Erosion, Weathering, and Landforms

Agents of Erosion and Landforms

• Water (rivers, streams, floods) forms valleys, canyons, and river deltas
• Wind (deserts) creates sand dunes and desert pavements
• Ice (glaciers) forms U-shaped valleys, fjords, and moraines
• Gravity (mass wasting) causes landslides and talus slopes

Mechanical Weathering Types

• Frost wedging occurs as water freezes and expands in cracks
• Unloading is exfoliation due to pressure release
• Thermal expansion is expansion and contraction from temperature changes
• Biological activity involves plant roots and animal activity breaking rocks
• Salt crystal growth happens as salt accumulates and forces cracks to expand

Chemical Weathering Types

• Dissolution occurs as minerals dissolve in water with mild acids like limestone dissolving in carbonic acid
• Oxidation involves a reaction with oxygen, forming rust in iron-rich rocks
• Hydrolysis is the reaction with water to form new minerals, such as feldspar to clay

Factors Affecting Weathering

• Dissolution, oxidation, and hydrolysis all chemically or physically affect weathering

Weathered Products of Granitic Rocks

• Clay minerals form from feldspar breakdown
• Quartz grains resist weathering
• Soluble potassium and silica are released into groundwater

Mechanical vs Chemical Weathering

• Mechanical weathering breaks rocks into smaller fragments without changing their chemical composition
• The resulting products are smaller rock particles similar in composition to the original rock like sand, silt, and clay.
• Chemical weathering alters a rock's mineral composition and creates dissolved ions in water and new minerals like clay from feldspar through hydrolysis

Climate's Role in Weathering

• Chemical weathering is most effective in warm, moist climates like tropical regions
• Water is essential for chemical reactions such as hydrolysis, oxidation, and dissolution
• High temperatures also increase reaction rates
• Physical (mechanical) weathering is most dominant in cold or dry climates
• Cold climates promote frost wedging, where water seeps into cracks, freezes, expands, and breaks rock apart
• Dry climates promote thermal expansion, where daily temperature fluctuations cause rocks to expand and contract, leading to cracking

Granite Weathering

• Igneous rocks rich in feldspars such as pink granites weather and break down to produce chemical elements that are then released into a solution

Half Dome Weathering

• Half Dome in Yosemite National Park, California, was formed through unloading and exfoliation
• Pressure release caused the outer layers of granite to peel away, creating a dome shape

Cementation Agents of Sedimentary Rocks

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

Gravel Material Sizes

• Boulder: > 256 mm
• Cobble: 64–256 mm
• Pebble: 4–64 mm
• Granule: 2–4 mm

Uses for Gravel

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

Economic Uses of Sedimentary Rocks

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

Categories of Sedimentary Rocks

• Detrital (Clastic) rocks are formed from rock fragments like sandstone, shale, and conglomerate
• Chemical rocks are precipitated from a solution like limestone, rock salt, and gypsum
• Organic rocks are formed from biological material like coal and chalk

Detrital/Clastic Rock Sizes

• Common names for detrital/clastic rocks corresponding to size range: boulder, cobble, pebble, granule, sand, silt, and clay

Economic Uses for Shale

• Shale is a raw material for manufacturing ceramics and construction materials used in pottery, bricks, and tile
• Fine-grained shales are used in China and porcelain production for high-quality ceramic products
• Shale is an important ingredient in Portland cement
• Oil shale contains organic material that can be processed to extract shale oil
• Shale formations like the Marcellus Shale are a source of natural gas and petroleum via fracking

Mineral Components of Detrital Rocks

• Quartz (SiO₂) is the most abundant and resistant mineral in detrital rocks
• Clay minerals are formed from chemical weathering of feldspar (e.g., kaolinite, illite, and montmorillonite)
• Feldspars: Common in detrital rocks but less stable than quartz
• Rock fragments: Pieces of pre-existing rocks, commonly found in breccia and conglomerates

Distance Transported Indicators

• Rounded grains indicate sediment transported over long distances as longer a rock fragment is carried by water, wind, or ice, more its edges are worn down due to abrasion
• Angular grains indicate sediment transported a short distance with not enough erosion to smooth sharp edges

Continental Environments (Land-Based)

• Rivers (Fluvial) deposit sand, gravel, and mud from flowing water
• Lakes (Lacustrine) are where fine-grained sediments like silt and clay settle in calm waters
• Deserts (Aeolian) are where wind deposits sand dunes and loess (fine silt)
• Glaciers (Glacial) deposit poorly sorted sediments, including boulders, sand, and clay

Marine Environments (Ocean-Based)

• Shallow Marine (Nearshore) forms sandstone, limestone, and mudstone in shallow waters, rich in fossils
• Deep Marine (Offshore) is where fine-grained muds, clays, and carbonate deposits accumulate in deep ocean basins

Transitional Environments (Between Land and Sea)

• Deltas form as sediments deposited where rivers meet the ocean or a lake
• Beaches are where sand-sized grains are deposited by wave action
• Lagoons contain mud and organic material accumulating in quiet waters behind barrier islands

Creation of Chemical Sedimentary Rocks

• Minerals precipitate directly from solution due to evaporation or chemical changes in Inorganic Processes
• Evaporites like rock salt (halite) and gypsum form when water evaporates, leaving behind mineral deposits
• Organisms extract dissolved minerals from water to build shells or skeletons in Biochemical Processes
• Limestone forms from the accumulation of calcite-rich shells and corals in marine environments

