CE11 - Lecture 4 - Weathering and Sedimentary Rocks

Weathering

The process by which rocks are broken down at Earth’s surface by physical, chemical, or biological means

Physical Weathering

Breaks rocks into smaller pieces without changing their composition (e.g., exfoliation, freeze-thaw, root wedging)

Chemical Weathering

Involves chemical reactions that alter or destroy minerals, often enhanced by water and warm temperatures

Exfoliation

Expansion and cracking of rock due to reduced pressure from overlying material being removed

Root Wedging

Roots grow into rock cracks, expanding them as the plant grows

Ice Wedging

Water seeps into cracks, freezes, expands, and breaks rock apart

Crystal Growth (Salt Weathering)

Salt crystals grow from evaporating water and exert pressure, breaking rock

Carbonic Acid

Formed from water and carbon dioxide, a key agent in chemical weathering

Oxidation

Chemical reaction where minerals (especially those with iron) react with oxygen, forming oxides like rust

Erosion

The removal and transport of weathered material by agents like water, wind, or gravity

Transportation

Movement of sediments from the place of erosion to deposition, influenced by medium velocity

Deposition

Occurs when the transporting medium loses energy and drops its sediment load

Lithification

The transformation of sediments into solid rock through burial, compaction, and cementation

Clastic Sedimentary Rocks

Formed from solid fragments (clasts) of pre-existing rocks

Chemical Sedimentary Rocks

Formed from minerals precipitated from solution, often from evaporation

Conglomerate

Clastic rock made of rounded pebbles and cobbles, indicating high-energy environments

Breccia

Clastic rock with angular clasts, indicating little transport and nearby origin

Sandstone

Composed mainly of sand-sized particles, often quartz-rich

Shale

A fissile mudrock composed mainly of silt and clay, forms in calm environments

Limestone

Chemical or biochemical rock mainly made of calcite or aragonite, often forms in marine settings

Dolomite (Dolostone)

Carbonate rock composed mostly of dolomite, typically formed from altered limestone

Chert

Rock composed of microcrystalline silica, often from shells of radiolarians or diatoms

Rock Salt

Evaporite rock composed mainly of halite, formed from evaporated saline water

Evaporites

Rocks formed by the evaporation of water, leaving behind salts and minerals (e.g., gypsum, halite)

Bedding

Visible layers of sedimentary rock representing different depositional events

Cross-Bedding

Inclined layers within beds, formed by wind or water movement

Ripple Marks

Small-scale ridges on sediment surfaces formed by wind or water

Graded Bedding

A bed with a vertical change in grain size, typically coarser at the bottom and finer at the top

Mudcracks

Cracks formed when muddy sediment dries and shrinks

Karst

Topography formed by the dissolution of soluble rocks, featuring caves and sinkholes

Principle of Original Horizontality

Sediments are deposited in flat, horizontal layers

Principle of Superposition

In a sequence of sedimentary layers, the oldest is at the bottom

Principle of Inclusions

Rock fragments inside another rock are older than the surrounding material

Principle of Faunal Succession

Fossil organisms succeed one another in a recognizable order through geological time

Slaking

Breakdown of weakly cemented rocks like shale when exposed to water or air

Residual Soil

Weathered material that remains in place after complete breakdown of rock

Imbrication

Overlapping arrangement of particles that indicate flow direction in sediment

Cementation

Process where dissolved minerals crystallize and bind sediment grains together

Compaction

Process where sediments are squeezed together under pressure, reducing pore space

Alluvial Deposits

Sediments deposited by rivers, often in valleys with steep sides