Igneous Rocks Overview

These flashcards cover key concepts and terminology related to igneous rocks, including their formation, classification, textures, and identification procedures.

Igneous Rocks

Aggregates of one or more minerals formed through cooling and crystallization of magma or lava.

Felsic

Igneous rocks containing >70% silica; typically light or pink in color, often found in continental crust.

Intermediate

Igneous rocks containing ~60% silica; typically gray or salt and pepper appearance, associated with subduction zones.

Mafic

Igneous rocks containing 40-50% silica; usually dark gray, making up the oceanic crust.

Ultramafic

Igneous rocks containing <40% silica; dark green or almost black, primarily found in mantle rocks.

Intrusive Rocks

Rocks formed by the slow cooling of magma intruding into other rocks.

Phaneritic Texture

Texture characterized by large crystals, indicating slow cooling of intrusive rocks.

Porphyritic Texture

Texture involving a mixture of crystal sizes, resulting from two periods of cooling.

Extrusive Rocks

Rocks formed by the rapid cooling of magma on the Earth's surface.

Aphanitic Texture

Texture with small crystals due to fast cooling, characteristic of extrusive rocks.

Vesicular Texture

Texture resulting in holes from trapped gas, typical of certain extrusive rocks.

Tuffaceous

Referring to rocks that contain fragments of tuff (ash) less than 4mm in size.

Breccia

A type of sedimentary rock composed of angular fragments greater than 4mm in size.

Glassy Texture

Resulting from instantaneous cooling, leading to a lack of crystals in the rock.

Volcanism

The eruption of magma onto the Earth's surface, largely controlled by plate motion.

Tectonic Boundaries

Regions where plates meet, influencing the types of igneous rocks formed.

Igneous Rock Identification

Procedure to identify igneous rocks based on their composition and texture.

Igneous Rock Key

A tool or table used to assist in the identification of igneous rock samples.

Granite

An intrusive igneous rock with a phaneritic texture, primarily felsic in composition.

Diorite

An intrusive igneous rock with a phaneritic texture, typically intermediate in composition.

Gabbro

An intrusive igneous rock with a phaneritic texture, mainly mafic in composition.

Peridotite

An ultramafic intrusive rock composed primarily of olivine and pyroxene.

Rhyolite

An extrusive igneous rock with an aphanitic texture, primarily felsic in composition.

Andesite

An extrusive igneous rock that is intermediate in composition with an aphanitic texture.

Basalt

An extrusive igneous rock that is mafic in composition with an aphanitic texture.

Pumice

An extrusive igneous rock characterized by a vesicular texture, often light in color.

Scoria

An extrusive igneous rock with a vesicular texture, typically darker and denser than pumice.

Obsidian

A glassy extrusive igneous rock formed from rapid cooling of lava.