3.rocks 2015
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Department of Civil Engineering
H24GGY Geology for Civil Engineers
Lecture 3: Rocks
Formation, types and characteristics
Prof Stuart Marsh Professor of Geospatial Engineering
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Department of Civil Engineering 3rd Rock from the Sun
Formed from the solar nebula 4600 Million Years Ago
All rocks that we have today came from this material
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Department of Civil Engineering From Rock to Earth
An active planet
- Geology is dynamic
- Rock is constantly
being recycled...
created, changed
and destroyed by
geologic processes
- Geologic timescales
much greater than
human lifetimes, so
make it seem static
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Department of Civil Engineering
Differentiation of Crust, Atmosphere & Oceans
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Department of Civil Engineering Earliest Crustal Rock
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Department of Civil Engineering Elements and Minerals
geocaching.com
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Department of Civil Engineering The Geological Cycle
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Department of Civil Engineering Igneous Rocks
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Department of Civil Engineering Classification
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Department of Civil Engineering Granite
- Feldspar, Quartz
- Coarse, crystalline
- Massive, uniform
structure: strong
- Weathers to clays
and quartz sands
- Lack of internal
structure leads to
distinctive Tors
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Department of Civil Engineering Basalt
- 50% Feldspar, mica
and Fe-Mg silicates
- Fine, interlocking
crystals, some voids
- Strength depends on
associated deposits
- Iron content rusts;
weathers to clays
- Structures (columns,
pillows) from cooling
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Department of Civil Engineering Sedimentary Rocks
Finer sediments transported further
The process of laying down sediments results in structures such as bedding, channel infill, grading of grain size and dune formation
Lithification occurs on burial via increased temperature and pressure by three processes: cementation, re-crystallisation and compaction
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Department of Civil Engineering Classification
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Department of Civil Engineering Sandstone
- Desert, river, marine
- Quartz grains in a
quartz/clay/calcite
cement, pore space
- Medium grained
- Strength depends on
cementation strength
- Crumbles to sand
- Deposition structures
preserved like beds
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Department of Civil Engineering Clay Rocks
- Lakes and marine
- Structure-less mass
of clay minerals like
illite + quartz grains
- Fine grained
- Strength depends on
lithification (older are
strong), weathering
- Reverts to mud
- Mostly structure-less
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Department of Civil Engineering Limestone
- Lakes and marine
- 95% calcite, some
clay, dolomite, flint
- Variable grain size,
often with shells etc.
- Strength depends on
lithification (older are
strong) and structure
- Soluble in rainwater
- Massive or bedded
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Department of Civil Engineering Evaporites
- Lakes and marine
- Halite, gypsum
- Crystaline, fine or
coarse depending on
mode of deposition
- Often weak, subject
to water dissolution
by rain, groundwater
- Can be massive or
bedded
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Department of Civil Engineering Metamorphic Rocks
Metamorphism happens through re-crystalisation (e.g. in Marble), growth of new minerals from old
(e.g. in schist clay becomes mica) and by the development of fabric through the alignment of minerals
Heating by igneous rock leads to thermal metamorphism;
high pressure during tectonics to dynamic metamorphism;
high temperature and pressure to regional metamorphism
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Department of Civil Engineering Classification
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Department of Civil Engineering Hornfels
- Thermal metamorphic
- Quartzite, if created
from sandstone, but
marble if limestone
- Crystaline, fine grain
- Generally very strong,
hence often used as
building stone
- Very slow weathering
to clays
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Department of Civil Engineering Schist
- Regional/dynamic
metamorphic
- Coarse grained mica,
chlorite, and quartz
- Alignment of mica is
cause of schistosity
- Generally very weak,
also anisotropic due
to schistosity; shears
- Slow alteration to clay
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Department of Civil Engineering Further Reading
Foundations of Engineering Geology A C Waltham
Chapter 1: Geology and Civil Engineering
Chapter 2: Igneous Rocks
Chapter 3: Surface Processes
Chapter 4: Sedimentary Rocks
Chapter 5: Metamorphic Rocks