Petrology Flashcards

Flashcards covering key vocabulary from a petrology lecture, focusing on the origin, structure, and composition of igneous, sedimentary, and metamorphic rocks.

Petrology

The branch of science concerned with the origin, small-scale structure, and composition of rocks.

Igneous Rocks Origin

Derived from the Latin word 'ignis' meaning fire; formed from hot molten material by cooling and crystallization of magma.

Magma

Molten rock beneath the Earth's surface.

Lava

Molten rock that erupts onto the Earth's surface.

Volcanic Rocks

Rocks formed by cooling and crystallization of lava at the surface. Lava cools faster, resulting in smaller crystal grain sizes.

Plutonic Rocks

Rocks formed at considerable depths (7-10 km) below the Earth's surface.

Hypabyssal Rocks

Rocks formed at intermediate depths (up to 2 km) below the Earth's surface.

Igneous Rock Texture

The mutual relationship of different mineralogical constituents in a rock, determined by size, shape, and arrangement.

Holocrystalline

Termed as phaneritic; implies distinctly crystallized forms easily recognized by the unaided eye.

Holohyaline

Very fine in size and glassy or non-crystalline in nature.

Granularity

Average dimensions of different constituent minerals, used to describe the grain size of the rock.

Coarse-grained

Average grain size is above 5mm and easily identified with the naked eye.

Medium-grained

Average grain size lies between 5mm and 1mm.

Fine-grained

Average grain size is less than 1mm.

Fabric

Describes the shape and arrangement of mineral grains (euhedral, subhedral, anhedral).

Panidiomorphic

Fully developed crystal shapes.

Hypidiomorphic

Crystals displaying euhedral, subhedral, or anhedral characteristics.

Allotriomorphic

Most crystals are anhedral or irregular shapes.

Equigranular Textures

Textures where the majority of constituent crystals are broadly equal in size.

Inequigranular Textures

Textures where the majority of constituent minerals show a marked difference in their relative grain size.

Directive Textures

Textures that indicate the result of flow of magma during the formation of rocks.

Intergrowth Textures

Describes the situation where two or more minerals crystallize simultaneously in a limited space, resulting in intergrown crystals.

Intergranular Textures

Textures where crystals formed at earlier stages create polygonal or trigonal spaces filled by other minerals.

Graphic Texture

A texture with conspicuous and regular intergrowth between quartz and feldspar crystals resembling cuneiform writing.

Granophyric Texture

A less well-defined intergrowth of quartz and alkali feldspar in an igneous rock, often somewhat radiating.

Spherulitic Textures

Spherical intergrowths of radiating quartz and feldspar.

Intergranular

The angular interstices between plagioclase grains are occupied by grains of ferromagnesian minerals

Intersertal

The angular spaces between larger crystals are occupied by glass, or glass and small crystals.

Concordant Bodies

Does not cross cut country rock structure, bedding or metamorphic fabric.

Discordant Bodies

Cross cuts country rock structure.

Concordant Bodies

Intrusions in which the magma has been injected and cooled along or parallel to the structural planes of the host rocks.

Sill

A sheet of magma injected along or between the bedding planes of a sedimentary sequence.

Phacoliths

Occupy positions in the troughs and crests of bends (folds).

Laccoliths

A dome shaped intrusive body that has intruded between layers of sedimentary rock.

Lopoliths

A concordant body with a roughly flat top and a shallow convex base, may have a feeder dike or pipe below.

Discordant Bodies

Intrusive bodies that have been injected into strata without being influenced by their structural disposition, traverse across or oblique to bedding planes.

Dyke

A vertical or near-vertical intrusive igneous rock body that cuts across rock beds.

Cone Sheets

Assemblages of dyke-like injections, which are generally inclined towards common centers.

Ring Dykes

Typically arcuate, closed, and ring-shaped outcrops.

Volcanic Neck

A tubular roughly vertical body that may have been a feeder vent for a volcano.

Batholiths

Huge bodies of igneous masses that show both concordant and discordant relations with the country rock.

Granite

Plutonic light colored igneous rocks with abundance of quartz and felspar orthoclase as essential minerals.

