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Map showing the North and South Arch lava
fields, in red, in relation to the Hawaiian Islands. The axes of
the trough and arch are encircling the islands. |
Hawaiian trough and flexural arch
The enormous weight of the Hawaiian volcanoes on the ocean crust bends the lithosphere down, much like a piece of plywood on sawhorses would bend if you were to stand on it. Around the islands is a moat, the ~5500m deep Hawaiian Trough. Beyond the down-warped area is an arch, levered up 200m or so above the average depth of the ocean crust, just like the edges of the plywood would be lifted above the height of the sawhorses. Vast flows of flood basalts have been found along the North Arch, the axis of which is about 250 km north of Oahu, and also on the South Arch, south of the island of Hawaii. These flood basalts are intriguing because the locations are far from the center of the hot-spot plume, and the eruptions produced enormous volumes of very fluid lavas.
On the North Arch, the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Many of the vent lavas are highly vesicular, apparently because of degassing of CO2. The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa (Kauai) and Honolulu (Oahu) Volcanics, and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.
Our research on Hawaiian flexural arch
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Volcaniclastic deposits indicate explosive eruptions
NORTH ARCH - Submarine explosive eruptions are generally considered to become less likely with increasing depth due to the increasing hydrostatic pressure of the overlying water column. Volcaniclastic deposits from the North Arch volcanic field, north of Oahu, have textural characteristics of explosive fragmentation yet were erupted in water depths greater than 4,200 m.
The most abundant volcaniclastic samples from North Arch are clast-supported with highly vesicular, angular pyroclasts. They are most likely near-vent pyroclastic fall deposits formed in eruption columns of limited height. Interbedded with highly vesicular pillow lava, they form low (50 to 200 m), steep-sided cones around the vents. Less common are stratified samples with graded bedding; one such sample includes a layer of roughly aligned, platy, bubble-wall glass fragments (resembling littoral limu o Pele) that may have been deposited by density currents. In addition to bubble-wall glass shards, numerous glass fragments with spherical, delicate spindle and ribbon shapes, and Pele's hair-like glass strands occur in the finer size fraction (<0.5 mm) of some samples. They are probably more distal fallout. Another sample, consisting of glass fragments dispersed in a marine clay matrix, was apparently reworked and deposited farther from the vents by bottom currents.
Glass compositions include low-(~0.4-0.6 wt%) and medium-K2O (>0.6 wt%) alkalic basalt, basanite, and nephelinite. Sulfur and chlorine abundances are high, reaching a maximum of 1,800 and 1,300 ppm, respectively. The ubiquitous presence of limu o Pele fragments, regardless of glass composition, suggests that bursts of Strombolian-like activity accompanied most eruptions. Coalescing vesicles observed in larger pyroclasts and some pillow lava suggests accumulation of volatiles. Since the great hydrostatic pressure makes steam expansion impossible, a volatile-rich, supercritical magmatic fluid probably drove the eruptions. If these volatile-rich magmas had erupted in shallow water or subaerially, tall fountains would most likely have resulted. The great hydrostatic pressure (>40 MPa) limited fountain and eruption column heights.
Reference: A.S. Davis and D.A. Clague (2006) Volcaniclastic deposits from the North Arch volcanic field, Hawaii: explosive fragmentation of alkalic lava at abyssal depths, Bulletin of Volcanology, 68(3): 294-307. [Abstract] [Article]
Source components of North Arch lavas
NORTH ARCH AND OAHU - Hawaiian volcanoes, dominantly shields of tholeiitic basalt, form as the Pacific Plate migrates over a hot spot in the mantle. As these shields migrate away from the hot spot, highly alkalic lavas, forming the rejuvenated stage of volcanism, may erupt from vents scattered on the tholeiitic shields after an interval of erosion lasting for 0.25-2.5 million years. These lavas occur on the shields of Haleakala (Maui), Ko'olau (Oahu), Kauai and Ni'ihau. Alkalic lavas with geochemical characteristics similar to rejuvenated-stage lavas erupted on the sea floor north of Oahu along the Hawaiian Arch, forming the North Arch Volcanic Field.
The variable Tb/Yb, Sr/Ce, K/Ce, Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the North Arch and the rejuvenated-stage Honolulu Volcanics were controlled during partial melting by residual garnet, clinopyroxene, Fe-Ti oxides and phlogopite. However, the distinctively high Ba/Th and Sr/Nd ratios of lava forming the North Arch and Honolulu Volcanics reflect source characteristics. These characteristics are also associated with shield tholeiitic basalt; hence they arise from the Hawaiian hot spot, which is interpreted to be a mantle plume. Inversion of the batch melting equation using abundances of highly incompatible elements, such as Th and La, requires enriched sources with 10-55% clinopyroxene and 5-25% garnet for North Arch lavas. The 87Sr/86Sr and 143Nd/144Nd ratios in lavas forming the North Arch and Honolulu Volcanics are consistent with mixing between the Hawaiian plume and a depleted component related to mid-ocean ridge basalts. Specifically, the enrichment of incompatible elements coupled with low 87Sr/86Sr and high 143Nd/144Nd relative to bulk Earth ratios is best explained by derivation from depleted lithosphere recently metasomatized (new minerals replaced old ones by nearly simultaneous dissolution and deposition) by enriched incipient melt (<2% melting) from the Hawaiian plume. In contrast, the isotopic ratio of Os, an element that is compatible during mantle melting, has a large range in the published data (0.134-0.176), which reflects heterogeneity caused by various proportions of pyroxenite veins residing in a depleted peridotite matrix.
