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SOIL FORMING
ROCKS
Lecture # 03
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ROCKS
• What are rocks?
• How are rocks formed?
• How are rocks classified?
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Rocks Defined
• A rock may be defined as
“an aggregate of one or more
minerals”
[An aggregate of single mineral calcite
gives rise to crystalline limestone
Granite is a combination of several
minerals]
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Classification of Rocks
By origin, 3 great
divisions of rocks are
recognized:
• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks
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Igneous Rocks
• Igneous (Latin) means “made from fire or heat”
• When volcanoes erupt and liquid rock comes up
to earth's surface, new igneous rock is made
• When the rock is liquid & inside the earth, it is
called “magma”
• Magma comes from the Earth’s core which is
molten rock
• Igneous rocks form when molten magma cools
and turns into solid rock
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Classification of Igneous Rocks
Based on mode of origin
1. Intrusive or Plutonic [Granite, Diorite,
Gabbro, Peridotite]
2. Extrusive or Volcanic [Basalt, Rhyolite,
Andesite]
Intrusive rocks named after Pluto, the Roman God of the underworld.
Volcanic rocks named after Vulcan, the Roman God of fire.
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Vesicular rocks
Amygdaloidal rocks
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Classification of Igneous Rocks
Based on chemical composition
1. Acidic (65-75% SiO2) [Granite,Rhyolite]
2. Neutral (55-65% SiO2) [Andesite]
3. Basic (45-55% SiO2) [Basalt, Diorite]
4. Ultra basic (40-45% SiO2) [Picrite, Gabbro,
Limestone]
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GRANITE

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DIORITE
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Sedimentary Rocks
• Sedimentary rocks cover 75% of the
earth’s surface
• Most of the rocks found on the Earth’s
surface is sedimentary even though
sedimentary rocks only make up less than
5% of all the rocks that make up Earth
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Sedimentary Rocks
• When rocks are exposed to the elements – air,
rain, sun, freeze / thaw cycle, plants – erosion
occurs and the little bits of rock worn away get
deposited as sediments
• Over time, these sediments harden as they get
buried by more sediments and turn into
sedimentary rocks
• Sedimentary rocks are usually formed in layers
(strata)
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Sedimentary Rocks
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Kinds of Sedimentary Rocks
Based on appearance
Sedimentary Rock Composition
Conglomerate Pebbles, boulders cemented in
a matrix
Sandstone Sand grains cemented
Shale Clay hardened into a rock
Limestone Made of calcium carbonate
Gypsum, etc. When seawater evaporates
Porphory Jagged rock bits cemented
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Kinds of Sedimentary Rocks
Based on origin
Type Description Example
Residual Formed in situ Laterite
Transported Deposited as solids
in suspension
Sandstone,
Shale
Deposited by
chemical
precipitation
Limestone
Iron stone
Deposited through
agency of organic
matter
Peat
Phosphatic
deposits
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Kinds of Sedimentary Rocks
Based on grain size
• Rudaceous: Rocks consisting chiefly of
boulders [Conglomerate]
• Arenaceous: Rocks consisting chiefly of
sand grade materials [Sandstone]
• Silt rocks: Rocks consisting chiefly of silt-
sized particles [Silt stone]
• Argillaceous: Rocks consisting chiefly of
clay sized particles [Shale]
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SHALE
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GYPSUM
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LIMESTONE
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Metamorphic Rocks
• Rocks that have changed
• Greek "meta" and "morph” “to change form”
• Originally these were igneous or sedimentary
• Metamorphism brought about by water, heat,
pressure or chemically active liquids and gases
• Changes brought about are both physical and
chemical
• Hydro-, thermo-, dynamo-metamorphism
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Metamorphic Rocks
• The least common of the 3 kinds of rocks
• Igneous or sedimentary rocks transformed by
great heat or pressure
• Slate is transformed shale, splits into smooth
slabs
• Schist, the most common metamorphic rock
• Marble is transformed limestone
• Quartzite is very hard
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Metamorphic Rock
Sedimentary Rock Metamorphic Form
Sandstone Quartzite
Limestone Marble
Shale Slate
Clay Schist
Granite Granite gneiss
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QUARTZITE
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MARBLE
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SLATE

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Key References
Daji: A Text Book of Soil Science
Any book on Geology
www.geology.com
7-Feb-23 © ARR