UNIVERSITY OF SOUTH AFRICA
College of Agriculture and Environmental Sciences
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CMT3700: Construction Materials and Testing
Assignment 01 — Semester 1, 2026
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
CMT3700
Module Code:
Construction Materials and Testing
Module Name:
Assignment 01
Assignment:
2026
Due Date:
50 Marks
Total Marks:
Submitted in partial fulfilment of the requirements for CMT3700 — UNISA 2026
, UNISA | CMT3700 Construction Materials and Testing – Assignment 01
Question 1: Rock Classification and Soil Testing
1.1 Formation Processes and Textures of Intrusive and Extrusive Igneous Rocks
Igneous rocks are formed through the cooling and solidification of magma, either below the
Earth’s surface or on it (USGS, 2024). The key distinction between intrusive and extrusive
types lies in where that cooling takes place, and the speed of that process directly determines
the resulting texture.
1.1.1 Intrusive (Plutonic) Igneous Rocks
Intrusive igneous rocks form when magma rises through the Earth’s crust but does not
reach the surface. The magma accumulates in large underground chambers called magma
chambers, where it cools very slowly over thousands to millions of years (Britannica, 2026).
This slow cooling gives mineral crystals a long time to grow, producing a coarse-grained
or phaneritic texture in which individual crystals are clearly visible to the unaided eye (Geo-
sciences LibreTexts, 2025). Intrusive bodies occur in several structural forms: a batholith is
a large discordant body covering over 100 square kilometres; a dyke is a tabular, discordant
intrusion cutting across existing strata; a sill is a concordant, sheet-like intrusion parallel to
existing rock layers; and a laccolith is a dome-shaped body formed where magma domes up
overlying strata (GeoScience LibreTexts, 2025).
Examples of intrusive igneous rocks:
• Granite: A felsic intrusive rock rich in quartz and alkali feldspar, with minor plagioclase
and biotite. Its coarse, interlocked crystal structure makes it highly durable. Granite un-
derlies much of the Bushveld Igneous Complex in South Africa and is widely used as a
construction material (USGS, 2024).
• Gabbro: A mafic intrusive rock composed principally of pyroxene, olivine, and plagioclase
feldspar. It is the intrusive equivalent of basalt and occurs widely in the Merensky Reef,
South Africa, where it hosts platinum-group mineral deposits (USGS, 2024).
1.1.2 Extrusive (Volcanic) Igneous Rocks
Extrusive igneous rocks form when magma reaches the Earth’s surface through volcanic
vents or fissures and erupts as lava. On contact with air or water, lava cools rapidly, leaving lit-
Page 2 of 15
College of Agriculture and Environmental Sciences
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
CMT3700: Construction Materials and Testing
Assignment 01 — Semester 1, 2026
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄⋄
CMT3700
Module Code:
Construction Materials and Testing
Module Name:
Assignment 01
Assignment:
2026
Due Date:
50 Marks
Total Marks:
Submitted in partial fulfilment of the requirements for CMT3700 — UNISA 2026
, UNISA | CMT3700 Construction Materials and Testing – Assignment 01
Question 1: Rock Classification and Soil Testing
1.1 Formation Processes and Textures of Intrusive and Extrusive Igneous Rocks
Igneous rocks are formed through the cooling and solidification of magma, either below the
Earth’s surface or on it (USGS, 2024). The key distinction between intrusive and extrusive
types lies in where that cooling takes place, and the speed of that process directly determines
the resulting texture.
1.1.1 Intrusive (Plutonic) Igneous Rocks
Intrusive igneous rocks form when magma rises through the Earth’s crust but does not
reach the surface. The magma accumulates in large underground chambers called magma
chambers, where it cools very slowly over thousands to millions of years (Britannica, 2026).
This slow cooling gives mineral crystals a long time to grow, producing a coarse-grained
or phaneritic texture in which individual crystals are clearly visible to the unaided eye (Geo-
sciences LibreTexts, 2025). Intrusive bodies occur in several structural forms: a batholith is
a large discordant body covering over 100 square kilometres; a dyke is a tabular, discordant
intrusion cutting across existing strata; a sill is a concordant, sheet-like intrusion parallel to
existing rock layers; and a laccolith is a dome-shaped body formed where magma domes up
overlying strata (GeoScience LibreTexts, 2025).
Examples of intrusive igneous rocks:
• Granite: A felsic intrusive rock rich in quartz and alkali feldspar, with minor plagioclase
and biotite. Its coarse, interlocked crystal structure makes it highly durable. Granite un-
derlies much of the Bushveld Igneous Complex in South Africa and is widely used as a
construction material (USGS, 2024).
• Gabbro: A mafic intrusive rock composed principally of pyroxene, olivine, and plagioclase
feldspar. It is the intrusive equivalent of basalt and occurs widely in the Merensky Reef,
South Africa, where it hosts platinum-group mineral deposits (USGS, 2024).
1.1.2 Extrusive (Volcanic) Igneous Rocks
Extrusive igneous rocks form when magma reaches the Earth’s surface through volcanic
vents or fissures and erupts as lava. On contact with air or water, lava cools rapidly, leaving lit-
Page 2 of 15