Reference: H.-J. Yang, F.A. Frey, and D.A. Clague (2003) Constraints on the source components of lavas forming the Hawaiian North Arch and Honolulu Volcanics, Journal of Petrology, 44(4): 603-627. [Abstract] [Article]
Eruption style and flow emplacement
NORTH ARCH – The North Arch Volcanic Field surrounds numerous Cretaceous volcanic ridges, circular flat-topped volcanoes, and low-relief regions of the sediment-covered seafloor. New SeaBeam bathymetric maps that cover about 1/3 of the flow field reveal nearly 100 volcanic structures ranging from low shields to steep cones. One shield is modified by a pit crater approximately 1.1x1.25 km and 300 m deep. A lava flow in the SE part of the volcanic field covers about 3,600 km2, has an estimated volume of 36-72 km3, and apparently erupted from a 75 km long NNW-trending fissure system. A 108 km long flow advanced north in a graben parallel to the Cretaceous mid-ocean ridge that formed the crust; its surface gradient is 1.9 m/km (or 1º slope).
Shinkai 6500 submersible dive 502 explored one of the composite volcanoes and found dense alkalic basalt sheet flows erupted after vesicular basanite pillow basalts and fragmental hyaloclastite that make up the steep-sided cone. Dive 503 collected alkalic basalt sheet flows and pillow basalt from the top 122 m of the southern wall of a pit crater that formed by collapse caused by a decrease in magma volume from a shallow storage chamber located 1-2 km below the surface. The volume change may have been caused by loss of gas bubbles from the stored magma when replenishment ceased at the end of the eruption. The surficial drapery-folded sheet flow is covered by only a few cm of sediment, indicating that it is younger than the 0.5-1.5 Ma ages previously estimated for North Arch flows and vents.
Vigorous eruption of highly vesicular lava constructed steep-sided cones of pillow basalt and hyaloclastite whereas steady eruption of dense lava that had lost its bubbles prior to eruption formed low relief lava shields. Vesiculation, even fragmentation, of these alkalic magmas at water depths as great as 4300 m is caused by exsolution of much of their high initial volatile content, mainly CO2 with less H2O. One hyaloclastite sample contains thin glass bubble-wall fragments similar to those formed by mild explosions as seawater boils following mixing of magma and seawater. Low-viscosity dense lava is emplaced as sheet and channelized tube-fed flows that travel down gradients as small as 1.5 m/km (0.08º) and flow as far as 108 km from their vents.
Reference: D.A. Clague, K. Uto, K. Satake, and A.S. Davis (2002) Eruption style and flow emplacement in the submarine North Arch Volcanic Field, Hawaii, In: Hawaiian Volcanoes: Deep Underwater Perspectives, E. Takahashi, P.W. Lipman, M.O. Garcia, J. Naka, and S. Aramaki (eds), Geophysical Monograph 128, American Geophysical Union, 65-84.
Volcanism at the edge of the plume
NORTH ARCH – Submarine lavas erupted onto the Hawaiian Arch show that Hawaiian volcanism is over a much larger area than previously recognized. The North Arch field comprises 25,000 km2 of ~0.5-1.15 Ma, volatile-rich, olivine-phyric alkalic lavas.
Olivine accumulation and fractionation were the major post-melting processes that affected the composition of the lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, relatively low-extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting and there is little evidence for residual hydrous phases, sulfides or Fe-Ti oxides. The mantle source for the North Arch lavas had Sr, Nd, and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a the source magmas being a mixture of lithosphere and hot-spot plume. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, the South Pacific lithosphere upon which the islands later erupted was contaminated by plumes that formed large oceanic plateaus (e.g., Ontong Java), and that this mixed source was subsequently partially melted as it passed near the Hawaiian plume.
Reference: F.A. Frey, D. Clague, J.J. Mahoney, and J.M. Sinton (2000) Volcanism at the edge of the Hawaiian plume: petrogenesis of submarine alkalic lavas from the North Arch Volcanic Field, Journal of Petrology, 41(5), 667-691. [Abstract]
Degassing of deep submarine-erupted alkalic lavas
NORTH ARCH - Glasses from North Arch volcanic field lavas were analyzed for H2O, CO2, Cl, S, Fe3+/ΣFe, and noble gases to investigate the role of volatiles in the generation, evolution, and degassing of these alkalic series lavas. The relationship between SiO2 and nonvolatile incompatible elements is predictable, but the behavior of the volatile components is much more irregular.
The complex data point to a simple result, namely that parental magma compositions can be derived by variable extents of melting of a homogeneous source followed by olivine crystallization and degassing at 400 bar. If the parental liquids are produced by 1.6-9.0% partial melting (±20% relative), then mantle volatile contents are estimated to be 525±75 ppm H2O, 1300±800 ppm CO2 and 30±6 ppm Cl.
Reference: J.E. Dixon, D.A. Clague, P. Wallace, and R. Poreda (1997) Volatiles in Alkalic Basalts from the North Arch Volcanic Field, Hawaii: Extensive Degassing of Deep Submarine-erupted Alkalic Series Lavas, Journal of Petrology, 38(7): 911-939. [Abstract] [Article]
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