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.

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