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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, frost wedging, root wedging, salt
wedging, thermal expansion, and animal attack. Common categories
of chemical weathering are these:
- 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 loses 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
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), and (3) 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 sedimentary rocks (examples: breccia, conglomerate, arkose,
sandstone, shale, and siltstone)
- biochemical sedimentary rocks (examples: limestone, including
fossiliferous limestone and chalk, and chert)
- chemical sedimentary rocks (examples: the evaporites gypsum
and halite, travertine, dolostone, and several varieties of chert)
- organic sedimentary rocks (example: coal)
Sedimentary rocks occur in layers called beds or
strata, which may display special features such as cross beds, 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,
sand dune, lake, or river), or it may have been a marine environment
(a delta, 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.
A word of advice: though the content covered in this
chapter is not conceptually difficult, you will encounter considerable
new vocabulary. The matching sections that follow offer one means
of mastering new terms; you may also wish to make up flash cards
or develop mnemonic aids to memory.
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 6 is all about,
changed rocks—metamorphic rocks.
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