Petrology: Igneous Rocks I (Texture, Structure, and Composition)

A set of vocabulary flashcards defining the key terms, textures, structures, and mineralogical components of igneous rocks as presented in the Petrology lecture notes.

Igneous Bodies

A term referring to the shape of an igneous rock body relative to its position of formation within the Earth's crust.

Extrusive / Volcanic Rocks

Igneous rocks that form and solidify on the surface of the Earth, characterized by fast to very fast cooling rates.

Hypabyssal / Shallow Intrusion

Also known as minor intrusions, these are igneous bodies measuring meters to tens of meters in size that form at shallow depths near the surface.

Plutonic / Deep Intrusion

Large-scale igneous bodies (plutons or batholiths) measuring tens to hundreds of kilometers that form deep within the crust with very slow cooling rates.

Lava Pahoehoe

A type of basaltic lava characterized by a smooth or ropy surface texture.

Lava Blocky

A type of lava typically associated with andesitic to dacitic compositions.

Pillow Lava

Magma that solidifies underwater, forming characteristic rounded shapes.

Dyke

A discordant shallow intrusion that cuts across the layering of the host rock.

Sill

A concordant shallow intrusion that is parallel to the layering of the host rock.

Xenolith

A fragment of earlier rock, often the host rock, contained within an igneous body.

Holocrystalline

A degree of crystallization where the rock is composed entirely of crystals.

Hypocrystalline

A degree of crystallization where the rock is composed of both crystals and volcanic glass.

Holohyaline

A degree of crystallization where the rock is comprised mostly of volcanic glass.

Phaneritic

An equigranular texture where the crystal grains are relatively uniform, coarse, and can be distinguished by the naked eye.

Aphanitic

An equigranular texture where the crystal grains are uniform but so fine that they are difficult to distinguish without magnification.

Porphyritic

An inequigranular texture where larger crystals (phenocrysts) are surrounded by a finer-grained groundmass.

Vitrophyric

An inequigranular texture where larger crystals (phenocrysts) are surrounded by a glassy groundmass.

Pegmatite

An extremely coarse-grained igneous texture (crystals > $30\,mm$) resulting from very slow cooling and high water content in very deep intrusions.

Diabase

A mafic, hypabyssal rock characterized by needle-like plagioclase and pyroxene phenocrysts surrounded by an aphanitic groundmass.

Euhedral

A crystal fabric description where the mineral is bounded by its own well-formed crystal faces.

Vesicular

A rock structure containing small holes created by trapped gas during solidification, such as in scoria or pumice.

Amygdaloidal

A structure where original gas holes (vesicles) have been filled with secondary minerals like quartz, calcite, or zeolite.

Sheeting Joint

A type of structure formed by the removal of overburden or pressure release, appearing as layered cracks.

Columnar Joint

Hexagonal or polygonal rock columns formed specifically due to the cooling and contraction of an igneous body.

Essential Minerals

Primary minerals that are abundant and necessary for the naming and classification of a rock, such as quartz in granite.

Accessory Minerals

Primary minerals present in quantities < $5\%$ that do not typically influence the primary naming of the rock.

Secondary Minerals

Minerals formed after the initial solidification of the rock through processes like hydrothermal alteration or weathering, such as chlorite or epidote.

Felsic Minerals

Light-colored, acidic minerals rich in silica (Si), including quartz, K-feldspar, Na-plagioclase, and muscovite.

Mafic Minerals

Dark-colored, basic minerals rich in iron (Fe) and magnesium (Mg), such as olivine, pyroxene, amphibole, and biotite.