Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 66

The surface of Earth consists of about 75% sedimentary rocks. These rocks are recognized readily by the most noticeable feature of stratification or layers, often enriched with fossils. There is a wide range of sedimentary rocks depending upon the origin of their source material. West (1995) stated that 99% of all sedimentary rocks consist of shales (46%), sandstones (32%), and limestones (22%). The engineering properties of sedimentary rocks vary greatly, but these types of rocks are not used at high temperatures. Deep down in the earth, sedimentary rocks are subjected to metamorphic process due to heat and pressure.

Particles in the form of sand and silt produced from erosion of rocks are carried out by streams and rivers to the oceans or large lakes. The transported particles are consolidated by processes known as lithification. The Ancient Greek word “lithos” meaning rock and the suffix “ific” derived from Latin are combined to make “lithification” for describing the processes in which water‐saturated unconsolidated sediments compact under pressure to become “sedimentary rocks.” Lithification is therefore a time‐dependent process of the removal of fluid from pores through compaction and cementation. Millions of years of lithification processes of compaction, cementation, and crystallization of sediments deposited in water produce sedimentary rocks.

Lithification should not be confused with the word petrification that involves the replacement of organic material by silica over a long period of time, such as in the development of fossils (Monroe et al. 2006). However, there is a counterpart of the lithification processes in the case of the densification of snow or ice particles deposited on Earth's surface. In this case, air trapped in unconsolidated and porous snow deposits is removed under pressure and the mass undergo morphological changes by compaction, grain growth, and cementation (sintering). The primary difference is the extremely high thermal states of snow deposits. This subject will be covered later. Very similar processes are also involved in making ceramics and powder metallurgy.