increasing frequency and intensity, the ground vibrations called volcanic tremor, localized uplift of the surface, ground cracks, and anomalous gas emissions. Of all the possible hazards from a future volcanic eruption in the Yellowstone region, by far the least likely would be another explosive caldera-forming eruption of great volumes of rhyolitic ash. Abundant evidence indicates that hot magma continues to exist beneath Yellowstone, but it is uncertain how much of it remains liquid, how well the liquid is interconnected, and thus how much remains eruptible. Any eruption of sufficient volume to form a new caldera probably would occur only from within the present Yellowstone caldera, and the history of postcaldera rhyolitic eruptions strongly suggests that the subcaldera magma chamber is now a largely crystallized mush. The probability of another major caldera-forming Yellowstone eruption, in the absence of strong premonitory indications of major magmatic intrusion and degassing beneath a large area of the caldera, can be considered to be below the threshold of useful calculation.
[1] The longest distance between subaerial shield volcanoes in the Hawaiian Islands is between the islands of Kaua'i and O'ahu, where a field of submarine volcanic cones formed astride the axis of the Hawaiian chain during a period of low magma productivity. The submarine volcanoes lie ∼25-30 km west of Ka'ena Ridge that extends ∼80 km from western O'ahu. These volcanoes were sampled by three Jason2 dives. The cones are flat topped, <400 m high and 0.4-2 km in diameter at water depths between ∼2700 and 4300 m, and consist predominantly of pillowed flows. Ar-Ar and K-Ar ages of 11 tholeiitic lavas are between 4.9 and 3.6 Ma. These ages overlap with shield volcanism on Kaua'i (5.1-4.0 Ma) and Wai'anae shield basalts (3.9-3.1 Ma) on O'ahu. Young alkalic lavas (circa 0.37 Ma) sampled southwest of Ka'ena Ridge are a form of offshore secondary volcanism. Half of the volcanic cones contain high-SiO 2 basalts (51.0-53.5 wt % SiO 2 ). Pb than any Hawaiian tholeiitic lava. West Ka'ena tholeiitic lavas have geochemical and isotopic characteristics similar to volcanoes of the Loa trend. Hence, our results show that the Loa-type volcanism has persisted for at least 4.9 Myr, beginning prior to the development of the dual, subparallel chain of volcanoes. Several West Ka'ena samples are similar to higher SiO 2 , Loa trend lavas of Ko'olau Makapu'u stage, Lāna'i, and Kaho'olawe; these lavas may have been derived from a pyroxenite source in the mantle. The high Ni contents of olivines in West Ka'ena lavas also indicate contribution from pyroxenite-derived melting.
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