Igneous rocks, field relations

Field Characteristics of Lavas

Lava (from the Italian lavare ‘to wash’) was originally applied to streams of water, and in the eighteenth century in Neopolitan dialect to streams of molten rock from Vesuvius. The term now is used both for the molten material that erupts from volcanoes and to the rock that forms on solidification of this material. Most lavas are silicate liquids and range in composition from komatiite, an ultramafic lava rich in Mg, Fe, and Ca, to rhyolite, a felsic lava rich in Si, Al, and alkali elements. Basalt, a mafic lava, is the most common type.

The main types of lava and their chemical and physical characteristics are given in Table 1. There is a close relationship between the physical properties of lavas and their chemical compositions. Ultramafic and mafic lavas erupt at high temperatures and have relatively high densities and low viscosities (i.e., they are highly fluid). They tend to have low concentrations of dissolved water and other volatile phases....