Rocks, the Rock Cycle, and Igneous Rocks

Vocabulary practice flashcards covering the major rock groups, the rock cycle, magma characteristics, igneous textures, mineral compositions, and specific igneous rock types.

Igneous Rocks

Rocks that form as molten rock (magma or lava) cools and solidifies.

Sedimentary Rocks

Rocks formed from sediments derived from the weathering of preexisting rocks that accumulate in layers at Earth's surface and undergo lithification.

Metamorphic Rocks

Rocks formed by the alteration of preexisting igneous, sedimentary, or other metamorphic rocks, primarily driven by heat and pressure.

Rock Cycle

A geologic model that visualizes the interrelationships among different parts of the Earth system, illustrating how rocks constantly form, change, and reform over long time spans.

Magma

Molten rock that forms from the partial melting of rocks deep beneath Earth's surface, serving as the parent material of igneous rocks.

Lava

Magma that has reached Earth's surface.

Melt

The liquid portion of magma.

Volatiles

Dissolved gases in magma that vaporize at surface pressure, most commonly including water vapor ($H_2O$), carbon dioxide ($CO_2$), and sulfur dioxide ($SO_2$).

Crystallization

The cooling process of magma that results in the systematic arrangement of ions into orderly crystalline patterns.

Silicon-Oxygen Tetrahedron

The basic building block of silicate minerals, formed when silicon and oxygen atoms link together during the early stages of magma crystallization.

Intrusive Igneous Rocks (Plutonic Rocks)

Igneous rocks formed when magma crystallizes at depth beneath Earth's surface, later exposed after uplifting and erosion of overlying rocks.

Extrusive Igneous Rocks (Volcanic Rocks)

Igneous rocks formed by the cooling and solidification of lava or volcanic debris on Earth's surface.

Texture

The overall appearance of a rock based on the size, shape, and arrangement of its mineral crystals.

Aphanitic Texture

A fine-grained igneous texture characterized by microscopic crystals that cooled rapidly and are too small to be identified without a microscope.

Phaneritic Texture

A coarse-grained igneous texture resulting from slow cooling, composed of mineral grains large enough to be identified without a microscope.

Porphyritic Texture

An igneous texture featuring two distinctly different crystal sizes, formed when some minerals grow large before the magma moves to a different environment where the remaining minerals crystallize quickly.

Phenocrysts

Large, conspicuous crystals embedded within the finer-grained matrix of an igneous rock displaying a porphyritic texture.

Groundmass

The matrix of smaller crystals surrounding larger phenocrysts in an igneous rock with a porphyritic texture.

Vesicular Texture

An extrusive igneous texture containing spherical or elongated voids left by gas bubbles that escaped as lava solidified.

Glassy Texture

An igneous texture formed by very rapid cooling, during which ions are frozen in place before they can unite into an orderly crystalline structure.

Pyroclastic (Fragmental) Texture

An igneous texture produced by the consolidation of individual rock fragments, ash, or molten blobs ejected during an explosive volcanic eruption.

Pegmatitic Texture

An exceptionally coarse-grained igneous texture that forms in the late stages of magma crystallization within rocks called pegmatites.

Light Silicates (Felsic Minerals)

Generally light-colored silicate minerals lacking iron and magnesium, with a specific gravity of approximately $2.7$, including feldspars, quartz, muscovite, and clays.

Dark Silicates (Mafic Minerals)

Generally dark-colored silicate minerals containing iron and/or magnesium, with a specific gravity between $3.2$ and $3.6$, including olivine, pyroxene, amphibole, and biotite.

Felsic (Granitic) Composition

An igneous composition high in silica ($SiO_2$) and feldspar, composed almost entirely of light silicates like quartz and potassium feldspar; the major constituent of continental crust.

Granite

A coarse-grained (phaneritic) felsic igneous rock containing $10-20\%$ quartz, roughly $50\%$ potassium feldspar, and minor amounts of dark silicates.

Rhyolite

A fine-grained (aphanitic), buff to pink or light gray extrusive equivalent of granite composed primarily of light-colored silicates.

Obsidian

A dark-colored, glassy volcanic rock with a felsic chemical composition similar to granite, formed when silica-rich lava cools very rapidly at Earth's surface.

Pumice

A frothy, light-colored volcanic glass with a vesicular texture formed by gas escaping from lava, often displaying fine intertwined glass shards and capable of floating in water.

Intermediate (Andesitic) Composition

An igneous composition containing $25\%$ or more dark silicate minerals (amphibole, pyroxene, and biotite mica), typically associated with volcanic island arcs.

Andesite

A medium-gray, fine-grained (aphanitic) volcanic rock of intermediate composition that commonly exhibits a porphyritic texture.

Diorite

A coarse-grained (phaneritic) intrusive equivalent of andesite that resembles gray granite but lacks visible quartz crystals, often showing a salt-and-pepper appearance.

Mafic (Basaltic) Composition

An igneous composition dominated by dark silicates and calcium-rich plagioclase feldspar, higher in density than granitic rocks, which comprises the ocean floor and volcanic islands.

Basalt

A very dark green to black, fine-grained extrusive igneous rock composed predominantly of pyroxene and calcium-rich plagioclase feldspar, constituting the upper layers of oceanic crust.

Gabbro

A dark green to black, coarse-grained (phaneritic) intrusive equivalent of basalt, composed mostly of pyroxene and calcium-rich plagioclase feldspar.

Ultramafic Composition

A rare igneous rock composition made almost entirely of ferromagnesian minerals, primarily olivine and pyroxene.

Peridotite

An ultramafic, coarse-grained igneous rock composed mostly of olivine and pyroxene, representing the main constituent of Earth's upper mantle.