•Bedrock may be completely covered by vegetation, sand, mud, gravel, soil,
water, asphalt, concrete, or buildings and be unfamiliar.
•Particularly rare to see bedrock in the midwestern United States, where ice-age
glaciers deposited debris on top of bedrock in the past million years.
A.3) The Basis of Rock Classiﬁcation
•18th century beginnings: attempts to classify rocks sensibly–not all rocks are the
•Classiﬁcation schemes help organize information and remember signiﬁcant details
about materials or objects.
•Also help to recognize similarities and differences between them.
•End of 18th century: wide acceptance of modern genetic scheme for rock
•Focuses on the origin (genesis) of rocks.
•Genetic Scheme: recognizes three basic
groups of rock:
•1) Igneous rocks, formed by freezing
(solidiﬁcation) of molten rock).
•2) Sedimentary rocks, formed either by
cementing together of fragments
(grains) broken off preexisting
rocks; or by precipitation of mineral
crystals out of water solutions at or
near Earth’s surface.
•3) Metamorphic rocks, formed when preexisting rocks change character in
response to a change in pressure and temperature conditions.
•Occurs in the solid state, doesn’t require melting.
•Different rock types form in different geologic settings.
•Each of these groups contains many different individual rock types, distinguished by
•1) Grain size: dimensions of individual “grains” in a rock can be measured in mm
•Some are so small they can’t be seen with a microscope, some are large enough
to be seen with the naked eye (or are actually quite large).
•Grains can be equant (same dimensions in all directions) or inequant
(dimensions are not the same in all directions).
•2) Composition: rocks are masses of chemicals; proportions of different chemicals
that make up a particular rock are its composition.
•The proportion of chemicals affects the proportion of different minerals that
constitute a rock.
•3) Texture: the arrangement of grains in a rock, as grains connect to one another.
•Whether inequant grains are aligned parallel to each other.
•4) Layering: some rock bodies appear to contain distinct layering, deﬁned by
bands of different compositions or textures, or by the alignment of inequant
grains so they trend parallel to one another.
•Different types of layering occur in different kinds of rocks.