Weathering Processes and Granite Weathering

Weathering Processes

• Unloading: Exfoliation occurs as pressure is released from rocks.
• Salt crystal growth: Salt accumulation expands cracks in rocks.
• Dissolution: Minerals dissolve in water containing mild acids, such as limestone dissolving in carbonic acid.
• Hydrolysis: A reaction with water forms new minerals, like feldspar transforming into clay.

Weathering of Granitic Rocks

• Clay minerals result from feldspar breakdown.
• Quartz grains remain as they are resistant to weathering.
• Soluble potassium and silica are released into the groundwater.

Key Concepts in Weathering and Erosion

• Mass Wasting: Rock, soil, and debris move down slopes due to gravity, often triggered by water, earthquakes, or human activities.
• Chemical Weathering: Rocks break down via chemical reactions involving water, acids, and oxygen, altering their composition.
• Erosion: Weathered material (sediments) is transported from one location to another by wind, water, ice, or gravity.
• Exfoliation Dome: Large, rounded rock formations are created by exfoliation, commonly seen in granite landscapes like Yosemite National Park.
• Talus Slope: Piles of loose rock fragments accumulate at the base of cliffs due to rockfalls and weathering.
• Carbonic Acid Formation: Carbon dioxide (CO₂) reacts with water (H₂O) to form carbonic acid (H₂CO₃), aiding chemical weathering by dissolving rocks like limestone.

Cementation Agents in Sedimentary Rocks

• Calcite (CaCO₃)
• Silica (SiO₂)
• Iron oxide (Fe₂O₃)

Uses for Gravel

• Gravel is used as a construction material for roads, concrete, and landscaping.

Economic Uses of Sedimentary Rocks

• Sandstone: Used in building materials and glass production.
• Limestone: Used for cement, crushed stone, agricultural lime, and steel production.
• Shale: Used in bricks, tiles, pottery, and cement.

Sedimentary Rock Categories

• Detrital (Clastic): Formed from rock fragments, such as sandstone, shale, and conglomerate.
• Chemical: Precipitated from solution, such as limestone, rock salt, and gypsum.
• Organic: Formed from biological material, such as coal and chalk.

Economic Uses for Shale

• Shale is a raw material for ceramics and construction materials like pottery, bricks, and tiles.
• Fine-grained shales are used in china and porcelain production.
• Shale is an important part of Portland cement.
• Certain shales contain organic material that can be processed into shale oil.
• Shale formations, such as Marcellus Shale, are major sources of natural gas and petroleum.

Mineral Components of Detrital Rocks

• Quartz (SiO₂): The most abundant and resistant mineral due to its hardness and chemical stability.
• Clay minerals: Formed from the chemical weathering of feldspar (e.g., kaolinite, illite, and montmorillonite).
• Feldspars: Common but less stable than quartz, often breaking down into clay minerals.
• Rock fragments: Pieces of pre-existing rocks, common in breccia and conglomerates.

Marine Environments

• Shallow Marine (Nearshore): Sandstone, limestone, and mudstone are formed, often rich in fossils.
• Deep Marine (Offshore): Fine-grained muds, clays, and carbonate deposits accumulate.

Transitional Environments

• Deltas: Sediments are deposited where rivers meet the ocean or lakes.
• Beaches: Sand-sized grains are deposited by wave action.
• Lagoons: Mud and organic material accumulate in quiet waters behind barrier islands.

Sedimentation Processes

• Inorganic Processes: Minerals precipitate directly from solution due to evaporation or chemical changes.
• Example: Evaporites like rock salt (halite) and gypsum form when water evaporates.
• Biochemical Processes: Organisms extract dissolved minerals from water to build shells or skeletons, which later become rock.
• Example: Limestone forms from the accumulation of calcite-rich shells and corals in marine environments.
• Fossil-rich limestone forms in warm, shallow marine environments.
• Halite (rock salt) forms in evaporative environments.

