igneous rocks 1

8/11/2019 Igneous Rocks 1

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I gneous Rocks

Mount St. Helens


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St. Helens (USA)

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Crater

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It remained quit for about a century

As magma moved up it resulted into bulging of the mountain a domeformed

Erupted on May 18, 1980. With a small magnitude earthquake 5.2

Hot rocks + ash flowed down with temp 600C and speed up to 1000 km/hr. More than 620 sq.km of forest was leveled and tress were swept away.

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VOLCANO:A gap in the EarthsCrust through whichthe molten rocks or magma comes out to the surfaceor it is a conical mountain formed around the vent

through which molten mass comes out to the surface


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Historical Events


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Eruption of Mt. Vesuvius, Italy in AD79 Mt. Vesuvius erupted in AD

79,a kind of pyroclasticflow

Destroyed the Roman townsof Pompeiiand Herculaneum,killing >16,000 people

Archeologists excavated the

remains of some 2000 peoplesuffocated in the eruption

Plaster casts of molds ofthe victims reveal adults,children, and dogs in theirdeath positions.

Pompeii was buried by up to3 m of ash and pumice, andHerculaneumwas excavatedfrom beneath 20 mofvolcanic debris.


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Mt. Tambora, Indonesiaerupted in 1815 killed 10,000 people


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Mount Pele, St. Pierre in 1902, French Caribbean

Island of Martinique


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Mount ST. HELENS, 1980, US

During the eruption, much of the northern side of the compositevolcano was blown away, and the altitude of the summit was reducedby approximately 400 m.

9,677 feet before; 8,363 feet after; 1,314 feet removed

Before After


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1883 Krakatau Eruption, Indonesia

On of the deadliest event in Indonesia 26, Aug 1883 started to emit ash and then

exploded

Explosion was heard as far as 4600 km It resulted into tsunami

Effect was felt world-wide

~20 km3 of debris was ejected duringeruption

Temp around the globe was dropped by 1C


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Mt. Pinatubo, Philippines

Mt. Pinatubo eruption in 1991, ejected vastamounts of volcanic ash and sulfur dioxide

up to about 30 km into the atmosphere. Extremely fine particles (aerosols), fromthe eruption remained in the upperatmosphere and circled the Earth for morethan a year.

This dust cloud temporarily lowered theaverage global temperature. The Mt.Pinatubo eruption was the second largest in

the twentieth century.


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Eyjafjallajkull Volcano, Iceland

Volcanic ash plume blanketed the Icelandic countryside following theeruption of Eyjafjallajkull Volcano beneath the Icelandic icecap.

Enhanced color satellite image highlighting a volcanic ash plume movingfrom Iceland towards the United Kingdom on April 15, 2010; all

flights in and out ofBritians

airports were grounded.


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Mt. Fuji: with steep conical mound emitstephra and lava


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3,776 m high, last erupted in AD1707


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FLUIDITY OF MAGMA

Fluidity or Viscosity of magma depends on content

(%) of Silica

Silica Rich-Known as Acidic magma-More viscous, so do notspreads and piles up atone place

Silica poor- Known as Basic magma- Less viscous, movesfaster and occupieslarger area

However, the viscosity of magma is considerably influenced by

temperature too. When temperature is low it is more viscous

and when High temperature - less viscous.


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Igneous Rocks

Extrusive Intrusive


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FORMS OF INTRUSIVE IGNEOUS ROCKS

Commonly observed forms of Plutonic(intrusive) rocks observed in the field

are: dykes, sills, laccoliths, volcanicnecks, batholiths etc.

Based on the attitudes of theassociated country rocks the formsare called either as Concordant orDiscordant.


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All igneous rocks are formed by cooling of magma

Either by extrusion on the surface or by intrusion

These are termed as Extrusive Igneous Rocks and Intrusive Igneous Rocks

Stocks Batholiths LACCOLITHS

Dike

Sills

Volcanic pipe

Composite volcanoVolcanic neck

Cinder cone Lava flow


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Extrusive and Intrusive Rocks

These rocks contains larger as well as smaller size of

minerals

The size of the mineral depends on cooling time of magma

Magma when flows on the surface cools faster, henceallowing insufficient time for large crystals to grow Extrusive rocks which are fine grained

Magma which intrude the underlying rocks below thesurface (Intrusive) cools slowly, provides sufficient timeto form large mineral grains/crystal Intrusive rocks arecoarse grained


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DYKES: Dykes and sills are the most common forms ofthe intrusive igneous bodies

They are discordant

Cut across the bedding of therocks in which they intrude

Vertical to steeply inclined and

sheet-like body (extensive inlateral dimension)

Thickness vary widely from aninch up to hundreds of feet

Injected through fractures,joints, and weak planes


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SILLS:Sills are relatively thin tabular sheet-like bodiesthat penetrates parallel to the bedding planes, henceconcordant in nature.

Laterally it may extendsfor 100s of km andcovers area up to 10 kmor more.

Lateral extend mainlydepends on thetemperature, degree of

fluidity or viscosity,weight of overlyingsediment column.


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LACCOLITHS:It is a concordant body, with flat bottom and

convex upward. It is dome shaped.

When viscous magma is injected rapidly along the bedding, as itcannot spreads it pushes up the overlying layers and keep on

piling up.

It causes folding of the overlying rock layers. JN Malik

B h l k d f b d


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BATHOLITHS: are the largest kind of intrusive bodies,irregular in shape and occupies large area

Their sides are sloping away from each other which makes them largerextending downwards to a greater depth

Their occurrence is commonly associated with the mountain-building process

The Batholiths - are either granites or granodiorites in composition


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Stocks: Are smallerirregular bodieswith 10 km inmaximum dimension,and are associatedwith batholiths


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BYSMALITH: It is cylindrically shaped body

It is developed when highly viscous magma (acidic magma) isinjected, because the lateral spreading along the bedding is less,it acquires to move upwards and form cylindrical shape.

