UNIVERSITY OF SOUTH AFRICA
College of Science, Engineering and Technology
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
Engineering Geology
Comprehensive Examination Answers
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
Engineering Geology
Module:
Examination
Assessment Type:
100
Total Marks:
2026
Date:
Submitted in partial fulfilment of the require-
ments for Engineering Geology — UNISA 2026
,UNISA | Engineering Geology Exam Answers
Question 1: Mineralogy and Rock Formation
1.1 Bowen’s Reaction Series – Discontinuous Series Diagram
Bowen’s reaction series describes the sequence in which minerals crystallise from a cool-
ing magma. The discontinuous series involves distinct mineral phases that do not share a
continuous structural transition – each new mineral that forms is chemically and structurally
different from the one before it (Bowen, 1922).
Crystallises
STAGE 1: ≈1200◦ C – Ultramafic ⋆ FIRST
Mineral: Olivine [(Mg,Fe)2 SiO4 , Composition
No. 1]
Coarse-grained: Peridotite / Dunite
Fine-grained: Komatiite (rare)
Rock colour: Dark green to black
HIGH TEMPERATURE −→ LOW TEMPERATURE
STAGE 2: ≈1100◦ C – Mafic
Mineral: Pyroxene [e.g. Augite – CaMgSi2 O6 ,
Composition No. 2]
Coarse-grained: Gabbro
Fine-grained: Basalt
Rock colour: Dark grey to black
STAGE 3: ≈1000◦ C – Intermediate
Mineral: Amphibole [e.g. Hornblende –
Ca2 (Mg,Fe)5 Si8 O22 (OH)2 , Composition No. 3]
Coarse-grained: Diorite
Fine-grained: Andesite
Rock colour: Grey to dark grey
STAGE 4: ≈800◦ C – Felsic
Mineral: Biotite Mica
[K(Mg,Fe)3 (AlSi3 O10 )(OH)2 , Composi-
tion No. 4]
Coarse-grained: Granite
Fine-grained: Rhyolite
Rock colour: Light grey to pink
Late: <700◦ C – Muscovite, K-
Feldspar, Quartz (felsic residuals)
Figure 1: Bowen’s Discontinuous Reaction Series – showing temperature stages, magma
compositions, primary minerals, intrusive/extrusive rock equivalents, and colour progression.
Olivine (⋆) has the highest melting point and crystallises first.
Key Distinction
The discontinuous series (left branch in classic diagrams) produces minerals that are
structurally distinct at each stage: Olivine → Pyroxene → Amphibole → Biotite. Each
transition involves a complete change in crystal structure, not just composition. This
contrasts with the continuous plagioclase series (right branch), where structure stays
Page 2 of 23
, UNISA | Engineering Geology Exam Answers
the same but Ca/Na ratios shift gradually.
Page 3 of 23
College of Science, Engineering and Technology
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
Engineering Geology
Comprehensive Examination Answers
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
Engineering Geology
Module:
Examination
Assessment Type:
100
Total Marks:
2026
Date:
Submitted in partial fulfilment of the require-
ments for Engineering Geology — UNISA 2026
,UNISA | Engineering Geology Exam Answers
Question 1: Mineralogy and Rock Formation
1.1 Bowen’s Reaction Series – Discontinuous Series Diagram
Bowen’s reaction series describes the sequence in which minerals crystallise from a cool-
ing magma. The discontinuous series involves distinct mineral phases that do not share a
continuous structural transition – each new mineral that forms is chemically and structurally
different from the one before it (Bowen, 1922).
Crystallises
STAGE 1: ≈1200◦ C – Ultramafic ⋆ FIRST
Mineral: Olivine [(Mg,Fe)2 SiO4 , Composition
No. 1]
Coarse-grained: Peridotite / Dunite
Fine-grained: Komatiite (rare)
Rock colour: Dark green to black
HIGH TEMPERATURE −→ LOW TEMPERATURE
STAGE 2: ≈1100◦ C – Mafic
Mineral: Pyroxene [e.g. Augite – CaMgSi2 O6 ,
Composition No. 2]
Coarse-grained: Gabbro
Fine-grained: Basalt
Rock colour: Dark grey to black
STAGE 3: ≈1000◦ C – Intermediate
Mineral: Amphibole [e.g. Hornblende –
Ca2 (Mg,Fe)5 Si8 O22 (OH)2 , Composition No. 3]
Coarse-grained: Diorite
Fine-grained: Andesite
Rock colour: Grey to dark grey
STAGE 4: ≈800◦ C – Felsic
Mineral: Biotite Mica
[K(Mg,Fe)3 (AlSi3 O10 )(OH)2 , Composi-
tion No. 4]
Coarse-grained: Granite
Fine-grained: Rhyolite
Rock colour: Light grey to pink
Late: <700◦ C – Muscovite, K-
Feldspar, Quartz (felsic residuals)
Figure 1: Bowen’s Discontinuous Reaction Series – showing temperature stages, magma
compositions, primary minerals, intrusive/extrusive rock equivalents, and colour progression.
Olivine (⋆) has the highest melting point and crystallises first.
Key Distinction
The discontinuous series (left branch in classic diagrams) produces minerals that are
structurally distinct at each stage: Olivine → Pyroxene → Amphibole → Biotite. Each
transition involves a complete change in crystal structure, not just composition. This
contrasts with the continuous plagioclase series (right branch), where structure stays
Page 2 of 23
, UNISA | Engineering Geology Exam Answers
the same but Ca/Na ratios shift gradually.
Page 3 of 23
