Geology: Weathering, Sedimentary Rocks, Metamorphism, and Mountain Building

Comprehensive practice flashcards defining essential geological vocabulary across rock weathering, soil horizons, sedimentary rock formation, metamorphism, and mountain deformation structures.

Weathering

The set of physical and chemical processes that break solid, intact rock into loose clasts (fragments/grains) and/or dissolved ions.

Exfoliation

A type of physical jointing where fractures form parallel to the rock surface due to changes in temperature and/or pressure, causing slabs of rock to peel away.

Frost Wedging

A physical weathering process in which water trapped within rock joints freezes and expands, forcing rock faces apart and shattering blocks free.

Salt Expansion

A physical weathering process occurring when the evaporation of saline water in rock pores precipitates and grows salt crystals, prying individual grains apart and forming honeycomb weathering structures.

Hydrolysis

A chemical weathering reaction in which minerals react chemically with water or acidic solutions to transform into entirely new minerals.

Saprolite

Fresh bedrock that has undergone chemical weathering and mineral alteration by atmospheric agents and living organisms while retaining its original rock structure.

Spheroidal Weathering

A weathering pattern in which corners and edges of rectangular joint blocks weather faster than flat faces, gradually rounding the blocks into spherical shapes.

Soil Horizons

Distinct vertical layers of soil: O (organic layer), A (dark mix of organic matter and mineral clasts where leaching occurs), E (light-colored zone of substantial leaching), B (subsoil zone of ion and clay accumulation), and C (weathered parent material/saprolite).

Clastic Sedimentary Rock

Sedimentary rock comprising coherent aggregates of solid fragments or grains (clasts) derived from preexisting rocks, cemented together by precipitated groundwater minerals.

Lithification

The step-by-step geologic transformation of loose sediment into solid rock through compaction, consolidation, and cementation.

Arkose

An immature sandstone containing a significant proportion of feldspar grains

Wacke

A poorly sorted sandstone consisting of sand-sized grains and lithic rock fragments suspended within an abundant fine clay matrix.

Breccia

A coarse-grained clastic sedimentary rock composed of angular, sharp-edged rock fragments lithified from fallen debris or talus.

Conglomerate

A coarse-grained clastic sedimentary rock containing smooth, rounded pebbles and cobbles deposited in areas where rapid fluid flow slows down.

Sediment Maturity

The degree to which nonresistant minerals have been broken down and grains have become well-sorted and rounded; mature sediments contain only resistant minerals such as quartz.

Biochemical Limestone

A sedimentary rock formed from the accumulation and lithification of calcium carbonate ($\text{CaCO}_3$) shells and skeletal remains of organisms such as corals, clams, snails, and plankton.

Micrite

A very fine-grained biochemical limestone composed of clay-sized calcite clasts and microscopic skeletal material.

Coquina

A biochemical limestone shell hash

Bedded Chert

(Biochemical) A hard, cryptocrystalline quartz rock possessing conchoidal fracture, formed on abyssal ocean floors from the accumulated siliceous skeletons of plankton.

Dolostone

A carbonate rock composed predominantly of the mineral dolomite, formed when magnesium-bearing groundwater chemically reacts with preexisting limestone.

Coal

An organic sedimentary rock formed from the compaction and alteration of plant debris in oxygen-poor swamp environments when carbon concentration exceeds $35\%$.

Oil Shale

An organic sedimentary rock consisting of fine clay mixed with $15\%\text{ to }75\%$ kerogen, derived from buried plankton and algae.

Evaporites

Chemical sedimentary rocks precipitated from evaporating bodies of saltwater; gypsum precipitates when $80\%$ of water evaporates, and halite precipitates when $90\%$ evaporates.

Travertine

A chemical limestone precipitated from groundwater in caves or hot springs as carbon dioxide ($\text{CO}_2$) bubbles out of solution due to pressure release.

Cross-Bedding

A sedimentary structure characterized by internal layers inclined at an angle to the main bedding planes, formed on the leeward slip faces of migrating ripples and dunes.

Turbidity Current

A dense, fast-moving underwater cloud of mixed sediment and water rushing down a submarine slope, which deposits graded beds (layer of sediment) & turbidites.

Transgression

The inland migration of the shoreline caused by a relative rise in sea level, resulting in deeper marine depositional facies superimposing over shallower facies.

Protolith

The original, unmetamorphosed parent rock (igneous, sedimentary, or metamorphic) from which a metamorphic rock is derived through solid-state changes.

Neocrystallization

A metamorphic process whereby chemical reactions grow completely new mineral types with different atomic structures than those in the protolith.

Pressure Solution

A metamorphic mechanism under compression & a water film coating grains. Ions detach where grains press together, move in the fluid, precipitate elsewhere. Causes grains become shorter

Metasomatism

A process where a rock's overall chemical composition changes due to hydrothermal fluids dissolving old ions and bringing in different new ones.

Foliation

The planar alignment of platy mineral grains (ALIGNMENT/LAYERING), alternating compositional bands, or flattened clasts produced by differential stress during metamorphism.

Mylonite

A fine-grained, strongly foliated metamorphic rock produced by intense shearing, friction, and grain-size reduction within a ductile shear zone.

Index Minerals

Specific metamorphic minerals whose initial appearance serves as a diagnostic indicator of the temperature and pressure grade of metamorphism.

Isograd

A line plotted on a geologic map connecting points of equal metamorphic grade, demarcated by the first appearance of a specific index mineral.

Metamorphic Facies

A set of metamorphic mineral assemblages (new minerals formed during metamorphic process) that develop under a specific, common range of temperature and pressure conditions.

Thermal Metamorphism

Metamorphism driven by high temperatures adjacent to a pluton or magma intrusion in the absence of differential stress, producing nonfoliated rocks such as hornfels.

Regional Metamorphism

Large-scale metamorphism occurring along convergent plate margins or continental collisions where elevated temperatures, confining pressure, and differential stress generate foliated rocks over extensive areas.

Exhumation

The geological process by which deeply buried rocks return toward Earth's surface (typically at rates of $<1\,\text{cm/yr}$) via crustal upward flow, orogenic collapse, and erosion.

Strike and Dip

Strike is the angle between a horizontal line and true north; dip is the angle between horizontal plane & steepest slope, measured perpendicular to strike.

Reverse Fault and Thrust Fault

Dip-slip faults in which the hanging-wall block slides upward relative to the footwall; characterized as reverse when having a steep dip and as thrust when the dip angle is $<30^\circ$.

Oblique-Slip Fault

A fault exhibiting both strike-slip (horizontal) and dip-slip (vertical) components of motion, such as left-lateral plus reverse or right-lateral plus normal displacement.

Extensional Fault System

A series of related normal faults merging into a low-angle detachment fault at depth, producing tilted fault blocks, keystone-shaped down-dropped grabens, and half grabens.

Fold Components

The anatomical elements of folded rock: the hinge (line of greatest curvature), the limbs (flanks of the fold with less curvature), and the axial plane (plane intersecting hinges across successive layers).

Anticline

An arch-like fold structure in which the limbs dip outward away from the hinge line, exposing the oldest strata along the central hinge core.

Syncline

A trough-like fold structure in which the limbs dip inward toward the hinge line, exposing the youngest strata in the central core.

Isostasy

The state of gravitational equilibrium between Earth's lithosphere and asthenosphere, such that thick crustal mountain roots sink deepest and float highest above the mantle.

Orogenic Collapse

The lateral spreading and thinning of an overthickened mountain belt under its own weight when ductile mid-crustal rocks flow and brittle upper crust breaks along normal faults.