|
Guide to Reading
In this chapter we encounter the second basic rock type: sedimentary rocks. Formed from detritus, mineral crystals, and shells, sediments and sedimentary rocks cover 80% of Earth’s surface and are part of a multitude of different environments. Past environments have influenced the types of sedimentary rocks created, and current sedimentary rocks influence the looks, characteristics, and resources of current environments.
The chapter starts by explaining that sedimentary rocks are created at or near Earth’s surface in one of three general ways: (1) cementing together loose grains of rock, (2) precipitating ions from water solution, or (3) concentrating skeletal material of aquatic organisms.
The rock grains needed to create sedimentary rocks are the result of the breakdown (disintegration) and chemical change (decomposition) of existing rock by physical (mechanical) weathering and chemical weathering. Several types of physical weathering are discussed, including jointing, exfoliation, frost wedging, root wedging, salt wedging, thermal expansion, and animal attack. Common categories of chemical weathering are offered next. These are:
- dissolution, which is just the plain dissolving of a solid in water
- hydrolysis, in which water facilitates the chemical change of minerals
- oxidation, in which an element looses some electrons and which may or may not directly involve oxygen
- hydration, in which water absorbed into the crystal structure may cause the mineral to expand
The above chemical processes may happen with or without the involvement of organisms, and may destroy minerals within months or possibly not for millions of years.
A word of advice to the readers who are not chemists: don’t worry. Words are supplied for every formula and equation so you can understand what’s going on, looking at the formulas and equations and using simple arithmetic will let you keep track of all the ingredients, and some of the reactions are as simple as iron rusting (iron oxidizing).
A discussion of soils comes next. Soil science is complex and can be the subject matter for numerous courses. Here the author offers the simple basics: (1) why soil is more than just broken down rock, (2) the physical structure of typical soils (zones and horizons), (3) factors influencing soil development (climate, substrate composition, slope steepness, drainage, time, and vegetation type), and (4) soils’ relations to environments, using the relations of pedalfer, pedocal, and laterite as examples.
A large part of the chapter is devoted to classifying and describing common sedimentary rocks. There are four main classifications:
- clastic (detrital) sedimentary rocks (examples: breccia, conglomerate, arkose, sandstone, shale, siltstone, mudstone, and graywacke)
- biochemical sedimentary rocks (examples: limestone, including fossiliferous limestone, micrite and chalk, and chert)
- chemical sedimentary rocks (examples: the evaporites gypsum and halite, travertine, dolostone, and several varieties of chert)
- organic sedimentary rocks (examples: coal and oil shale)
Sedimentary rocks occur in layers called beds or strata, which may display special features such as cross beds, bioturbation, graded beds, ripple marks, mud cracks, and fossils.
The very existence of a certain type of sedimentary rock is a clue to its past environment. It may have been a terrestrial environment (possibly glacial valley, mountain stream, mountain front alluvial fan, sand dune, lake, or river), or it may have been a marine environment (a delta, coastal beach sands, shallow-marine clastic area, shallow-marine carbonate area, or deep-ocean water.) The sequence of sedimentary beds can even tell the geologist whether the sea was encroaching on the land (transgression) or receding (regression) during the time of the sediment deposition.
The chapter ends by relating sedimentary rock formation and distribution to that grand unifying concept, plate tectonics. Once again you read about rifts, passive continental margins, intracontinental areas, and foreland basins. As all of these develop, sea level changes. Sequences of sedimentary rock record the times oceans have encroached on land (transgressions) and the times the water has backed off (regressions).
By chapter’s end you have covered two of the three major rock types, igneous and sedimentary. What is characteristic of the third type? The more you learn about geology the more you’ll realize Earth is a very dynamic place. Even solid rock doesn’t stay the same forever. And that’s what Chapter 8 is all about, changed rocks—metamorphic rocks. |
