UNIVERSITY OF SOUTH AFRICA (UNISA)
College of Science, Engineering and Technology
⋄
CMT3700 ASSIGNMENT 01
Semester 1 Assignment 01 — 2026
⋄
Module Code: CMT3700
Module Name: Construction Materials Technology
Assignment No.: Assignment 01
Due Date: 2026
Semester: Semester 1, 2026
Submitted in partial fulfilment of the requirements for Construction Materials Technology
at the University of South Africa.
,UNISA | CMT3700 Construction Materials Technology — Assignment 01
Question 1: Igneous Rocks 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 molten rock material
known as magma (National Geographic, 2024). The conditions under which this cooling
takes place determine whether the resulting rock is classified as intrusive or extrusive, and
these conditions are also the primary factor governing crystal size and overall rock texture
(Johnson et al., 2025).
Intrusive (Plutonic) Igneous Rocks
Intrusive igneous rocks, also called plutonic rocks, form when magma rises from the Earth’s
mantle into the lower or middle crust but does not reach the surface. The magma becomes
trapped in large underground chambers, where it cools very slowly over thousands to millions
of years (National Park Service, 2024). Because the surrounding rock acts as an insulating
blanket, the heat loss is gradual, and mineral crystals have ample time to grow to sizes that
are visible to the naked eye.
The resulting texture is described as phaneritic (coarse-grained): individual mineral grains
are clearly distinguishable without a microscope and may range from a few millimetres to
several centimetres in length (Johnson et al., 2025). Intrusive rocks are generally denser than
their extrusive equivalents because the tightly interlocking crystal structure leaves little pore
space. They are exposed at the Earth’s surface only after millions of years of overlying rock
removal through uplift and erosion, which is why they are commonly found in the cores of
ancient mountain ranges (National Park Service, 2024; Geology Base, 2024).
Occurrences of intrusive bodies include batholiths (the largest type, covering hundreds of
square kilometres), stocks, dikes, and sills.
Two examples of intrusive igneous rocks:
1. Granite: A light-coloured, coarse-grained intrusive rock composed primarily of quartz,
alkali feldspar, and minor amounts of mica (biotite or muscovite) and hornblende. Its
speckled appearance results from the varying colours of these interlocking minerals.
Granite forms deep in continental batholiths and is among the most widely used con-
struction and monument materials in the world (National Park Service, 2024).
Page 1 of 15
, UNISA | CMT3700 Construction Materials Technology — Assignment 01
2. Gabbro: A dark-coloured, coarse-grained intrusive rock composed mainly of pyroxene,
olivine, and plagioclase feldspar. It is the intrusive equivalent of basalt in terms of com-
position but differs entirely in texture due to its slow cooling history. Gabbro makes up a
significant portion of the oceanic crust and is used as a decorative stone commercially
sold under the name “black granite” (Johnson et al., 2025).
Extrusive (Volcanic) Igneous Rocks
Extrusive igneous rocks, also called volcanic rocks, form when magma reaches the Earth’s
surface through volcanic vents or fissures and is expelled as lava. Once exposed to the atmo-
sphere or ocean water, lava cools very rapidly, sometimes within hours or days (Geology Base,
2024). This rapid cooling prevents mineral crystals from growing to any significant size.
The resulting texture is described as aphanitic (fine-grained): individual mineral grains are too
small to see with the unaided eye and require a petrographic microscope for identification
(Johnson et al., 2025). In extreme cases where cooling is almost instantaneous, no crystals
form at all and the rock has a glassy texture (e.g., obsidian). Some extrusive rocks display a
vesicular texture, where gas bubbles trapped in the rapidly solidifying lava create a porous,
frothy structure (e.g., pumice). A porphyritic texture occurs when magma undergoes two
stages of cooling: large crystals (phenocrysts) develop during a slow initial phase under-
ground, and these are then surrounded by a fine-grained groundmass formed during rapid
cooling at the surface (Johnson et al., 2025; National Geographic, 2024).
Two examples of extrusive igneous rocks:
1. Basalt: The most abundant extrusive rock on Earth, basalt is dark grey to black, fine-
grained, and composed primarily of pyroxene, plagioclase feldspar, and olivine. It forms
from mafic (iron-and-magnesium-rich, low-silica) lava and is the dominant rock of the
oceanic crust and of shield volcanoes such as those found in Hawaii (Geology Base,
2024; National Park Service, 2024).
2. Rhyolite: A light-coloured, fine-grained extrusive rock with a composition similar to gran-
ite (rich in silica, quartz, and alkali feldspar). Rhyolite forms from highly viscous felsic
lava during explosive volcanic episodes. It often contains visible shards of volcanic glass
and may display a porphyritic texture. A notable example is the material erupted during
the 1980 Mount St. Helens eruptions in the United States (Geology Base, 2024; University
of Nevada, 2024).
