from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Plural form of extrusive.
Sorry, no etymologies found.
It is regarded as a product of primary deposition, under the influence of hot solutions related in some way to the igneous flows; but whether the solutions were magmatic, originating in the lavas or below, or whether they were meteoric waters rendered hot by contact with the extrusives, and thereby made effective in leaching copper from them, is not clear.
These are called effusives or extrusives, or lavas or volcanics, to distinguish them from intrusives formed below the surface.