Earth Structure, Mineralogy, Petrology, and Metamorphism Vocabulary

Vocabulary flashcards covering key geologic terms including Earth's structure, mineralogy, petrology, sedimentary processes, and metamorphism.

Differentiation

The process in protoplanets where heating from collisions and radioactive decay causes internal layering; dense iron and nickel sink to form a planet core, while lighter elements rise and volatile gases escape.

Iron catastrophe

An event during differentiation where iron and nickel sink to form the core, lighter elements rise to form outer layers, and volatile gases escape to the atmosphere.

d'' zone

A layer within the Earth's interior described as a slab graveyard.

Moho

The boundary between the Earth's crust and the lithospheric mantle.

Mineral

A naturally occurring, crystalline solid with a definite (but not fixed) chemical composition that is generally inorganic.

Amorphous

Descriptive term for minerals that display structural disorder rather than long-range crystalline order.

Unit cell

The smallest repeating unit of a crystal lattice that serves as the building block of a crystal.

Isometric / Cubic System

The crystal system characterized by having the highest possible degree of symmetry in three-dimensional space.

Index of Refraction ($n$)

A value comparing the speed of light in a vacuum to the speed of light in a material, expressed as $n = \frac{\text{Velocity in vacuum}}{\text{Velocity in object}}$.

Polarizer

A microscope optical device (also called an analyzer) that restricts unpolarized light so it vibrates in a single direction.

Plane-Polarized Light (PPL)

Microscopic illumination using only the lower polarizer (oriented east-west), which decreases overall light intensity.

Cross-Polarized Light (XPL)

Microscopic illumination using both upper (north-south) and lower (east-west) polarizers, creating a dark field unless an anisotropic thin section is present.

Pleochroism

An optical property observable under PPL where a mineral changes color as the microscope stage is rotated due to differential wavelength absorption.

Relief

An optical feature in PPL determined by refractive index ($n$) that describes how strongly a mineral stands out against the epoxy and surrounding minerals.

Euhedral

A crystal habit formed by unimpeded growth in open space, characterized by well-formed flat faces and sharp, geometric edges.

Anhedral

A crystal habit formed in crowded environments, lacking flat faces and exhibiting irregular or rounded outer edges.

Subhedral

A crystal habit displaying intermediate features, with a mix of well-formed flat faces and rounded or incomplete boundaries.

Anisotropic mineral

A mineral that interacts with light differently along different crystallographic directions under PPL and XPL (includes all crystal systems except cubic).

Isotropic mineral

A mineral with a uniform refractive index ($n$) in all directions that remains extinct under XPL (includes gas, liquids, and cubic system minerals).

Parallel extinction

Extinction behavior where an anisotropic mineral becomes dark when its cleavage or crystal length is parallel to the microscope cross-hairs.

Inclined extinction

Extinction behavior where an anisotropic mineral becomes dark when its features are aligned at an angle relative to the microscope cross-hairs.

Birefringence

The numerical difference in refractive index ($n$) between the fastest and slowest crystal directions in an anisotropic mineral.

Bird's eye extinction

A distinctive pebbly or grainy extinction texture visible under XPL in micas like biotite.

Undulose extinction

A wavy, irregular extinction pattern visible under XPL, common in metamorphosed quartz.

Polysynthetic twinning

Parallel, alternating light and dark stripe patterns characteristic of plagioclase under XPL.

Tartan twinning

A cross-hatched or grid-like plaid extinction pattern characteristic of microcline under XPL.

Ionic bonds

Chemical bonds formed by electron transfer between atoms with nearly full and nearly empty outer shells.

Covalent bonds

Durable chemical bonds formed by electron sharing between atoms with half-filled outer shells.

Nesosilicates

Silicate structure containing isolated tetrahedra bonded only by interstitial cations, with a silica-to-oxygen ratio of $1:4$ (e.g., olivine).

Sorosilicates

Silicate structure featuring paired tetrahedra (dimers) sharing a single oxygen, with a silica-to-oxygen ratio of $1:3.5$ (e.g., epidote).

Cyclosilicates

Silicate structure featuring rings of tetrahedra sharing two corner oxygens, with a silica-to-oxygen ratio of $1:3$ (e.g., tourmaline).