Causes breaking of overlying rock layers.


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Variety of Igneous Rock

Formation

Identification criteria


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BOWEN REACTION SERIES

Discontinuous Series Continuous Series

Ferro-Magnesium Non Ferro-Magnesium

(Mafic) minerals (Feldspar) Minerals

Olivine (isolate SiO4) (Mg, Fe)2 SiO4 Ca-Plagioclase (Anorthite)

CaAl2Si2O8

Pyroxene (Single chain) Na-Ca Plagioclase(e.g., Augite, Hypersthene)

Ca(Mg,Fe,Al) (Si Al)2 O6

(e.g., Labradorite)

NaCaAlSi3O8

Amphibole(double chain)(Mg7Si8O22(OH)2 e.g., Hornblende)

Na- Plagioclase (e.g., Albite)

Na AlSi3O8

Bitotie

Potash Feldspar (e.g., Orthoclase) KAlSi3O8

Muscovite

Quartz

14000C

8000C

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KINDS OF IGNEOUS ROCK


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Rh li A d it B ltBasalt (Olivine rich)

A h iti (fi i d)


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Decrease in Silica % (darker)

Type of magma:

Rhyolite

Granite

Andesite

Diorite

Basalt

Gabbro Dunite, Peridotite(Olivine 90%)

Aphanitic (fine grained)

Phaneritic (coarse grained)

Increase in Silica %

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TEXTURES OF IGNEOUS ROCKS

Based on granularity or grain size: grain size depends on physical

conditions that prevailed during the

time of Crystallization of magma

Slow cooling = larger mineral grains

Fast cooling = smaller mineral grains


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PHANERITIC TEXTURE

Is characterized by LARGE SIZE MINERALSwhich can be easilyseen by naked eye (size at least 2mm or greater)

Commonly associated with the INTRUSIVE (PLUTONIC)IGNEOUS ROCKS, because magma in the crust cools at slowerrate and have enough time to result into large mineral grains e.g.Granite; Pegmatite

Rh lit Andesite BasaltBasalt (Olivine rich)



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Type of magma:

Rhyolite

Granite

Andesite

Diorite

Basalt






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DIORITE Plagioclase Feldspar

Amphibole

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GABBRO Plagioclase Feldspar

Amphibole

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Rhyolite Andesite BasaltBasalt (Olivine rich)



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Type of magma:

Rhyolite

Granite

Andesite

Diorite

Basalt






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APHANITIC TEXTURE

Is characterized by

FINE GRAINEDMINERALS, which canbe seen undermicroscope (size


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Pumice

It forms during fast cooling process of magma containing gas

Comprised of vesicles that represents gas bubbles that were

trapped during the rapid cooling of magma. Abundant vesicles and the thin layers give the rock a very low

specific gravity (about 1 g/cm3or less). Allowing with an abilityto float on water

Continental drif t:The super-continent Pangaeabegan to break up about


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225-200 million years ago.

Fragmented into the numerous continents

Panthalassa

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Volcanic Rocks in India (Deccan) The occurrence of volcanic rocks

seen in India e.g., Deccan Traps location: Maharastra; some partof Gujarat and MP

This volcanic area occupies about200,000 square miles of area of

India Were formed near the end of the

Cretaceous period (65 Ma) due tovolcanic activity when the Indiancontinent started drifting from

the Main Gondwana land These traps gave good building

stone and also yielded very fertileclayey loam, particularly suited tocotton cultivation


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This happens when slow cooling is followed by rapid cooling.

Phenocrysts= larger crystals, matrixor groundmass= smaller

crystals

PORPHYRITIC TEXTURE

Is a distinctive mixture of large and fine grained mineral grains

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Rhyolite Andesite BasaltBasalt (Olivine rich)



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Type of magma:

Rhyolite

Granite

Andesite

Diorite Gabbro Dunite, Peridotite(Olivine 90%)

p ( g )



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ANDESITE (PORPHYERITIC TEXTURE)


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Phenocrysts of Amphibole

Groundmass (Plagioclase

feldspar (Na/Ca))

If contains more than 50% large size

mineral are termed as PORPHYRY

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Obsidian Volcanic rock Glass: Rapid cooling of lava causes


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Obsidian p gminerals to form tiny crystals or glass

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Pyroclasts


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Pyroclasts The rock fragments thrown out during volcanic eruption are called

Pyroclasts

Based on shape and size they are categorized as:

The smallest are called ash (32mm).

lapilliash


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A large pyroclasts are known as volcanic bombs.Typical shape with head and a tail. Bombs can resultinto severe damage if they strike

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We are fortunate

for not having such

volcanos in India

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Few famous monuments made up of


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pIgneous Rocks

Carving of the faces of four US presidents: Mount Rushmore is among theworld's most recognizable GRANITEmonuments.

Mt. Rushmore was selected because it is the highest peak in the region andcomprise the hardest Granite (Batholith).

The George Washington sculpture was dedicated in 1934, with Thomas Jefferson

following in 1936, Abraham Lincoln in 1937 and Theodore Roosevelt in 1939

Moai statues carved from


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Moai statues carved fromBasalt, Chile, South America

Machu Picchu - a city in Andes Mountains,


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y ,Peru (built between AD 1460 and 1470)

Most of the structures are built of granite

G it


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Granite The great pyramid of Giza :

Some stones are of Granite

Tanjore Big temple : worldsfirst Granite temple

Mahabalipuram, Granite

igneous rocks 1

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