Shield volcanoes built almost entirely of lava flows

Shield volcanoes, built almost entirely of lava flows, occur at mid-ocean ridges and hot spots. They have gentle topographic profiles and the lava is fluid. Lava fountains and flows may be spectacular. The Hawaiian shield volcanoes threaten sensitive coastal ecosystems, transportation and communication networks, and populated regions. Volcanic emissions are of interest to scientists who study the atmosphere. Underwater volcanoes heat the ocean and change the sea floor topography. These changes can cause the global sea level to rise and affect weather patterns. Tsunamis caused by debris avalanches pose a threat to the Hawaiian coast. Earthobserving satellites, such as NOAA's Geostationary Operational Environmental Satellites (GOES), gather and relay valuable data which are crucial in efforts to minimize losses from hazardous volcanic activity.

Stratovolcanoes (also known as composite volcanoes) are built of successive layers of ash and lava. The magma (molten rock) within the volcano is viscous and often contains trapped gas, causing explosive eruptions. The clouds of ash from the volcanic eruptions present a hazard to aviation. We use imagery from polar orbiting and geostationary satellites to detect the boundaries of a volcanic ash cloud, and estimates the altitude and movement of the ash plume. NOAA distributes Volcanic Ash Advisory Statements (VAAS) to warn aircraft pilots of hazardous ash clouds. Particles erupted into the stratosphere during major eruptions can change global temperature by several degrees, altering weather patterns and effecting global agriculture. Approximately 85% of stratovolcanoes are located around the Pacific Ocean, forming what is called the "Ring of Fire. " Stratovolcanoes occur at the margins of tectonic plates, large sections of Earth's crust that move together. The continental plates, composed of less dense material, override the oceanic plates. Magma generated from the subducting plate rises and squeezes into cracks, eventually reaching the surface in a volcanic eruption.

RECORD RIDE: Pete Cabrinha, off Maui, Hawaii, in January, surfing the 70 -ft. monster that won him the 2004 Billabong XXL Award. It is given annually to the rider of the biggest wave

Earth' s oceanic and continental crust is broken up into about ten major plates and several minor ones. Each plate is about 100 -150 kilometers thick. They are all moving in different directions related to one another. An abundant amount of energy is dispersed as the plates move apart, collide, and slide by each other at the plate margins. Thus, it is at these plate boundaries where the most momentous geologic marvels occur such as mountain building, earthquakes, and volcanic activity. Volcanoes are one of nature’s most awesome forces. These natural vents and fissures erupt molten rock and gases from deep inside the earth. Unlike most volcanoes created in plate boundaries, these volcanoes are far from such things. Many of them, like the Hawaiian hot spot, lie in the center of a plate. Secondly, they make up a very small fraction, less than one percent, of all the volcanic activity in the world. Thirdly, they produce linear island chains which have a distinctive age progression. And lastly, lavas that are generated by the volcanoes are also unique. They are composed of basalts and are richer in alkali metals such as sodium, potassium, and lithium. (Abbott, Macdonald, & Peterson, 1983)

The plume hypothesis survived largely as a belief system and had to be extensively modified to account for unexpected observations. ” G. R. Foulger and J. H. Natland, “Is ‘Hotspot’ Volcanism a Consequence of Plate Tectonics? ” Science, Vol. 300, 9 May 2003, p. 921. “It seems that we must abandon the convenient concept of fixed hotspots as reference points for past plate motions. ” Ulrich Christensen, “Fixed Hotspots Gone with the Wind, ” Nature, Vol. 391, 19 February 1998, p. 740. “It was later shown, however, that the Pacific hotspots move relative to those in the Atlantic at rates of 1– 2 cm yr-1. This is less than the speed of fast-moving plates (10 cm yr-1), but enough to make the hotspot frame of reference suspect. ” The Cook-Austral and Marquesas chains, for example, are marked by gross violations of a simple age-distance relationship and by extreme variations of isotopic signature, inconsistent with a single volcanic source. The Hawaiian-Emperor chain provides a more consistent age sequence, but there is no systematic variation of heat flow across the Hawaiian swell, contradicting the simple hotspot model (Keith, 1993).

