When rock is exposed to water, air, and surface conditions, weathering processes go to work. These include both chemical and physical (also known as mechanical) weathering. Chemical weathering transforms rock by changing its chemical composition; carrying away ions in solution; altering the chemical formula and physical structure of the original mineral grains; and disrupting the relationships between the mineral grains in the original rock, thus weakening the rock physically as well. Chemical weathering reactions usually involve the parent mineral and water (and sometimes a dissolving agent in the water, such as carbonic acid or dissolved oxygen), and frequently result in the creation of a secondary or residual mineral as a byproduct of weathering. Sometimes the parent or primary mineral dissolves completely and leaves no solid residue.
Physical weathering processes break the rock into smaller fragments. As the book points out, the smaller the fragments, the larger the total amount of surface area that is available to come in contact with moisture and dissolving agents, and therefore breaking the rock into smaller pieces also ensures that chemical weathering will be more effective. The extent to which a rock is affected by weathering is therefore also partly a function of its original physical structure: if it has a lot of joints and fractures where water was able to seep in, weathering will take place more rapidly. If it is relatively massive and unjointed, weathering will be less active.
Although chemical and physical weathering can lead directly to the production of smaller rock fragments that are available to be eroded and then transported and deposited as sediment, an intermediate stage that is common in most environments is the evolution of soil. Soil is not just dirt; it is a complex mixture of solid mineral grains (including both original, or primary minerals, and weathering products, or secondary minerals); organic matter (plant fragments or complex organic molecules synthesized by living organisms, as well as the breakdown products of accumulated organic matter); and pore spaces occupied by a mixture of water and air or soil gases. Soils do evolve over time and are affected by the original composition of the parent rock, the climatic conditions at the site, the length of time over which the material has been exposed at the surface, and local site conditions such as slope angle. There is a fairly complex system for classifying different soils, but we will touch on this only briefly. You should be aware that a typical soil has a recognizable sequence of layers or horizons that develop in place through the action of chemical and physical processes over time, chiefly as a result of chemical reactions with moisture that seeps downward from the surface. A good soil scientist can tell you a lot about the age and history of a soil just by examining these horizons and recording their properties.
Weathering results not only in the production of soil and rock fragments, but also in the dissolution of some original minerals and the carrying away in solution of some of the ions from the original minerals. Some of these dissolved materials may eventually reach the ocean and remain in solution, but in many cases they will later precipitate out of solution to form new minerals. Mineral grains formed by precipitation of materials that were transported in dissolved form are referred to as chemical sediments, and many sedimentary rocks - such as limestone or rock salts - are made from chemical sediments. Conversely, solid particles that are eroded from the surface, transported to another environment, and then deposited, are referred to as clastic sediments. This basic distinction is of great significance when we classify sedimentary rocks, as most sedimentary rocks are either clastic or chemical in origin.
The creation of sedimentary rocks therefore involves several important steps: first, weathering of the parent rock. Next, erosion (for solid particles) and transportation to the environment of deposition; or solution (for chemical sediments) and transportation in dissolved form, followed by chemical precipitation of a new mineral in the environment of deposition. Typically you make sediment into rock by a series of processes that are collectively known as lithification: burial and compression, followed by the cementing together of the individual sediment particles as pore waters circulate through in the subsurface environment and chemical cement precipitates out of solution. Chemical and physical changes that occur after burial are also referred to collectively as diagenesis. This term is not found in your textbook but is commonly used by geologists.
Sedimentary rocks are characterized in a number of different ways: by the types of minerals present; by the particle sizes that are present (particularly important for clastic sediments); by sedimentary structures that are diagnostic of the depositional environment; and by fossils. For a geologist, a sedimentary rock typically contains a wealth of information that may allow a detailed reconstruction of what was going on in the environment where the sediments were deposited.
Geologists also examine sequences of different kinds of sedimentary rocks that may be found in different environments, and sometimes these also provide important information: for example, you may find sedimentary rocks diagnostic of shallow marine conditions, barrier beaches, and lagoon or marsh environments in fairly close proximity to one another, and the sequence of all of these together also provides information about the general coastal and nearshore environment in which these sediments were originally deposited. Sedimentary rocks that represent a natural sequence like this are referred to as facies. (We will see that term again in connection with metamorphic rocks, but in that case it will have a somewhat different meaning.)
Chapter 6