Linear Volcanic Chains in Oceans: Possible Formation Mechanisms

摘要

Possible formation mechanisms of linear volcanic chains in oceans are considered with particular emphasis placed on tectonic processes in the lithosphere. Nonparallel patterns of volcanic chains, as well as irregular variations in volcanism ages, may be due to the formation of sigmoid fractures that appear in certain stress fields. The tectonic stress may control the dimensions of volcanic chains, their lengths, and the volcanism intensity. At the same time, certain assumptions are necessary. For example, to explain shallow magmatism, it must be assumed that the temperature of the asthenosphere is close to the melting point of mantle material, although the asthenosphere may be highly variable in the degree of enrichment. Hence, even insignificant variations in the temperature, volatile contents, or bulk composition may provoke large-volume melting. It is shown that the rotation of the Earth causes additional displacements of plates relative to the underlying mantle. While a fertile fragment exists in the mantle, such an inhomogeneity remains stationary relative to the moving plate and the melting of this inhomogeneity may result in the growth of volcanic uplift. The global stress field determined by plate boundaries and an intraplate factor controls the distribution of the stress fields, which are responsible for the formation of volcanic chains. It is concluded that the available data on the age progressions and character of linear volcanic chains within oceanic plates provide no grounds for any single hypothesis explaining the formation of these chains. The most universal hypothesis seems to be the explanation based on shallow tectonic processes. The localization and formation mechanism of volcanic chains are determined by the stress field in the lithosphere, thermal compression and expansion, the specific features of the plate structure, melt dynamics, and the occurrence of fertile material in the mantle rather than by temperature. The volume of volcanic eruptions depends on the degree of fertility of the mantle material; the presence of volatiles; the plate thickness; and, to a lesser extent, the temperature. At the same time, the formation of such large volcanic uplifts as Hawaii and Iceland may be explained in terms of the classic plume hypothesis. Thus, it is suggested that the formation of linear volcanic chains is a polygenetic process resulting from the combination of different geody-namic factors. Further detailed investigation will give rise to new geodynamic models.