Structure of igneous rocks

Structure of igneous rocks

1. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Md. YousufGazi
Lecturer
Department of Geology
University of Dhaka

2. Thestructuresofigneousrocksarelargescale
features,whicharedependentonseveralfactors
like:
(a)Compositionofmagma.
(b)Viscosityofmagma.
(c)Temperatureandpressureatwhichcoolingand
consolidationtakesplace.
(d)Presenceofgasesandothervolatiles.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

3. 1.Vesicular and Amygdaloidal Structure
2. Block Lava and Ropy Lava
3.Pillow Lava
4.Jointing, Sheet Structure
5.Columnar Structure
6.Pegmatites
7.Spherulitic
8.Orbicular
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Sevencommonigneousrocksstructures:

4. Whenlavasheavilychargedwithgasesandothervolatiles
areeruptedonthesurface,thegaseousconstituent'sescapes
fromthemagmaasthereisadecreaseinthepressure.
Nearthetopoftheflowsinthematerial,emptycavities,
bubbles,vesiclesareformed,whichmayelliptical,spherical,
cylindricalorirregularinshape.
Theindividualopeningsareknownasvesiclesandthe
structureasawholeisknownasvesicularstructure.
If,however,thevesiclesthusformedaresubsequentlyfilled
inwithsomelow-temperaturesecondaryminerals,suchas
calcite,zeolite,chalcedonyetc.,theseinfillingsarecalled
“amygdales”.
1. Vesicular and Amygdaloidal Structure
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

5. 2. Block Lava and Ropy Lava
Sincelavasofacidiccomposition,duetotheir
highviscosity,donotflowtogreaterdistances,
aftersolidificationtheyarefoundtoofferavery
roughsurface.Suchlavaflowsareknownas
blocklava.Itisalsoknownas'aa'structure.
Lavasofbasiccompositionarequitemobile
becauseoftheirlowviscosityandtheycanflow
togreaterdistancesandaftersolidificationoffers
verysmoothsurface.Suchlavaflowsareknown
asropylavaandarealsoknownas'pahoehoe'
structure.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

6. Adistinctiveformoflavaflowinwhichthesolidifiedrock
formsroundedmasseswithaglassyexterior.
Pillowlavaisfoundwherelavaflowsintotheoceanora
lakeandquicklycools.
Theliquidlavaformsaspherewhenitentersthecoldwater
asaresultofsurfacetension.Thesehotballsofquickly
solidifyinglavasettletothegroundquickly.
Astheysettle,theyflattenandbecomepillowlikeinform.As
moreandmorepillowssettletotheseafloor,theweight
flattensthemevenmore.Pillowlavasthereforeformpiles
aroundtheventorentrypointofalavaflow.Theymakeup
muchoftheoceaniccrust.
3.PillowLava
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

7. •Jointsaredivisionalplaneswhicharefound
inallkindsofigneousrockimpressedupon
themmainlybyforceactingfromoutside.
Jointsplainsmaybeverticalorhorizontal.
•Thehorizontaljointsplanesaresometimesso
closelyspacedastoproduceasheet
structure.Thesheetsarecommonlythinner
asthesurfaceofthegroundisapproached
andtheyusuallyshowsomedegreeof
parallelism.
4. Jointing, Sheet Structure
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

8. Withuniformcoolingandcontractionina
homogenousmagma,thepartingplanestend
totakeonaregularorprismaticform,
Characterizedbythedevelopmentoffour,
fiveorsixsidedprisms,whichmaybe
intersectedbycross-joints.
5. Columnar Structure
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

9. Pegmatitesareextremeigneousrockthatform
duringthefinalstageofamagma’s
crystallization.Theyareextremebecausethey
containexceptionallylargecrystalsandthey
sometimescontainmineralsthatarerarely
foundinothertypesofrocks.
Tobecalleda“pegmatite,”arockshouldbe
composedalmostentirelyofcrystalsthatareat
leastonecentimeterindiameter.Graniteand
Syenitemoreoftenshowthistypeofstructure.
6. Pegmatites
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

10. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
•Spheruliticaggregate(Spherulites)areradiating
arraysoffibrous(needle-like,acicular)crystalsthat
arecommoninglassyfelsicvolcanicrocks.
•Spherulitesare typically two-mineral aggregates (mainly
quartz and feldspar), formed by initial Spheruliticgrowth of
one mineral and later crystallization of a second mineral
from the liquid or glass between the fibres.
7. Spherulitic

11. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
•Orbiculargranite(alsoknown
asorbicularrockororbiculite)isan
uncommonplutonicrocktypewhichis
usuallygraniticincomposition.
•Theserockshaveauniqueappearance
duetoorbicules-concentricallylayered,
spheroidalstructures,probablyformed
throughnucleationaroundagrainina
coolingmagmachamber
8. Orbicular