Comprehensive Study Guide on Materials of the Earth: Magma, Rocks, and Weathering

Course Outcomes and Expected Activities

  • Course Outcome 2 (CO2): Focuses on the Materials of the Earth. The primary goal is to elucidate the formative processes, physical and chemical traits, and the classification of rocks.

Fundamental Nature of Rocks

  • Definition: Rocks are naturally occurring, coherent aggregates of minerals, glass, or organic material.

    • Naturally-occurring: Indicates that only materials formed by natural processes are considered rocks. Artificial materials like concrete are not rocks.

    • Coherent: A rock must be held together as a solid mass and not broken into loose pieces; a pile of unattached minerals is not a rock.

    • Aggregate: A rock consists of one or more minerals that have grown or are stuck together.

  • Examples of Rocks: Granite, Conglomerate, and Marble.

  • Scientific and Economic Importance:

    • Rocks provide insights into the formative processes of the Earth, as well as its history and evolution.

    • They have significant industrial and economic uses, serving as sources for building materials, metals, glass, and fertilizers.

Basis of Geologic Classification

  • History of Classification: Geologists in the 18th century struggled to classify rocks. Classification serves to organize information and retain significant details about materials.

  • The Genetic Scheme: Accepted by the end of the 18th century, this scheme focuses on the origin of rocks. It was first described by James Hutton (1726–17971726\text{--}1797).

  • Definition of Genetic Scheme: A classification of formative processes and a correlation of these processes with rock types and the intrinsic characteristics they developed.

The Rock Cycle

  • Definition: The rock cycle represents how one type of rocky material transforms into another through various formative processes. It shows how rocks are formed, broken down, and processed in response to changing conditions.

  • System Interactions: Processes involve the interactions of the geosphere with the hydrosphere, atmosphere, and/or biosphere.

  • Rock Cycle in Plate Tectonics (Convergent Boundaries):

    • Magma is created by the melting of rock at subduction zones.

    • Less dense magma rises and cools to form igneous rock.

    • Igneous rock exposed at the surface is weathered into sediment.

    • Sediments are transported to low-lying areas, buried, and hardened into sedimentary rock.

    • Sedimentary rock is heated and squeezed at depth to form metamorphic rock.

    • Metamorphic rock may eventually heat up and melt to form magma again.

  • Major Processes in Rock Formation:

    • Igneous Processes: Melting and crystallization.

    • Sedimentary Processes: Weathering and erosion.

    • Metamorphic Processes: Burial involving heat and pressure.

Nature and Formation of Magma

  • Definitions:

    • Magma: Molten rock inside the Earth, usually rich in silica (SiO2SiO_2) and containing dissolved gases.

    • Lava: Magma that has been ejected and flows on the Earth's surface.

  • Heat Sources: Heat transforms rocks into magma. Most of this heat comes from the core via conduction or convection through the mantle and crust.

  • Factors Affecting Magma Formation:

    • Geothermal Gradient: The rate at which temperature increases with increasing depth. However, this is typically not enough to melt rock alone because the melting points of minerals increase with pressure.

    • Pressure Changes (Decompression Melting): Melting occurs when rising mantle rock is subjected to lower pressure, reducing its melting point.

    • Presence of Water (Flux Melting): Water becomes highly reactive at high temperatures. At sufficient pressures and temperatures, reactive water vapor can reduce the melting point of rocks by over 200 ∘C200\,^\circ\text{C}.

Magma Evolution and Magmatic Paths

  • Magmatic Paths: Describe how magma changes in composition and character as it cools and crystallizes.

  • Four Major Evolution Paths:

    1. Partial Melting: The process by which magma composition varies because different minerals melt at different temperatures.

    2. Magma Mixing: The composition of a magma body changes as it mixes with another separate magma body.

    3. Assimilation (Contamination): Hot magma incorporates material from the surrounding host rock (country rock). Host rock is dislodged, melts, and becomes part of the magma body.

    4. Differentiation (Fractional Crystallization): The process where different ingredients separate from an originally homogeneous mixture. In crystal settling, earliest-formed minerals settle downward, changing the composition of the remaining melt.

Bowen's Reaction Series

  • Definition: Minerals crystallize from a cooling magma in a predictable order over a large temperature range.

  • Discontinuous Branch: As mafic magma cools, it crystallizes minerals in this order: Olivine →\rightarrow Pyroxene →\rightarrow Amphibole →\rightarrow Biotite. These minerals are removed, enriching the remaining magma in silica.

  • Continuous Branch: Plagioclase feldspar evolves gradually from calcium-rich (Anorthite) to sodium-rich (Albite).

  • Final Crystallization: As temperature decreases further, Potassium feldspar, Muscovite, and finally Quartz crystallize.

