Rocks-Igneou%20Rocks.pdf

Al-Qassim University
College of Engineering
Civil Engineering Department
CE 285
Introduction to Geotechnical Engineering
Dr. Sherif M. ElKholy

ROCKS

• Rocks are defined as: “A Mixture of Minerals”.
• All Rocks contain similar minerals but with different
percentages as shown in the following table as an example:

• If Rocks are of similar composition, Why are there
different Rocks with different mechanical properties?
Because, the types of the rock DOES NOT only depend on its
composition but also on:
a- The way those minerals occur in nature
(Chemical actions/reactions between them).
b- The mode of formation of those minerals.
c- Weathering process (Effect of environment).

ROCK GROUPS
Igneous Rocks Sedimentary Rocks Metamorphic Rocks

IGNEOUS ROCKS
• Also known as CRYSTALIZE MAGMAS.
• They are formed by the Cooling and Solidification of
Molten Rock (known as Magma).
• They are characterized form other rocks by:
– They are composed of tightly interlocking crystals.
– Due to even distribution of pressure within the
liquid Magma, minerals of Igneous Rocks are free
to grow in any direction, therefore, they show
random crystal orientation.

IGNEOUS ROCKS

IGNEOUS ROCKS

IGNEOUS ROCKS

SEDIMENTARY ROCKS
• They are formed from pieces (sediment) of other rock melt
or are precipitated from water.
• Those pieces (sediment) come from the weathering, erosion
and transportation of mountainous rocks.
• Therefore, any original crystals have either decomposed or
else have been worn away to more or less rounded grains.
• The SR therefore always show Layered Formation.

SEDIMENTARY ROCKS
• Unlike metamorphic rocks and many igneous rocks
Sedimentary Rocks are Characterized By:
– They are formed near the surface of earth.
– They usually show remains of plants and animals
which existed during periods of sedimentation.
– The layers of sedimentation give an idea about: the
environment during sedimentation and the media
that transported the original sediments (air, water or
ice).

SEDIMENTARY ROCKS

SEDIMENTARY ROCKS

METAMORPHIC ROCKS
• Formed from other rocks by changes in temperature
and/or pressure.
• These changes occur commonly in the presence of solution.
• They are characterized by:
– The formation of new minerals, however the
original rocks never melted.
– Since they form under direct pressure, the formed
crystals tend to grow in a definite direction giving
the MR a banded or laminated appearance.

METAMOROPHIC ROCKS

ROCK CYCLE
• Rocks (i.e. minerals that make them up) are stable only in
the environments in which they form.
• Thus, rocks are in a constant state of change.
• Using the same ions over and over again, nature keeps on
making new rocks all the time in what is known as ROCK
CYCLE.
• Igneous, sedimentary, and metamorphic rocks may
weather at the surface to produce new sedimentary rocks
and minerals.

ROCK CYCLE
• Similarly sedimentary, metamorphic and igneous rocks may
melt deep in the crust to form new igneous rocks and
minerals, i.e the possibilities are complex and endless.
• Each step in the cycle will be accompanied by significant
change in the physical and mechanical properties of the
formed rocks.
• However, the actual elements in the rock remain essentially
the same but in a different mineralogical and texture form
(homogenous, heterogeneous, isotropic, anisotropic….etc.

ROCK CYCLE

IGNEOUS ROCKS
• Also known as CRYSTALIZE MAGMAS.
• They are formed by the Cooling and Solidification
of Molten Rock (known as Magma).
• Magma are very hot (500 – 1700 o ) molten rocks originate at
depths of about 200 km within the earth.
• Magma consists of: the main elements of the silicate
minerals (O, al, ca,…..) + some gases (especially water
vapor).

Extrusive (volcanic)
Igneous Rocks
IGNEOUS ROCKS
IGNEOUS ROCKS
Intrusive (plutonic)
Igneous Rocks
Hypabyssal Igneous
Rocks

Extrusive (Volcanic) Igneous Rock
• They are called extrusive because magmas extrude (flow
out) onto the surface from within the earth.
• When magmas reach the surface of the earth and loose the
gas constituents, they are called Lava.

Extrusive (Volcanic) Igneous Rock
• They are formed by Rapid Cooling of Magmas when they
come out at the earth’s surface following eruptions of
volcanoes.
• The are characterized by: being Fine Grained. Why?
Due to the Rapid Cooling Process of Magmas.

Intrusive (Plotonic) Igneous Rock
• They are called intrusive because magmas commonly cut
across or are injected into the surrounding areas.
• Formed by Slow Cooling of Magmas within the earth, at
depths of few kms, where we can not observe the rockforming
process.

Intrusive (Plotonic) Igneous Rock
• Although they are formed within the earth, they might be
noticed at the surface due to the erosion process that exposes
the formerly deep-seated parts of the earth’s crust (removing
the overlaying layers of rocks).
• They are characterized by: being Coarse Grained. Why?
Due to the Slow Cooling Process of Magmas.

Hypabyssal Igneous Rock
• Formed in minor intrusions (dykes and sills) below earth
surface and have cooled more slowly than extrusive rocks
but faster than intrusive rocks.
• Therefore, they are characterized by: being medium-sized
Grains. Why?
Due to the Medium Cooling Process of Magmas.

Igneous Rock

Cooled very
quickly
Cooled quickly
Igneous Rock
Glassy
surface
Fine < 0.6 mm
Medium 0.6 – 2.0 mm
Cooled slowly Coarse > 2.0 mm

Rock Texture:
Igneous Rock
• The most important factors affecting igneous rocks texture
are:
- The Rate of Magmas Cooling
- The Composition of Magmas
• e.g. :
– The higher content of Fe & Ms (more fluid) cooled
quickly results in: Rock with crystalline texture.
– Under the same conditions of cooling but with
higher content of silicate (more viscous) results in:
Rock with glassy texture.

Rock Texture:
Igneous Rock
• Terms used to describe igneous rock texture:
– Aphanitic: Very fine-grained texture.
– Phaneritic: Texture where individual crystals can be
noticed.
– Prophyritic: Texture where large crystals embedded in
a matrix of smaller crystals.
– Glassy: Entirely glass-like texture.
– Vesicular: Aphanitic texture where gases escaped
resulting in voids in the rock surface.

IGNEOUS ROCKS

Igneous Rock

Igneous Rock

Classification of Igneous Rock

Home Work
• Define the three major groups of rocks.
• Define IR and mention what are they composed of.
• Discuss how the formation process of IR affects the their
texture.
• Chose “True” or “False” for the following statements:
– Igneous rocks are characterized by crystallized
texture.
– Sedimentary rocks are characterized by layered
formation.
– In IR, crystals tend to grow in a definite direction to
give the rocks a banded or laminated formation.

Home Work
• Describe these types of IR textures:
a- Aphanitic
b- Vesicular
c- Porphyritic
• Granite and Basalt are both igneous rocks, mention the
difference between them on the basis of the formation
process and texture.