Diorite

Intermediate type of igneous rock of plutonic origin; coarse to medium grained and holocrystalline.

Andesite

Family of fine-grained, extrusive igneous rocks that are usually light to dark gray in color, typically found in lava flows produced by stratovolcanoes.

Syenite

Igneous, plutonic, even-grained rocks in which alkali felspars are the most common felspars; textures broadly similar to those of granites.

Dolerites

Formed as shallow sills and dykes, equivalents of gabbros of plutonic origin and basalts of volcanic origin; mostly medium to fine grained rocks.

Basalts

Volcanic igneous rocks formed by rapid cooling from lava flows from volcanoes either over the surface or under water on oceanic floors.

Pegmatites

Exceptionally coarse-grained igneous rocks mostly inequigranular; exhibit great variation in their mineral composition.

Aplites

Igneous rocks of plutonic origin but characterized with a fine-grained, essentially equigranular texture.

Lamprophyre

Any of a group of dark gray to black intrusive igneous rocks that generally occur as dikes; Panidiomorphic, fine-grained and holocrystalline.

Peridotites

Coarse-grained, dark-colored, ultramafic igneous rocks; generally form sills and dykes of moderate size; dominant rock of the upper part of the Earth’s mantle.

Sedimentary Rocks

Secondary rocks formed by accumulation, compaction, and cementation of sediments.

Clastic Rocks

Sedimentary rocks made up of pieces (clasts) of pre-existing rocks.

Weathering

Decay and disintegration of rocks, breaking them into smaller fragments.

Transport of Sediments

Transportation of sediments to a suitable place for transformation into a rock mass.

Diagenesis

Transformation of loose sediments to solid cohesive rock masses under pressure or cementation.

Chemically Formed Rocks

Non-clastic rocks that form from chemical reactions.

Organically Formed Rocks

Non-clastic rocks formed by the death and decay of organisms.

Breccia

A mechanically formed sedimentary rock, also known as Rudite, consisting of angular fragments of heterogeneous composition.

Conglomerates

Sedimentary rock made of rounded gravels also belonging to the Rudaceous group.

Sandstone

Mechanically formed sedimentary rock of arenaceous group that are composed mostly of sand grade particles.

Shale

A fine-grained sedimentary rock of argillaceous (clayey) composition often characterized with a distinct fissility (parting) parallel to bedding planes.

Limestone

Most common sedimentary rocks from the non-clastic group formed both bio-chemically and mechanically largely made up of calcite.

Dolomite

Sedimentary rock composed of calcium magnesium carbonate.

Coals

Metamorphosed sedimentary rocks of carbonaceous character.

Iron Ores

Sedimentary iron deposits formed as chemical precipitates.

Gypsum

Sedimentary rock composed of the mineral gypsum formed from water evaporation

Rock Salt

Sedimentary rock composed of mineral halite (NaCl).

Flint and Chert

Rocks of siliceous composition consisting chiefly of chalcedony and extremely fine-grained sediments.

Tillite

A sedimentary rock of glacial origin.

Metamorphic Rocks

Rocks formed through processes involving the alteration of pre-existing igneous and sedimentary rocks by heat, pressure, and chemically active fluids.

Crystalloblastic Textures

Textures newly imposed upon the rock during metamorphism.

Palimpsest Textures

Textures present in the parent rock and retained despite metamorphic changes.

Foliated Rock

Metamorphic rocks showing development of conspicuous parallelism (foliation).

Non-Foliated Rock

Metamorphic rocks characterized with total or nearly total absence of foliation.

Slate

An extremely fine-grained metamorphic rock characterized by a slaty cleavage.

Schists

Megascopically crystalline foliated metamorphic rocks characterized by a typical schistose structure.

Gneiss

A megascopically crystalline foliated metamorphic rock characterized by segregation of constituent minerals into layers or bands.

Quartzite

Granular metamorphic rocks composed chiefly of intersutured grains of quartz.

Marble

A granular metamorphic rock composed chiefly of recrystallized limestone (made of mineral calcite).