Metamorphic Rocks Lab review

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Sedimentary Rock Identification

Quartz Sandstone

  • Type: Clastic

  • Grain Size: Medium sand sized grains

  • Sorting: Well sorted

  • Depositional Environments: Continental, transitional, marine (anywhere sand is deposited)

Coquina

  • Type: Biochemical

  • Composition: Cemented shell fragments

  • Reactivity: Reactive to HCl (hydrochloric acid)

  • Depositional Environment: Transitional (e.g., beaches)

Conglomerate

  • Type: Clastic

  • Grain Size: Gravel (and other sizes)

  • Shape: Well rounded (indicating transported sediment)

  • Sorting: Poorly sorted

  • Depositional Environment: Continental

Arkose

  • Type: Clastic

  • Grain Size: Medium sand sized grains

  • Shape: Angular grains (indicating short transport)

  • Sorting: Poorly sorted (again indicating short transport)

  • Color: Commonly pink to light red

  • Depositional Environment: Continental

Greywacke

  • Type: Clastic

  • Composition: Sand sized quartz and rock fragments mixed with clay

  • Appearance: Salt and pepper look

  • Depositional Environment: Marine

Mudstone

  • Type: Clastic (though clasts aren’t visible)

  • Grain Size: Silt or clay sized grains

  • Types: Includes siltstone, shale, and claystone (specific names to look for)

  • Depositional Environments: Transitional, marine

Micrite

  • Type: Chemical/Biochemical (can form through either process)

  • Composition: Composed of calcium carbonate

  • Reactivity: HCl reactive

  • Appearance: Massive, microcrystalline lime mudstone

  • Fracture: Conchoidal fracture

  • Depositional Environment: Marine

Chert

  • Type: Chemical/Biochemical

  • Composition: Composed of SiO2 (silicon dioxide)

  • Characteristics: Hard, dense, cryptocrystalline, conchoidal fracture

  • Depositional Environment: Marine

Shale

  • Type: Clastic (a specific kind of mudstone)

  • Grain Size: Clay sized clasts

  • Texture: Smooth-feeling mixture of clay, laminated (layered)

  • Depositional Environments: Continental, transitional, marine

Lignite

  • Type: Biochemical

  • Composition: Composed of plant material

  • Characteristics: Low density, blocky, matte luster

  • Type: Low grade coal

  • Depositional Environment: Continental

Fossiliferous Limestone

  • Type: Biochemical

  • Composition: Composed of calcium carbonate

  • Reactivity: HCl reactive

  • Characteristics: Contains abundant fossils in a massive calcareous matrix

  • Depositional Environment: Marine

Rock Salt

  • Type: Chemical

  • Composition: Composed of halite

  • Characteristics: Crystalline with a salty taste

  • Depositional Environment: Continental

Lab 4: Metamorphic Rocks

Metamorphic Rocks

  • Definition: Rocks that have been changed by exposure to heat and pressure.

  • Parent Rock/Protolith: The preexisting rock that gets metamorphosed.

  • Types of Protoliths: Can be sedimentary, igneous, or metamorphic.

Effects of Metamorphosis on the Protolith

  • Key Changes:

    • Deformation

    • Rotation and/or recrystallization of pre-existing minerals

    • Crystallization of new index minerals

  • Chemical Mobilization: Built by chemical elements mobilized during the metamorphic process.

  • Deformative Processes:

    • Deformation

    • Recrystallization

    • Crystallization of index minerals

Important Note

  • No Melting: The protolith does not melt during metamorphism.

  • Transitions: Solid state transitions with no overall change in composition; still composed of the same molecules, just in different formations.

  • Metaphor: "It got squished but didn’t melt!" Real-life metamorphism is more complex than this point.

Metamorphic Grades

  • Definition: Changes happen at varying levels of intensity (grades).

  • Manifestation of Changes: Changes to the protolith become more apparent with increasing grade.

  • Influencing Factors: Composition of the protolith affects how rocks change during metamorphism.

  • Grade Classifications:

    • Low grade: Minor changes often limited to mechanical flattening or low-level recrystallization.

    • High grade: Major changes; the metamorphic rock may not resemble its parent sedimentary rock.

Major Types of Metamorphism

Contact Metamorphism

  • Definition: Develops in rocks in proximity to an intrusive body of magma (subsurface).

  • Trigger: Change in temperature.

  • Effects: Favors recrystallization and produces non-foliated (non-layered) metamorphic rocks.

Regional Metamorphism

  • Definition: Develops in rocks experiencing shear forces (differential pressure) due to mountain building or tectonic plate movement.

  • Effects: Shear forces can induce reorientation and recrystallization of their minerals, leading to foliated metamorphic rocks.

Index Minerals

  • Definition: New minerals that crystallize under specific metamorphic conditions of pressure and temperature.

  • Examples:

    • Garnet

    • Chlorite

    • Talc

    • Kyanite

  • Application: We can determine the grade of contact metamorphism by which index minerals are present.

  • Quartz and Calcite: Present at all levels of metamorphism and are NOT index minerals.

Metamorphic Textures

  • Divisions: Two broad groups
    a. Foliated
    b. Non-foliated

Foliated Metamorphic Rocks

  • Description: Characterized by the layering of minerals.

  • Types of Foliated Textures:

    • Slatey Cleavage:

    • Characteristics: Parallel alignment of microscopic clay minerals, oriented perpendicular to the greatest pressure; rocks cleave along parallel surfaces; typical of low-grade regional metamorphism; very flat surfaces.

    • Schistosity:

    • Characteristics: Subparallel platy minerals recrystallized to visible sizes; micas and amphiboles are common; appearance is somewhat shiny; indicative of intermediate grade.

    • Gneissic Banding:

    • Characteristics: Ion migration by solid mass transfer leading to irregular banding in minerals; typical of high-grade regional metamorphism; if temperature or pressure increases further, melting will begin, creating an IGNEOUS rock; features discrete layering.

The Metamorphic Continuum

  • Visual Representation: A continuum showing increasing heat and pressure, leading to increasing metamorphism.

  • Progression:

    • No metamorphism: Sedimentary rock

    • Low-grade metamorphism: Shale → Slate

    • Medium-grade metamorphism: Phyllite

    • High-grade metamorphism: Schist → Gneiss

Non-Foliated Texture

  • Characteristics: High degree of crystallinity; uniform masses of recrystallized minerals of one mineral species; crystals tend to be uniform in size without specific orientation.

  • Typical of: Contact metamorphism, e.g., marble.

Instructions for Lab Exercise

  • Task: Identify if the rock is foliated or non-foliated.

  • Observation: Review the sample from various angles.

  • For Foliated Rocks: What is the texture? Use the description of texture and composition to identify the rock.

  • Identify: The protolith, the metamorphic grade (in the “possible metamorphic environment” column), and the metamorphic environment.

Homework Assignments

  • Activities: Complete questions 2a-c.

  • Submission: Submit pages 4-6 and 4-7 on Blackboard as a single PDF document.

  • Clarification: The review lab is for personal benefit; nothing is due from that lab.

  • Reminder: Midterm exam next week – bring a laptop for the exam.

  • Study Recommendations: Review posted PowerPoints, quizzes, homework questions, and chapter introductions covered.