Coal Formation

• Bituminous coal forms from peat and lignite under increasing heat and pressure over millions of years as a soft, black fuel source.
• Anthracite coal forms when bituminous coal undergoes further metamorphism, increasing its carbon content and energy efficiency as a hard, shiny coal with high heat output and low impurities.
• Plant Material: Dead plants accumulate in swampy and tropical environments with little oxygen.
• Peat: Materials partially decay and build up in wetlands and bogs.
• Lignite (Brown Coal): Burial and compaction increase heat and pressure, forming a low-grade coal.
• Bituminous Coal: Continued pressure and heat transform lignite into a denser, higher-energy coal.
• Anthracite Coal: High-grade metamorphism further removes impurities, producing the hardest, most energy-rich coal.

Oil and Gas Formation

• Derived from marine organic matter buried under heat and pressure.
• Typically forms in porous reservoir rock (sandstone, limestone) beneath an impermeable cap rock (shale).

Metamorphic Rock Formation

• Confining pressure: Equal pressure from all directions (deep burial).
• Differential stress: Unequal pressure, forming foliated textures (mountain-building).
• Contact metamorphism: Occurs near magma intrusions.
• Regional metamorphism: Occurs in mountain-building zones.

Types of Metamorphism

• Contact metamorphism occurs due to high temperature near magma, producing localized changes.
• Regional metamorphism is caused by heat and pressure over large areas, producing the most metamorphic rock.
• Regional metamorphism produces the greatest volume of metamorphic rock.

Grades of Metamorphism

• Low-grade metamorphism: Shale transforms into slate.
• Medium-grade metamorphism: Slate transforms into schist.
• High-grade metamorphism: Schist transforms into gneiss.

Foliated vs. Non-Foliated Rocks

• Foliated rocks form when minerals are subjected to directed pressure, causing them to align in parallel layers (e.g., Schist).
• Non-foliated rocks form when pressure is uniform, causing minerals to recrystallize into larger, interlocking crystals instead of aligning in layers (e.g., Marble).
• Slate → Schist → Gneiss is an example of progressive metamorphism resulting in foliated rocks.
• Marble (from limestone) and Quartzite (from sandstone) are examples of non-foliated rocks.

Changes in Texture and Mineralogy

• Heat: Interaction with magma and high temperatures (between 200–800°C) cause minerals to recrystallize.
• Pressure (Stress): Confining pressure results in denser rock structures, while differential stress causes minerals to align, leading to foliation.
• Chemically Active Fluids: Water and volatiles promote ion migration and recrystallization, enhancing mineral growth.
• Progressive Metamorphism: Increasing temperature and pressure lead to the transformation of minerals into more stable forms (e.g., clay minerals in shale change into mica in schist).

Metamorphic Rock Examples

• Foliated Metamorphic Rocks (due to directed pressure):
  • Slate: Fine-grained, splits easily, forms from shale.
  • Schist: Strongly foliated, "platy" texture, forms from slate.
  • Gneiss: Banded texture due to mineral segregation, forms from schist.
• Non-Foliated Metamorphic Rocks (due to uniform pressure or heat):
  • Marble: Large calcite crystals, forms from limestone.
  • Quartzite: Fused quartz grains, forms from sandstone.

Metamorphic Grades

• Low-Grade Metamorphism:
  • Rock Example: Slate
  • Characteristics: Fine-grained, retains original rock features, splits easily along planes.
  • Forms from shale under low heat and pressure conditions.
• Medium-Grade Metamorphism:
  • Rock Example: Schist
  • Characteristics: Strongly foliated, visible platy minerals like mica, shiny surface.
  • Forms from slate with increased temperature and pressure.
• High-Grade Metamorphism:
  • Rock Example: Gneiss
  • Characteristics: Strong banding due to mineral segregation, coarse-grained, obliteration of original features.
  • Forms from schist under extreme heat and pressure conditions.

Economic Uses for Metamorphic Rocks

• Slate: Used in slate roofs, commonly found in Europe and northern parts of the U.S.
• Marble: Used as a building stone due to its durability and aesthetic appeal, available in a variety of colors.
• Quartzite: Known for its hardness, making it useful in construction and industrial applications; commonly used for countertops, flooring, and decorative stones.