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Rocks
These are great masses of hard materials
that make up the outer part of the earth. They
are aggregate of minerals which may occur as
single or in groups of minerals. A mineral is a
naturally occurring chemical substance hat is
neither a plant nor an animal. Geologists divide
all rocks into main groups. When geologists
speak of rocks, they usually do not mean a
small stone. They mean the great mass that
make up a mountain or that underlies a large
region.
1. IGNEOUS ROCKS (from Latin igneuos – fire).These are formed from rock
materials so hot that it was liquid. Most of the liquid rock material cooled
and hardened below the surface of the earth. Some of the liquid rock broke
through the surface, flowed out and then hardened.
Igneous rocks underlie all other rocks of the earth. It forms the base of the
continent and the cores of mountains. It lies beneath the sediments of the
oceans. An igneous rock mass that can be seen on the surface may once
have been the core of the mountain. It may be rock that reached the surface
in a molten state. But, whenever it was formed deep within the earth or on
the surface, all igneous rock was once liquid.
2 Kinds of molten materials
a. Magma – are liquid rock that lies in underground pockets in huge
quantities. It is found 60 to 200 kilometers below earth’s surface, at or
near the crust / mantle boundary.
b. Lava – are magma that moves upward to the earth’s surface. They are
released during volcanic eruptions or flows out through fractures onto
earth’s surface.
IGNEOUS
ROCKS
2 General Types of Magma
Basaltic magma – flows readily and forms thin layers of dark colored
lava that cover large areas. This magma makes up much of the
ocean floor. Temperatures of basaltic magma range from 900 to
12000 at the surface. IT contains a high percentage of iron and
magnesium. The presence of these elements produces igneous
rocks that are generally quite dense and dark in color.
Granitic magma – is thick and stiff due to its high silica content. Its
temperature is generally below 8000C at the surface. It contains a
high percentage of aluminum and potassium or sodium. These
produce less dense, lighter-colored igneous rocks.
2 Kinds of Igneous Rocks as to Form:
Intrusive igneous rocks - formed when magma forces its way
into pre-existing rocks and solidifies slowly. Solidification of
these rocks occurs inside the earth. The rock contains iron magnesium minerals.
Examples:
- peredotite – is composed of all living.
- gabbro basalt – is formed after more coaling of the
peredotite.
- diorite – is made of sodium with some iron- magnesium.
- granite – the last rock to formed which contains quartz
with small amount of sodium.
INTRUSIVE INGEOUS ROCKS
PEREDOTITE
GRANITE
GABBRO BASALT
DIORITE
Extrusive igneous rocks (fine - grained rocks) –
formed when magma goes out into the earth’s surface
and cools there rapidly into fine crystals. They may be
blown out when volcanoes erupt or may come out quietly
from cracks of rocks.
Examples:
- basalts – are dark in dense and can be recognize
even if the
minerals cannot be identified.
- rhyolite – fine – grain equivalent to granite.
- andesite – is the fine - grain equivalent to diorite.
EXTRUSIVE INGEOUS ROCKS
ANDESITE
BASALT
RHYOLITE
2. SEDIMENTARY ROCKS – are made of loose Earth materials called
sediments. These rocks are formed from weathered igneous, metamorphic
or other sedimentary rocks that are found on the earth’s surface.
Many sedimentary rocks are deposited in a series of layers which are recognized
by differences in color, grain, size, and composition. The oldest layer at the
bottom and the youngest bed on top. Fossils, especially of the marine life, are
common in sedimentary rocks. Fossils, are the remains on traces of once living
organism preserved in the earth’s rocks. Bones, shells and prints of plants and
animals may be preserved as fossils.
Features of Sedimentary Rocks:
Ripple marks – are wavy features of some sand stone which are common in
beaches.
Concretions– are ball – like objects or irregularly shaped masses of
cementing
material that are collected around a nucleus.
Geodes – are ball – like objects found in some sedimentary rocks.
SEDIMENTARY ROCKS
Classification of Sedimentary Rocks:
1. Clastics – are made from fragments of rocks, minerals and broken shells.
- Conglomerate – are mixtures of rounded pebbles of any kind and
shape. Clay and sand grains also may be present. Less than 2mm
in
diameter.
- Breccia – similar to conglomerate except that its fragments are
sharp,
angular pieces. Less than two mm in size.
- Sandstone – usually consist of small quartz grains, but maybe
composed of calcite, feldspar, or pieces of other rocks, sandstone
feel
gritty and the grains that compose them are easy to see 0.0625 to
2mm
in size.
- Siltstone – is like sandstone but with much smaller grains which
can be
felt but can be seen only with magnification 0.004 to 0.06mm.
CLASTICS SEDIMENTARY ROCKS
BRECIA
SANDSTONE
CONGLOMERATE
SILTSTONE
SHALE
2. Non – clastics – are sedimentary rocks that are
deposited from solution or by organic process.
- Limestone – composed of calcite (CaCO3)
- Flint or Chert – made of silica
- Rock Salt – made of halite
- Rock Gypsum or Alabaster – made of
gypsum
- Coal – fragments of plants or carbon
compounds.
NON-CLASTICS
SEDIMENTARY ROCKS
LIMESTONE
FLINT OR CHERT
ROCK GYPSUM
OR ALABASTER
ROCK SALT
COAL
3. METAMORPHIC ROCKS – are formed beneath the surface of the
earth by the alteration of pre-existing igneous or sedimentary rocks.
Alteration maybe due to high temperature, various chemical
substances or a combination of pressure and shearing stresses
caused by the movement of the earth’s crust. Meta is a Greek word,
meaning to transform or change.
Classification of Metamorphic Rocks:
1. Foliated rocks – are banded rocks that show layers
a. Slates – are finely foliated and tend to split also along the
parallel lines into plates of uniform thickness. They are made
from shale and break into very thin slabs which can be used as
roofing materials and flagstones. Blackboards of many
classrooms are used to be made of shale.
b. Phyllites – show foliation midway between that of schists
and shale.
c. Schists – are rocks that show some internal change and
definite foliation. The most common metamorphic rocks.
d. Gneisses – are the result of high temperatures and
pressure.
2. Non – foliated – are massive rocks and lack banding. These
rocks usually contain only one mineral and have undergone internal
changes.
a. Marble - is a metamorphosed limestone. Its crystals are harder
and larger than limestone.
b. Quartzite – forms from the sedimentary rock sandstone.
c. Serpentinite – has a marble streaks formed from a parent rock
that contains calcium.
FOLIATED METAMORPHIC ROCKS
SLATES
PHYLLITES
SCHIST
Gneisses
NON-FOLIATED
METAMORPHIC ROCKS
MARBLE
QUARTZITE
SERPENTINITE
Rock Cycle Concept
This refers to the forming of earth materials into
new rocks by responding to changing environment.
Records indicate that the responses and adjustments
have continued throughout the entire history of the
earth. Igneous rocks become transformed to
sedimentary rocks can be metamorphosed together
with igneous rocks and metamorphic rocks may melt to
form magma. Crystallization of magma will produce
igneous rocks thus the cycle continues. Any type of rock
can be weathered or eroded. Igneous rocks sometimes
are metamorphosed without passing through a
sedimentary stage. However, sedimentary rocks cannot
be transformed to igneous rocks.