This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
This is the change effected in surrounding rocks by igneous magmas. There is a difference between the effects produced by a surface lava flow and those caused by a plutonic intrusive. In the former case the results are usually not very striking, because of the way in which a lava stream surrounds itself with non-conducting scoriae, and are such as may be referred to the action of dry heat. Bituminous coal is changed into a natural coke by the removal of its volatile constituents; clay may be baked into a hard red rock, looking like earthenware, and limestone changed to quick lime, by driving off the C02. Plutonic intrusions, on the other hand, are more efficient agents of change, because they are presumably of a higher temperature and retain their heat longer, and because the vapours and gases which they contain cannot escape into the atmosphere, but strongly affect the invaded rocks. The rock invaded and metamorphosed may be either sedimentary, igneous, or already metamorphic, and the effects may be very marked, or surprisingly small; indeed, it is often quite impossible to say why the changes should be so insignificant. Magmas which contain an abundance of the mineralizing vapours (see p. 287) produce much more effect than those with only a small quantity of such vapours.
For this reason acid magmas are more effective than basic. Much, too, depends upon the nature of the invaded rock; sediments which contain large percentages of alumina and lime are much more readily and profoundly changed than those which are.made up almost entirely of silica. The distance to which the zone of change extends is wider when the intrusive mass cuts across the strata than when it follows the bedding-planes, so that a dyke or stock is more effective than a sill.
We may now consider some examples of contact metamor-phism, and, for this purpose, shall select only the changes of sedimentary rocks; for those of the other classes require a treatment too minute and refined for an elementary work. We may note, in passing, however, that some of the veins given off from granite stocks, which have invaded other igneous rocks, are probably of a metamorphic nature and due to the penetration of vapours.
In a series of strata which have been invaded by an igneous magma, we find a gradual change from the unmodified rock which lies beyond the reach of the transforming agencies, to that at the actual contact with the igneous mass. Along this line of contact the strata are so thoroughly reconstructed that often only a microscopical examination will distinguish the changed sediment from the igneous rock. A siliceous sandstone or conglomerate develops no new minerals in the change, or only in insignificant quantity from the impurities present. The bulk of the material simply crystallizes and forms the white rock, quartzite. Clay rocks undergo more radical change and are usually divisible into distinct zones; the outermost zone is unchanged; in the intermediate one the shale is changed to a dense slate spotted with biotite, magnetite, or other dark minerals. The spotted slate passes gradually into mica schist, a rock made up of flakes of mica, with some quartz and felspar, arranged in rudely parallel planes.
At the contact the rock is converted into hornfels,1 which is a very dense substance, looking like trap, and filled with numerous sili-cated minerals, such as hornblende, felspar, and many others which were not enumerated in the chapter on the rock-forming minerals.
Limestones are crystallized by the heat into marble, a dense aggregation of crystalline grains of calcite, usually with obliteration of the bedding-planes and of any fossils which the rock may have originally contained. Pure limestone gives rise to white marble, but as most limestones contain impurities, they develop, when metamorphosed, a large variety of minerals, such as biotite, garnet, amphiboles, pyroxenes, etc. Beds of bituminous coal are baked into a natural coke, as in Virginia and North Carolina, or changed to anthracite, as in Colorado, or even to graphite in the contact zone, and limonite is converted into magnetite.
1 Also called hornstone, but as this term is used for flint, it is better to retain it in the latter sense only.
Among investigators of the subject there is much difference of opinion as to how far there is an actual migration of material from the plutonic magma into the enclosing rock walls. When there is shattering along the contact, or fissures and crevices are opened in the country rock, material, both in solution and in a state of fusion, is introduced. Cementation is the deposition of mineral matters from solution in the interstices between the granules of the rock. Quartz, calcite, iron oxides, felspars, mica, augite, and other minerals maybe thus introduced, and sometimes the quantity of new material brought into the rock is very large. Injection is the penetration of a rock by molten substances which may fill up all the minute crevices. The distinction between cementation and injection is not a very sharply marked one, because superheated water and molten magmas appear to mix in all proportions. The difference between the two processes seems thus to be largely a question of the quantity of water present.
In some examples even into the unruptured walls fluorine and boron have penetrated, and probably the escaping hydro-fluosilicic acid has introduced silica and some bases for a short distance from the contact.
Contact metamorphism, as its name implies, is a local phenomenon, but a widely ramifying and complex system of igneous intrusions may change large areas of sedimentary rocks.