Igneous rock classification revisited: 5. Grain size problems

摘要

For much of the last century, geologists spoke of 'plutonic' and 'volcanic' rocks rather than 'coarse-grained' and 'fine-grained' igneous rocks, simply because it dodged the issue of having to decide numerical figures for coarseness and fineness. Rosenbusch introduced the terms plutonic and volcanic in 1887 that quickly replaced the earlier coarse and fine-grained terminology advocated by Zirkel. It was said that 'everybody' knew what plutonic meant. But did they? During drilling offshore for oil, crystalline rocks were found and labelled 'plutonic' because they were coarse-grained. This led to a plutonic intrusion being identified. Had the rocks been classified as coarse-grained with the possibility that they might be a thick sill or lava pile (as they were later proved to be) with significant sediments underneath, drilling might have continued. The misleading classification was a costly error. This true story focussed the mind of the Subcommission.