Skip to main content
Geosciences LibreTexts

3: Intrusive Igneous Rocks (Exercises)

Q3.1 Rock around the rock-cycle clock

Sedimentary rock is buried deeper to make metamorphic rock, the metamorphic rock is uplifted and during this process the material overhead is eroded so that it can be exposed at surface. The metamorphic rock is then eroded to make more sediments, which are deposited and then buried to make sedimentary rock. This would likely take at least 60 million years.

Q3.2 Making magma viscous

Responses will vary

Q3.3 Rock types based on magma composition

SiO2 Al2O3 FeO CaO MgO Na2O K2O Type?
55% 17% 5% 6% 3% 4% 3% intermediate (although the SiO2 level is borderline, there is too little FeO, MgO and CaO to be mafic)
65% 14% 4% 5% 4% 3% 3% intermediate (although the SiO2 level is borderline, there is too much MgO and CaO to be felsic)
74% 14% 3% 3% 0.5% 5% 4% felsic
47% 14% 8% 10% 8% 1% 2% mafic


Q3.4 Porphyritic minerals

a) only olivine phenocrysts

b) pyroxene and amphibole phenocrysts, along with plagioclase with a composition that is about half-way between the Ca-rich and the Na-rich end-members.

Q3.5 Mineral proportions in igneous rocks

a) ~25% K-feldspar, ~30% quartz, ~35% albitic plagioclase and ~10 biotite/amphibole

b) ~65% plagioclase and ~35% biotite/amphibole (most likely amphibole)

c) ~45% anorthitic plagioclase, ~25% amphibole and ~35% pyroxene d) ~50% pyroxene and ~50% olivine.

Q3.6 Pluton problems

a) stock

b) dyke (it cuts across bedding and the granite)

c) sill (it is parallel to bedding),

d) is a dyke, e is a sill.