3 - Sedimentary, Metamorphic, and Igneous Rocks

Metamorphism

The process by which rocks change in mineralogy and texture due to heat, pressure, and chemically active fluids, while remaining essentially solid.

Protolith (Parent Rock)

The original rock that exists before metamorphism begins (e.g., shale, basalt, limestone).

Factors Controlling Metamorphism

Parent rock composition, temperature, pressure (type and amount), fluids present, and time available for reactions.

Metamorphic Grade

A general term for the relative temperature and pressure conditions under which metamorphic rocks form (low-grade vs. high-grade).

Prograde Metamorphism

Occurs when temperature and pressure progressively increase, leading to higher metamorphic grade.

Retrograde Metamorphism

Occurs when temperature and pressure decrease, causing high-grade minerals to revert to lower-grade forms.

Foliation

A planar texture in metamorphic rocks caused by the parallel alignment of platy or elongated minerals under directed pressure (e.g., slate, schist).

Non-foliated Metamorphic Rock

Metamorphic rock lacking a directional fabric, typically forming in environments without strong directed pressure (e.g., marble, quartzite).

Slate

Fine-grained, low-grade foliated metamorphic rock with excellent slaty cleavage; derived from shale or mudstone.

Phyllite

A fine-grained foliated metamorphic rock with a silky sheen, intermediate between slate and schist.

Schist

A medium- to coarse-grained foliated metamorphic rock with visible platy minerals (e.g., mica) defining a schistosity.

Gneiss

A coarse-grained foliated metamorphic rock with compositional banding of light and dark minerals.

Marble

A non-foliated metamorphic rock composed of recrystallized calcite or dolomite; derived from limestone or dolostone.

Quartzite

A non-foliated metamorphic rock composed of recrystallized quartz grains; derived from quartz sandstone.

Hornfels

A fine-grained, non-foliated metamorphic rock formed by contact metamorphism.

Mylonite

A fine-grained, foliated metamorphic rock formed by intense shearing in ductile fault zones.

Regional Metamorphism

Large-scale metamorphism caused by high temperatures and pressures associated with mountain building (convergent plate boundaries).

Contact Metamorphism

Metamorphism caused primarily by heat from an intruding magma body, producing a localized aureole of non-foliated rocks.

Hydrothermal Metamorphism

Metamorphism driven by hot, ion-rich fluids circulating through rocks, often associated with magmatic intrusions.

Cataclastic (Dynamic) Metamorphism

Metamorphism caused mainly by high shear stress along fault zones, resulting in mechanical crushing and grinding.

Blueschist Metamorphism

High-pressure, low-temperature metamorphism typical of subduction zones, characterized by the mineral glaucophane.

Metasomatism

The process where the chemical composition of a rock is altered by the addition or removal of elements by fluids.

Anisotropic (Engineering)

Having physical properties (e.g., strength, permeability) that vary with direction; a key characteristic of foliated metamorphic rocks.

Bowen's Reaction Series

A sequence describing the order of crystallization of minerals from a cooling magma, explaining mineral stability during weathering and metamorphism.

Partial Melting

The process where only a portion of a rock melts due to different melting points of its constituent minerals, producing magma.

Decompression Melting

Melting triggered by a decrease in pressure as hot mantle rock rises, without a significant temperature increase.

Flux Melting

Melting triggered by the addition of volatiles (like water) which lower the melting point of rock.

Fractional Crystallization

The process where early-formed minerals are separated from the magma (e.g., by settling), changing the magma's composition.

Magma Composition Categories

Ultramafic (

Felsic Magma/Rock

High silica content, viscous, light-colored (e.g., granite, rhyolite); forms continental crust.

Mafic Magma/Rock

Low silica, high Fe/Mg, fluid, dark-colored (e.g., basalt, gabbro); forms oceanic crust.

Extrusive (Volcanic) Rock

Igneous rock formed from lava cooling rapidly at Earth's surface, resulting in fine-grained or glassy texture.

Intrusive (Plutonic) Rock

Igneous rock formed from magma cooling slowly beneath the surface, resulting in coarse-grained texture.

Pluton

A general term for a body of intrusive igneous rock.

Batholith

A large pluton with an exposed surface area greater than 100 km².

Stock

A pluton with an exposed surface area less than 100 km².

Dike

A tabular intrusive body that cuts across pre-existing rock layers.

Sill

A tabular intrusive body that parallels pre-existing rock layers.

Laccolith

A lens-shaped intrusive body that pushes overlying rock layers upward.

Columnar Jointing

A fracture pattern forming polygonal columns, common in cooling lava flows like basalt.

Weathering

The breakdown of rocks at Earth's surface by physical, chemical, or biological processes.

Mechanical (Physical) Weathering

The physical breakdown of rock into smaller pieces without changing its chemical composition (e.g., frost wedging, exfoliation).

Chemical Weathering

The alteration of rock minerals through chemical reactions (e.g., hydrolysis, oxidation, dissolution).

Hydrolysis

A chemical weathering process where minerals react with water, often forming clay minerals (e.g., feldspar to kaolinite).

Oxidation

A chemical weathering process where minerals react with oxygen, commonly forming iron oxides (rust).

Dissolution

A chemical weathering process where minerals completely dissolve in water (e.g., calcite in acidic water).

Erosion

The transportation of weathered material by wind, water, ice, or gravity.

Sediment

Weathered and eroded rock fragments (clasts) or chemical precipitates.

Lithification

The process converting loose sediment into solid rock via compaction and cementation.

Clastic Sedimentary Rock

Rock composed of transported solid fragments (clasts), classified by grain size (e.g., conglomerate, sandstone, shale).

Chemical Sedimentary Rock

Rock formed from minerals precipitated from solution (e.g., limestone, rock salt, chert).

Biochemical Sedimentary Rock

A subtype of chemical rock formed from biological processes (e.g., limestone from shells, coal from plants).

Udden-Wentworth Scale

A grain-size scale for sediments: clay, silt, sand, pebble, cobble, boulder.

Mudrock

A fine-grained clastic sedimentary rock with >75% silt and clay (includes shale and mudstone).

Shale

A fine-grained, laminated mudrock that breaks into thin layers (fissile).

Sandstone

A clastic sedimentary rock composed mainly of sand-sized grains, often quartz-rich.

Conglomerate

A clastic sedimentary rock with rounded gravel-sized clasts (pebbles, cobbles).

Breccia

A clastic sedimentary rock with angular gravel-sized clasts.

Limestone

A chemical/biochemical sedimentary rock composed primarily of calcite (CaCO3).

Dolostone (Dolomite)

A chemical sedimentary rock composed primarily of the mineral dolomite (CaMg(CO3)2).

Evaporite

A chemical sedimentary rock formed by the evaporation of water, leaving behind dissolved minerals (e.g., rock salt, gypsum).

Principle of Original Horizontality

Sediments are deposited in horizontal layers; tilting indicates post-depositional deformation.

Principle of Superposition

In an undisturbed sequence, the oldest rock layers are at the bottom and the youngest are at the top.

Cross-bedding

Inclined layers within a horizontal sedimentary bed, indicating deposition by water or wind currents.

Graded Bedding

A sedimentary layer showing a gradual change in grain size from bottom to top, typically from coarse to fine.

Slaking

The rapid breakdown of weak rock (especially shale) upon exposure to air and water after excavation.

Karst Topography

A landscape formed from the dissolution of soluble rocks like limestone, characterized by sinkholes, caves, and underground drainage.

Engineering Weathering Grade

A classification (I-VI) describing the degree of rock weathering, from fresh rock to residual soil, critical for geotechnical assessment