokai Volcano continued infrequent volcanic activity that formed a series of small coastal sea cliffs, now submerged, as the island subsided. Lavas exposed at the break-in-slope are subaerially erupted and emplaced tholeiitic shield lavas. Submarine rejuvenated-stage volcanic cones formed after the landslide took place and following at least 400–500 m of subsidence after the main break-in-slope had formed. The sea cliff on east Molokai is not the headwall of the landslide, nor did it form entirely by erosion. It may mark the location of a listric fault similar to the Hilina faults on present-day Kilauea Volcano. The Wailau landslide occurred about 1.5 Ma and the Kalaupapa Peninsula most likely formed 330±5 ka. Molokai is presently stable relative to sea level and has subsided no more than 30 m in the last 330 ka. At their peak, West and East Molokai stood 1.6 and 3 km above sea level. High rainfall causes high surface runoff and formation of canyons, and increases groundwater pressure that during dike intrusions may lead to flank failure. Active shield or postshield volcanism (with dikes injected along rift zones) and high rainfall appear to be two components needed to trigger the deep-seated giant Hawaiian landslides.
... Macdonald and Abbott [ 1970, p. 365], however, questioned the prodigious size of these features, compared with previously known landslides, and proposed that the blocky seamounts are in situ volcanoes. The results of three separate oceanographic investigations by Mathewson [1970], Langford and Brill [1972], and Andrews and Bainbridge [1972, p. 112] caused each of them to reject the landslide hypothesis, with the latter two studies fhvoring the in situ volcano concept. Later measurements of the magnetic stratigraphy of East Molokai volcano led This work showed that a large region (including the south Kona slump and the Alika slide complex) between the previously enigmatic, subaerially exposed Kealakekua and Kahuku faults was involved in seaward sliding that reached 80 km offshore [Normark et al., 1979]. ...
... The terrace was either built by (1) formation of a lava delta during the end of shield-building activity of East Molokai volcano, or (2) growth of a particularly vigorous carbonate reef during a period of stable shoreline conditions. The reef hypothesis is supported by the appearance on the terrace of mounds in echo records that are interpreted as reefs [Mathewson, 1970 [Clague and Dalrymple, 1987]. The landslide scar is largely untilled except that the small Kalaupapa volcano has grown within the upper part of it (Figure 8). ...
... Several major submarine canyons occur in the Wailau debris avalanche amphitheater and most of them head offshore from major canyons above sea level on East Molakai volcano (Figure 8) [Shepard and Dill, 1966;Mathewson, 1970]. Some of the canyons cross the Wailau 1300-m terrace and extend several hundred meters below it. ...
The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ridge has been delimited by systematic mapping of the Hawaiian exclusive economic zone using the side-locking sonar system GLORIA. These surveys show that slumps and debris avalanche deposits are exposed over about 100,000 km2 of the ride and adjacent seafloor from Kauai to Hawaii, covering an area more than 5 times the land area of the islands. Some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume, ranking them among the largest on Earth. The slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts, culminate near the end of subaerial shield building, and apparently continue long after dormancy. Consequently, landslide debris is an important element in the internal structure of the volcanoes. The dynamic behavior of the volcanoes can be modulated by slope failure, and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems. The landslides are of two general types, slumps and debris avalanches. The slumps are slow moving, wide (up to 110 km), and thick (about 10 km) with transverse blocky ridges and steep toes. The debris avalanches are fast moving, long (up to 230 km) compared to width, and thinner (0.05-2 km); they commonly have a well-defined amphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacement of debris avalanches may have produces high-level wave deposits (such as the 365-m elevation Hulopoe Gravel on Lanai) that are found on several islands. Most present-day submarine canyons were originally carved subaerially in the upper parts of debris avalanches. Subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters.
... Hawaiian volcanoes experience an initial period of isostatic subsidence, probably due to the great uncompensated mass extruded onto the crust during the shieldbuilding phase of volcanism (Hamilton, 1957;Furumoto and Woolard, 1965;Strange et al., 1965;Moore and Fiske, 1969;Watts, 1978). That isostatic subsidence continues long after upward growth of the volcano ceases is demonstrated by bathymetric and seismic refraction studies, revealing deep canyons apparently cut by stream erosion into the islands of Kauai, Oahu, and Molokai, now drowned by as much as 1800 m (5905 feet) of sea (Shepard and Dill, 1966;Mathewson, 1970;Andrews and Bainbridge, 1972;Coulbourn et al., 1974). Moreover, the direction of isostatic movement of the islands may not always be consistently downward. ...
... Macdonald and Abbott [ 1970, p. 365], however, questioned the prodigious size of these features, compared with previously known landslides, and proposed that the blocky seamounts are in situ volcanoes. The results of three separate oceanographic investigations by Mathewson [1970], Langford and Brill [1972], and Andrews and Bainbridge [1972, p. 112] caused each of them to reject the landslide hypothesis, with the latter two studies fhvoring the in situ volcano concept. Later measurements of the magnetic stratigraphy of East Molokai volcano led This work showed that a large region (including the south Kona slump and the Alika slide complex) between the previously enigmatic, subaerially exposed Kealakekua and Kahuku faults was involved in seaward sliding that reached 80 km offshore [Normark et al., 1979]. ...
