PETROLOGY
Definition of Petrology
Petrology is the branch of Geology focused on the study of rocks and their formation.
The term "petrology" derives from Greek words:
"petros" meaning "rock"
"logos" meaning "study"
Major Concepts of Petrology
Petrology employs various classical fields to analyze rocks:
Mineralogy: the study of minerals, their structure, properties, and classification.
Petrography: systematic description and classification of rocks, often utilizing microscopic techniques.
Chemical analyses: examining the chemical composition of rocks to determine their origin and evolution.
Sub-disciplines of Petrology
A. Experimental Petrology
Focuses on laboratory synthesis of rocks.
It investigates the physical and chemical conditions under which rocks form.
B. Petrography
Concentrates on the systematic classification and precise description of rocks using microscopic examination techniques.
Types of Rocks
Igneous Rocks
Form from the solidification of magma or lava.
Characteristics: Hard, typically no layering.
Types:
Intrusive: Formed from slow cooling of magma under the surface (e.g., Granite).
Extrusive: Formed from rapid cooling of lava above ground (e.g., Basalt, Obsidian).
Sedimentary Rocks
Form from compaction and cementation of sediment.
Characteristics: Crumbly, typically layered.
Types:
Clastic: Composed of fragments of other rocks (e.g., Sandstone).
Chemical: Formed from dissolved minerals (e.g., Limestone).
Organic: Composed of biogenic material (e.g., Coal, Chalk).
Metamorphic Rocks
Formed by the transformation of existing rocks (igneous or sedimentary) due to heat and pressure.
Characteristics: Can be foliated (layered) or non-foliated (uniform).
Examples:
Foliated: Slate, Schist, Gneiss.
Non-Foliated: Marble, Quartzite.
Branches of Petrology
Igneous Petrology
Studies the composition and texture of igneous rocks formed from molten rock or magma.
Sedimentary Petrology
Examines the composition and texture of sedimentary rocks formed from fragments or chemicals.
Metamorphic Petrology
Analyzes the changes in composition and texture of rocks under extreme conditions.
Importance of Petrology in Engineering
Provides foundational knowledge about rock properties essential for construction.
Supplies details on the strength, durability, color, and chemical composition of rocks suitable for various building purposes.
Influences material selection for construction based on physical properties.
Most Common Rocks Used in Construction
Basalt
Granite
Limestone
Sandstone
Slate
Laterite
Marble
Gneiss
Quartzite
Classification of Rocks
Rocks are classified based on:
Mineral Content: Variability in minerals present.
Composition: The chemical makeup of the rock.
Texture: The size and arrangement of the mineral grains.
Sizes: The dimensions of the rock fragments.
Colors: The visible shades and hues of the rocks.
Engineering Properties of Rocks
Common Engineering Considerations
Rocks with foliation should generally be avoided for construction due to weakness along layers.
Marble is preferred for building because of strength and aesthetic appeal.
Schists may pose landslide hazards, while massive gneisses are beneficial for creating large underground spaces for facilities.
Metamorphic Petrology
Metamorphic rocks form through
Chemical, mineralogical, and textural changes due to high temperature and pressure without melting.
The process of metamorphism involves:
Mineralogical adjustments.
Not involving weathering or melting.
Types of Metamorphism
Contact or Thermal Metamorphism
Occurs due to a rise in temperature in the host rock.
Regional Metamorphism
Results from tectonic movements during mountain-building events.
Burial Metamorphism
Happens at the bottom of thick sedimentary deposits.
Hydrothermal Metamorphism
Involves the alteration of rocks through hot, chemically-active fluids.
Classification of Metamorphic Rocks
A. Foliated Metamorphic Rocks
Have visible layers due to differential stress.
Examples: Slate, Phyllite, Schist, Gneiss.
B. Non-foliated Metamorphic Rocks
Lack layers due to equal pressure in all directions.
Examples: Marble, Quartzite, Hornfels, Novaculite.
Definitions and Uses of Common Rocks
Basalt
A fine-grained, dark-colored extrusive igneous rock.
Uses include: building blocks, road surfaces, countertops, and concrete aggregates.
Granite
A coarse-grained, light-colored intrusive igneous rock.
Widely used for countertops, flooring, and decorative applications.
Gabbro
A coarse-grained, dark-colored intrusive igneous rock.
Uses include countertops, road materials, and facing stones.
Pegmatite
An extremely coarse-grained intrusive igneous rock.
Industrial applications include glass and ceramics manufacturing.
Diorite
A coarse-grained intrusive rock consisting of feldspar and pyroxene.
Utilized for roads, drainage systems, and architectural features.
Syenite
Coarse-grained intrusive igneous rock.
Employed in construction and aggregate markets.
Dolerite
Texturally between gabbro and basalt.
Used in road mixed aggregates and masonry.
Sedimentary Petrology
Focuses on the composition and texture of sedimentary rocks that are formed from sediment particles.
Discusses processes such as erosion, deposition, compaction, and lithification leading to sedimentary rock formation.
Modes of transportation: glaciers, wind, water, and gravity are crucial in sediment deposition.
Chemical Sedimentary Rocks
Form through precipitation from water solutions.
Varieties of limestone include: Travertine, Chalk, Coquina, and Oolitic limestone.
Uses of Sedimentary Rocks in Engineering
Building Construction: Often used as structural materials.
Cement and Concrete Production: Integral for structural integrity in buildings.
Road and Pavement Construction: Sedimentary rocks play a crucial role.
Manufacturing: Used for brick and tile production and wallboard materials.
Summary Questions
What branch of Geology focuses on the study of rocks?
Name the three branches of Petrology.
What are the two sub-disciplines of Petrology?
Answers
Petrology
Igneous Petrology, Metamorphic Petrology, Sedimentary Petrology
Experimental Petrology, Petrography