5

Rock Definition

  • Rock: An agglomeration of minerals.

Learning Goals

  • Define rocks.

  • Describe the rock cycle.

  • Explain how igneous rocks fit into the rock cycle.

  • Distinguish between extrusive and intrusive rocks (coarse vs. fine-grained).

  • Identify felsic (granitic) vs. mafic (basaltic) rocks based on mineral composition.

  • Describe Bowen’s Reaction Series to understand igneous rock mineral composition.

Types of Rocks

  1. Igneous Rocks

    • Origin: Derived from the Latin word "igneous" meaning fire.

    • Formation: Crystallize from melted material (magma/lava).

    • Types:

      • Extrusive Rocks (Volcanic): Formed on Earth's surface; cool quickly, resulting in small crystals (e.g., basalt, obsidian).

      • Intrusive Rocks (Plutonic): Formed underground; cool slowly, leading to large crystals (e.g., granite).

  2. Sedimentary Rocks

    • Origin: From the Latin "sedimentum" which means to settle.

    • Formation: Deposition of weathered materials (broken or dissolved pieces).

    • Key processes include: weathering, erosion, and deposition.

  3. Metamorphic Rocks

    • Origin: From the Greek words "meta" (change) and "morph" (shape).

    • Formation: Transformed by high pressure, temperature, fluids, and time (essentially 'cooked').

The Rock Cycle

  • Elements of the rock cycle include:

    • Uplift, weathering, erosion, transportation, melting, cooling, burial, compaction, and cementation.

    • Pathways through the rock cycle can include:

    • Sedimentation > Igneous > Sedimentary

    • Metamorphic > Sedimentary > Igneous

Magma vs. Lava

  • Magma: Molten rock below the Earth's surface.

  • Lava: Magma that reaches the Earth’s surface.

Bowen's Reaction Series

  • Describes the sequence of mineral crystallization from cooling magma:

    • Temperature Dependence: Specific minerals crystallize at specific temperatures, from simple silicates to more complex forms.

    • Left Side: Various silicates from simple to complex structures.

    • Right Side: Variety of feldspar minerals from calcium-rich to sodium-rich.

Melting Processes

  1. Increase Temperature: Hotter materials tend to melt.

  2. Decrease Pressure: Known as decompression melting; lowering the pressure can melt rocks without raising the temperature.

  3. Add Water/Volatiles: Introducing water or other volatiles reduces the melting temperature of rocks.

Locations of Magma Formation

  1. Decompression Melting: Occurs at divergent boundaries and hot spots.

  2. Heat Transfer: Occurs at convergent boundaries where one plate subducts.

  3. Water Induction: Common in subduction zones where water is brought down with the descending plate.

Intrusive Igneous Bodies

  1. Plutons: Large masses of igneous rock that cool within the crust.

  2. Batholiths: Large (>100 km²) igneous bodies formed by multiple plutons.

  3. Dykes: Intrusive bodies that cut through surrounding rock layers.

  4. Sills: Intrusive bodies that follow the horizontal layers of sedimentary rock.

  5. Laccoliths: Dome-shaped intrusions that form when magma is injected between layers.