IGNEOUS ROCKSBy: Hasan javed

ROCK CYCLE

IGNEOUS ROCKSIgneous rocks are formed when magma(molten rock) solidifies at or 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.

IDENTIFICATION OF IGNEOUS ROCKS Igneous rocks names are based on: - Mineralogical composition - Textural classification• Textural classification:-Because of their larger grain size, plutonic rocks can be easily identified from 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.

SOME IGNEOUS ROCKS

BASALT

Mafic = Dark color

Fine grained= Small crystals

Extrusive = Small crystals = Formed on surface

DIORITE

Felsic = Light color

Coarse grained= Large crystals

Intrusive = Large crystals = Formed underground

SOME IGNEOUS ROCKS

PUMICE

Felsic = Light color

Fine grained= Small crystals

Extrusive = Small crystals = Formed above ground

GABBRO

Mafic = Dark color

Coarse grained= Large crystals

Intrusive = Large crystals = formed underground

COMPONENT OF IGNEOUS ROCKS

MINERAL CONTENTMagma type SiO2 content Classification General color

Rhyolitic > 70% Felsic Light

Andesitic ~ 60% Intermediate ½ light ½ dark

Basaltic < 50% Mafic Dark

0% Ultramafic Dark / Green

INTRUSIVE BODIES • 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 layering in the country rock.- Sill:- Sill is also a tabular structure but it is concordant i-e layering is parallel to country rock.

COUNTRY ROCK

- Pluton:- Pluton is a body of magma that crystallizes at a considerable depth with in the surface of earth.- Stock:- A stock is a small discordant with a small outcrop area < 100 square kilometers.- Batholiths:- if outcrop area exceeds than 100 square kilometers, this large discordant body is called batholith.- Diapirs:- These are the large blobs of magma , that are less dense than surrounding rocks and rises up to upper crust due to this low density.

MOST ABUNDANT IGNEOUS ROCK• Granite is the most abundant igneous rock in

mountain ranges. It is also the most abundant igneous rock found in lowland continents.

FORMATION OF MAGMAIf a rock is heated sufficiently , the rock begins to melt to form magma. Under ideal conditions, the rock can melt and yield a granitic magma at temperatures as low as 625 degree centigrade, whereas temperature of more than 1000 degree centigrade is required to form basaltic magma.- HEAT FOR MELTING ROCK :-Most of the heat that contributes to the generation of magma comes from the very hot earth’s core, which is brought to surface by:- Conduction through mantle and crust.- Convection currents- Or by geothermal gradient.

MELTING TEMPERATURE• 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 are deep inside earth require high pressure to melt than those at the surface. Water vapors High vapor pressure can significantly lower the melting temperature. Water pressure sealed in under high pressure by overlaying rocks help break crystal structures. Mixed minerals Few combinations of minerals can significantly lower the melting temperature from 100-200 degree centigrade.

BOWEN’S REACTION SERIES• 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 EVOLUTION• Magma can be evolved by:- Mixing of magma- Assimilation- Differentiation- Partial melting

IGNEOUS ACTIVITY BY PLATE TECHTONICSPlate tectonics occur at three regions where igneous activity may happen.- Divergent boundary(associated with gabbro &

basalt)- Convergent boundary(associated with granite &

andesite)- Subduction zone (zone where felsic igneous rocks

are formed)

DIVERGENT BOUNDARY

CONVERGENT BOUNDARY

VOLCANO • 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 without solidifying but when it does, its called lava.• Pyroclast:- The rock fragments burst into the air by the gases entrapped by Lava. Pyroclastic debris is known as tephra .• Lava flows:- Outpouring of lava extended to earth’s surface.

LAVA FOUNTAIN & PYROCLAST

WHERE DOES MAGMA FORM?

VOLCANIC VENT• The opening of a volcano where the magma

escapes to reach the earth’s surface

CRATERS• The steep walled depression around a volcanoes

vent.

HOW DO VOLCANOES FORM ?- heat and pressure changes the rock to melt

forming liquid magma.

- Magmas less dense than rock, forced slowly towards earth’s surface.

WHERE DO VOLCANOES OCCURR?

• 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

TYPES OF VOLCANOES• Shield volcanoes:- Shield volcanoes are broad gently sloping cones constructed of solidified lava. During eruption the lava spreads quickly due to its low viscosity.• Cinder cones:- Cinder cone is a volcano constructed of pyroclast ejected from the central vent.• Strato volcano:- Its also called composite volcano. It is formed from the alternative layering of pyroclastics and rock solidified from the lava flows.

SHIELD VOLCANO

CINDER CONE

DISTRIBUTION OF COMPOSITE VOLCANO- Circum pacific belt- Mediterranean belt

VOLCANIC DOME• Volcanic domes are steep sided dome or spine

shaped masses of volcanic rocks formed from viscous lava that solidifies in or above the volcanic vent.

COLUMNAR STRUCTURE• Volcanic rock in parallel, usually vertical columns,

mostly six sided; also called columnar jointing.