Tham khảo tài liệu 'brownstein s., et al. barron's .(barrons)(669s)(1997) episode 1 part 5', ngoại ngữ, ngữ pháp tiếng anh phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 112 Reading Comprehension Questions part of the earth s crust but they are generally covered at the surface by a relatively thin layer of sedimentary or metamorphic rocks. Igneous rocks are distinguished by the following characteristics 1 they contain no fossils 2 they have no regular arrangement of layers and 3 they are nearly always made up of crystals. Sedimentary rocks are composed largely of minute fragments derived from the disintegration of existing rocks and in some instances from the remains of animals. As sediments are transported individual fragments are assorted according to size. Distinct layers of such sediments as gravels sand and clay build up as they are deposited by water and occasionally wind. These sediments vary in size with the material and the power of the eroding agent. Sedimentary materials are laid down in layers called strata. When sediments harden into sedimentary rocks the names applied to them change to indicate the change in physical state. Thus small stones and gravel cemented together are known as conglomerates cemented sand becomes sandstone and hardened clay becomes shale. In addition to these other sedimentary rocks such as limestone frequently result from the deposition of dissolved material. The ingredient parts are normally precipitated by organic substances such as shells of clams or hard skeletons of other marine life. Both igneous and sedimentary rocks may be changed by pressure heat solution or cementing action. When individual grains from existing rocks tend to deform and interlock they are called metamorphic rocks. For example granite an igneous rock may be metamorphosed into a gneiss or a schist. Limestone a sedimentary rock when subjected to heat and pressure may become marble a metamorphic rock. Shale under pressure becomes slate. 4. The primary purpose of the passage is to A differentiate between and characterize igneous and sedimentary rocks B explain the factors that may cause rocks to change in form C show how the
