Chapter3 1igneousrock
Chapter3 1igneousrock
Chapter3 1igneousrock
ENGINEERING GEOLOGY
CHAPTER 3
THE STUDY OF ROCKS
INTRODUCTION
(4) Hydrothermal - Some are formed by crystallization from hot aqueous fluids (watch this
http://www.youtube.com/watch?v=e4H4W0jiP2w sulfur mining indonesia)
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ROCK CYCLE
https://youtu.be/7m8tevimgco
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ROCK CYCLE
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https://youtu.be/EGK1KkLjdQY
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CHAPTER 3.1:
IGNEOUS ROCKS
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IGNEOUS ROCKS
• Defined as rocks which are normally crystalline in nature having solidified
from an original molten state or magma that exists for long period of
time beneath the surface of earth.
• Igneous rocks can be derived from the (1) cooling of molten magma or of
(2) solidification of lava from volcanic eruption.
• MAGMA is molten rock material generated in the certain zones deep inside
the earth's crust and possible in the upper zones of the mantle.
• Magma moves from deeper zones to higher zones of the crust through
forceful injections into fractures and faults in the adjacent rocks.
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What Is Magma?
• Magma is hot molten mobile rock.
• The most abundant magma is a melt of silicate composition and this can carry
suspended crystals and gases which bubble out in air.
• Magmas are less dense than surrounding rocks, and therefore it will move upward.
• If magma makes it to the surface it will erupt and later crystallize to form an
extrusive or volcanic rock.
• If it crystallizes before it reaches the surface it will form an igneous rock at depth
called a plutonic or intrusive igneous rock.
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Extrusive/
Volcanic
Intrusive/
Plutonic
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Andesit
Magma
The thicknes
may be differ
for different
site
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Temperature: 1200 - Temperature: 1000 Temperature: 800 -
1000°C - 800°C 650°C
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Magma Type (Basaltic) - 1000C -
1200C
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Magma Type (Andesitic) - 800C -
1000C
Andesite: Has a porphyritic texture Diorite: This specimen clearly shows the
familiar "salt and pepper" appearance of
diorite, produced by white plagioclase
contrasting with black hornblende and
biotite.
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Magma Type (Rhyolitic) - 650C - 800C
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INTRUSIVE EXTRUSIVE
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VERY FAST COOLING –
CELLULAR/VESICULAR
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Types of Igneous Rocks
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Rock Texture
• In Igneous rocks, the cooling history and environment is the function of the
formation of textures.
• Magmas located deep within the Earth's crust cools slowly and thus the
individual minerals grains may grow.
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Rock Texture
• Consist of:
1.Phaneritic texture
2.Aphanetic texture
3.Glassy texture
4.Porphyritic texture
5.Vesicular Texture
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(1)Phaneritic texture
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Examples of phaneritic rocks; phaneritic texture, consists of large grains and can be
seen unaided
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(2) Aphanetic texture
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Aphanetic texture consists of grains too small to be seen without a
microscope
BASALT
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(3) Glassy texture
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A glassy texture develops when lava material cools very fast
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(4) Porphyritic texture
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Andesite: Has a porphyritic texture.
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Phenocrysts
Matrix/Ground mass
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(5) Vesicular Texture
• The image below shows a basalt with vesicles, hence the name
"vesicular basalt".
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Vesicular rocks
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Pumice
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Scoria
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Summary of Igneous Rocks Structures
Mode of
Rock Types Rock Textures
formation
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Chemical and Mineralogical Composition
• Rocks that are rich in silica are called silicic or felsic (Acid), rocks and those that
are low in silica content are called mafic rocks (Basic).
• Color provides a valuable clue for identification igneous rocks because the silicic
rocks are mainly composed of lightly colored minerals like quartz and feldspar,
whereas the mafic rocks are dark colored because of the abundance
ferromagnesian minerals. The dark colored ferromagnesian minerals are rich in iron
and magnesium, include olivine, pyroxene and hornblende.
• The major igneous rock types fall into categories of high, intermediate and low silica
content.
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Minerology of Igneous Rock
Felsic/Light
colour Mafic/Dark colour
ULTRA
ACID BASIC (45
INTERMEDIATE BASIC (< 45%
(>63% silica) – 55%)
silica)
Crystalline Feldspar
Texture Orthoclase - Plagioclase
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Formation of Igneous Rocks
• Intrusive rocks which cool and solidify under pressure and at great depths are usually
wholly crystalline in texture, since the conditions of cooling are conducive to crystal
formation.
• Such rocks occur in masses of great extent, often going to unknown depths.
• Although originally formed deep underground, intrusive rocks are now widely exposed
because of earth movement and erosion processes.
• Rocks that make up plutons usually have phaneritic texture because the cooling time was
sufficient to allow the formation of large crystals.
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Types of Plutons
• Plutons differ in terms of size, shape and relationship to the
rocks that were intruded by the magma, which are older rocks
known as country rocks.
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DIKE AND SILL FORMATION
https://youtu.be/6loGYTCBVqo
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Dykes
• Tabular or wall like mass.
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Laccoliths
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https://youtu.be/6loGYTCBVqo
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Batholiths
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Types of plutons
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Extrusive Processes
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Examples of Igneous rock
• Granite
• Basalt
• Gabbro
• Diorite
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Granite (Intrusive)
• Granite characterized by a granular texture, has feldspar and
quartz (at least 20%) as its two most abundant minerals.
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Colour differ based on the mineral.
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Basalt (Extrusive)
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Crystallization of Magma
• There are two crystallization sequences that are observed as the melt
cools.
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First sequence - crystallization of plagioclase
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Second sequence - crystallization of ferromagnesian mineral
• As the temperature decreases, no change in the olivine crystals occurs until a critical temperature is
reached.
• At this point, augite rather than olivine begins to crystallize and the early-formed olivine crystals
react with the liquid to form augite.
• These reactions are different from the continuous reaction of plagioclase because entirely new
minerals with different internal structures form at specific temperatures.
• For this reason the ferromagnesian crystallization sequence is called a discontinuous reaction series.
• The same type of reaction occurs between augite and liquid to form hornblende at a lower
temperature.
https://www.youtube.com/watch?v=0XgpYXVN-Kk
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Thank you for your attention!!!