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- 1. IGNEOUS ROCKS ANDITS TYPES Centre for Geographical Information System (GIS) University of the Punjab, Lahore
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- 3. IGNEOUS ROCKS Igneous rocksare formed by the cooling and solidification of magma. “Magma” is a hot, viscous, siliceous melts, containing gases. It comes from great depth below the earth’s surface. When magma comes out upon the earth’s surface, it is called “lava” Igneous rocks vary greatly by texture, Colour, mineral assemblage. However are formed through the cooling and solidification of Magma or lava
- 4. The composition ofigneous rocks depends upon the composition of the magma, from which they are originated. Magma are of two types; i) Acid Magma It is rich in Si, Na, K and poor in Ca, Mg, and Fe ii) Basic Magma It is rich in Ca, Mg, and Fe, and poor in Si, Na, and K.
- 5. Igneous Rocks canbe classified according to their Mode of occurrence Texture size Mineralogy Chemical composition Geometry of the igneous body Classification of IGNEOUS ROCKS Intrusive rocks Solidify by Magma at Depths Coarse-grained. Phaneritic Granite, Diorite, Gabbro Extrusive rocks Solidify by lava at surface Fine-grained. Aphanite Rhyolite, Andesite, Basalt
- 6. Extrusive Igneous rocks Extrusiveigneous rocks form when lava cools on Earth’s surface, where they have been “extruded.” Extrusive igneous rocks are fine grained. Fine grained igneous rock is called an aphanite (from the Greek word meaning invisible). Intrusive Igneous rocks Intrusive igneous rocks form when magma cools within existing rocks in Earth’s crust. Intrusive igneous rocks are coarse grained. Coarse-grained igneous rock is called a phanerite (from the Greek word meaning visible).
- 7. Classification of IGNEOUSROCKS Igneous Rocks can be classified according to their Mode of occurrence Texture size Mineralogy Chemical composition Geometry of the igneous body The two most obvious textural features of an igneous rock are the size of its mineral grains and how the mineral grains are packed together. On the basis of texture, igneous rocks can be classified as following.
- 8. Classification of IGNEOUSROCKS on the basis of Texture size 1. Vitric or glassy texture (Glassy rocks) Rapid cooling or quenching of the magma Extrusive igneous rocks that are largely or wholly glassy are called obsidian or high silica (Rhyolite) glass. Another common variety of glassy igneous rock is pumice, a mass of glassy bubbles of volcanic origin. Volcanic ash is also mostly glassy because the fragments of magma cooled too quickly to crystallize.
- 10. Classification of IGNEOUSROCKS on the basis of Texture size 2. Aphanitic - Fine grain size (<1mm) Individual mineral grains (crystals) can't be seen with naked eyes. The fine-grained texture indicates rapid cooling, and is typical of volcanic rocks. Examples Basalt, Rhyolite, Andesite
- 12. Classification of IGNEOUSROCKS on the basis of Texture size 3. Phaneritic :visible grains (1-10 mm) Individual mineral grains (crystals) are visible with the naked eye. The coarse-grained texture indicates slow cooling, and is typical of intrusive rocks. Examples: Granite, Diorite, Gabbro
- 14. Classification of IGNEOUSROCKS on the basis of Texture size 4. Pegmatitic - Very large crystals (more than1cm) many over 2 cm or larger Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 meters across have been found.
- 16. Classification of IGNEOUSROCKS on the basis of Texture size 5. Porphyritic Describes a rock in which 50% or more is coarse mineral grains and are surrounded by a fine-grained mineral matrix. The mixture of grain size indicates non-uniform cooling (slow cooling followed by a period of rapid cooling). Terms for the textural components: a. Phenocrysts - the large crystals b. Groundmass or matrix - the finer crystals surrounding the large crystals. Types of porphyritic textures: a. Porphyritic-aphanitic b. Porphyritic-phaneritic
- 18. Classification of IGNEOUSROCKS on the basis of Texture size 6. Vesicular – Contains tiny holes called vesicles. Formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water. Examples Vesicular Basalt Pumice Scoria
- 20. Classification of IGNEOUSROCKS on the basis of Texture size 7. Pyroclastic or Fragmental – Pyro means fire and Klastos means broken. Pyroclastic texture denotes a rock made up of broken volcanic particles (pyroclasts) that are fused by heat or cemented together by finer grained material into a rock. May resemble rhyolite or andesite, but close examination shows pieces of fine- grained rock fragments in it. May also resemble a sedimentary conglomerate or breccia, except that rock fragments are all fine-grained igneous or vesicular. • Tuff - made of volcanic ash •Volcanic breccia - contains fragments of fine-grained igneous rocks that are larger than ash.
- 22. Classification of IGNEOUSROCKS Igneous Rocks can be classified according to their Mineralogy and chemical composition a) Ultrabasic Rocks: These rocks contain less than 45% silica, e.g. peridotite. b) Basic Rocks: These rocks contain silica between 45% and 55%, e.g. basalt. c) Intermediate Rocks: These rocks contain silica between 55% and 65%, e.g. diorite. d) Acid Rocks: These contain more than 65% silica, e.g. granite