Igneous rocks are formed by the cooling and solidification of molten rock either below or above the Earth's surface. There are three main types - mafic rocks like basalt that are rich in magnesium and iron; felsic rocks like granite that contain more silica and lack magnesium/iron; and intermediate rocks with mixtures like andesite. Igneous rocks are also classified based on their texture and mineral crystal size as extrusive volcanic rocks formed above ground, hypabyssal rocks formed below ground, or intrusive plutonic rocks formed deep underground where larger crystals can form.

1. What are igneous rocks?
 Igneous rocks are primary rocks.
 They are formed due to solidification of molten mass lying below or above the
earth’s surface. The inner layers of the earth are at high temperatures causing
silicates to melt.
 This magma is forced up as a volcanic eruption and spreads over the surface of
the earth where it solidifies.
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2. Types of Igneous rocks
MAFIC ROCKS
 This type is rich in Magnesium and Iron
 Most mafic minerals are dark in colour.
 Common rock forming mafic minerals are
pyroxene and olivine.
 Mafic rocks include: basalt, diabase,
gabbro.
INTERMEDIATE ROCKS
 They are a composition of felsic and mafic
rocks.
 This type has roughly even mixtures of
felsic and mafic minerals.
 Most are grey in colour.
 Examples are: andesite and diorite
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3. FELSIC ROCKS
 Felsic rock is a modifier describing
igneous rocks. They lack Magnesium and
Iron.
 They are rich in elements that form
feldspar and quartz.
 They are white, light grey, or pink.
 Examples are granite (coarse grained) and
rhyolite (fine grained)
ULTRA MAFIC ROCKS
 These have an extremely low silica
composition because they are made up of
olivine and pyroxene.
 This type contains a very low amount of
silica and are common in the mantle.
 An example is kimberlite.
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4. Classification of Igneous rocks
 Igneous rocks are classified according to how much crystalline substance is
contained.
 The properties of an igneous rock depend on its formation and the minerals
present in it.
 The texture of the rock is decided by the size of the mineral crystals within a rock.
 Igneous rocks are grouped into: extrusive rocks, hypabyssal rocks, and intrusive
rocks.
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5. Extrusive (Volcanic) rocks
 They are formed when magma exits and
cools above or near the earth’s surface.
 These are rocks that form at erupting
volcanoes and oozing fissures.
 Examples include basalt, obsidian, and
pumice.
 The large holes in pumice are caused by
gases that escape as lava cools down.
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6. Properties of extrusive rocks
 They have fine grained textures because
their rapid cooling on the surface does
not provide enough time for large crystals
to grow.
 They are usually dark coloured, such as
obsidian, shown beside.
 Some are lightweight, like pumice.
 Many extrusive rocks contain intrusive
components.
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7. Hypabyssal (Intermediate) rocks
 These rocks are also called mesotype
rocks.
 They are mixture of felsic and mafic.
 Furthermore, they are roughly even
textures of felsic and mafic minerals.
 Examples are andesite, diorite, and dacite.
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8. Properties of hypabyssal rocks
 They have both light and dark colours but
are usually grey in colour.
 They have a porphyritic texture
characterized by visible crystals floating in
a fine grained groundmass.
 They are found in continental or oceanic
plates.
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9. Intrusive (Plutonic) rocks
 These rocks are formed by slow
solidification of magma deep below the
earth’s surface.
 Are characterized by large crystals.
 A common example is granite.
 Many minerals in granite give it various
colours.
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10. Properties of intrusive rocks
 They have a coarse grained texture
because the cooling of magma deep in
the earth is much slower than the cooling
process at the surface, hence large crystals
develop.
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11. References
 The Field Description of Igneous Rocks, 2nd Edition; Dougal Jerram, Nick Petford.
 Igneous Rocks and Processes- A practical guide; Robin Gill. Wiley-Blackwell.
 Petrology of the Igneous Rocks, 13th Edition: F. H. Hatch, A. K. Wells, M. K. Wells.
 Igneous Rocks- A Classification and Glossary of Terms, 2nd Edition;
Recommendations of the International Union of Geological Sciences
Subcommission on the Systematics of Igneous Rocks.
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