How the Earth Works: Minerals and Igneous Rocks

Igneous

form from crystallization of molten material. Have interlocking crystals.

Sedimentary

form from lithification of fragments of preexisting rocks

Metamorphic

form from alteration of preexisting rock from application of heat, pressure, or both

Gneiss

a high grade metamorphic rock

Intrusive

form underground

Extrusive

form at Earth’s surface

Phaneritic

grains can be seen with the naked eye (found in intrusive igneous rocks)

Aphanitic

grains are microscopic (found in extrusive rocks)

Porphyritic

large, visible crystals embedded in a finer-grained groundmass

Pluton

Any body of intrusive igneous rock that cooled and crystallized below Earth's surface

Discordant

cut across existing rock layers/bedding

Concordant

bodies squeeze in parallel to existing layers.

Dike

A discordant, tabular (sheet-like) intrusion that cuts across the layering of the host rock, often filling vertical or steeply-angled fractures

Sill

A concordant, tabular intrusion that squeezes in parallel to existing rock layers (bedding planes

Laccolith

A concordant intrusion where the magma is thick/viscous enough that it domes up the overlying rock layers rather than spreading flat, creating a mushroom- or blister-like bulge.

Stock

A discordant, irregularly shaped intrusive body with a surface exposure less than 100 km² (about 40 square miles).

Batholith

a massive, discordant intrusive body exceeding 100 km² in surface exposure. Batholiths are often composites — many individual plutons that intruded over time and merged into one large mass.

Flood basalts

enormous volumes of low-viscosity basaltic lava erupted rapidly from fissures rather than a single central volcano, spreading out as broad, flat sheets that can cover huge areas.

Pāhoehoe Lava

Smooth, ropy, billowy-surfaced lava. Forms when basalt is hot and fluid allowing the outer skin to solidify into thin, wrinkled folds while the still-molten interior keeps moving underneath

Aa Lava

Rough, jagged, clinkery, blocky surface. Forms when basalt is cooler, more viscous, or moving faster/more turbulently.

Pillow lava

Forms when basalt erupts or flows underwater

Felsic Lava

is molten rock erupted at Earth's surface that is rich in silica, usually violent

Pyroclastic Eruptions

Produce ash and are volcanic bombs

Intermediate Eruptions

Produce medium viscosity lava and are explosive. Usually associated with the most violent hazards

Caldera

A large, roughly circular depression formed when a volcano's summit collapses into an emptied magma chamber below, usually after a major eruption.

Crater

A bowl-shaped depression at a volcano's summit or vent, formed by explosive ejection of material or simple accumulation of debris around the opening

Central vent

The main conduit through which magma rises and erupts at a volcano's summit, typically located at the center of the cone

Cinder Cone

A small, steep-sided volcanic cone built from loose pyroclastic fragments

Shield Volcano

A broad, gently-sloping volcano built from low-viscosity basaltic lava flows that spread far from the vent before solidifying

Composite Volcano

A tall, steep-sided, symmetrical cone built from alternating layers of lava flows, ash, and pyroclastic debris, produced by intermediate magma

Pyroclastic deposits

Accumulations of fragmented volcanic material

Pyroclastic Flows

A fast-moving, ground-hugging current of hot gas, ash, and volcanic debris that races down a volcano's slopes

Volcanic Ash

Fine, fragmented particles of pulverized rock, minerals, and volcanic glass less than 2mm in diameter

Lahar

A destructive mudflow or debris flow composed of volcanic material mixed with water, often triggered by melting snow/ice during an eruption

Nuée Ardente

another term for a pyroclastic flow — a fast-moving, superheated avalanche of gas, ash, and volcanic debris that races down a volcano's slopes

Phreatic Explosion

steam-driven volcanic explosion caused when groundwater or surface water contacts hot rock or magma, flashing instantly to steam and rupturing the surrounding rock.

CO 2 “Eruption”

sudden, non-volcanic release of carbon dioxide gas, most notably from volcanic lakes where CO₂ has accumulated in deep water from underlying magmatic sources

Hot Springs

A surface pool or spring of groundwater heated by nearby magma or hot rock at depth, then rising back to the surface.

Mudpot

A type of hydrothermal feature where limited groundwater mixes with volcanic gases and acid to break down surrounding rock into a bubbling, muddy slurry

Geyser

A hot spring that periodically erupts, ejecting a column of boiling water and steam into the air.

Element

substance that cannot be broken down into simpler substances by chemical means; defined by its number of protons

Mineral

A naturally occurring, inorganic solid with a definite chemical composition and an ordered crystalline structure.

Rock

A naturally occurring aggregate of one or more minerals consolidated into a solid mass

volcanic necks

An erosion-resistant pillar of solidified magma that once filled the central conduit (vent) of a volcano, left standing after the surrounding softer volcanic cone or ash has eroded away.

glassy

texture describing a smooth, vitreous surface, similar in appearance to man-made glass

pyroclastic

composed of fragmented volcanic material

toxic gases

Harmful gases released during volcanic eruptions or emitted continuously from vents, fumaroles, and magma at depth

climatic changes

Short- and long-term changes to global or regional climate caused by volcanic eruptions

Crystalline solid

(table salt, or halite)

Vesicular

Texture where a rock pitted with many holes and cavities