Geology Reference
In-Depth Information
Exercise 1 • Earth Materials, Geologic Time, and Geologic Processes
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TABLE 1.1 Mineral Identification
Sample
Number
Specific
Gravity
Fracture or
Cleavage
Other
Properties
Name of
Mineral
Chemical
Composition
Color
Streak
Luster
Hardness
-9
9
Igneous Rocks
Igneous rocks are formed by the crystallization of a
molten silicate-rich liquid known as magma. Magma
that cools relatively slowly beneath the Earth's surface
forms
plutonic
or
intrusive
igneous rocks, whereas
magma that crystallizes at or near the Earth's surface
as lava forms
volcanic
or
extrusive
igneous rocks. The
rate at which magma cools determines the texture of
igneous rocks. The composition of the magma deter-
mines the mineral composition of the rock. These two
properties, texture and composition, are the basis for
classifying and identifying igneous rocks.
Igneous rock texture
Interlocking mineral grains that
can be distinguished with an
unaided eye
Individual mineral grains are
small and cannot be
distinguished with an
unaided eye
Two sizes of minerals, with
large crystals (phenocrysts)
imbedded in fine- or
coarse-textured groundmass
Rocks contain many cavities or
voids, giving a texture similar
to a hard sponge
Rocks made from volcanic
materials that have flown
through the air, such as ash
(tephra)
Texture that resembles glass
Coarse
(Phaneritic)
Fine
(Aphanitic)
Porphyritic
Ves :ular
Pyroclastic
Glassy
FIGURE 1.1
The rock system.
