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Rocks

Igneous Rocks
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Sedimentary Rocks

Metamorphic Rocks

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Igneous Rock: Also called as Parent
Rock

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Formed by Intrusive: Lava

Types
Formation solidfication cools within
and earth surface
crystallisation Extrusive: Lava

of lava cools down on
earth surface
Intrusive: Diorite, Gabbro, Granite and Pegmatite

Extrusive: Andesite, Basalt, Obsidian, Pumice, Rhyolite and Scoria

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• Also called Plutonic Igneous Rocks
• Forms when magma trapped deep inside the earth

Intrusive • Magma cools down in millions of years and hence it is coarse
grained
• Magma cools and forms rock through process of crystallisation
• Coarse Grained

• Rock formed from lava erupted out from erupting volcanoes and
oozing fissures

Extrusive • Formed by quick cooling and it is fine grained
• Often hot gas bubbles trapped.
• Fine Grain

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Geological Importance
• their minerals and global chemistry give information about the composition of
the mantle, from which some igneous rocks are extracted, and the temperature
and pressure conditions that allowed this extraction, and/or of other pre-existing
rock that melted;
• their absolute ages can be obtained from various forms of radiometric dating and

thus can be compared to adjacent geological strata, allowing a time sequence of
events;
• their features are usually characteristic of a specific tectonic environment,
allowing tectonic reconstitutions;
• Some special circumstances they host important mineral deposits (ores): for
example, tungsten, tin, and uranium are commonly associated
with granites and diorites, whereas ores of chromium and platinum are
commonly associated with gabbros.

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Classification basis Chemical composition
• felsic igneous rocks containing a high silica content, greater than 63%
SiO2 (examples granite and rhyolite)
• intermediate igneous rocks containing between 52 - 63% SiO2 (example
andesite and dacite)

• mafic igneous rocks have low silica 45 - 52% and typically high iron -
magnesium content (example gabbro and basalt)
• ultramafic rock igneous rocks with less than 45% silica.
(examples picrite, komatiite and peridotite)
• alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with
a molar ratio of alkali to silica greater than 1:6.
(examplesphonolite and trachyte)

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Sedimentary rocks Clastic Sedimentary rocks

Misc
Formation

Types
are formed from Chemical only form about 8
sediment grains percent of the
deposited by water, Organic rocks on Earth that
wind or ice. They cover the other
are always formed types of rocks like a

in layers, called thin coat of paint.
“beds” or “strata”,
and quite often
contain fossils.

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6 Main kinds of Sedimentary rocks

• Sandstone is a soft stone that is made when sand grains cement together.
Sometimes the sandstone is deposited in layers of different colored sand.
• Shale is clay that has been hardened and turned into rock. It often breaks apart in
large flat sections.

• Limestone is a rock that contains many fossils and is made of calcium carbonate
&/or microscopic shells.
• Gypsum, common salt or Epsom salt is found where sea water precipitates the salt
as the water evaporates.
• Conglomerate rock has rounded rocks (pebbles, boulders) cemented together in a
matrix.
• Breccia has jagged bits of rock cemented together in a matrix.

Some other examples of sedimentary rocks are:
• Fossil Fuels / coal
• Ironstone

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Type of Sedimentary rocks

• Clastic: your basic sedimentary rock. Clastic sedimentary rocks are
accumulations of clasts: little pieces of broken up rock which have piled up
and been "lithified" by compaction and cementation. The kinds of clastic
sedimentary rocks are called: Conglomerates, breccias, sandstone,

mudstone, siltstone.
• Chemical: many of these form when standing water evaporates, leaving
dissolved minerals behind. These are very common in arid lands, where
seasonal "playa lakes" occur in closed depressions. Thick deposits of salt
and gypsum can form due to repeated flooding and evaporation over long
periods of time.
• Organic: any accumulation of sedimentary debris caused by organic
processes. Many animals use calcium for shells, bones, and teeth. These
bits of calcium can pile up on the seafloor and accumulate into a thick
enough layer to form an "organic" sedimentary rock.