  • Rock Types Produced:

    • Ultramafic: High temperature; contains Olivine.

    • Mafic: Contains Pyroxene and Calcium-rich Plagioclase.

    • Intermediate: Contains Amphibole and equal Calcium/Sodium Plagioclase.

    • Silicic (Felsic): Low temperature; contains Quartz, Potassium feldspar, and Sodium-rich Plagioclase.

Volcanism and Earth Subsystems

  • Volcanism: Occurs when magma makes its way to the Earth's surface.

  • Subsystem Impacts:

    • Atmosphere: Originally created from gases released during eruptions.

    • Hydrosphere: Produced by the condensation of volcanic water vapor.

    • Biosphere: Influenced positively (fertile soils from ash) and negatively (destruction of life, climate changes, mass extinctions).

  • Eruption Types:

    • Effusive Eruptions: Calm oozing of low-viscosity (runny, usually basaltic) magma. Low gas content. Forms shield volcanoes (e.g., Kilauea).

    • Explosive Eruptions: Violent ejection of ash, gas, and pyroclasts. High-viscosity (thick, usually andesitic to rhyolitic) lava. High gas content. Forms stratovolcanoes or calderas (e.g., Mount Pinatubo).

Products of Volcanic Eruptions

  • Lava Flows:

    • Mafic Lava: Low viscosity; includes Pahoehoe (smooth/ropy), A'a (jagged/blocky), and Lava Tubes.

    • Flood Basalts: Very low viscosity; flows from fissures over large areas.

    • Columnar Jointing: Parallel, six-sided vertical columns formed during cooling.

    • Submarine Lava (Pillow Structure): Formed as lava flows into water.

    • Intermediate/Felsic Lava: Thick and viscous; flows only short distances.

  • Pyroclastic Materials:

    • Classified by size: Dust, ash, cinders, lapilli, blocks, and bombs.

    • Pyroclastic Flows: Rapidly moving downslope mixtures of gas and debris.

Volcanic Landforms and Features

  • Shield Volcanoes: Broad and gently sloping; composed of solidified basaltic flows.

  • Cinder (Cone) Volcanoes: Small and steeply sloping; composed of piles of loose cinders (mostly basaltic).

  • Composite Volcanoes (Stratovolcanoes): Moderately to steeply sloping; alternating layers of pyroclastic debris and lava flows. Primarily intermediate composition (Andesite). Common at convergent boundaries and the Ring of Fire.

  • Extrusive Features:

    • Lava Domes: High viscosity, degassed felsic lavas (e.g., Obsidian).

    • Calderas: Volcanic depressions at least 1 km1\,\text{km} in diameter caused by violent eruptions (e.g., Crater Lake, Taal Crater).

  • Intrusive (Plutonic) Features:

    • Volcanic Neck: Solidified magma in the throat of a volcano.

    • Dike: Tabular structure that cuts across country rock layering.

    • Sill: Tabular structure that parallels country rock layering.

    • Plutons: Large, blob-shaped intrusive bodies.

    • Stocks: Small plutons (exposed area <100\,\text{km}^2).

    • Batholiths: Large plutons (exposed area >100\,\text{km}^2).

Igneous Rock Classification

  • Based on Location:

    • Extrusive (Volcanic): Formed at the surface; cools quickly (fine-grained).

    • Intrusive (Plutonic): Formed underground; cools slowly (coarse-grained).

  • Based on Texture:

    • Aphanitic (Fine-grained): Crystals too small to see with the naked eye.

    • Phaneritic (Coarse-grained): Crystals large enough to see.

    • Pegmatitic: Extremely coarse-grained (crystals >5\,\text{cm}).

    • Porphyritic: Two distinct crystal sizes (phenocrysts and groundmass).

    • Glassy: No crystals; extremely rapid cooling (e.g., Obsidian).

    • Vesicular: Contains gas bubble cavities (e.g., Scoria, Pumice).

    • Pyroclastic: Consolidated fragments like Tuff or Volcanic Breccia.

  • Based on Composition (Silica Content):

    1. Felsic (Granitic): >65\%\,SiO_2. Light-colored; rich in Aluminum, Sodium, Potassium. Examples: Rhyolite (extrusive), Granite (intrusive).

    2. Intermediate (Andesitic): 55–65% SiO255\text{--}65\%\,SiO_2. Examples: Andesite (extrusive), Diorite (intrusive).

    3. Mafic (Basaltic): 45–55% SiO245\text{--}55\%\,SiO_2. Dark-colored; rich in Iron, Magnesium, Calcium. Examples: Basalt (extrusive), Gabbro (intrusive).

    4. Ultramafic: <45\%\,SiO_2. Almost entirely ferromagnesian minerals. Examples: Komatiite (extrusive), Peridotite (intrusive).