Page 2 of 15
College of Science, Engineering and Technology
⋄
CMT3700 ASSIGNMENT 01
Semester 1 Assignment 01 — 2026
⋄
Module Code: CMT3700
Module Name: Construction Materials Technology
Assignment No.: Assignment 01
Due Date: 2026
Semester: Semester 1, 2026
Submitted in partial fulfilment of the requirements for Construction Materials Technology
at the University of South Africa.
,UNISA | CMT3700 Construction Materials Technology — Assignment 01
Question 1: Igneous Rocks 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 molten rock material
known as magma (National Geographic, 2024). The conditions under which this cooling
takes place determine whether the resulting rock is classified as intrusive or extrusive, and
these conditions are also the primary factor governing crystal size and overall rock texture
(Johnson et al., 2025).
Intrusive (Plutonic) Igneous Rocks
Intrusive igneous rocks, also called plutonic rocks, form when magma rises from the Earth’s
mantle into the lower or middle crust but does not reach the surface. The magma becomes
trapped in large underground chambers, where it cools very slowly over thousands to millions
of years (National Park Service, 2024). Because the surrounding rock acts as an insulating
blanket, the heat loss is gradual, and mineral crystals have ample time to grow to sizes that
are visible to the naked eye.
The resulting texture is described as phaneritic (coarse-grained): individual mineral grains
are clearly distinguishable without a microscope and may range from a few millimetres to
several centimetres in length (Johnson et al., 2025). Intrusive rocks are generally denser than
their extrusive equivalents because the tightly interlocking crystal structure leaves little pore
space. They are exposed at the Earth’s surface only after millions of years of overlying rock
removal through uplift and erosion, which is why they are commonly found in the cores of
ancient mountain ranges (National Park Service, 2024; Geology Base, 2024).
Occurrences of intrusive bodies include batholiths (the largest type, covering hundreds of
square kilometres), stocks, dikes, and sills.
Two examples of intrusive igneous rocks:
1. Granite: A light-coloured, coarse-grained intrusive rock composed primarily of quartz,
alkali feldspar, and minor amounts of mica (biotite or muscovite) and hornblende. Its
speckled appearance results from the varying colours of these interlocking minerals.
Granite forms deep in continental batholiths and is among the most widely used con-
struction and monument materials in the world (National Park Service, 2024).
Page 1 of 15
, UNISA | CMT3700 Construction Materials Technology — Assignment 01
2. Gabbro: A dark-coloured, coarse-grained intrusive rock composed mainly of pyroxene,
olivine, and plagioclase feldspar. It is the intrusive equivalent of basalt in terms of com-
position but differs entirely in texture due to its slow cooling history. Gabbro makes up a
significant portion of the oceanic crust and is used as a decorative stone commercially
sold under the name “black granite” (Johnson et al., 2025).
Extrusive (Volcanic) Igneous Rocks
Extrusive igneous rocks, also called volcanic rocks, form when magma reaches the Earth’s
surface through volcanic vents or fissures and is expelled as lava. Once exposed to the atmo-
sphere or ocean water, lava cools very rapidly, sometimes within hours or days (Geology Base,
2024). This rapid cooling prevents mineral crystals from growing to any significant size.
The resulting texture is described as aphanitic (fine-grained): individual mineral grains are too
small to see with the unaided eye and require a petrographic microscope for identification
(Johnson et al., 2025). In extreme cases where cooling is almost instantaneous, no crystals
form at all and the rock has a glassy texture (e.g., obsidian). Some extrusive rocks display a
vesicular texture, where gas bubbles trapped in the rapidly solidifying lava create a porous,
frothy structure (e.g., pumice). A porphyritic texture occurs when magma undergoes two
stages of cooling: large crystals (phenocrysts) develop during a slow initial phase under-
ground, and these are then surrounded by a fine-grained groundmass formed during rapid
cooling at the surface (Johnson et al., 2025; National Geographic, 2024).
Two examples of extrusive igneous rocks:
1. Basalt: The most abundant extrusive rock on Earth, basalt is dark grey to black, fine-
grained, and composed primarily of pyroxene, plagioclase feldspar, and olivine. It forms
from mafic (iron-and-magnesium-rich, low-silica) lava and is the dominant rock of the
oceanic crust and of shield volcanoes such as those found in Hawaii (Geology Base,
2024; National Park Service, 2024).
2. Rhyolite: A light-coloured, fine-grained extrusive rock with a composition similar to gran-
ite (rich in silica, quartz, and alkali feldspar). Rhyolite forms from highly viscous felsic
lava during explosive volcanic episodes. It often contains visible shards of volcanic glass
and may display a porphyritic texture. A notable example is the material erupted during
the 1980 Mount St. Helens eruptions in the United States (Geology Base, 2024; University
of Nevada, 2024).
Page 2 of 15