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Clastic Rock: Conglomerates Vs Breccia
• Conglomerates are made from bits of rounded gravel that were deposited by the
agent, usually water. Over time the wet pile of sediments becomes so heavy that the
gravel gets compacted together, and minerals in the water cement the bits of gravel
together. The only difference between conglomerates and breccias is: Conglomerates

are made from rounded gravel and breccias are make from angular gravel.
• Like “ripple marked” sandstone, mudstone can have marks made by water on
it. Mudstone is made up of very small particles than sand also called as slate as formed
from very small particles . Siltstone is another example even smaller than Mudstone.
• Sandstone is basically the same. Piles of sand are piled up by water or wind and over
time the piles get so heavy the sand is pressed together and cemented by minerals
found in ground water. There are some interesting things that can be found in
sandstone. Fossils are often found in sandstone.
• Sometimes sand dunes will become so large they harden into rock preserving the
hump shape of the sand dune. There is a very famous fossil that was found in China in
one of these preserved sand dunes. The fossil is called the “fighting dinosaurs.”

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Clastic Rock: Conglomerates Vs Breccia
• Biochemical sedimentary rocks are formed from the debris of life. For
example, limestone is formed from out of decayed animal shells. Animals use
calcium to form their shells. After the animal dies, the shell falls apart and the
calcium combines with other elements and minerals and hardens into rock.

Fossils are very common in this type of rock. I have found fossil trilobites in
this rock. Another example of this type of rock is coquina. Coquina is formed
from pieces of seashells cemented together

Chemical Formation
• Chemical sedimentary rocks are formed when water evaporates and leaves
behind minerals that harden into rock. A great example of this kind of rock is
salt. So remember when you're eating salt, you're eating a rock. It's a rock
called halite.

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Formation Metamorphic rocks form deep
within the Earth when heat and
pressure are applied to either

igneous rocks or sedimentary
rocks. This heat and pressure in
essence cooks the rocks,
changing their structure
substantially

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Process- • Metamorphic rocks are formed when rocks are changed under heat and intense pressure.
• Of the rocks that form the continental land mass, most are metamorphic rocks. There are three
different ways rocks can be changed by heat and pressure. The process of changing is called
I metamorphism.

• Scientists call the third way metamorphic rocks form a tectonic process. Sometimes I think
scientists like to use long words to make it harder for non-scientists to figure out what they
are talking about. Here's what they are talking about: The large plates that make up the crust
of the earth are always moving. Sometimes they slam into each other. Sometimes they grind
past each other. Sometimes the plates are pulling apart forming large cracks called rifts.
Process - When plates pull apart it is called a divergent boundary. On some boundaries one plate dives
under another plate. Geologists call this type of boundary a convergent boundary. When the
plates bump and grind past each other, the geologists call this boundary a transform fault
II boundary. A famous example of this type of fault in the United States is called the San Andres
Fault. At that plate boundary, the North American plate is moving mostly south and the
Pacific plate is moving mostly north, which means that in 15 million years Los Angeles and
San Francisco will be neighbors. I wonder if that will solve the debate about which is the
better town. In any case, metamorphic rocks are formed from the pressure and heat caused
by the plates crashing into each other.

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1. Classified by texture and composition.
2. Rarely has fossils.
3. May react with acid.

4. May have alternate bands of light and dark minerals.
5. May be composed of only one mineral, ex. marble & quartzite.
6. May have layers of visible crystals.
7. Usually made of mineral crystals of different sizes.
8. Rarely has pores or openings.
9. May have bent or curved foliatio

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Metamorphic rocks have been changed over time by extreme pressure and
heat.
Metamorphic rocks can be formed by pressure deep under the Earth's surface,
from the extreme heat caused by magma or by the intense collisions and friction
of tectonic plates.
Uplift and erosion help bring metamorphic rock to the Earth's surface.
Examples of metamorphic rocks include anthracite, quartzite, marble, slate,

granulite, gneiss and schist.
Anthracite is a type of coal with a high carbon count, few impurities and with a
high luster (meaning it looks shiny).
Marble is a metamorphic rock that is formed from the sedimentary rock
limestone.
Quartzite is a metamorphic rock that is formed from the sedimentary rock
sandstone.
Slate is a metamorphic rock that is formed from the sedimentary rock
mudstone.
Granulite is a metamorphic rock that is formed from the igneous rock basalt.

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