Formation of Rocks
1. Rhyolite—finegrained texture,
extrusive formation,
SiO2 and feldspar
minerals, granitic
composition
2. Obsidian—compact
glassy texture,
extrusive formation,
SiO2 (quartz)
3. Pumice—frothy
glassy texture,
extrusive formation,
quartz
1. As the rate at which
molten rock cools
increases, the size of
the mineral grains
increases.
2. You can determine if an
igneous rock has had
an intrusive (coarsegrained) or extrusive
(fine or glassy) origin by
its texture.
4. A light colored igneous
rock is less dense than
a dark colored igneous
rock.
5. The main difference
between lava (on) and
magma (beneath) is its
location in relation to
the Earth’s surface.
6. A porphyritic texture is
an adjective that
describes an igneous
rock with appreciable
phenocrysts (prominent
embedded crystals).
Conclusion: Igneous rocks
are classified on the basis
of texture and composition.
Clastic—Texture
1.
2.
3.
Conglomerate—rounded
gravel fragments, formed
via compaction and
cementation
Sandstone—quartz,
compaction and
cementation
Shale—clay, compaction
and cementation
Chemical—Composition
4. Fossiliferous Limestone—
organic calcite (CaCO3 ),
evaporation method of
lithification
1.
3.
4.
The size range for gravel is
over 2 mm. The minimum
and maximum dimensions
for sand is 1/16 to 2mm.
You can distinguish a clastic
(grain size) from a
chemically (crystalline or
shells) formed sedimentary
rock by the texture.
The sequence of events in
the formation of an
evaporite can be described
as dissolved substances
precipitate, or separate
from solution when water
evaporates or boils off
leaving a solid.
5.
The crystalline texture of a
chemical limestone (formed
by evaporation) can be
distinguished from an
organic limestone shelly
texture (formed by
cementation).
6. The sequence of events in
the lithification of
sandstone can be described
by ordering the processes of
weathering, erosion,
deposition, compaction, and
cementation.
Conclusion: Sedimentary rocks
are classified by texture,
composition, and mode of
formation.
Foliated
1. Schist—contact
metamorphism, phyllite
parent rock
2. Gneiss—contact
metamorphism, granite
origin
Nonfoliated
3. Quartzite—regional
metamorphism,
sandstone origin
1. Metamorphic
rocks formed by
contact
metamorphism
are usually not
dense and
resistant because
they are changed
as a result of
being in contact
with hot magma.
3. You seldom find fossils in
metamorphic rocks because
these are fragile imprints of
plant and animal remains easily
deformed beyond recognition
under pressure.
4. Pressure often causes minerals in
metamorphic rocks to rearrange
in layers.
5. Quartzite tends to be
hard and resistant
because it is a regional
metamorphic rock
formed by forces acting
over wide areas under
extreme conditions.
Conclusion: Metamorphic
rocks are classified on the
basis of texture (foliated or
nonfoliated) and
composition (parent rock)
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Rock Identification Lab