Thermal Metamorphism and Its Products

Abstract

Thermal metamorphism includes pyrometamorphism, contact metamorphism optalic metamorphism, and pneumatolytic metamorphism, but these classes are not sharply marked off, and pass by insensible graduations one into the other. By pyrometamorphism is here meant the effects of the highest degree of heat possible without actual fusion, acting under relatively-dry conditions. Contact metamorphism takes place at lower temperatures, and mineral transformations are facilitated by abundance of rock moisture aided by magmatic emanations. The changes produced with positive additions of material from magmatic sources are dealt with later under the heading of pneumatolytic metamorphism (p. 324). The character and extent of the changes produced by the above kinds of metamorphism depend on a number of variable factors. The greater the size of an intrusive mass the greater, in. general, the degree of metamorphism effected in the surrounding rocks. Thus, the contact aureole around large granite batholiths may be several miles wide. In small sills and dykes, however, the alteration may be limited to a few feet or inches immediately adjoining the contacts.