Igneous rocks are formed when magma(molten rock) solidifies ator under the surface of earth.o TYPES There are two types of igneous rocks:- Intrusive igneous rocks(also called plutonic rocks) Formed when magma solidifies under the surface of earth. Composed of coarse grained rocks , with grain size > 1mm. The crystalline grains of plutonic rocks are commonly interlocked in mosaic pattern.- Extrusive igneous rocks(also called volcanic rocks) Formed when magma solidifies at the surface of earth. Composed of fine grained rocks, with grain size < 1mm.
Igneous rocks names are based on: - Mineralogical composition - Textural classification• Textural classification:-Because of their larger grain size, plutonic rocks can be easily identifiedfrom volcanic rocks. Both differ in grain size as well as cooling rate.• Mineralogical composition:-Rocks can be mine rally classified as:- Felsic rocks(having higher silica content i-e up to 70% & light colored minerals) includes granite-rhyolite.- Mafic rocks(composed of ferromagnesian silicates & are dark colored, contains low silica content ~50%) includes gabbro-basalt.- Intermediate rocks(containing almost equal amount of felsic & mafic minerals with silica content b/w 53-65%) includes diorite-andesite.- Ultramafic rocks(rich in iron and magnesium silicates with silica content < 45%) includes peridotite.
BASALTMafic = Dark colorFine grained= Small crystalsExtrusive = Small crystals = Formedon surfaceDIORITEFelsic = Light colorCoarse grained= Large crystalsIntrusive = Large crystals = Formedunderground
PUMICEFelsic = Light colorFine grained= Small crystalsExtrusive = Small crystals = Formedabove ground GABBRO Mafic = Dark color Coarse grained= Large crystals Intrusive = Large crystals = formed underground
Magma type SiO2 content Classification General colorRhyolitic > 70% Felsic LightAndesitic ~ 60% Intermediate ½ light ½ dark Basaltic < 50% Mafic Dark 0% Ultramafic Dark / Green
• Intrusions or intrusive structures are bodies of intrusive rocks whose names are based on their shape and size as well as their relationships to the surrounding rocks.• Intrusion can either be deep or shallow depending on the depth at which magma cools/solidifies.- Dike:- Dike is a tabular, discordant intrusive structure.Discordant means that body is not parallel to any layeringin the country rock.- Sill:- Sill is also a tabular structure but it is concordant i-elayering is parallel to country rock.
- Pluton:- Pluton is a body of magma that crystallizes at a considerabledepth with in the surface of earth.- Stock:- A stock is a small discordant with a small outcrop area < 100square kilometers.- Batholiths:- if outcrop area exceeds than 100 square kilometers, this largediscordant body is called batholith.- Diapirs:- These are the large blobs of magma , that are less dense thansurrounding rocks and rises up to upper crust due to this lowdensity.
• Granite is the most abundant igneous rock in mountain ranges. It is also the most abundant igneous rock found in lowland continents.
If a rock is heated sufficiently , the rock begins to melt toform magma. Under ideal conditions, the rock can melt andyield a granitic magma at temperatures as low as 625degree centigrade, whereas temperature of more than1000 degree centigrade is required to form basalticmagma.- HEAT FOR MELTING ROCK :-Most of the heat that contributes to the generation ofmagma comes from the very hot earth’s core, which isbrought to surface by:- Conduction through mantle and crust.- Convection currents- Or by geothermal gradient.
• The temperature at which solid rocks start to change their form into magma (or starts melting).- FACTORS:-- Pressure- Water vapors- Mixed minerals Pressure The melting temperature increases with increasing pressure. The rocks that aredeep inside earth require high pressure to melt than those at the surface. Water vapors High vapor pressure can significantly lower the melting temperature. Waterpressure sealed in under high pressure by overlaying rocks help break crystalstructures. Mixed minerals Few combinations of minerals can significantly lower the melting temperaturefrom 100-200 degree centigrade.
• Explains the order in which minerals crystallize out of magma solution.
Left side - Iron-rich minerals - Discontinuous Crystallization Right side- Feldspar minerals- Continuous crystallization
• Magma can be evolved by:- Mixing of magma- Assimilation- Differentiation- Partial melting
Plate tectonics occur at three regions where igneousactivity may happen.- Divergent boundary(associated with gabbro & basalt)- Convergent boundary(associated with granite & andesite)- Subduction zone (zone where felsic igneous rocks are formed)
• A hill or a mountain formed by the extrusion of lava or ejection of rock fragments from vent.• Lava:- Magma does not always reach the surface withoutsolidifying but when it does, its called lava.• Pyroclast:- The rock fragments burst into the air by the gasesentrapped by Lava. Pyroclastic debris is known as tephra .• Lava flows:- Outpouring of lava extended to earth’s surface.
• The opening of a volcano where the magma escapes to reach the earth’s surface
• The steep walled depression around a volcanoes vent.
heat and pressure changes the rock to melt formingliquid magma.Magmas less dense than rock, forced slowly towardsearth’s surface.
• They often occur when Earth’s tectonic plates: • Move apart – divergent boundary • Move towards each other – convergent boundary • At “Hot Spots” – where crust of Earth is thinner than normal
• Shield volcanoes:- Shield volcanoes are broad gently sloping conesconstructed of solidified lava. During eruption the lavaspreads quickly due to its low viscosity.• Cinder cones:- Cinder cone is a volcano constructed of pyroclastejected from the central vent.• Strato volcano:- Its also called composite volcano. It is formed from thealternative layering of pyroclastics and rock solidified fromthe lava flows.