Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 65

SEDIMENTARY

METAMORPHIC

IGNEOUS

Rocks form due to the solidification of lava on Earth's surface and that of magma inside the earth. Lava refers to molten rock material that is extruded onto the surface during a volcanic eruption. Molten rock is known as magma when it lies below the ground surface. There are three basic types of rocks: sedimentary, metamorphic, and igneous; these types are briefly described in the following section.

We cannot define a rock as a solid in terms of a melting point. The melting point of rocks cannot be defined in a simplistic manner because they consist of different minerals with different melting points. In the field of engineering geology, a rock is defined not only in terms of the thermal state, but also as a hard solid that consists of compacted aggregation of minerals that remains intact in water and cannot be excavated without blasting (West 1995). In terms of T _m, the definition of solid rock is, therefore, not as simplistic as one may think – as in polycrystalline pure metals, ice, and many ceramics.