Volcanism, Seismology, Earth's Interior, and Mountain Building

Comprehensive vocabulary flashcards covering volcanic products and structures, faults and earthquake mechanics, seismic wave propagation, Earth's interior boundaries, gravity anomalies, and structural geology and mountain building.

Basaltic Lava Flow

A highly fluid, low-silica, low-viscosity, and very hot molten rock flow capable of traveling long distances across the ground.

Pahoehoe

A basaltic lava flow characterized by a smooth, glassy, and ropy surface texture that forms when the cooled skin of hot flowing basalt is rolled into ridges.

A'a'

A basaltic lava flow that solidifies into a jagged, sharp, and angular surface of rubbly blocks as cooling lava breaks up during movement.

Columnar Jointing

A distinctive pattern of vertical fractures with hexagonal cross sections that forms as thick, basaltic lava flows cool and contract.

Pillow Basalt

Glassy mounds of rounded basalt blobs created when submarine lava eruptions are rapidly quenched by water, common along mid-ocean ridges.

Lava Dome

A bulbous, steep-sided mass formed when highly viscous, silica-rich rhyolitic lava plugs the volcanic vent instead of flowing away.

Pyroclastic Flow

A fast-moving avalanche of superheated gas, ash, and volcanic debris racing down the slope of a volcano at speeds up to $300\,km/h$ ($186\,mph$) and temperatures of $500\text{--}1000^\circ\text{C}$.

Tephra

Unconsolidated pyroclastic debris of any particle size ejected from an erupting volcano.

Tuff

Lithified volcanic ash that may accumulate as snow-like air-fall deposits or weld together while hot into rock known as ignimbrite.

Lahar

A destructive volcanic mudflow or debris flow formed when water mixes with volcanic ash and larger blocks.

Crater

A circular, bowl-shaped depression atop a volcano, measuring up to $500\,m$ across and $200\,m$ deep, formed as erupted lava piles up around a central vent.

Caldera

A gigantic, steep-walled, flat-floored volcanic depression formed by the inward collapse of a volcano when its underlying magma chamber empties.

Shield Volcano

A broad, gently sloped, dome-shaped volcano built by successive eruptions of low-viscosity basaltic lava that flows over expansive areas.

Cinder Cone

A steep, cone-shaped volcano composed of loose, lapilli-sized pyroclastic ejecta accumulated at the angle of repose around a central vent.

Stratovolcano (Composite Volcano)

A large, steep-sided, cone-shaped volcano constructed from alternating layers of felsic to intermediate lava flows, tephra, and debris.

Flood Basalts

Vast, thick accumulations of low-viscosity basaltic lava erupted from fissures over large geographic provinces, often associated with mantle plumes interacting with rifting lithosphere.

Footwall Block

The rock body situated directly below an inclined fault plane.

Hanging-Wall Block

The rock body situated directly above an inclined fault plane.

Normal Fault

A dip-slip fault formed under crustal extension/tension where the hanging-wall block moves downward relative to the footwall block.

Reverse Fault

A steep-angled dip-slip fault formed under compressional stress where the hanging-wall block moves upward relative to the footwall block.

Thrust Fault

A gently inclined reverse fault dipping at less than $35^\circ$, which accommodates crustal shortening and thickening during compressional mountain building.

Strike-Slip Fault

A near-vertical fault along which one block slides horizontally past another with little to no vertical displacement.

Elastic Strain

The recoverable, non-permanent bending that occurs in rock under tectonic stress prior to brittle rupture and earthquake generation.

Asperities

Microscopic to macroscopic surface irregularities or bumps along a fault plane that interlock and resist slip until tectonic stress overcomes friction.

Stick-Slip Behavior

The stop-and-start pattern of fault movement where friction prevents motion during stress buildup (stick) until friction is briefly overwhelmed by slip.

P-Waves (Primary Waves)

Compressional body waves that travel fastest through rock (approximately $6\,km/s$ in the crust) by particle motion parallel to wave propagation; can pass through solids, liquids, and gases.

S-Waves (Secondary Waves)

Shear body waves that travel slower than primary waves (approximately $4\,km/s$) by moving material perpendicular to wave propagation; can propagate only through solids.

Love Waves (L-Waves)

Surface seismic waves generated by S-waves intersecting Earth's surface that cause the ground surface to undulate laterally back and forth like a snake.

Rayleigh Waves (R-Waves)

Surface seismic waves generated by P-waves intersecting Earth's surface that cause the ground to ripple in an elliptical, rolling wave-like motion.

Vertical-Motion Seismograph

An instrument designed to detect and record up-and-down earthquake vibrations using a suspended inertia weight on a spring.

Horizontal-Motion Seismograph

An instrument designed to detect and record side-to-side earthquake ground shaking using a pivoting pendulum weight.

Epicenter

The point on Earth's surface situated directly above an earthquake's underground focus or hypocenter.

Mercalli Intensity Scale

A subjective scale that assigns Roman numerals ($\text{I}$ to $\text{XII}$) to evaluate the severity of shaking and physical damage caused by an earthquake at a given location.

Moment Magnitude Scale ($M_w$)

A logarithmic measure of earthquake size based on the product of rock rigidity, fault rupture area, and amount of displacement, recognized as the most accurate scale for large events.