Plumes: hypothetical entities considered to be strong, active upwellings in contrast to passive upwellings caused by plate divergence. Plumes are assumed to provide magma to hotspots such as ocean islands. Plume heads are assumed by some authors to be responsible for surface uplift, breaking of the lithosphere and large igneous provinces (LIPs). The modern thinking about plumes is substantially different; plumes are considered to reflect a secondary mode of convection unrelated to (and little affected by) plate-scale convection. The idealized plume has two components: a plume head, supposedly responsible for very short-lived, massive igneous events, and a narrow plume tail which generates long-lived hotspot tracks. The source of mantle plumes is a thermal boundary layer deep in the mantle, perhaps the core-mantle boundary, although some have argued for a shallower source. The complex picture of the Earth that emerges in the plume model is compounded by uncertainties in the number of hotspots, the depth of origin of plumes, whether hotspots are fixed, the composition of plumes, the amount of melting in plumes, and the relationship of plumes to

Plume hypothesis has acquired the status of an unchallengeable dogma and an obvious fact (H. C. Seth, 1998). The popular and widespread notion that hotspot tracks are simply the products of one or more plumes beneath moving plates is actually far from reality. Few predictions and requirements of the mantle plume model seem to be fulfilled in the actual geology. The complete plume hypothesis is untestable. The narrow plume tails, 10200 km in diameter, and extending deep into the mantle are below the resolution of geophysical techniques and cannot be resolved by numerical or theoretical computation. They give no signal and have no measurable effect. The geophysical effects (bathymetry, geoid, tomography) of the large flattened plume heads are little different from alternative models of upper mantle structure. Large scale hot regions in the upper mantle can be generated by a variety of mechanisms. Hawaii should have the most readily resolvable conduit as it is situated away from ridge systems and is supposedly the strongest plume. Investigators “searched for low-velocity anomalies in the lower mantle beneat the hotspot, but found no low-velocity anomaly which correlated with the surface expression of volcanism

Unfortunately for the hotspot model, measurements along the axis of the Hawaiian swell suggested an increase in heat flow with distance away from the supposed site of the plume. The swell may not be dynamically supported, but merely represents a thick section of basalt” or of “thickened peridotite predating Hawaiian volcanism. The size of the swell does not decline along the Hawaiian chain, and there is no corresponding swell associated with the Emperor chain. While Hawaii is thought be the strongest currently active plume, its effects are small compared to those expected during the mid-Cretaceous superplume event invoked for the formation of many of the intraplate edifices on the Pacific seafloor. Uplift associated with a Cretaceous event has been observed, but fails to reach the amounts predicted in the plume model The Hawaiian Islands were considered the best example of a ‘Fixed Hotspot Plume’ phenomenon. Not explained were the large chains of submarine volcanoes scattered over a large area next to the Hawaiian chain, but not in line with it. One adjacent chain of volcanoes is actually perpendicular to the Hawaiian chain. It is now recognized that if hotspots exist, they must move.

Gravity anomalies and seafloor fabric suggest that the volume and location of volcanism in this region is controlled by stress in the lithosphere rather than the locus of narrow plumes rising from the deep Earth” (V p. 479 Hawaiian landslides have been catastrophic Volcanic activity and gentle erosion have not been the only forces to shape the Hawaiian islands. Landslide debris has now been mapped off of all the islands. Enormous amounts of material have traveled great distances, indicating that the slides were truly catastrophic. The Nuuanu and Wailau landslides, shown in the image, tore the volcanoes forming eastern Oahu and Molokai in half, and deposited blocks large enough to have been given names as seamounts. Tsunamis generated during these slides would have been devastating around the entire Pacific Basin

Replacing plume theory “There are essentially two models which have dealt with a shallow origin for the sources of intraplate volcanism on a global scale. [One is the] concept of an enriched `perisphere' layer residing between the lithosphere and Mid-ocean ridge basalts (MORB)-source. The perisphere includes the upper part of the asthenosphere and thermal boundary layer [at the base] of the continental mantle. The layer undergoes continuousenrichment from subduction processes, but is essentially static and hence encounters difficulty ingenerating long-lived volcanism as along the Hawaiian chain” (IX p. 157). “Melts from enriched mantle are most evident at new or slowly rifting regions, infant subduction zones, new backarc basins, slabwindows, and mid-plate environments away from spreading induced upwelling. Enriched mantle istherefore probably shallow. [The perisphere] is physically isolated from the depleted mantle not by itsstrength but by its weakness and buoyancy. It has the chemical characteristics often attributed tocontinental lithosphere (or plume heads)” (II p. 125). “The perisphere/asthenosphere is probably laterally and vertically inhomogeneous” (III p. 119).

