Petrology

Petrology is the study of rocks and their formation processes. Rocks provide a tangible record of Earth's history and the processes that have shaped our planet. Understanding the three major rock types and their interrelationships is fundamental to geology.

Igneous Petrology

Formation of Igneous Rocks

Igneous rocks form from the crystallization of molten rock material (magma). The cooling rate and composition determine the final rock characteristics:

Magma Generation

Primary magmas form through three main processes:

1. Decompression melting: Reduction in pressure causes melting (e.g., mid-ocean ridges) $T_{solidus} = f(P, H_2O)$

2. Flux melting: Addition of volatiles (especially water) lowers the melting point (e.g., subduction zones)

3. Heat addition: Addition of heat to rock causes melting (e.g., mantle plumes)

Classification of Igneous Rocks

Chemical Classification (TAS Diagram)

Based on SiO₂ and (Na₂O + K₂O) content:

• Ultramafic: < 45% SiO₂ (e.g., peridotite, komatiite)
• Mafic: 45-52% SiO₂ (e.g., basalt, gabbro)
• Intermediate: 52-63% SiO₂ (e.g., andesite, diorite)
• Felsic: > 63% SiO₂ (e.g., rhyolite, granite)

Textural Classification

Rate of cooling controls crystallization and texture:

• Phaneritic: Coarse-grained, crystals visible to naked eye (plutonic rocks)
• Aphanitic: Fine-grained, crystals not visible (volcanic rocks)
• Porphyritic: Large crystals in fine-grained matrix (two-stage cooling)
• Glassy: No crystals, rapid cooling (obsidian)
• Vesicular: Containing gas bubbles (scoria, pumice)

Bowen's Reaction Series

Norman Bowen determined the order of mineral crystallization from cooling magma:

Discontinuous Series

Olivine → Pyroxene → Amphibole → Biotite

• Each mineral changes composition, disappears at lower temperatures

Continuous Series

Plagioclase feldspar from Ca-rich (anorthite) to Na-rich (albite)

• Same mineral structure, changing composition

Igneous Structures

Intrusive Bodies

• Dike: Vertical, cuts across country rock
• Sill: Horizontal, parallel to layering
• Batholith: Massive intrusive body > 100 km²
• Stock: Smaller intrusive body < 100 km²

Extrusive Features

• Lava flow: Surface extrusion of fluid magma
• Pyroclastic deposits: Explosive volcanic ejecta
• Volcanic neck: Solidified conduit of eroded volcano

Sedimentary Petrology

Formation of Sedimentary Rocks

Sedimentary rocks form through the processes of weathering, erosion, transportation, deposition, and lithification:

Weathering Processes

Physical Weathering

• Frost wedging: Expansion of freezing water
• Root wedging: Plant roots expanding cracks
• Thermal expansion: Daily heating/cooling cycles
• Unloading: Removal of overburden causes expansion

Chemical Weathering

• Hydrolysis: Reaction with water $KAlSi_3O_8 + H^+ + H_2O \rightarrow K^+ + Al_2Si_2O_5(OH)_4 + H_4SiO_4$ (K-feldspar → kaolinite clay + dissolved silica)

• Oxidation: Reaction with oxygen $4FeO + O_2 \rightarrow 2Fe_2O_3$

• Carbonation: Reaction with carbonic acid $CaCO_3 + H_2CO_3 \rightarrow Ca^{2+} + 2HCO_3^-$

Classification of Sedimentary Rocks

Detrital/Clastic Rocks

Based on grain size:

• Conglomerate: >2mm grains (rounded)
• Breccia: >2mm grains (angular)
• Sandstone: 1/16 to 2mm grains
• Siltstone: 1/256 to 1/16 mm grains
• Shale/Mudstone: <1/256 mm grains

Chemical/Biochemical Rocks

• Limestone: CaCO₃, often with fossils
• Dolomite: CaMg(CO₃)₂
• Chert: SiO₂
• Rock salt: Halite (NaCl)
• Rock gypsum: CaSO₄·2H₂O

Sedimentary Structures

Depositional Structures

• Cross-bedding: Formed by moving water/sand dunes
• Graded bedding: Grains decrease in size upward
• Ripple marks: Formed by water/wind action
• Mud cracks: Drying of clay-rich sediments

Post-depositional Structures

• Fossils: Preserved remains of ancient life
• Concretions: Localized cementation
• Sedimentary dikes: Soft-sediment injection