Wadati-Benioff Zone

A planar, sloping zone of intermediate- and deep-focus earthquakes that delineates the trajectory of a subducting lithospheric slab descending into the mantle down to $660\,km$.

Liquefaction

The transformation of saturated, water-rich sediment into an unstable slurry during seismic shaking as increased pore-water pressure destroys frictional grain contacts.

Sand Volcano

A small cone or deposit of sand expelled onto the surface when liquefied pressurized subterranean sand squirts through fissures.

Tsunami

A series of massive, long-wavelength ocean waves generated by vertical displacement of the seafloor during a subduction megathrust earthquake or volcanic collapse.

Wave Front

The continuous boundary or surface marking the leading edge of expanding seismic wave energy separating agitated rock from undisturbed rock.

Mohorovičić Discontinuity (Moho)

The seismic boundary separating Earth's crust from the denser, peridotitic mantle, identified by the abrupt acceleration of seismic waves passing through it.

Low-Velocity Zone (LVZ)

A zone within the upper mantle beneath oceanic lithosphere at depths of $100\text{--}200\,km$ where partial melting (up to $\sim 6\%$) reduces seismic-wave velocities.

P-Wave Shadow Zone

A band on Earth's surface between $103^\circ$ and $143^\circ$ from an earthquake epicenter where direct P-waves do not arrive due to refraction at the core-mantle boundary.

S-Wave Shadow Zone

The broad region between $103^\circ$ and $180^\circ$ from an earthquake epicenter where no direct S-waves arrive, demonstrating that Earth's outer core is liquid.

Geoid

An irregular equipotential surface of Earth's gravity that accounts for variations in gravitational acceleration produced by uneven mass distribution inside the planet.

Gravity Anomaly

The difference between the observed gravitational pull at a location and the value predicted by the smooth reference spheroid.

Isostasy (Isostatic Equilibrium)

The condition of gravitational balance in which Earth's crust floats on the denser, plastically deforming asthenosphere according to Archimedes' principle.

Orogenesis

The tectonic process of mountain-building, producing elongate, curvilinear belts of deformed, igneous, and metamorphic rocks.

Stress

The magnitude of force applied per unit area across a surface of rock, categorized into compression, tension, or shear.

Strain

The non-dimensional measure or percentage change in shape, length, or volume of a rock body resulting from applied stress.

Strike and Dip

Planar geometric measurements where strike is the compass bearing of a horizontal line on a tilted plane, and dip is the downward angle of maximum inclination perpendicular to strike.

Plunge and Bearing

Measurements describing linear geologic structures, consisting of the compass direction of the line's trend (bearing) and the angle it tilts downward from horizontal (plunge).

Joints

Planar fractures in brittle rock along which no observable displacement has occurred, typically forming in sets under tensile stress during exhumation.

Vein

A mineral-filled fracture formed when dissolved ions carried by hydrothermal or groundwater solutions precipitate out in open rock fractures.

Slickensides

Polished, grooved fault surface lineations produced by the friction and grinding of rocks sliding past one another.

Mylonite

A fine-grained, foliated metamorphic rock formed by ductile shear deformation in shear zones deep below the brittle-ductile transition.

Anticline

An arch-shaped fold in rock strata where the limbs dip downward away from the hinge, exposing the oldest rock layers along the axial core.

Syncline

A trough-shaped fold in rock strata where the limbs dip inward toward the hinge, exposing the youngest rock layers along the axial center.

Plunging Fold

A fold whose hinge line is inclined at an angle to the horizontal rather than running parallel to the ground surface.

Dome

A circular to elliptical structural fold resembling an overturned bowl, in which older strata are exposed in the center and beds dip outward.

Basin

A circular to elliptical structural fold resembling an upright bowl, in which younger strata occupy the center and beds dip inward.

Flexural-Slip Fold

A fold formed in layered rock where individual beds slide past one another without thinning, similar to bending a deck of cards.

Tectonic Foliation

Planar fabric or mineral alignment developed in deformed rocks as grains flatten, elongate, or recrystallize perpendicular to compression.

Accretionary Prism

A wedge-shaped mass of marine sediment and oceanic crust scraped off the subducting plate and thrust onto the overriding plate at a trench.

Exotic Terrane

A crustal block or island fragment with a distinct geologic and paleomagnetic history that was transported and sutured onto an active continental margin.

Craton

The stable, ancient interior of a continent that has not undergone deformation or mountain building for at least $1\,Ga$.

Cratonic Shield

An expansive area within a craton where ancient Precambrian igneous and metamorphic crystalline basement rocks crop out directly at the surface.

Cratonic Platform

The portion of a craton where thin layers of relatively flat-lying Phanerozoic sedimentary rocks cover the underlying Precambrian basement.

Taconic Orogeny

An early Appalachian mountain-building episode occurring $460\text{--}420\,Ma$ caused by the collision of an island arc (exotic terrane) with eastern North America.

Acadian Orogeny

The second Appalachian orogenic pulse occurring around $370\,Ma$, produced as exotic continental blocks were carried into a doubly dipping subduction zone and sutured to North America.

Alleghenian Orogeny

The final major orogeny of the Appalachian sequence around $270\,Ma$, resulting from the collision between Africa and North America to assemble the supercontinent Pangea.

Wilson Cycle

The cyclic tectonic process conceptualized by J. Tuzo Wilson documenting the cyclical opening, expansion, and closing of an ocean basin followed by continental collision and stabilization.