Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 67

A complete change of physical form or substance is called metamorphosis. Thermomechanical processes occurring on pre‐existing rocks within Earth's crust produce irreversible physical changes in their texture, structure, and mineralogical characteristics; and the product is known as a metamorphic rock. Metamorphism can take place near igneous intrusions, called contact metamorphism, or over large areas, called regional metamorphism. The latter occurs as a result of tectonic plate movements.

Two primary types of rocks are seen in regionally metamorphosed rocks: foliated and nonfoliated. Marble and quartzite are common examples of nonfoliated metamorphic rocks. Marble is metamorphosed limestone, and quartzite is metamorphosed sandstone. Slates, phyllites, schists, and gneisses are examples of foliated metamorphic rocks. Mechanically, they exhibit anisotropy because foliated layers may display weakness as well as strength depending on their material characteristics. Naturally, permeability, strength, deformation, and hence seismic response are affected by direction of foliation in foliated metamorphic rocks (West 1995).