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)
Aphanitic (fine grained)
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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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DIORITE Plagioclase Feldspar
Amphibole
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GABBRO Plagioclase Feldspar
Amphibole
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Rhyolite Andesite BasaltBasalt (Olivine rich)
Aphanitic (fine grained)
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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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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)
Aphanitic (fine grained)
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Decrease in Silica % (darker)
Type of magma:
Rhyolite
Granite
Andesite
Diorite Gabbro Dunite, Peridotite(Olivine 90%)
p ( g )
Phaneritic (coarse grained)
Increase in Silica %
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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
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