GROUP-6-PETROLOGY.

Introduction to Petrology

• Petrology: The scientific study of rocks.

Types of Rocks

• Three Main Types:

  • Sedimentary: Formed by the accumulation of sediments.

  • Igneous: Result from the solidification of molten rock (magma).

  • Metamorphic: Pre-existing rocks that have been altered through heat and pressure.

The Rock Cycle

• Processes: A web of processes that describes how each of the three major rock types (igneous, sedimentary, metamorphic) forms and breaks down.

• Key Processes in Rock Cycle:

  • Weathering: Breakdown of rocks at the Earth’s surface.

  • Erosion: The movement of sediments.

  • Melting: Transition of solid rock to molten rock (magma).

  • Cooling: Solidification of magma into igneous rock.

  • Compaction: Process where sediments are squeezed together.

  • Cementation: Minerals precipitate from water and bond sediment particles together.

Igneous Rocks

• Definition: Rocks formed from the cooling and solidification of magma, derived from the Latin word ignis meaning "fire rock".

• Types of Igneous Rocks:

  • Intrusive Igneous Rocks:

    • Formed from magma that solidifies beneath the Earth's surface.

    • Often exhibit large visible crystals due to slow cooling (e.g., granite, diorite).

    • Also known as Plutonic rocks.

  • Extrusive Igneous Rocks:

    • Formed when magma reaches the surface during volcanic eruptions.

    • Typically have small crystals due to rapid cooling (e.g., basalt, obsidian).

Mineral Composition of Igneous Rocks

• Felsic Minerals: Characterized by high levels of feldspar and silica.

• Mafic Minerals: Composed mainly of magnesium and ferrous iron.

Common Types of Igneous Rocks

• Examples:

  • Basalt

  • Granite

  • Gabbro

  • Pumice

Sedimentary Rocks

• Also known as "derived rocks" created from the accumulation of sediments.

Sediments

• Definition: Fragments of rock broken down by weathering.

• Classification (from smallest to largest):

  • Clay

  • Silt

  • Sand

  • Pebble

  • Cobble

  • Boulder

Sediment Transport

• Refers to the movement of solid particles by agents such as wind, water, or ice.

Sediment Deposition

• Process of laying down sediments that are carried by wind, water, or ice.

Lithification

• Process transforming loose sediment into solid rock.

• Key Processes:

  • Compaction: Sediments are pressed together under pressure.

  • Cementation: Minerals fill the spaces between sediment grains to bond them.

Classification of Sedimentary Rocks

• Clastic Sedimentary Rocks: Composed of particles from other rocks.

• Biogenic Sedimentary Rocks: Formed from biological material.

• Chemically Precipitated Sedimentary Rocks: Formed when minerals precipitate from solution.

Metamorphic Rocks

• Formed from existing rocks (igneous, sedimentary, or earlier metamorphic) altered by heat, pressure, or mineral-rich fluids.

• This transformation occurs through a process called metamorphism.

Types of Metamorphism

• Contact Metamorphism: Occurs when rocks are heated by nearby magma.

• Regional Metamorphism: Changes that happen over large areas under high pressure and temperature.

• Dynamic Metamorphism: Associated with high shear stress, often near fault zones.

Agents of Metamorphism

• Heat: Causes chemical reactions and mineral changes.

• Pressure: Can be directional or uniform, leading to changes in rock texture.

• Chemically Active Solutions: Fluids can facilitate metamorphic reactions.

Foliated vs. Non-Foliated Metamorphic Rocks

• Foliated Rocks: Have a layered or banded appearance (e.g., slate, schist, gneiss).

  • Characterized by aligned minerals/planes of weakness within the rock.

• Non-Foliated Rocks: Have a more uniform texture; not layered (e.g., marble, hornfels).

  • Typically contain crystals of similar sizes throughout.

Examples of Metamorphic Rocks

• Slate: Fine-grained, foliated, derived from shale.

• Schist: Medium to coarse-grained, with visible minerals.

• Marble: Metamorphosed limestone.

• Gneiss: Alternating bands of light and dark minerals.