Field Identification of Lake District Volcanic Rocks

Abstract

The magma of the volcanoes of island arcs and continental margins originates along subduction zones. It is initially derived from the mantle and oceanic crust and is low in silica, but as it rises through the granitic rocks of the continental crust it becomes contaminated by these silica-rich rocks and assumes an average composition of andesite (figure 7). There is however considerable chemical variation from basalt at one extreme to rhyolite at the other depending on local conditions, and there is also considerable variations in the form of the extrusive material, from lava flows to explosive fragmental material and volcanic mudflows. The silica-poor basalts and basaltic andesites form the more fluid magmas and are frequently found as lava flows, although breccia (large fragments) and tuff (finer-grained ash), formed when rocks disintegrated by explosion, are also common. The opposite is true of the more viscous silica-rich rhyolite and dacite magma which frequently occur in the form of disastrously explosive ignimbrite (eruptions of incandescent ash) although lavas of this composition are also common.