EASC 1000 - Topic 4: Sedimentary Rocks and Weathering Processes

Vocabulary practice flashcards covering sedimentary rock classification, physical and chemical weathering processes, sediment textures, and lithification.

Clastic Sedimentary Rock

A sedimentary rock composed of eroded, loose rock and mineral fragments from pre-existing rocks that have been deposited and cemented together.

Organic Sedimentary Rock

A sedimentary rock composed of the carbon-rich remains of once-living plants and other organisms.

Biochemical Sedimentary Rock

A sedimentary rock composed of cemented shells and mineral skeletons grown by organisms.

Chemical Sedimentary Rock

A sedimentary rock composed of minerals that precipitated directly from dissolved ions in water.

Biogenic Sedimentary Rock

A combined classification group that encompasses both biochemical and organic sedimentary rocks.

Weathering

The breakdown of solid rock into loose, unconsolidated sediment of any particle size when exposed to air, moisture, and living organisms.

Physical Weathering

The suite of processes that break intact rock into unconsolidated clasts without altering the mineral composition; it is typical in cold, dry environments.

Jointing

The formation of natural fractures in rock bodies as rocks formed at depth in the crust are exhumed, uplifted to the surface, and subjected to changing pressure and temperature.

Exfoliation Joints

Curved sheet-like fractures running roughly parallel to the ground surface that form as overburden pressure is unloaded, allowing outer slabs of rock to peel away.

Frost Wedging

A mechanical weathering process in which water fills open joints in a rock and expands by $9\%$ upon freezing, splitting the rock apart.

Root Wedging

A physical weathering process in which growing and expanding plant roots exert outward pressure inside fractures, forcing rock blocks apart.

Thermal Expansion Weathering

Physical fracturing and joint development driven by the repeated heating and cooling of rock caused by wildfires, lightning, or solar radiation.

Chemical Weathering

Chemical reactions that alter or break down existing minerals into new minerals and dissolved ions through exposure to water and atmospheric gases.

Dissolution

A chemical weathering reaction involving the complete (congruent) breakdown and separation of a mineral into constituent dissolved ions in water.

Hydrolysis

An incongruent chemical reaction between minerals and water wherein $H^+$ and $OH^-$ replace mineral cations, altering silicate minerals (such as feldspar) into clay minerals and releasing soluble cations.

Oxidation

A chemical weathering redox reaction in which an element (most commonly iron) reacts with dissolved oxygen ($\text{O}_2\text{(aq)}$), converting $\text{Fe(II)}$ into $\text{Fe(III)}$ to form iron oxides (e.g., hematite) or oxyhydroxides (e.g., goethite).

Hydration

The absorption and addition of water molecules directly into a mineral's crystal structure, resulting in expansion and conversion into a new hydrous mineral (e.g., anhydrite to gypsum).

Differential Weathering

The variation in the rate of weathering across different rock types or minerals caused by differences in resistance, structure, and composition (e.g., weak shale eroding faster than strong sandstone).

Gravel

Unconsolidated sediment composed of pea-sized or larger clasts with diameters greater than $2\,\text{mm}$.

Sand

Clastic sediment consisting of grains measuring between $0.0625\,\text{mm}$ and $2\,\text{mm}$, roughly the size of a pinhead.

Silt

Fine clastic sediment with grain diameters ranging from $0.0039\,\text{mm}$ to $0.0625\,\text{mm}$ ($3.9\,\mu\text{m} - 62.5\,\mu\text{m}$), roughly the size of a grain of table salt.

Clay

The finest grain-size category of clastic sediment, consisting of particles smaller than $0.0039\,\text{mm}$ ($< 3.9\,\mu\text{m}$), comparable in size to particles of flour.

Breccia

A coarse-grained clastic sedimentary rock composed of angular to subangular, gravel-sized ($> 2\,\text{mm}$) clasts cemented together.

Conglomerate

A coarse-grained clastic sedimentary rock formed from cemented, rounded to subrounded gravel-sized clasts ($> 2\,\text{mm}$).

Siltstone

A clastic sedimentary rock formed by the compaction and cementation of lithified silt-sized particles ($0.0039\,\text{mm} - 0.0625\,\text{mm}$).

Shale

A fine-grained clastic sedimentary rock formed from clayey mud that characteristically breaks into very thin, platy sheets.

Mudstone

A fine-grained clastic sedimentary rock formed from lithified clayey mud that breaks into blocky chunks rather than splitting into thin sheets.

Angularity

A textural descriptor describing the smoothness or sharpness of clast corners and edges, reflecting the duration and distance of sediment transport.

Sphericity

A textural measure indicating how closely the three-dimensional shape of a sedimentary clast approaches the shape of a sphere.

Sorting

A measure of the uniformity of grain sizes present in a clastic sediment or sedimentary rock, describing whether grains are all similar in size (well-sorted) or variable (poorly sorted).

Sediment Maturity

The degree to which a sediment has evolved compositionally and texturally, moving from an immature mixture of unstable and lithic grains toward a mature assemblage dominated by well-sorted, rounded, resistant quartz.

Evaporite

A chemical sedimentary rock formed by the inorganic precipitation of salts (e.g., halite, gypsum, sylvite) from water bodies undergoing high evaporation rates.

Travertine

A chemical limestone ($\text{CaCO}_3$) precipitated from oversaturated or degassed groundwater and hydrothermal fluids, commonly found at hot springs and as cave speleothems.

Tufa

A variety of chemical limestone formed by the precipitation of calcium carbonate from cold groundwater springs and seeps, often found in alkaline lake settings.

Chert

A dense chemical or biochemical sedimentary rock composed of microcrystalline silica ($\text{SiO}_2$) that precipitated directly or replaced other materials.

Banded Iron Formation

A distinctive Precambrian chemical sedimentary rock consisting of alternating microcrystalline layers of chert and iron-bearing oxide minerals.

Diagenesis

The full suite of physical, chemical, and biological processes that convert loose sediment into sedimentary rock and modify it at temperatures below $150\,^{\circ}\text{C}$.

Lithification

The primary stage of diagenesis that transforms loose, unconsolidated sediment into solid rock via compaction and cementation.

Compaction

The step in lithification where the weight of overlying layers expels water and air from pore spaces, pressing sediment grains tightly together.

Cementation

The step in lithification in which dissolved minerals (such as quartz or calcite) precipitate out of groundwater into pore spaces between clasts, binding them into solid rock.