Hot areas of the upper mantle may be due to the absence of cooling rather than the importation of plume heads from great depth in the mantle” (III p. 108). “A moving plate, overriding a hot region of the mantle, and being put into tension, will behave, in many respects, as if it were being impacted from below by a giant plume head” (III p. 120). “Hot cells are an alternative to plume heads” (III p. 120). “Rifting causes massive magmatism if the break occurs over hot cells. CFB may result from the upwellings of already hot, even partially molten, mantle” (III p. 99

Geologists have long assumed that the Hawaiian Islands owe their existence to a "hotspot" – stationary plumes of magma that rise from the Earth's mantle to form Mauna Loa, Kilauea and Hawaii's other massive volcanoes According to conventional views, the North-South age progression of this chain indicates that the Pacific plate has moved northward over a fixed hotspot that is currently spewing out Hawaii. The motion of the plate shifted to the northwest ~43 mya as indicated by the bend in the chain. Ppaleomagnetic data (2003) suggest it really was the "hotspot" that moved south as the Emperor chain was being formed. 1. When a rock forms, atoms of potassium start decaying into argon at a constant rate, regardless of changes in the rock's temperature, chemistry or pressure. By measuring the number of potassium-derived argon atoms in the samples, researchers were able to estimate that the submerged volcanoes formed some 47 million to 81 million years ago. 2. The magnetite behaves like miniature compass needles: The closer they are to the Earth's magnetic pole, the steeper their position. Researchers were able to verify the latitudes at which the seamounts formed by determining the angles at which the magnetite had frozen 3. Using these data, the research team concluded that the "fixed" Hawaiian hotspot probably crept southward between 81 million and 47 million years ago at a rate of about 44 millimeters a year, "changing our understanding of terrestrial dynamics. 4. Given the central role the Hawaiian-Emperor bend has played as an example of [tectonic] plate motion change, these observations now question whether major plates can undergo large changes in direction rapidly, and whether plate boundary forces alone can play a dominant role in controlling plate motions. These data sets indicate a much more active role of mantle convection in controlling the distribution of volcanic islands

Tarduno et al (Science, 2003) in journal Science disputes that longstanding paradigm by concluding that the fixed hotspot in the Pacific was not stationary after all. research suggests that the Hawaiian hotspot actually drifted southward between 47 and 81 million years ago during the Late Cretaceous to Early Tertiary This study raises fundamental questions about how the mantle works and how plates work. What's really going on here? Why do these hotspots drift, and why do they suddenly stop? We know that the mantle moves, now we have to find out how deep the motion goes

Fujiyama in Japan. An extraordinary lenticular altocumulus cloud (cloudcap) hovers like a spacecraft over a stratovolcano

Fragments of limu o Pele and Pele's hair, formed from lava bubbles during eruption. Photo © 2001 MBARI

Debris from Giant Slumps or landslides off Oahu and Molokai extends hundreds of kilometers offshore

Large landslides don't just fall to the base of the mountain the way small ones do; they often go great distances, some up to 30 times the distance they fell. Well known on a small scale as a Bingham Fluid, Melosh suggests that fluidization at the base of large landslides reduces friction to near zero (acoustic fluidization).

Slope failure on Kilauea's Submarine south flank Offshore observations along the submarine south flank of Kilauea volcano have revealed the subsurface structure of active submarine slope failure and the remnants of an ancient landslide. New multichannel reflection data and high-resolution bathymetry provide this evidence, and suggest a dynamic interplay among slope failure, regrowth, and volcanic spreading. Disrupted strata along the upper reaches of Kilauea's flank denote a coherent slump, correlated with the active Hilina fault zone on land. The slump comprises mostly slope sediments, underlain by a detachment 3 -5 km deep. Extension and subsidence along the upper flank is compensated by uplift and folding of the slump toe, which surfaces about midway down the submarine flank. Reference: J. K. Morgan, G. F. Moore, and D. A. Clague (2003) Slope failure and volcanic spreading along the submarine south flank of Kilauea volcano, Hawaii, Journal of Geophysical Research, 108(B 9): 2415. [Abstract] [Article]

