Igneous Rocks Lecture

Vocabulary flashcards covering intrusive and extrusive igneous rocks, intrusive structures, volcanic features, and textural classifications.

Igneous Rocks

Rocks that form from molten rock (magma) crystallizing below Earth's surface or from volcanic activity.

Plutonic Rocks

Intrusive igneous rocks that form from cooling magma at depth beneath Earth's surface; they are usually coarse-grained and cool slowly.

Extrusive Igneous Rocks

Igneous rocks that form from volcanic activity at or near Earth's surface; they are usually fine-grained and cool quickly.

Batholith

A major-scale, usually discordant intrusive body with a map area $> 100\,\text{km}^2$, made of many stocks on the scale of an entire mountain range.

Stock

A fairly large igneous intrusion with a map area $< 100\,\text{km}^2$ that cuts across pre-existing rock layers on the order of an individual mountain peak.

Lopolith

A large, concordant, dish-shaped or inverted mushroom-shaped intrusive body that sags down in the middle due to dense rocks.

Laccolith

A concordant, intermediate-scale mushroom-shaped intrusion that puffs up in the center due to gases.

Sill

A tabular, concordant igneous intrusion that spreads out parallel to pre-existing rock layers.

Dike

A tabular, discordant igneous body that exhibits a cross-cutting relationship with pre-existing country rock.

Pahoehoe Flows

Smooth, ropey lava flows that are dominantly basalt with low viscosity and gas content.

Aa Flows

Rough and irregular lava flows, also referred to as block flows.

Baked Zones

Oxidized zones formed due to contact with a high-temperature lava flow.

Lava Dome

Thick, steep-sided flows formed from dacite or rhyolite characterized by high viscosity and low gas content.

Tephra

Fragmental volcanic material or pyroclastic particles broken during the eruption of magma.

Ash

Pyroclastic particles categorized by a particle size of $< 2.0\,\text{mm}$.

Lapilli

Pyroclastic particles categorized by a particle size of $2\text{--}64\,\text{mm}$.

Blocks and Bombs

Pyroclastic particles categorized by a particle size of $> 64\,\text{mm}$.

Pyroclastic Flow

A fast-moving, high-density flow of hot ash, crystals, blocks, and/or pumice (also known as nuée ardente or ignimbrite) that follows topographic lows.

Cinder Cones

Volcanoes formed from explosive eruptions, made mostly of cinders and rock particles with a narrow base and steep sides.

Shield Volcanoes

Dome-shaped volcanic mountains formed by quiet flows of runny lava over a large area, such as Mauna Loa ($4\,\text{km}$ above sea level).

Composite Volcanoes

Cone-shaped volcanic mountains built up of alternating layers of rock particles and lava from alternating violent and quiet eruptions.

Crater

A funnel-shaped pit or depression located at the top of a volcanic cone.

Caldera

A great central depression formed when volcanic walls collapse, the central top explodes/collapses, or through crater erosion and enlargement.

Phaneritic Texture

An igneous rock texture where crystals are visible with the naked eye, typical of plutonic or intrusive rocks.

Aphanitic Texture

An igneous rock texture where crystals are too small to be seen with the naked eye, typical of volcanic or extrusive rocks.

Porphyritic Texture

An igneous rock texture featuring two different dominant grain sizes, consisting of large crystals (phenocrysts) in a finer groundmass.

Fragmental Texture

An igneous rock texture composed of disaggregated igneous material, characteristic of pyroclastic rocks.