Engineering Geology
Igneous rocks
Hussien Al - deeky
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Engineering Geology
The Geology Definition of
Rocks
In Geology Rock is defined as the solid material forming the outer rocky shell or
crust of the earth.
There are three major groups of rocks by its origin:
(1)Igneous rocks: cooled from a molten state ; e.g., granite, basalt
(2)Sedimentary rocks: deposited from fluid medium; the products of weathering of
other rocks in water ; e.g., sandstone, mudstone…;
(3)Metamorphic rocks : formed from pre-existing rocks by the action of heat and
pressure . e.g., dolomite, marble …;
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Engineering Geology
Rock Cycles
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Engineering Geology
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Engineering Geology
The Engineering Definition of
Rocks
Thus, in pure geological sense rock is defined as the essential part of the earth’s
crust. Geologists concern about the origin, classification, history, and the spatial
aspects of rocks. So, geologically speaking, ice, sand, marble, coal, basalt, can be
simply regarded as rocks
. However, the Engineering Geologists have a different, and relatively narrower view
of rocks. The Engineering Definition of Rocks Rock is the hard and durable
material
By an excavation point of view, Rocks are the earth materials that cannot be
excavated without blasting . This definition clearly excludes other kinds of
earth materials such as soils, and glacial till s, etc. Here is another
engineering definition of rocks : The earth materials that do not slake
when soaked into water . For example, a thick loess deposit is regarded as
rock geologically and regarded as soil in engineering.
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Engineering Geology
Igneous rocks
• Igneous rocks form by cooling and crystallization of
molten rock
• Molten material residing below Earth’s surface is known
as magma, whereas the same material at the surface is
called lava.
• Igneous rocks formed of cooled lava or volcanic ejected
are common, but most molten material cools below
Earth’s surface,
producing bodies of
igneous rock known
as plutons.
• Intrusive and Extrusive
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Engineering Geology
Properties of Magma and Lava
• All igneous rocks derive either directly or
indirectly from magma. Lava is magma that has
reached Earth’s surface.
• Plutonic (intrusive) igneous rocks form as
magma cools and crystallizes within Earth.
• Volcanic (extrusive) igneous rocks form by
cooling and crystallization
of lava or by consolidation
of pyroclastic material,
such as volcanic ash,
ejected from volcanoes
It is not possible to study magma directly
• However, studying lavas can tell us a lot
• Magmas have a range of compositions
• Characterized by high temperatures
• Have the ability to flow
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Engineering Geology
Magma Composition
• Silicate minerals are by far the most abundant minerals in the crust and silica is the
most abundant constituent of magma. • The bulk chemical composition of magma is
dominated by the most abundant minerals Si - Al - Fe - Ca - Mg - Na - K - H - O
• These major elements occur as oxides (SiO2) • SiO2 = ~45 to 75% of rocks •
Water and CO2 make up 0.2 - 3 %
• Minor and trace elements make up the remainder
How hot are magma and lava?
• Erupting lavas range in temperature from 1000° to
1200°C. Magma must be even hotter, but direct
measurements are not possible.
• Rock is a poor heat conductor.
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Engineering Geology
Cooling rates
• Intrusive (plutonic) rocks cool slowly while
extrusive (volcanic) rocks cool quickly
• The cooling rate determines whether or not
crystals form
• So cooling and
crystallization
determine the
texture of the
rock
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Engineering Geology
Texture
• Texture refers to the size, shape and arrangement of
minerals’ grains and is an important characteristic
of igneous rocks. Grain size records cooling
history.
• 1-An aphanitic texture consists of an aggregate of
very small mineral grains, too small to be seen
clearly with the naked eye. Aphanitic textures
record rapid cooling at or very near Earth’s surface
and are characteristic of extrusive (volcanic)
igneous rocks.
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Engineering Geology
2-Plutonic textures: Phaneritic
• A phaneritic texture consists of an
aggregate of large mineral grains, easily
visible without magnification . Phaneritic
textures record slow cooling within Earth
and are characteristic of intrusive
(plutonic) igneous rocks
Engineering Geology
3-Glassy texture
• Very rapid cooling of lava produces a “glassy
texture”. The lava cools so quickly that atoms do
not have time to arrange in an ordered three dimensional
network typical of minerals. The result
is natural glass, or obsidian
Engineering Geology
4-Vesicular textures
• Gases trapped in cooling lava can result in
numerous small cavities, vesicles, in the solidified
rock.
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Engineering Geology
5-Pyroclastic texture (fragmental textures)
• Igneous rocks formed of mineral and rock fragments
ejected from volcanoes by explosive eruptions have
Pyroclastic (fragmental) textures. The ejected ash
and other debris eventually settles to the surface
where it is consolidated to form a Pyroclastic
igneous rock. • Much of this material
consists of angular
pieces of volcanic
glass measuring up to 2mm
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Engineering Geology
6-Porphyritic texture
• Igneous rocks comprised of minerals of two or more
markedly different grain sizes have a porphyritic texture.
The coarser grains are called phenocrysts and the
smaller grains groundmass. Porphyritic textures result
from changes in cooling rate and include both aphanitic
porphyrys and phaneritic porphyrys.
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Engineering Geology
Classifying Igneous Rocks
• Most igneous rocks can be classified on the basis of
Texture ,mineral composition and color.
• Compositional equivalents
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Engineering Geology
Engineering Considerations of Igneous Rocks
(1)Fine-grained igneous rocks cannot be used as aggregates in
Portland cement due to volume expansion caused by the Alkali-silica
reaction. Solutions include:(a)Can be used in low alkali
cement;(b)Non-reactive aggregates go with the high alkali
cement;(c)Add pozzolans, coal-ashes, etc. in the aggregate-cement
mixture to minimize the reaction.
(2)Coarse-grained igneous rocks (e.g., granite, syenite, etc.) are not
for aggregates for constructions because its low abrasion resistance;
but fine-grained igneous rocks (e.g., basalt) are good for aggregates
(e.g., basalt as paving aggregates goes with asphalt.
(3)Sitting of foundations needs to avoid weathered rocks (e.g., dams,
bridge piers, etc.);
(4)Igneous rocks are good for dimension stone (tombstone etc.)
because their resistance to weathering but need avoid fractures
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Engineering Geology
Gabbro and basalt
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Engineering Geology
Diorite and Andesite
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Engineering Geology
Granite and Rhyolite
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Engineering Geology
Red Obsidian
Tuff
Pumice
Scoria
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