Stair steps of flat-topped cones
seen in the bathymetric data off the island of
Ni'ihau (west of Kauai)
Folded, ropy sheet flow, upended to become a tumulus off Ni'ihau
Styles of eruptions at hot spot volcanoes
The eruptive styles of Hawaiian volcanoes change as they evolve. During the shield building stage, large volumes of fluid, tholeiitic lava erupt and build broad, shield-shaped mountains. Fire fountains, surface flows of pahoehoe and 'a'a lava, and flows within lava tubes are typical of the eruptions. An example is Mauna Loa on the Big Island. As the volcano ages and moves away from the hot spot, the magma fractionates and eruptions of the post-shield stage contain more alkalic lavas. The summits become peppered with smaller cinder and spatter cones. An example is Haleakala on Maui. The final eruptive phase is the rejuvenated phase. The lavas are strongly alkalic and the eruptions are often violent and build more steep-sided cones of layered ash and pyroclastic debris. Examples are Diamond Head (see photo on the Volcanic Hazards page) and Hanauma Bay on Oahu.
Discoveries of submarine lava ponds and flood basalts imply that lava may stay molten for a long time, despite the almost infinite heat sink of the ocean. Fragmental rocks (ash, hyaloclastites, and volcaniclastic rocks), limu o Pele, and spatter imply that explosive eruptions take place even in the deep sea, where hydrostatic pressure is so great that steam bubbles should not be able to expand. These observations indicate volcanic processes that contradict conventional wisdom.
Hot spot volcanoes often have long rift zones that radiate from a summit caldera, along which smaller vents and fissures occur. Pu'u O'o, which has been erupting for more than 20 years, is a vent along Kilauea's east rift zone. The minerals found in the erupted lavas offer clues about processes within the rift zones of the volcanoes.
Our research on volcanic processes at hot spot volcanoes
Postshield stage volcanism at Mahukona Volcano
MAHUKONA VOLCANO - Age spectra from 40Ar/39Ar incremental heating experiments yield ages of 298+25 ka and 310+31 ka for transitional composition lavas from two cones on submarine Mahukona Volcano, Hawaii. These ages are younger than the inferred end of the tholeiitic shield stage and indicate that the volcano had entered the postshield alkalic stage before going extinct. Previously reported elevated helium isotopic ratios of lavas from one of these cones were incorrectly interpreted to indicate eruption during a preshield alkalic stage. Consequently, high helium isotopic ratios are a poor indicator of eruptive stage, as they occur in preshield, shield, and postshield stage lavas. Loihi Seamount and Kilauea are the only known Hawaiian volcanoes where the volume of preshield alkalic stage lavas can be estimated.
Honolulu Volcanics: rejuvenated stage cones
OAHU - Lavas and volcaniclastic deposits were observed and collected from 4 submarine cones that are part of the rejuvenated stage Honolulu Volcanics on Oahu, Hawaii. The locations of these and a few additional, but unsampled, vents demonstrate that nearly all the vents are located on or very close to the shoreline of Oahu, with the most distal vent just 12 km offshore. The clastic samples and outcrops range from coarse breccias to cross-bedded ash deposits and show that explosive volcanism at depths between about 350 and 590 m depth played a part in forming these volcanic cones. The eruptive styles appear to be dominantly effusive to strombolian at greater depths, but apparently include violent phreatomagmatic explosive activity at the shallower sites along the submarine southwest extension of the Koko Rift. The compositions of the recovered samples are broadly similar to the strongly alkalic subaerial Honolulu Volcanics lavas, but the submarine lavas, erupted further from the Koolau caldera, have slightly more radiogenic Sr isotopic ratios, and trace element patterns that are distinct from either the subaerial Honolulu Volcanics or the submarine North Arch lavas. These patterns are characterized by moderate to strong positive Sr and P anomalies, and moderate to strong negative Cs, Rb, U, Th, Zr, and Hf anomalies. Most samples have strong negative K and moderate negative Ti anomalies, as do all subaerial Honolulu Volcanics and North Arch samples, but one group of samples from the Koko Rift lack this chemical signature. The data are consistent with more garnet in the source region for the off-shore samples than for either the on-shore Honolulu Volcanics lavas. New Ar–Ar ages show that eruptions at the submarine vents and Diamond Head occurred between about 0.5 Ma and 0.1 Ma, with the youngest ages from the Koko Rift. These ages are in general agreement with most published ages for the formation and suggest that some much younger ages reported previously from the Koko Rift are probably erroneously young.
Reference: D.A. Clague, J.B. Paduan, W.C. McIntosh, B.L. Cousens, A.S. Davis, J.R. Reynolds (2006) A submarine perspective of the Honolulu Volcanics, Oahu, Journal of Volcanology and Geothermal Research, 151: 279-307. [Abstract] [Article] [Appendices]
Flat-topped and pointed cones
HAWAII, MAUI, NI'IHAU - High-resolution bathymetric maps show that submarine flat-topped volcanic cones are common on the submarine rift zones of Kilauea, Kohala, Mahukona, and Haleakala volcanoes. Samples show these cones are tholeiitic basalt erupted during the shield-building stage. Similarly shaped flat-topped cones off of Ni'ihau are alkalic basalt erupted during the rejuvenated stage. These flat-topped cones appear to have formed during effusive eruptions lasting years to decades, and apparently form as continuously overflowing submarine lava ponds. There is a delicate balance between lava supply and cooling of the floating crust to sustain these ponds. Pointed cones of alkalic basalt and hawaiite erupted during the post-shield stage are found on Hilo Ridge, Mahukona, Hana Ridge, and Ni'ihau. Lava that is volatile-rich or erupted in shallow water will produce fragmental and highly vesicular lava that will accumulate to form these steep, pointed cones.
Reference: D.A. Clague, J.G. Moore, and J.R. Reynolds (2000) Formation of submarine flat-topped volcanic cones in Hawaii. Bulletin of Volcanology, 62: 214-233. [Article]
Next: Explosive eruptionsQuestions? Comments? Please contact Jenny Paduan