Metamorphic Rocks Practice

Metamorphic Rocks

Metamorphism Definition

  • Metamorphism means to "change form".

  • Key factors include:

    • Elevated temperatures

    • Elevated pressures

  • Results in changes in:

    • Mineralogy

    • Sometimes chemical composition

  • Every metamorphic rock has a parent rock (the original rock from which it formed).

  • Parent rocks can originate from:

    • Igneous rocks

    • Sedimentary rocks

    • Other metamorphic rocks

Examples of Parent Rocks and Their Metamorphic Equivalents

  • Parent Rock: Shale

    • Characteristics:

    • Loosely packed clay minerals

    • Metamorphic Rock: Slate

    • Characteristics after metamorphism:

      • Tightly packed chlorite and mica minerals

      • Formed under low-grade metamorphism at low temperatures and pressures

  • Parent Rock: Granodiorite

    • Characteristics:

    • Randomly oriented minerals

    • Metamorphic Rock: Folded Gneiss

    • Characteristics:

      • Formed under high-grade metamorphism with strong compressional forces, high temperatures, and pressures

      • Displays deformed layers of segregated minerals

Agents of Metamorphism

  • There are four main agents of metamorphism:

    1. Heat

    2. Pressure

    3. Differential stress

    4. Chemically active fluids

1. Heat

  • Importance: It is the most significant agent of metamorphism.

  • Provides energy essential for chemical reactions.

  • Recrystallization: Process of forming new, stable minerals larger than the original.

  • Sources of Heat:

    • Geothermal gradient: Increase in temperature with depth, approximated at 25°C per kilometer.

    • Magma plumes or bodies of magma can also contribute heat.

  • Various metamorphic environments include:

    • Subducting sediments: Metamorphosed due to increased pressure and temperature.

    • Shallow crustal rocks: Metamorphosed by heat from nearby magma bodies.

    • Rocks in large sedimentary basins: Encounter low-grade metamorphic conditions at the bottom of sediment piles.

  • Temperature ranges:

    • Low temperature (~300°C) leads to low-grade metamorphism.

    • High temperature (~1200°C) corresponds to idealized geothermal gradients.

  • Note on geothermal gradients:

    • Low geothermal gradients in subduction zones observed due to the cold oceanic crust descending.

    • High geothermal gradients found where rising magma delivers heat to the upper crust.

2. Pressure

  • Confining Pressure:

    • Forces applied equally in all directions which cause rocks to become compacted.

  • Compressional Stress:

    • Rocks are shortened in one direction and elongated in another direction.

    • In high-pressure and-temperature environments, rocks behave in a ductile manner (flow-like) and undergo deformation such as stretching, flattening, or folding.

3. Differential Stress

  • Differential Stress:

    • Forces are unequal in different directions.

    • Greatest stress experienced along one direction, leading to potential deformation of rock materials.

4. Chemically Active Fluids

  • Water under pressure can remain liquid at temperatures greater than 100°C.

  • Dissolution of Minerals: Minerals can dissolve in water, and the movement of groundwater can mobilize dissolved minerals.

  • In certain environments, fluids can transport mineral matter over considerable distances, affecting metamorphic processes.

Hydrothermal Processes

  • Cold seawater can percolate into hot newly formed crust.

  • Black smoker: Hydrothermal vents that emit hot, mineral-rich water.

    • Occurs at mid-ocean ridges, where hot water rises to the seafloor.

  • Hydrothermal Metamorphism: Occurs at shallow crustal depths where active geothermal features such as geysers and hot springs exist.

    • Examples include hydrothermal vein deposits and pegmatite deposits around igneous bodies or plutons.

Metamorphic Texture

  • Foliation:

    • Metamorphic rocks may display a preferred orientation of minerals, with mineral grains aligned parallel to each other.

  • After metamorphism due to differential stress, minerals may become aligned, contrasting with their indistinct orientations prior to metamorphism, under confining pressure.

Foliation Process

  • Foliation can develop through the following processes:

    • Rotation of minerals

    • Recrystallization of minerals

    • Flattening of spherical grains which may elongate perpendicular to the direction of maximum stress.

Types of Foliated Textures

  1. Rock (Slaty) Cleavage:

    • Rocks split into thin slabs.

  2. Schistosity:

    • Minerals (typically micas) are visible to the naked eye and layered.

  3. Gneissic Texture (Banding):

    • During high-grade metamorphism, minerals separate into alternating light and dark bands.

