Subsidence and GPS measurements have been done on lava deltas formed on the coast during the Pu‘u ‘Ō‘ō - Kūpa‘ianahā eruptions of Kīlauea Volcano to see the evolution of these forms and the eventual hazards for tourists. When the flow of lava stops, benches formed below the old sea cliff continue to be unstable during several years. Landslides of the pyroclastic basement involve subsidence and collapse, partial or complete, of the delta. For example, the East Kupapa‘u bench, built between April 2001 and January 2002, has been subsided of 12 cm/year afterwards. Progressively, by compaction, the fan of pyroclasts stabilizes and so the delta. The bench can be qualified as mesastable when there is a relative stability. For instance, between 2000 and 2002, the subsidence only reaches from 0 to 3 mm/year on the 1995 bench. Movements are mostly concentrated on the outer part of the delta. Thus, on the benches we surveyed, the external margin subsides from 0,6 to 8,6 cm/year and gently moves seaward from 5 to 27 cm/year, with opening of cracks more or less parallel to the coastline. This movement certainly traduces underlying slides. Paroxysmic events cause collapses of the outward margin, building new benches, which can sometimes entirely disappear into the sea. Underlying slides, pressure of breaking waves, decompression and gravity create slow decollement of the cliff along the cracks too, up to breakpoint with toppling. The delta stabilization explains the decreasing retreat of the coast, shown by the GPS measurements done on the coast of the Pu‘u ‘Ō‘ō - Kūpa‘ianahā eruptions between 1997 and 2002. Firstly, numerous collapses of the bench result in a quick retreat of the coast. From approximately 100 m/year, the rate quickly lessens to 10 m/year or so, two years after the end of the bench building. Then, the rate stays high but reduces slowly during more or less 10 years to reach about 2,5-5 m/year. The stability of the major part of the delta prevents from large collapses, which only occur sporadically and to a lesser degree in the unstable outer part. When the delta is totally stable, the retreat of the coast is lower and only made by wave attack, which provokes topple and rockslides of the strongly fissured rock. Instability of inactive lava deltas, mostly on the external margin, causes hazards for tourists who walk on them. The dangerousness is especially high because of the absence of hazard perception, contrary to active lava deltas.
Hawaiian volcanoes are exceptional examples of intraplate hotspot volcanism. Hotspot volcanoes, which frequently host large eruptions and related earthquakes, flank‐failure landslides, and associated tsunamis, can present severe hazards to populated regions. Many studies have focused on subaerial parts of Hawaiian volcanoes, but the deep‐water flanks of the edifices, which can reach 5700 m below sea level, remain poorly understood because they are so inaccessible. In 1998 a collaborative program between Japan and the United States was initiated to explore the evolution of Hawaiian volcanoes, including their growth and degradation.
New mapping and 60 new radiocarbon ages define the age and distribution of latest Pleistocene and Holocene (past 13,000 years) lava flows at Haleakalā volcano, Island of Maui. Paleomagnetic directions were determined for 118 sites, of which 89 are in lava flows younger than 13,000 years. The paleomagnetic data, in conjunction with a reference paleosecular variation (PSV) curve for the Hawaiian Islands, are combined with our knowledge of age limitations based on stratigraphic control to refine age estimates for some of the undated lava flows. The resulting volumetric rate calculations indicate that within analytical error, the extrusion rate has remained nearly constant during the past 13,000 years, in the range 0.05–0.15 km 3 /kyr, only about half the long‐term rate required to produce the postshield strata emplaced in the past ∼1 Myr. Haleakalā's eruptive frequency is similar to that of Hualālai volcano on the Island of Hawai‘i, but its lava flows cover substantially less area per unit time. The reduced rates of lava coverage indicate a lower volcanic hazard than in similar zones at Hualālai.
The State's geology is presented on eight full-color map sheets, one for each of the major islands. These map sheets, the illustrative meat of the publication, can be downloaded in pdf format, ready to print. Map scale is 1:100,000 for most of the islands, so that each map is about 27 inches by 36 inches. The Island of Hawai`i, largest of the islands, is depicted at a smaller scale, 1:250,000, so that it, too, can be shown on 36-inch-wide paper. The new publication isn't limited strictly to its map depictions. Twenty years have passed since David Clague and Brent Dalrymple published a comprehensive report that summarized the geology of all the islands, and it has been even longer since the last edition of Gordon Macdonald's book, Islands in the Sea, was revised. Therefore the new statewide geologic map includes an 83-page explanatory pamphlet that revisits many of the concepts that have evolved in our geologic understanding of the eight main islands. The pamphlet includes simplified page-size geologic maps for each island, summaries of all the radiometric ages that have been gathered since about 1960, generalized depictions of geochemical analyses for each volcano's eruptive stages, and discussion of some outstanding topics that remain controversial or deserving of additional research. The pamphlet also contains a complete description of map units, which enumerates the characteristics for each of the state's many stratigraphic formations shown on the map sheets. Since the late 1980s, the audience for geologic maps has grown as desktop computers and map-based software have become increasingly powerful. Those who prefer the convenience and access offered by Geographic Information Systems (GIS) can also feast on this publication. An electronic database, suitable for most GIS software applications, is available for downloading. The GIS database is in an Earth projection widely employed throughout the State of Hawai`i, using the North American datum of 1983 and the Universal Transverse Mercator system projection to zone 4. 'This digital statewide map allows engineers, consultants, and scientists from many different fields to take advantage of the geologic database,' said John Sinton, a geology professor at the University of Hawai`i, whose new mapping of the Wai`anae Range (West O`ahu) appears on the map. Indeed, when a testing version was first made available, most requests came from biologists, archaeologists, and soil scientists interested in applying the map's GIS database to their ongoing investigations. Another area newly depicted on the map, in addition to the Wai`anae Range, is Haleakala volcano, East Maui. So too for the active lava flows of Kilauea volcano, Island of Hawai`i, where the landscape has continued to evolve in the ten years since publication of the Big Island's revised geologic map. For the other islands, much of the map is compiled from mapping published in the 1930-1960s. This reliance stems partly from shortage of funding to undertake entirely new mapping but is warranted by the exemplary mapping of those early experts. The boundaries of all map units are digitized to show correctly on modern topographic maps.
