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Discuss the classification of Rocks.

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1. Igneous Rocks

-Rocks formed when molten material from the earth’s interior cools and solidifies on or beneath the earth’s surface.

Types of Igneous Rocks

a) Intrusive Igneous Rocks
-Rocks formed when magma cools and solidifies below the earth’s surface e.g. granite, diorite, gabbro, peridotite.
-Have coarse texture as a result of slow cooling giving minerals more time to form large crystals.

-Are classified further into two

(i) Hypabyssal rocks- intrusive igneous rocks which are near the earth’s surface.
(ii) Plutonic rocks-intrusive igneous rocks which are deep below the surface.

b) Extrusive Igneous Rocks
-Rocks formed when lava solidifies on the earth’s surface.
-Have fine texture due to fast cooling giving minerals less time to collect together to form larger crystals
2. Sedimentary Rocks

-Rocks formed when particles of other rocks are laid down and compressed into layers or when plant and animal remains are buried and compressed and compacted.

- When they are laid down a layer is formed.
- As deposition continues additional layers are formed which compress the lower layers into a hard mass.

Types of Sedimentary Rocks
a) Mechanically Formed Sedimentary Rocks
-Sedimentary rocks formed when weathered igneous or metamorphic rocks are deposited and compacted e.g. sandstone and shale.

b) Organically formed Sedimentary Rocks
-Sedimentary rocks formed when animal and plant or animal remains are buried, compressed and compacted.

Classification of Organically Formed Sedimentary Rocks

(i) Calcareous rocks-rich in calcium carbonate e.g. chalk and limestone.

Coral rocks are formed from remains of sea polyps which extract lime from the sea, build shells for protection, attach themselves to each other and rocks to live in colonies, then die and shells to form coral rocks.

(ii) Ferruginous Rocks-rich in iron e.g. ironstone.

(iii) Siliceous Rocks-rich in silica e.g. diatomite.

(iv) Carbonaceous Rocks-rich in carbon e.g. coal.

c) Chemically formed Sedimentary Rocks

-Sedimentary rocks formed when materials dissolved in water chemically react forming new substances then water evaporated leaving layers of those salts.

Classification of Chemically Formed Sedimentary Rocks

(i) Carbonates e.g. trona and dolomite

(ii) Sulphates-sulphate compounds

(iii) Chlorides e.g. halite

(iv) Silicates e.g. flint

(v) Iron stones e.g. haematite and limonite.

3. Metamorphic Rocks

-Rocks which have changed their physical appearance and chemical properties as a result of subjection to great heat and pressure e.g.
- Gneiss from granite
- Slate from clay
- Marble from limestone
- Quartzite from sandstones

marto answered the question on March 22, 2019 at 10:15

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