Terry R. Naumann

The global distribution of human population and recent volcanism

Abstract This study quantifies the spatial relationship between the global distribution of human population and recent volcanism. Using recently compiled databases of population and Holocene volcanoes, we estimate that almost 9% (455 × 106 people) of the worlds 1990 population lived within 100km of an historically active volcano and 12% within 100km of a volcano believed to have been active during the last 10,000 years. The analysis also indicates that average population density generally decreases with distance from these volcanoes (within 200 km). In tropical areas, the elevation and fertile soils associated with volcanic regions can provide incentives for agrarian populations to settle close to potentially active volcanoes. In Southeast Asia and Central America higher population densities lie in closer proximity to volcanoes than in other volcanic regions. In Japan and Chile, population density tends to increase with distance from volcanoes. The current trends of rapid urbanization and sustained popul...

Generation of alkalic basalt by crystal fractionation of tholeiitic magma

Contemporaneous alkalic and tholeiitic basalts from Cerro Azul volcano, in the Galapagos archipelago, are related by fractional crystallization in the lower crust and upper mantle. The higher pressures enhance augite crystallization, which depletes the residual liquids in silica while enriching them in K and Na, just as predicted by phase-equilibria experiments. Such occurrences have also been reported from other ocean islands. The common aspect of these alkalic suites is that they occur at volcanoes that have relatively low magma-supply rates, which leads to the development of deep magma bodies and the production of alkali-olivine basalts by fractional crystallization of a tholeiitic parent. During more robust phases of magmatism, tholeiitic magmas ascend into the upper crust and produce a low-pressure tholeiitic differentiation trend.

Illegitimate magmas of the Galápagos: Insights into mantle mixing and magma transport

Roughly 1–2% of the flows erupted from flank vents of the western Galapagos shield volcanoes have anomalous compositions. We call these illegitimate magmas because of their uncertain parentage. Because some illegitimate magmas are compositionally indistinguishable from lavas of an adjacent volcano and erupt from the flank facing the adjacent volcano, such magmas apparently result from lateral intrusion of magma from the adjacent volcano. Other illegitimate magmas come from parts of the Galapagos plume that have incompletely mixed or result from unusually advanced melting of part of the mantle.

Widespread Secondary Volcanism Near Northern Hawaiian Islands

Hot spot theory provides a key framework for understanding the motion of the tectonic plates, mantle convection and composition, and magma genesis. The age-progressive volcanism that constructs many chains of islands throughout the worlds ocean basins is essential to hot spot theory. In contrast, secondary volcanism, which follows the main edifice building stage of volcanism in many chains including the Hawaii, Samoa, Canary, Mauritius, and Kerguelen islands, is not predicted by hot spot theory. Hawaiian secondary volcanism occurs hundreds of kilometers away from, and more than 1 million years after, the end of the main shield volcanism, which has generated more than 99% of the volume of the volcanos mass [Macdonald et al., 1983; Ozawa et al., 2005]. Diamond Head, in Honolulu, is the first and classic example of secondary volcanism.