Study Notes on Metamorphic Rocks and Rock Deformation

Unit 8 – Metamorphic Rocks

Overview

• This unit covers two short topics: Metamorphic rocks and Rock deformation.

Metamorphic Rocks

Definition

• Metamorphic Rocks: Rocks that have been altered in composition or texture by heat or pressure.
  • Different minerals and textures form at varying temperatures and pressures.

Classification by Grade

• High-Grade Rocks: Formed at high temperatures and pressures.
• Low-Grade Rocks: Formed at lower temperatures and pressures.
  • All metamorphic rocks are formed under conditions of high temperature or pressure.
  • Low-grade metamorphism occurs at the lower end of the temperature and pressure range; high-grade metamorphism occurs at the upper end.
  • Temperatures above a certain threshold lead to melting of the rock.

Types of Metamorphism

• Different types of metamorphism are categorized based on the methods of generating necessary pressures and temperatures.

1. Regional Metamorphism

• Occurs over large geographic areas, typically associated with convergent plate boundaries.
  • Examples:
  • Subduction zones: High temperatures from friction.
  • Continental collisions: High pressures from colliding plates.

2. Contact Metamorphism

• High-temperature metamorphism caused by magma heating adjacent rock without melting it.
  • Forms a halo of altered rock around the igneous intrusion known as an aureole.
  • Rock close to the intrusion is typically high-grade metamorphic rock; farther away results in lower-grade rocks.

3. Cataclastic Metamorphism

• Involves the crushing and smearing of rock along fault zones.
  • Commonly found in subduction zones and transform fault boundaries.

4. Hydrothermal Metamorphism

• Resulting from interaction with hot water, often occurring at mid-ocean ridges (e.g., Mid-Atlantic Ridge, East Pacific Rise).
  • Heated water alters the rock and can dissolve some minerals.
  • Hot water is less dense and rises, causing precipitation of minerals upon contact with cooler water, creating black smokers.

5. Burial Metamorphism

• Caused by the heat and pressure from deep burial of rock in sedimentary basins.
  • Sedimentary basins collect eroded sediments from nearby mountains, leading to increased crustal depth and temperatures (approximately 30ºC increase per kilometer of depth).
  • Depth-related effects: e.g., the deepest mines (~3 km deep) require cooling to prevent overheating miners.

6. Shock Metamorphism

• Caused by large meteorite impacts which create intense heat and pressure instantaneously.
  • Example: Meteor Crater, Arizona.

Pressure and Temperature Summary Graph

• High-temperature, low-pressure: typical of contact and hydrothermal metamorphism.
• Low-temperature, high-pressure: typical of cataclastic and regional metamorphism.
• Intermediate pressures and temperatures: characteristic of burial metamorphism.
• Localized high temperature and pressure: indicative of impact metamorphism.

Alteration Processes

1. Mineral Alteration: Involves structural change or element substitution.

   • Structural Alteration: Atoms remain the same but are rearranged; e.g., graphite converting to diamond (both are pure carbon, yet the arrangement of atoms differs).
   • Element Substitution: One atom is swapped for another, such as iron being replaced by magnesium, yielding a different mineral.

2. Textural Alteration: Includes recrystallization or stretching.

   • Under high temperature and pressure, atoms migrate to create larger crystals; crystals may rotate due to differential pressure.
   • Random alignment occurs under uniform pressure; alignment occurs perpendicular to greater force under differential pressure.

Effects of Alteration

• Foliation: Flat or wavy lines resulting from oriented mineral grains.
  • Serves as the first characteristic for classifying metamorphic rocks.

Classification of Metamorphic Rocks

• Based on mineral composition and texture (foliation).

• Foliated Rocks: Have oriented mineral grains; often exhibit cleavage, which refers to a tendency to break along parallel planes.

  • Example rock progression:
  • Slate: Low-grade rock with tiny oriented micro-crystals and good cleavage.
  • Phyllite: Higher-grade with glossy sheen and oriented crystals.
  • Schist: High-grade rock with visible, oriented crystals grouped into bands.
  • Gneiss: Highest grade with large, conspicuous banding before melting.

• The process of metamorphism can lead from non-metamorphic rocks like shale to higher-grade metamorphic rocks as temperature and pressure increase.

Rock Coloration Details

1. Slate: Notable for strong cleavage, used in paving stones.
2. Phyllite: Shows glossy sheen with emerging visibility of tiny crystals.
3. Schist: Exhibits visible, oriented crystals organized in bands.
4. Gneiss: Distinguishable by large crystals that exhibit banding.

Non-Foliated Metamorphic Rocks

• Lack clearly oriented minerals; often formed by burial, contact, or hydrothermal metamorphism.
  • Examples:
  • Quartzite: Resulting from metamorphosed quartz-rich sandstone.
  • Marble: Metamorphosed limestone, varying in color and pattern.
  • Greenstone: Typically altered basalt through hydrothermal alteration, colored by chlorite.

Unique Textures in Metamorphism

1. Porphyroblasts: Large crystals embedded within a fine-grained matrix; e.g., garnet in garnet schist.
2. Deformational Texture: Maintaining recognizability despite smearing or stretching; shown by stretched pebble conglomerates.

Rock Deformation

Introduction

• Rock deformation techniques contextualize dating methods, leading to better understanding of rock history.

Key Concepts

• Folding: Bending without breaking (plastic deformation).
• Faulting: Breaking and displacement on one side of the rupture; a break without movement is termed a joint.

Factors Influencing Folding and Faulting

• Combination of rock properties and physical conditions determine deformation type.
  • Brittle Minerals: Tend to break (e.g., chalk).
  • Ductile Minerals: Tend to bend (e.g., copper sheets).

Impacts of Pressure

1. Confining Pressure: High uniform pressure encourages folding; examples include ice at the base of a glacier able to bend due to high pressure versus shattering under low pressure.
2. Temperature: Higher temperatures enable softening and facilitate folding under confining pressure.
3. Strain Rate: Rapid movement causes faulting while slow movement often leads to folding.

Types of Stress

• Identified stress types yield different features in rock deformation:
  1. Compression Stress: Inward forces producing shortening, often causing folds at convergent boundaries.
  2. Tension Stress: Pulling forces resulting in stretching at divergent boundaries.
  3. Shear Stress: Opposite, parallel forces causing lateral movement, commonly at transform fault boundaries.

Folding Features

• Limbs: Layers extending upward or downward from a fold’s base.
• Syncline: Bottom of a fold, limbs point upward.
• Anticline: Top of a fold, limbs pointed downward.

Erosion Effects on Fold Structures

• Once eroded, older layers are revealed in anticlines, while synclines expose younger layers.
• Visual aids include cross-sections showing relative age and surface maps of geological formations.

Structures Beyond Simple Folds

• Domes and Basins: Formed under compressional forces;
  • In a dome, limbs point down, oldest layers in the center.
  • In a basin, limbs point up, youngest layers in the center.

Faulting Observations

Definitions

• Hanging Wall: Wall that can suspend a lantern, found above a fault.
• Footwall: Wall on which one could stand, found below a fault.

Fault Types

• Reverse Faults: Form under compression; hanging wall moves upward.
• Normal Faults: Form under tension; hanging wall moves downward.
• Strike-Slip Faults: Form under shear stress, lateral movement.

Geological Tools

• Faults can reveal the tectonic history through the types of movements (stretched, compressed, lateral). Hazard mapping for earthquake preparedness and oil drilling strategies are also influenced by fault knowledge.