Single chain Inosilicates

Silicate structure where tetrahedra form single chains sharing two oxygens, linked by cations, with a silica-to-oxygen ratio of $1:3$ (e.g., pyroxene).

Double chain Inosilicates

Silicate structure formed by two interconnected single chains sharing corner oxygens, with a silica-to-oxygen ratio of $1:2.75$ (e.g., amphibole).

Phyllosilicates

Sheet silicate structure formed by tetrahedra sheets linked by aluminum octahedra, with a silica-to-oxygen ratio of $1:2.5$ (e.g., micas and clays).

Tectosilicates

Three-dimensional silicate framework where every oxygen atom is shared between tetrahedra, giving a silica-to-oxygen ratio of $1:2$ (e.g., quartz, feldspar).

Solidus

The temperature boundary below which a rock is completely solid and above which partial melting initiates.

Liquidus

The temperature boundary above which a material is completely liquid.

Flux Melting

Melting driven by the addition of volatiles like water to hot mantle rock, lowering the solidus temperature (characteristic of subduction zones).

Decompression Melting

Melting induced by a decrease in pressure on hot rock without the addition of heat, lowering the solidus conditions (characteristic of mid-ocean ridges and rifts).

Fractional crystallization

The separation and removal of early-formed crystals from a cooling magma chamber, progressively causing the remaining melt to become more felsic.

Assimilation

The incorporation and melting of surrounding country rock into a magma chamber, altering the magma composition toward higher silica contents.

Obduction

A tectonic process in which dense fragments of oceanic crust and mantle are overthrust onto lighter continental lithosphere.

Wilson cycle

A cyclic tectonic model describing the opening and closing of ocean basins over geological time.

Dike

A sheet-like, planar igneous intrusion that cuts vertically across pre-existing rock layers.

Sill

A tabular, planar igneous intrusion injected horizontally parallel to host rock layering.

Pluton

A generic term for a subsurface body of intrusive igneous rock, categorized as a batholith (large) or stock (area $< 100\,\text{km}^2$).

Tuff

A porous, lightweight rock formed from lithified and cemented fine volcanic ash and fragments.

Pyroclastic flow

A rapidly moving, high-density avalanche of hot gas, volcanic ash, pumice, and rock debris traveling down a volcanic slope.

Ignimbrite

A volcanic deposit formed by the consolidation of material laid down by pyroclastic density currents.

Diagenesis

The set of chemical, physical, and biological changes that transform loose sediment into consolidated rock at temperatures below $150\text{--}200\,^\circ\text{C}$.

Bedload

Coarse sedimentary material (sand, gravel, boulders) transported along or near a stream bed via sliding, rolling, or saltation.

Suspended load

Fine sediment particles (silt and clay) held aloft within the turbulent body of a moving fluid.

Dissolved load

Chemical ions carried in solution in water derived from mineral chemical weathering.

Ooids

Spherical carbonate grains formed by concentric layers of calcium carbonate precipitated around a central nucleus.

Micrite

Fine-grained microcrystalline carbonate mud produced by mechanical breakdown or biochemical precipitation.

Sparite

Coarse, clear crystalline carbonate cement precipitated in pore spaces from aqueous fluids.

Alluvium

Unconsolidated sediment transported and deposited by flowing water in a stream system.

Colluvium

Loose sediment accumulated at the base of a slope due to gravity-driven mass movement.

Diamictite

A generic term for any lithified, poorly sorted sedimentary rock containing clasts of diverse sizes in a mud/sand matrix.

Metamorphism

Solid-state textural and mineralogical alterations in a rock induced by changes in temperature and pressure without chemical alteration of the bulk rock.

Metasomatism

Metamorphic alteration accompanied by substantial chemical change in bulk composition due to hydrothermal fluid flow.

Protolith

The unmetamorphosed parent rock from which a metamorphic rock is derived.

Prograde metamorphism

Metamorphic mineral changes occurring in response to conditions of increasing temperature and pressure.

Retrograde metamorphism

Metamorphic mineral changes occurring under conditions of decreasing temperature and pressure.

Neocrystallization

The growth of entirely new mineral species in a metamorphic rock as pre-existing minerals become unstable and break down.

Mylonite

A fine-grained, strongly foliated metamorphic rock produced by intense ductile shear along fault zones at depth.

Blueschist

A high-pressure, low-temperature metamorphic rock typically formed in subduction zones.