Metamorphic Petrology

Metamorphic Processes

Metamorphism involves changes in mineral assemblage and texture due to:

• Temperature: Increases reaction rates
• Pressure: Affects mineral stability
• Fluids: Facilitate metasomatism
• Time: Determines degree of reactions

Metamorphic Grades

Low Grade (200-350°C)

• Slate: Fine-grained, cleavage develops
• Zeolite facies: Contains zeolite minerals

Medium Grade (350-550°C)

• Schist: Coarse-grained with schistosity
• Greenschist/Amphibolite facies: Contains metamorphic amphiboles

High Grade (550-700°C+)

• Gneiss: Banded texture with light and dark layers
• Granulite facies: No hydrous minerals

Metamorphic Textures

Planar Textures

• Slaty cleavage: Microscopic mica alignment
• Schistosity: Coarse mineral alignment
• Gneissic banding: Composition layering

Linear Textures

• Lineation: Linear mineral alignment
• Stretching lineation: Elongated mineral grains

Metamorphic Environments

Contact Metamorphism

• Aureoles: Zones around igneous intrusions
• Baked margins: High temperature, low pressure
• Hornfels: Contact metamorphic rock

Regional Metamorphism

• Orogenic belts: Mountain building processes
• Barrovian sequence: Prograde series in pelitic rocks
• High-pressure/low-temperature: Blueschist facies in subduction zones

Dynamic Metamorphism

• Fault zones: Cataclasis and frictional heating
• Mylonites: Ductilely deformed rocks

Metamorphic Reactions

Dehydration Reactions

$3Al_2Si_2O_5(OH)_4 \rightarrow Al_2O_3 \cdot 2SiO_2 + 3H_2O$ (Pseudomorph → corundum + quartz + fluid)

Fluid-Mediated Reactions

$CaAl_2Si_2O_8 + 2Mg_2SiO_4 \rightarrow Ca_2Mg_3Si_3O_{12} + Al_2SiO_5$ (Anorthite + forsterite → grossular + kyanite)

The Rock Cycle

Interconnections

The rock cycle illustrates the continuous transformation between rock types:

Time Scales

• Igneous processes: Years to millions of years
• Sedimentary processes: Years to millions of years
• Metamorphic processes: Thousands to millions of years

Plate Tectonic Controls

• Divergent boundaries: Mainly igneous rock formation
• Convergent boundaries: All rock types possible
• Transform boundaries: Predominantly metamorphic
• Continental interiors: Mainly sedimentary

Real-World Application: Subduction Zone Rock Formation

Subduction zones are natural laboratories where all three rock types form in close proximity.

Rock Assemblages in Subduction Zones

# Subduction zone rock formation analysis
subduction_parameters = {
    'slab_age': 80,        # Ma (age of oceanic plate)
    'subduction_rate': 6,  # cm/year
    'slab_temperature': 800,  # °C at 100 km depth
    'overriding_plate': 'continental'
}

# Estimate metamorphic conditions
pressure_at_100km = 3.0  # GPa (approximate)
temperature = subduction_parameters['slab_temperature']  # °C

print(f"Subduction zone conditions at 100 km depth:")
print(f"Pressure: {pressure_at_100km} GPa")
print(f"Temperature: {temperature} °C")

# Rock types that form
rock_types = {
    'metamorphic': ['blueschist', 'eclogite', 'gneiss'],
    'igneous': ['andesite', 'dacite', 'rhyolite'],
    'sedimentary': ['turbidites', 'mudstones', 'limestones']
}

print(f"\nMetamorphic rocks: {rock_types['metamorphic']}")
print(f"Igneous rocks: {rock_types['igneous']}")
print(f"Sedimentary rocks: {rock_types['sedimentary']}")

# Estimate igneous rock composition based on water content
water_content = 4  # % in subducted slab (high water content)
if water_content > 3:
    magma_type = "Explosive andesite/dacite"
    explosivity = "High"
else:
    magma_type = "Effusive basaltic"
    explosivity = "Low"

print(f"\nMagma composition prediction: {magma_type}")
print(f"Volcanic explosivity: {explosivity}")
print("This explains why subduction zone volcanism is typically explosive")

Pressure-Temperature-Time (P-T-t) Paths

Metamorphic rocks record the thermal and pressure history of their formation.