Weathering and Erosion

  • Weathering: Destructive processes changing physical/chemical character of rocks at the surface.

  • Erosion: Physical picking up of particles by water, ice, or wind.

  • Transportation: Movement of eroded particles.

  • Deposition: Accumulation of sediments in a new location.

  • Mechanism Types:

    • Mechanical (Physical): Breaking rocks without chemical change. Includes Pressure Release (exfoliation), Frost Action (frost wedging/heaving), Plant Growth, and Thermal Variation.

    • Chemical: Decomposition via exposure to water and gases (Oxygen, CO2CO_2). Includes Oxidation and Acid dissolution. Slightly acidic rainwater attacks feldspars to produce clay minerals.

  • Rates of Weathering:

    • Differential Weathering: Different rates due to different compositions (e.g., Shale weathers faster than Sandstone).

    • Spheroidal Weathering: Uniform weathering leading to rounded shapes.

Soil (Pedolith)

  • Definition: Layer of weathered, unconsolidated material on bedrock. Constituents include clay minerals, organic matter, water, and quartz.

  • Soil Horizons:

    • O: Organic matter.

    • A (Topsoil): Dark, rich in organic matter, high biological activity.

    • E (Eluviated): Zone of leaching.

    • B (Subsoil): Zone of accumulation (clays, iron oxides).

    • C: Partially weathered parent material (bedrock).

  • World Soil Orders: Includes Alfisols, Andisols, Aridisols (deserts), Entisols (young), Gelisols (permafrost), Histosols (wet/organic), Inceptisols, Mollisols, Oxisols (tropical/heavily weathered), Spodosols, Ultisols, and Vertisols.

Sedimentary Rocks

  • Sediment Classification by size:

    • Boulder: >256\,\text{mm}

    • Cobble: 64–256 mm64\text{--}256\,\text{mm}

    • Pebble: 2–64 mm2\text{--}64\,\text{mm}

    • Sand: 1/16–2 mm1/16\text{--}2\,\text{mm}

    • Silt: 1/256–1/16 mm1/256\text{--}1/16\,\text{mm}

    • Clay: <1/256\,\text{mm}

  • Lithification: Conversion of loose sediment into rock via Compaction and Cementation.

  • Types of Sedimentary Rocks:

    • Detrital: Formed from cemented fragments (e.g., Breccia, Conglomerate, Sandstone, Shale).

    • Chemical: Formed by precipitation (e.g., Rock Salt, Gypsum, Chert).

    • Biochemical: Formed from shells/biological debris (e.g., Limestone, Coquina, Chalk).

    • Organic: Remnants of plants (e.g., Coal).

  • Sedimentary Structures: Bedding (most common), Cross-bedding (indicates ripples/dunes), Ripple Marks, Graded Bedding, Mud Cracks, and Fossils.

Metamorphism

  • Definition: Solid-state changes to rocks due to increased heat, pressure, or reactive fluids. Occurs in Earth's interior.

  • Agents:

    • Temperature: Stable ranges for minerals; exceeding them causes recrystallization.

    • Pressure: Confining pressure (equal in all directions) and Differential Stress (compressive or shearing).

    • Fluids: Hot water acts as a transport agent for ions.

  • Foliation: Aligned mineral texture produced by differential stress.

    • Foliated Rocks: Slate →\rightarrow Phyllite →\rightarrow Schist →\rightarrow Gneiss (increasing grade).

    • Nonfoliated Rocks: Marble (from limestone), Quartzite (from sandstone), Hornfels (from shale).

  • Types of Metamorphism:

    • Contact: High temperature due to proximity to magma.

    • Regional: Large scale, high pressure, associated with mountain building.

    • Hydrothermal: Alteration by hot water, common at mid-ocean ridges.

    • Shock: Extreme pressure from meteor impacts.

  • Metamorphic Grade (Temperature Ranges):

    • Low Grade: 200–300 ∘C200\text{--}300\,^\circ\text{C}.

    • Medium Grade: 300–500 ∘C300\text{--}500\,^\circ\text{C}.

    • High Grade: 500–800 ∘C500\text{--}800\,^\circ\text{C}.

    • Ultra-High Grade: >800\,^\circ\text{C}.

Questions & Discussion

  • Volcanic Eruptions:

    • What are the factors that affect the strength and intensity of an eruption?

    • What are the possible products of a strong volcanic explosion?

  • Geological Formations:

    • How do formations like those in the Palawan Subterranean River, Bryce Canyon, or the Stone Forest compare and contrast?

    • What processes helped in their formation, and why is it relevant to explore these processes?

  • Plate Tectonics:

    • How do the rates of erosion and deposition give clues to the types of tectonic plate boundaries?