Nonfoliated Metamorphic Rocks

  • Metamorphic rocks that do not exhibit a layered appearance are classified as nonfoliated.

  • Typically form in environments with minimal compressional stress, leading to the development of equidimensional crystals rather than flat or tabular-shaped crystals.

  • Examples of nonfoliated rocks include:

    • Marble: Formed from limestone or dolostone; primarily consists of calcite.

    • Quartzite: Formed from quartz sandstone with its quartz grains fused together.

Comparison of Foliated vs. Nonfoliated Rocks

  • Foliated Rocks:

    • Typically associated with regional metamorphism, producing widespread metamorphic rock formations prevalent in mountain-building scenarios and continental collisions.

  • Nonfoliated Rocks:

    • Primarily result from contact metamorphism that occurs in low-pressure environments.

Metamorphic Rock Classification

Common Foliated Rocks

  • Slate:

    • Very fine-grained, resembling shale.

    • Commonly created from low-grade metamorphism of shale, mudstone, or siltstone.

  • Phyllite:

    • Platy minerals that are larger than those in slate but not large enough to be seen without aid.

    • Characterized by a glossy sheen and wavy surfaces.

  • Schist:

    • Medium- to coarse-grained, visible to the unaided eye.

    • Derived from shale under medium- to high-grade metamorphism; dominated by platy minerals, mainly micas.

  • Gneiss:

    • Medium- to coarse-grained with a banded appearance resulting from high-grade metamorphism.

    • Comprised of light-colored, feldspar-rich layers interspersed with dark ferromagnesian minerals.

Common Nonfoliated Rocks

  • Marble:

    • A crystalline compound originating from limestone or dolostone.

    • Mainly composed of calcite and frequently used in decorative applications and monuments.

  • Quartzite:

    • Formed from quartz sandstone.

    • Composed of interlocking quartz grains solidified together.

Visualization of Metamorphic Processes and Environments

  • Diagrammatic Representations: Include structural context of host rocks undergoing metamorphism in relation to igneous bodies and magmatic activities.

  • Regions of low-grade and high-grade metamorphism can be illustrated, alongside interpretations of underground processes like partial melting and rock deformation.

Summary of Metamorphic Rock Series

  • Low-Grade to High-Grade Metamorphism: Rocks are arranged indicating progression: Slate → Phyllite → Schist → Gneiss.

  • The illustrated metamorphic chain highlights the transformation through varying conditions of temperature and pressure affecting mineral structures and textures throughout geological timeframes.

Key Vocabulary and Processes in Metamorphism

Vocabulary

  • Metamorphism: The process of change in the form of rocks due to heat and pressure.

  • Parent rock: The original rock from which a metamorphic rock forms.

  • Foliation: The alignment of minerals within a rock due to pressure.

  • Nonfoliated: A term for metamorphic rocks that do not have a layered appearance.

  • Hydrothermal metamorphism: Changes in rocks due to hot, mineral-rich water.

  • Geothermal gradient: The rate at which temperature increases with depth in the Earth.

  • Slaty cleavage: A type of foliation where rocks can split into thin slabs.

  • Schistosity: A texture in metamorphic rocks characterized by visible platy minerals.

  • Gneissic texture: A banding pattern in metamorphic rocks, resulting from high-grade metamorphism.

Processes Involved in Metamorphism

  1. Heat: Essential for chemical reactions, leading to recrystallization of minerals. Sources include:

    • Geothermal gradient (approximately 25°C per kilometer).

    • Magma bodies contributing heat to surrounding rocks.

  2. Pressure: Influences the physical characteristics of rocks through:

    • Confining pressure (equal forces in all directions).

    • Compressional stress causing rock deformation.

  3. Differential Stress: Unequal forces in different directions lead to deformation. This stress is experienced along specific directions, resulting in various textures.

  4. Chemically Active Fluids: Fluids can dissolve minerals and transport them, significantly influencing metamorphic processes. Water, even above 100°C, can remain liquid under pressure, aiding in these transformations.

  5. Foliation Development: Foliation can arise through:

    • Rotation of minerals.

    • Recrystallization of minerals under stress.

    • Flattening of minerals influenced by maximum stress directions.

  6. Hydrothermal Processes: Cold seawater interaction with hot crust leads to:

    • Formation of deposits from mineral-rich fluids emitted by hydrothermal vents (black smokers).

    • Creation of hydrothermal metamorphic environments, such as around igneous bodies and geysers.

This summary of vocabulary and processes related to metamorphic rocks can assist in preparation for practice tests on this subject.