Limestone

• Limestone is the most abundant chemical sedimentary rock
• Limestone primarily contains the mineral calcite (CaCO₃)
• Limestone can form biochemically from shells and corals or inorganically as precipitated calcium carbonate such as travertine

Sedimentary Structures

• Cross Beds form in wind or water environments where sediment is deposited at an angle

Cross Beds Example

• Sand dunes found in desert/coastal environments
• River channels
• Shallow marine settings with strong currents
• Ripple Marks form by moving water (waves, rivers) or wind

Symmetrical Ripples

• Formed by waves in beach environments with equal movement back and forth

Asymmetrical Ripples

• Formed by currents in rivers or shallow marine settings using one-directional movement
• Mud Cracks form in areas where wet sediment dries and contracts

Mud Cracks Example

• Dried-up lake beds
• Deserts or tidal flats
• Floodplains that periodically dry out

Ripple Marks

• Ripple marks indicate the presence of moving water or wind

Ripple Mark Conditions

• Suggest shallow water conditions or wind-driven sediment movement

Symmetrical Ripple Marks

• Formed by wave action in shallow water environments like beaches
• Indicate back-and-forth movement of water, meaning tides or gentle waves were present

Asymmetrical Ripple Marks

• Formed by currents in rivers, streams, or shallow marine settings
• Indicate one-directional flow of water

Past Conditions

• Reveal that an ancient environment had moving water, tides, or wind-driven sand dunes

Environmental Formations

• Fossil-rich limestone forms in warm, shallow marine environments, such as coral reefs or carbonate platforms, where marine organisms accumulate and eventually lithify into rock
• Halite (rock salt) forms in evaporative environments, such as shallow inland seas, salt flats, or evaporating lakes

Coal Formation

• Bituminous coal is formed from peat and lignite under increasing heat and pressure over millions of years used as a soft, black coal and fuel source
• Anthracite coal forms when bituminous coal undergoes further metamorphism with high heat output/energy efficiency

Steps to Anthracite Coal Formation

• Plant Material begins as dead plants accumulate in swampy, tropical environments with little oxygen
• Peat transforms partially decayed plant material that builds up in wetlands and bogs
• Lignite (Brown Coal) forms as burial and compaction increase heat and pressure which forms a low-grade coal
• Bituminous Coal occurs when continued pressure and heat transforms lignite into a denser, higher-energy coal
• Anthracite Coal results from high-grade metamorphism further removes impurities, producing the hardest, most energy-rich coal

Oil and Natural Gas Formation

• Crude oil and natural gas are derived from marine organic matter buried under heat and pressure
• Typically forms in porous reservoir rock such as sandstone/limestone beneath an impermeable cap rock (shale)

Metamorphism

• Metamorphism occurs when pre-existing rocks undergo heat, pressure, and chemically active fluids
• Metamorphism occurs between 200°C and 800°C
• Confining pressure creates equal pressure in all directions from deep burial
• Differential stress involves unequal pressure, forming foliated textures such as mountain-building
• Contact metamorphism occurs near magma intrusions
• Regional metamorphism occurs in mountain-building zones

Contact vs Regional Metamorphism

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

Metamorphic Rock Quantities

• Regional metamorphism produces the greatest volume of metamorphic rock due to large-scale pressures and high temperatures associated with mountain formation

Progressive Grades of Metamorphism

• Low-grade metamorphism: Shale transforms into slate
• Medium-grade metamorphism: Slate transforms into schist
• High-grade metamorphism: Schist transforms into gneiss, where original features are obliterated

Foliated Rock Formation

• Foliated rocks form when minerals are subjected to directed pressure, which causes them to align in parallel layers; schist is an example with platy texture due to mineral alignment

Non-Foliated Rock Formation

• Non-foliated rocks form when pressure is uniform, causing minerals to recrystallize into larger, interlocking crystals instead of aligning in layers
• Marble is an example from limestone under heat and pressure

Rock Creation (Examples)

• Texture Changes: Pressure causes minerals to realign (foliation) or grow larger (non-foliated)
• Mineralogy Changes: Heat and chemically active fluids promote recrystallization, forming stronger minerals like garnets and rubies
• Foliated Rocks: Slate → Schist → Gneiss (progressive metamorphism)
• Non-Foliated Rocks: Marble (from limestone) and Quartzite (from sandstone)

Texture and Mineralogy Changes

• Heat interactions with magma and high temperatures (200–800°C) cause minerals to recrystallize
• Confining pressure is present during deep burial and results in denser rock structures
• Differential stress exists during mountain building and causes minerals to align leading to foliation
• Chemically Active Fluids (water and volatiles) promote ion migration and recrystallization, enhancing mineral growth
• Progressive Metamorphism involves increasing temperature and pressure which leads to the transformation of minerals into more stable forms such as clay minerals in shale changing into mica in schist

Types of Metamorphic Rocks

Foliated (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 (Due to Uniform Pressure or Heat)

• Marble: Large calcite crystals, forms from limestone
• Quartzite: Fused quartz grains, forms from sandstone

Grades of Metamorphism and Examples

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

Examples of Each Metamorphic Grade

Low-Grade

• Rock Example: Slate (formed from shale)
• Characteristics: Fine-grained, splits easily, minimal mineral transformation

Medium-Grade

• Rock Example: Schist (formed from slate)
• Characteristics: Strong foliation, visible platy minerals such as mica

High-Grade

• Rock Example: Gneiss (formed from schist)
• Characteristics: Strongly banded texture, minerals segregate into light and dark bands, original features are obliterated

Economic Uses

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