... The terrace was either built by (1) formation of a lava delta during the end of shield-building activity of East Molokai volcano, or (2) growth of a particularly vigorous carbonate reef during a period of stable shoreline conditions. The reef hypothesis is supported by the appearance on the terrace of mounds in echo records that are interpreted as reefs [Mathewson, 1970 [Clague and Dalrymple, 1987]. The landslide scar is largely untilled except that the small Kalaupapa volcano has grown within the upper part of it (Figure 8). ...
... Several major submarine canyons occur in the Wailau debris avalanche amphitheater and most of them head offshore from major canyons above sea level on East Molakai volcano (Figure 8) [Shepard and Dill, 1966;Mathewson, 1970]. Some of the canyons cross the Wailau 1300-m terrace and extend several hundred meters below it. ...
The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ridge has been delimited by systematic mapping of the Hawaiian exclusive economic zone using the side-looking sonar system GLORIA. These surveys show that slumps and debris avalanche deposits are exposed over about 100,000 km 2 of the ridge and adjacent seafloor from Kauai to Hawaii, covering an area more than 5 times the land area of the islands. Some of the individual debris avalanches are more than 200 km long and about 5000 km 3 in volume, ranking them among the largest on Earth. The slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts, culminate near the end of subaerial shield building, and apparently continue long after dormancy. Consequently, landslide debris is an important element in the internal structure of the volcanoes. The dynamic behavior of the volcanoes can be modulated by slope failure, and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems. The landslides are of two general types, slumps and debris avalanches. The slumps are slow moving, wide (up to 110 km), and thick (about 10 km) with transverse blocky ridges and steep toes. The debris avalanches are fast moving, long (up to 230 km) compared to width, and thinner (0.05-2 km); they commonly have a well-defined amphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high-level wave deposits (such as the 365-m elevation Hulopoe Gravel on Lanai) that are found on several islands. Most present-day submarine canyons were originally carved subaerially in the upper parts of debris avalanches. Subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters.
Analysis of geological and geophysical data collected on the Kohala Terrace reveals a complicated geological history for this section of the submarine terraces that are common along the Hawaiian Ridge. Evidence is found for extensive subsidence, on the order of 1250 m, due to normal faulting of the western flank of the island of Hawaii, as well as in situ submarine volcanism. The Kohala Terrace is composed of lava flows from Kohala, Mauna Kea, Hualalai and probably Mauna Loa volcanoes.
The numerous submarine and elevated terraces that fringe shorelines of the Hawaiian Islands have been used as classic examples of mid-ocean Quaternary eustatic terraces. Submarine canyons are important geomorphic features of island slopes. Later reef growth often partly masks both the terraces and canyons. Although difficult to match from one side of an island to the other, some of the terraces have been correlated to successions of higher and lower Quaternary sea levels determined elsewhere in the world. Subbottom seismic reflection profiling now permits a new view of the problem, especially as related to the most recent marine history of Oahu. The geophysical work allows a partial deciphering of former terraces, now buried by younger reefs and sand, and at the same time shows that the heads of submarine canyons do connect with subaerial valleys beneath the succession of Quaternary nearshore deposits. However, the work has disclosed so many additional buried terraces as to raise serious doubts whether it will be possible, without improved techniques of dating the deposits themselves, to unravel the history of Quaternary sea-level changes in Hawaii, much less to correlate them with events recorded elsewhere.
The development of concepts about the giant Nuuanu and Wailau landslides
have been dependant on bathymetric surveys of the landslide region
northeast of Oahu and Molokai. Improved technology of sonar surveying
and of navigation through the years has increased the precision of these
maps providing more details about the landslides. The landslides were
mapped by a U.S. Navy-Scripps Institution single-beam sonar survey in
1953-54, but the resulting map was not of sufficient detail to permit
recognition of the landslides. A more detailed U.S. Navy survey in 1958
utilized an improved radio navigation system and provided the
bathymetric detail necessary for recognition of the landslides. The
GLORIA side-scan sonar surveys during 1986 to 1991 produced an acoustic
reflectance image of the seafloor of the entire Hawaiian area and these
images clearly revealed the nature and size of the Nuuanu and Wailau
landslides and identified an additional 66 landslides longer than 20 km
in the Hawaiian region. In early 1998 Simrad multibeam high-resolution
surveys utilizing GPS navigation revealed new detail in the proximal
parts of the landslides. The 1998 and 1999 JAMSTEC multibeam surveys
produced detailed bathymetric maps of the entire landslide area for the
first time. In the Nuuanu landslide the exposed volume of the 54 blocks
larger than 1 km3 is 1400 km3. Refitting of the
blocks back to their apparent pre-slide positions shows that they do not
fit easily into the reentrants of the host volcanoes. A pre-landslide
seaward-extending bulge in the host volcano flanks could accommodate
this excess volume.
Geological studies of oceanic islands indicate that landslides play a large role in modifying the size and shape of islands. Sea floor mapping of the slopes of these islands shows that submarine landslides are common and widespread. Because surveys have concentrated on young volcanic islands (less than 28 My), little information is available about landslides on older atolls and guyots. Side‐scan sonar surveys are presented here of a Cretaceous age (100–70 My) guyot and an atoll. Both surveys show that landslides altering the carbonate caps are present on these features. Since the carbonate caps were formed long after volcanism ceased and the volcanic edifices had subsided, their modification suggests that landsliding continues to be a significant factor changing the shape and size of atolls and guyots. Early studies of atoll shapes indicate that few atolls are true ring reefs (about 5%). It is suggested that submarine landslides are the likely mechanism by which ring reefs are modified to irregularly shaped polygonal atolls.