Your Challenge: Rock Identification and Formation Path

Analyze an unknown rock sample and determine its formation conditions and history.

Goal: Use systematic rock identification techniques to determine the rock type and infer its formation conditions.

Sample Analysis

import math

# Observed properties of unknown rock sample
sample_data = {
    'texture': 'phaneritic',  # coarse-grained
    'mineral_composition': {
        'quartz': 25,      # percentage
        'orthoclase': 45,  # percentage
        'plagioclase': 20, # percentage
        'biotite': 10      # percentage
    },
    'structure': 'massive',  # no layering or banding
    'chemical_class': 'felsic',  # high silica content
    'density': 2.65,  # g/cm³
    'formation_environment_clues': 'coarse_grained suggests slow cooling'
}

# Rock identification matrix
rock_types = {
    'granite': {
        'texture': 'phaneritic',
        'composition': {'quartz': (20, 35), 'feldspar': (60, 70), 'mica': (5, 15)},
        'environment': 'intrusive',
        'cooling_rate': 'slow',
        'tectonic_setting': 'continental margin, orogeny'
    },
    'rhyolite': {
        'texture': 'aphanitic/porphyritic',
        'composition': {'quartz': (20, 35), 'feldspar': (60, 70), 'mica': (0, 10)},
        'environment': 'extrusive',
        'cooling_rate': 'fast',
        'tectonic_setting': 'continental rifting, calderas'
    },
    'diorite': {
        'texture': 'phaneritic',
        'composition': {'quartz': (0, 5), 'plagioclase': (65, 90), 'mafic': (10, 35)},
        'environment': 'intrusive',
        'cooling_rate': 'slow',
        'tectonic_setting': 'island arcs, continental magmatism'
    },
    'gabbro': {
        'texture': 'phaneritic',
        'composition': {'pyroxene': (40, 60), 'plagioclase': (40, 60)},
        'environment': 'intrusive',
        'cooling_rate': 'slow',
        'tectonic_setting': 'oceanic crust, ophiolites'
    }
}

# Calculate similarity scores
possible_rocks = []
for rock_name, rock_props in rock_types.items():
    score = 0
    
    # Texture match
    if sample_data['texture'] == rock_props['texture']:
        score += 25  # Texture is highly diagnostic
    
    # Composition match (approximate)
    if rock_name == 'granite':
        # Check if mineral percentages match granite composition
        expected_quartz = rock_props['composition']['quartz']
        expected_feldspar = rock_props['composition']['orthoclase'] + rock_props['composition']['plagioclase']
        
        if expected_quartz[0] <= sample_data['mineral_composition']['quartz'] <= expected_quartz[1]:
            score += 15
        if 60 <= (sample_data['mineral_composition']['orthoclase'] + sample_data['mineral_composition']['plagioclase']) <= 70:
            score += 20
        if 5 <= sample_data['mineral_composition']['biotite'] <= 15:
            score += 10
    
    possible_rocks.append((rock_name, score))

# Sort by best match
possible_rocks.sort(key=lambda x: x[1], reverse=True)

Analyze the unknown rock sample and determine its identity and formation conditions.

Hint:

• Consider texture, composition, and structure together
• Use mineral composition percentages to match rock classification
• Relate texture to cooling rate and environment
• Connect rock type to tectonic setting

# TODO: Identify the rock type
most_likely_rock = ""  # Name of identified rock
confidence_level = 0   # Percentage confidence in identification
formation_environment = ""  # Intrusive, extrusive, or metamorphic
cooling_history = ""   # Slow, fast, or variable cooling
tectonic_setting = "" # Continental margin, ocean floor, etc.
formation_depth = 0    # Estimated depth of formation (if intrusive)

# Print results
print(f"Rock identification: {most_likely_rock}")
print(f"Confidence level: {confidence_level:.1f}%")
print(f"Formation environment: {formation_environment}")
print(f"Cooling history: {cooling_history}")
print(f"Tectonic setting: {tectonic_setting}")
print(f"Estimated formation depth: {formation_depth} km")

# Additional observations
if sample_data['structure'] == 'banded':
    metamorphic_grade = "High-grade"  # Indicate if likely metamorphic
else:
    metamorphic_grade = "Not metamorphic"
    
print(f"Metamorphic characteristics: {metamorphic_grade}")

What does this rock tell you about the geological history of the area where it was found?