Spreading of Mauna Loa's flank MAUNA LOA - A transect of four ROV Tiburon dives across the submarine west flank of Mauna Loa volcano yields compelling evidence for volcanic spreading and associated hydrothermal circulation during volcano growth. A frontal bench at the toe of the flank, formerly thought to be a downdropped block of Mauna Loa, contains a mix of volcaniclastic lithologies, including distally derived siltstone, mudstone, and hyaloclastite. The bench is overlain by bedded gravels and subaerially erupted pillow flows derived from local shoreline-crossing lava flows. The volcaniclastic strata in the bench were offscraped, uplifted, and accreted to the edge of the flank, as it plowed seaward into the surrounding moat. The accreted strata underwent significant diagenesis, through deep burial and circulation of hydrothermal fluids expelled from porous sediments beneath the volcano. Timing constraints for bench growth and breakup suggest that catastrophic failure of the subaerial edifice ca. 250– 200 ka triggered volcanic spreading by reducing stresses resisting basal sliding and rift-zone inflation. Increased eruptive activity, and westward migration of Mauna Loa's southwest rift zone, gradually rebuilt the massive flank, arresting slip prior to detachment of the Alika 2 debris avalanche ca. 120 ka. Reference: J. K. Morgan and D. A. Clague (2003) Volcanic spreading on Mauna Loa volcano, Hawaii: Evidence from accretion, alteration, and exhumation of volcaniclastic sediments. Geology: Vol. 31, No. 5, pp. 411– 414. [Abstract] [Article

Conditions for landslides and canyon formation MOLOKAI - The main break-in-slope on the northern submarine flank of Molokai at 1500 to 1250 m depth is a shoreline feature that has been slightly modified by the Wailau landslide. Submarine canyons above the break-in-slope were subaerially carved. Where such canyons cross the break-in-slope, plunge pools may form by erosion from bedload carried down the canyons. West Molokai Volcano's continued infrequent eruptions 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 breakin-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. 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. Reference: D. A. Clague and J. G. Moore (2002) The proximal part of the giant submarine Wailau landslide, Molokai, Hawaii, Journal of Volcanology and Geothermal Research, 113: 259 -287. [Article]

Improvements in mapping landslides OAHU, MOLOKAI - The development of ideas on the giant Hawaiian landslides parallels improvements in the technology of bathymetric mapping and navigation. The landslides were first recognized in the 1960 s in a relatively detailed U. S. Navy single-beam sonar survey utilizing an improved radio navigation system. The GLORIA multibeam side-scan sonar system (1980 s) imaged unprecedented detail in the known landslides and revealed numerous other undiscovered ones. The JAMSTEC multibeam surveys (late 1990 s), utilizing GPS navigation, produced detailed maps of the entire landslide area for the first time. Reference: J. G. Moore and D. A. Clague (2002) Mapping the Nuuanu and Wailau landslides in 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, 223 -244. Volcaniclastic rocks on the flanks of landslide blocks

Volcaniclastic rocks on the flanks of landslide blocks OAHU, MOLOKAI - The rocks exposed on the steep slopes of giant landslide blocks in the Nuuanu and Wailau landslides are fragmental rocks: hyaloclastite and volcaniclastic breccias. They form as 1) secondary slope mantling of unlithified breccia consisting of clasts in a mud matrix; 2) hyaloclastite and breccia, all with zeolite cement, that form downslope of the shoreline where lava flows enter the sea and fragment; and 3) breccia formed by tectonic fragmentation of glassy submarine-erupted pillow basalt. Lavas erupted from single volcanoes are highly variable in mayor-element composition, even during their tholeiitic shield stage, making it difficult to identify which landslide block was derived from which volcano. Low-temperature fluids circulate through the fragmental deposits on the flanks of the volcanoes, partially altering the glass to palagonite and cementing the volcaniclastic rocks with Na- and K-rich zeolites. Spreading of the volcano early in its history along low-angle thrust faults laterally transports deep submarine pillow lava into the flank of the volcano where it crops out as tectonic breccia. The faults underlying the landslide blocks are within this tectonized core of the volcano, not simply within the shallow slope deposits of hyaloclastite and breccia. The Nuuanu landslide predates the 1. 5 Ma Wailau landslide. Reference: D. A. Clague, J. G. Moore, and A. S. Davis (2002) Volcanic breccia and hyaloclastite in blocks from the Nuuanu and Wailau landslides, 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, 279 -296.

Uplift of strata forming Papa'u seamount and offset of surface features along the western boundary of Kilauea indicate that the slump has been displaced ~3 km in a south-southeast direction. This trajectory matches coseismic and continuous ground displacements for the Hilina slump block on land, and contrasts with the southeast vergence of t he rest of the creeping south flank. To the northeast, slope sediments are thinned and disrupted within a recessed region of the central flank due to catastrophic slope failure in the recent past. Debris from the collapsed flank was shed into the moat in front of Kilauea, building an extensive apron. Seaward sliding of Kilauea's flank offscraped these deposits to build an extensive frontal bench. A broad basin formed behind the bench and above the embayed flank. Uplift and back tilting of young basin fill indicate recent, and possibly ongoing, bench growth. The Hilina slump now impinges upon the frontal bench; this buttress may tend to reduce the likelihood of future catastrophic detachment