Formation of igneous rocks

Formation of igneous rocks_Md. Yousuf Gazi

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

2. Bases of Classification
a.Chemical and Quasi Chemical
b.Mode of Occurrence
c.Texture
d.Mineralogical
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

3. Based on Silica(SiO
2) content
Types Silica(SiO
2) %Example
1. Acid/ Felsic>66% Granite
2. Intermediate52-66% Andesite
3. Basic 45-52% Gabbro
4. Ultra Basic<45% Peridotite
Chemical and Quasi-Chemical Classification
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

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

5. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Based on Texture

6. Saturated Minerals
Themineralswhicharecompatiblewith
quartzorprimarySiO
2
mineral(Tridymite,
Cristobalite),thesemineralsaresaturated
(mineralsofHighSilication)withrespectto
Si,e.gfeldspars,pyroxenes.
Classification Based on presence of Saturated Minerals
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Chemical and Quasi-Chemical Classification
Cristobalite

7. Unsaturated Minerals
Themineralswhicharenever
occurredwithaprimarysilica
mineral.Theseareunsaturated
(mineralsoflowSilication)
minerals,e.g.Mg-richolivine,
Nepheline.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

8. Types of Rock Containing
Saturated/Unsaturated minerals
Example
1.Over SaturatedFreesilica (Quartz) + Saturated
minerals
Granite, Rhyolite,
Granodiorite, Dacite
2. Saturated Only Saturated mineralsSyenite, Diorite,
Trachayte
3. Unsaturated Unsaturatedminerals Peridotite
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Classification Based on presence of Saturated Minerals

9. Classification based on Mode of Occurrence and Texture
1.IntrusiveIgneousRocks
IgneousrockswhichformbythecrystallizationofmagmaatadepthwithintheEartharecalled
intrusiverocks.Intrusiverocksarecharacterizedbylargecrystalsizes,i.e.,theirvisual
appearanceshowsindividualcrystalsinterlockedtogethertoformtherockmass.Thecoolingof
magmadeepintheEarthistypicallymuchslowerthanthecoolingprocessatthesurface,so
largercrystalscangrow.Rockswithvisiblecrystalsofroughlythesamesizearesaidtohave
aphaneritictexture.Example:Granite,Diorite,Gabbro
2.ExtrusiveIgneousRocks
IgneousrockswhichformbythecrystallizationofmagmaatthesurfaceoftheEartharecalled
extrusiverocks.Theyarecharacterizedbyfine-grainedtexturesbecausetheirrapidcoolingator
nearthesurfacedidnotprovideenoughtimeforlargecrystalstogrow.Rockswiththisfine-
grainedtexturearecalledaphaniticrocks.ThemostcommonextrusiverocksareRhyolite,
Andesite,Basalt.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

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

11. Classification of Igneous Rock based on Mineral Components
LightColoredMineral
Mineralswithalightcoloraregenerally
lackingintheelementsiron(Fe)and
magnesium(Mg).Anycolorsotherthangray,
black,brown(including"gold")anddark
greenwillbeconsideredtobe"light".The
light-coloredmineralslistedbelowareamong
themostcommonrock-formingminerals
1.Quartz
2.Feldsper
3.Feldspathoid
4.Muscovite
DarkColoredMinerals
Mineralswithadarkcoloraregenerally
relativelyrichintheelementsiron(Fe)and
magnesium(Mg).Colorsfortheseminerals
includegray,black,brown(including"gold")
anddarkgreen,andthesemineralswilloften
leaveagoodstreak.Thedark-coloredminerals
listedbelowareamongthemostcommonrock-
formingmineralsare
1.Pyroxene
2.Amphibole
3.Olivine
4.IronOxide
5.Apatite
6.Biotite
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

12. 1.Felsic
TheFelsicgroupconsistsofmineralscharacterizedbylightincolored,lowspecificgravityandof
comparativelylatecrystallization.Keymineralsinfelsicrocksaresodiumandpotassiumfeldspars,
quartz,feldspathoids,andmuscovite.Indeed,thetermfelsicisamnemonic,basedonthismineralogy,
formedfrom(fe)forfeldspar,(l)for(a.k.a.feldspathoid),and(s)forsilica,plus(-ic)asuffixmeaning
“havingthecharacterof.”ThecommonrocksonthisgroupareGranite,Rhyolite.
2.Mafic
Themaficgroupontheotherhandcomprisesmineralswhicharedarkincolor,heavyspecificgravityand
ofcomparativelyearlycrystallization.Maficrocksaredominantlycomposedofiron-andmagnesium-rich
silicates,specificallyolivine,pyroxenes,amphiboles,andbiotite.Thetermmaficcomesfrom(ma)
formagnesiumand(f)fromferrum,theLatinwordforiron,plus(-ic).Calcium-richplagioclase,
althoughnotaniron-magnesiumsilicate,isalsoacommonconstituentinmaficrocksbecausemafic
magmasareenrichedincalciumrelativetopotassiumandsodium.Commonexampleofthisclassare
basaltandgabbro.
1.Felsic
2.Mafic
Based on Colored minerals Igneous rocks classified by
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

13. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Vs.
Dark
Light