8-GO Metamorphic Rx

Types of Rocks

  • Igneous Rocks

    • Form by crystallization from magma (molten rock)

  • Sedimentary Rocks

    • Composed of materials eroded or dissolved from older rocks, minerals, and biological debris.

    • Provide records of Earth's history, evolution, and environments.

  • Metamorphic Rocks

    • Formed from preexisting rocks or sediments that undergo alteration due to environmental changes (temperature, pressure, stress) without melting.

Rock Formation Processes

  • Formation Factors

    • Minerals, texture, and fabric indicate how a rock formed.

    • Key environmental variables include:

      • Increased heat

      • Pressure

      • Differential stress

      • Hot water

      • Structural and tectonic history

Relationship Between Rock Types

  • Rocks are products of change and records of geological transformations over time.

  • Metamorphic rocks result from igneous, sedimentary, or other metamorphic rocks transformed under changing environmental conditions:

    • Equilibrium achieved with new temperature and pressure conditions.

    • Influenced by environmental variables like temperature, pressure, differential stress, and time.

Protolith

  • Definition: The original (parent) rock that metamorphoses into a new type.

    • Types include igneous, sedimentary, and metamorphic rocks.

    • In higher intensity metamorphism, the original protolith may become difficult to identify.

Environmental Variables in Metamorphism

  • The depth of Earth affects temperature and pressure, which increases with depth.

    • Rates of increase vary based on whether it's at:

      • Plate interiors vs. plate boundaries

      • Continental vs. oceanic lithosphere

      • Impact of rising magma or sinking lithosphere

Metamorphism in Various Settings

  • Seafloor Metamorphism

    • Involves interactions with cold seawater under varying pressure and temperature conditions.

    • Demonstrates varying types of metamorphism including high P-low T settings.

Rock Properties and Changes

  • Recrystallization: Changes in crystal size and shape due to temperature and pressure influences.

    • Example: Small calcite crystals in limestone may transform into large crystals in marble.

    • Notably, recrystallization may also occur through pressure application.

Types of Metamorphic Rocks

  • Examples:

    • Low-grade: Temperature above ~200°C

    • High-grade: Temperature approaching the melting point of the rock

  • Changes induced during metamorphism can affect:

    • Composition

    • Size and shape of mineral grains

    • Texture, density, and color of rocks

Environmental Influence on Metamorphism

  • Regional Metamorphism: Occurs over large areas due to heat and pressure at convergent boundaries, typically producing foliated rocks.

  • Contact Metamorphism: Involves localized thermal effects from nearby magma.

    • Non-foliated rocks like hornfels derived from shale, sandstone to quartzite, and limestone to marble.

Non-Foliated and Foliated Textures

  • Non-Foliated:

    • No visible bands; typically formed through contact metamorphism.

    • Characteristics include random mineral orientations and a massive appearance.

  • Foliated:

    • Layers or bands formed due to directional pressure.

    • Heat and pressure cause realignment and recrystallization of minerals.

  • Grades of Foliation:

    • Slate (fine-grained)

    • Schist (medium-grained)

    • Gneiss (coarse-grained)

Summary of Metamorphic Rock Properties

  • The extent of metamorphism correlates with the degree of heat and pressure applied, influencing mineral grains' realignment.

  • Metamorphic rocks like slate, phyllite, schist, and gneiss exhibit varying textures and compositions based on their parent rocks and metamorphic conditions.

Characteristics of Common Metamorphic Rocks

  • Slate

    • Fine-grained, excellent rock cleavage from shale.

  • Phyllite

    • Slightly coarser than slate with satin sheen.

  • Schist

    • Contains shiny micas and exhibits schistosity due to aligned minerals.

  • Gneiss

    • Exhibits banding due to segregation of light and dark minerals, indicating metamorphic processes.