Petrology 2

Types of Rocks

  • Three main types:

    1. Igneous

    2. Sedimentary

    3. Metamorphic

Igneous Rocks

  • Formation: Formed by the cooling & solidification of hot molten material called magma or lava.

  • Origin: Magma comes from deep within the Earth.

  • Etymology: The term ‘igneous’ is derived from the Latin word ‘ignis’ meaning ‘of fire.’

Sedimentary Rocks

  • Formation: Created from deposition of fragments (sediment) eroded from pre-existing rocks.

  • Components: Made of sand, pebbles, and mud.

  • Transport: Deposited through water, wind, or ice in environments like rivers, lakes, beaches, and oceans.

Metamorphic Rocks

  • Formation: Result from high pressure, temperature, or fluids acting upon previously formed rocks (igneous, sedimentary, or even earlier metamorphic rocks).

  • Process: Changes rock's mineralogy, structure, and texture without melting through processes such as burial and tectonic action.

Igneous Rock Examples

  • Modern Lava Flows: Notable examples include those from Hawaii.

  • Barren Island: Home to India's only active volcano.

  • Mount Pavagadh: Notable ignimbrite feature in Deccan Traps.

Intrusive & Extrusive Features

  • Intrusive: Formed from magma that cools slowly below the Earth's surface (e.g., granite, diorite).

    • Examples: Coarse-grained textures due to slow cooling.

  • Extrusive: Formed from lava that cools quickly on the Earth's surface (e.g., basalt, rhyolite).

    • Examples: Fine-grained textures due to rapid cooling.

Rock Cycle

  • Processes:

    • Weathering

    • Transportation

    • Deposition

    • Uplift and Exposure

    • Lithification

    • Metamorphism

  • Formations:

    • Igneous Rocks: From cooling magma (e.g., granite, basalt).

    • Sedimentary Rocks: From accumulated sediments (e.g., sandstone, limestone).

    • Metamorphic Rocks: From altered existing rocks (e.g., gneiss, marble).

Texture of Igneous Rocks

  • Determined by:

    • Size, shape, arrangement of mineral grains.

    • Types of Textures:

      • Aphanitic: Fine-grained due to rapid cooling; often has vesicles.

      • Phaneritic: Coarse-grained; identifiable crystals form during slower cooling.

      • Porphyritic: Large crystals in a fine matrix; indicates complex cooling history.

      • Glassy: Very rapid cooling leads to no crystal formation, forming obsidian.

      • Pyroclastic: Various fragments from explosive eruptions often resembling sedimentary rocks.

Igneous Compositions

  • Mafic Rocks (Basaltic):

    • Example: Basalt, aphanitic texture, contains primarily pyroxene and some olivine.

  • Intermediate Rocks:

    • Example: Diorite, intrusive, coarse-grained.

  • Felsic Rocks:

    • Example: Granite, phaneritic texture, contains quartz and K-feldspar.

Specific Rocks and Their Textures

  • Pegmatite: Very coarse-grained with crystals larger than 2 cm; usually granitic.

  • Basalt: Common extrusive rock, fine-grained.

  • Amygdaloidal Basalt: Contains bubbles, or amygdales filled with minerals like quartz and calcite.

  • Tourmaline Pegmatite: Contains large individual crystals, notable for rare minerals.

Study Points for Igneous Rocks

  1. Color

  2. Composition: felsic, mafic, or intermediate

  3. Grain size: fine, medium, coarse

  4. Mineral content

  5. Texture: aphanitic, porphyritic, glassy, vesicular, etc.

  6. Cooling history: extrusive vs. intrusive

  7. Rock naming conventions and classifications.