igneous rocks

Question 1

Mafic rocks

Answer 1

contain iron and magnesium
Magic comes from magnesium and ferric
mafic minerals are dark and dense

Hornblende-
found mostly in intermediate rocks
hard to identify as it is confused with augite
colour-dark with beige bits
hardness- 5.5-6
lustre- vitreous
streak- fine white powder
Augite-
A type of pyroxene, found mostly in magic and ultra mafic rock
colour- dark green to black
hardness- 5.5-6
lustre- vitreous
streak- fine white powder
Olivine-
found sometimes in mafic and ultramarine rocks
colour- green/yellow
hardness- 6.5-7
lustre- vitreous
streak- white colourless grey

Question 2

Silica percentage

Answer 2

the percentage sillica in a rock includes the quartz (SiO2) + all the other Si and O in the rock
more sillica lighter in colour
>66% Silicic
52-66% intermediate
45-52% mafic
<45% ultramafic

Question 3

Textures of igneous rocks

Answer 3

Flow banding- formed by friction as the Magma or lava slows down near an interface, aligning the minerals as it moves
Conchoidal- a fracture which results in a curved surface
Vesicular- a texture ferm for a rock containing vesicals.Vesicals were bubbles of gas which come out of solution as a result of pressure release
Porohyritic- a texture where large crystals called phenocrysts are completely surrounded by smaller crystals
equigranular- when crystals in an igneous rock are approximately the same size
Amygdaloidal- where there are large vesicals the have been filled with a secondary material
lava m- molten rock that colls at the surface
Ophiolites- Sections of the Earth oceanic crust that have been techtonicaly moved (abducted) onto continental crust

Question 4

silicic ( felsic) rocks

Answer 4

form from silicic lava/Magma
silicic magmas are generated by the melting of continental crust as convergent plate boundrys
mineral- quartz , feldspar and some micas (lighter in colour)
silicic magmas are viscous
form at lower Temps than other magmas (600-900°c)
Obsidian- glassy, texture-Conchoidal
pumice- fine, texture- Vesicular
Rhyolite- fine, texture- flow banding
granite and granodiorite- coarse, Porohyritic and equigranular

Question 5

intermediate igneous rocks

Answer 5

52-66 % Silica
more Grey’s, still more Silica tho
most common is andersite
made of Feldspars and some pyroxene+ only a little quartz
Magma is less viscous than silicic but still quite thick
temp higher than silicic lower than mafic

Andersite- fine, texture- Amygdaloidal, Vesicular,Porohyritic and equigranular
diorite- coarse, texture- equigranular, Porphyritic

Question 6

Mafic rocks

Answer 6

mafic magmas produce dark crystals
dark due to high amount of iron+ Mg
contain pyroxene and sometime olivine
no quartz, Silica is building other silicate mineral instead
45-52% Silica content
low viscosity
starts to solidify around 1000°C
most originate in mantle
basalt- fine, texture- equigranular,Vesicular, Amygdaloidal or Porphyritic
Dolorite- medium,Texture-equigranular,Porphyritic,orphyritic
Gabbro- coarse, texture- equigranular

Question 7

ultramafic rocks

Answer 7

Made almost entirely from ferromagnesium minerals
less than 45% Silica
some consist entirely of olivine
upper mantel is peridorite, extremely high melting point
sometime peridorite found as a ophiolite

peridorite- coarse, texture-equigranular, ophiolite

Question 8

Crystal grain size

Answer 8

determine by rate of cooling
slow=bigger crystals- intrusive
faster=smaller crystals growth- extrusive

Glassy- none
cooling occurs in hours, from lava extruded onto surface, cools sp quick no atomic order so no crystals form, Conchoidal fracture, volcanic glass
Fine- <1mm- hard to see with naked eye
extrusive,cooling takes weeks to months
fine crystals can occur in intrusive but only on chilled magmas
medium- 1-5mm- crystals seen with naked eye ,intrusive, form in minor intrusions (hyperbassal),cool over 1000s of years
coarse- >5mm- major intrusions at depth(botholeths or plutons >100km²
cool over millions of years

Question 9

Crystal grain shape

Answer 9

Euhedral- well formed crystals with good faces, form at depth slowly and must have been inimpeded
subhedral- some well formed faces but some poorly formed, I fast cooling magmas crystals more likely to be anhedral/subhedaral
anhedral- poorly formed crystals faces- “

Question 10

pegmatite

Answer 10

abnormally large crystals
phenocrysts/Porphyritic
form during final stage of Magma crystallization
pegmatites form quite quickly form from magama quite rich in water
this water makes then grow quick as it had lots of rare minerals in it right at the end of cooling it cools and forms massive minerals
very rare and valuable crystals