Dynamic Planet/Earthquakes and Volcanoes Vocabulary

This is a basic glossary of terms relating to earthquakes and volcanoes.

A

Aa:A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces.

B

Batholith: A great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through metamorphism.

Block fault: A structure formed when the crust is divided into blocks of different elevation by a set of normal faults.

C

Caldera: A large, circular depression in a volcanic terrain, typically originating in collapse, explosion, or erosion.

Central vent: The largest vent of a volcano, situated at the center of its cone.

Cinder cone: A steep, conical hill built up about a volcanic vent and composed of coarse pyroclasts expelled from the vent by escaping gases.

D

Density: The mass per unit volume of a substance, commonly expressed in grams/ cubic centimeter.

E

Earthquake: The violent oscillatory motion of the ground caused by the passage of seismic waves radiating from a fault along which sudden movement has taken place.

Elastic rebound theory: A theory of fault movement and earthquake generation that holds that faults remain locked while strain energy accumulates in the country rock, and then suddenly slip and release this energy.

Epicenter: The point on the Earth's surface directly above the focus or hypocenter of an Earthquake.

F

Fault: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement.

Fault-block mountain: A mountain or range formed as a horst when it was elevated between parallel normal faults.

Fault plane: The plane that best approximates the fracture surface of a fault.

Flood basalt: A plateau basalt extending many kilometers in flat, layered flows originating in fissure eruptions.

Focus (earthquake): The point at which the rupture occurs; synonymous with hypocenter.

Fold: A planar feature, such as a bedding plane, that has been strongly warped, presumably by deformation.

Fumarole: A small vent in the ground from which volcanic gases and heated groundwater emerge, but not lava.

H

Hypocenter: The point below the epicenter at which an earthquake actually begins; the focus.

I

Igneous rock: A rock formed by congealing rapidly or slowly from a molten state.

Intrusion: An igneous rock body that has forced its way in a molten state into surrounding country rock.

Intrusive rock: Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations.

Isostasy: The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which "floats" on the denser mantle. The theory that continents and mountains are supported by low-density crustal "roots."

J

Joint: A large and relatively planar fracture in a rock across which there is no relative displacement of the two sides.

L

Laccolith: A sill-like igneous intrusion that forces apart two strata and forms a round, lens-shaped body many times wider than it is thick.

Lahar: A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain or the water of a lake displaced by a lava flow.

Lava: Magma or molten rock that has reached the surface.

Lava tube: A sinuous, hollow tunnel formed when the outside of a lava flow cools and solidifies and the molten material passing through it is drained away.

Left-lateral fault: A strike-slip fault on which the displacement of the far block is to the left when viewed from either side.

Lithosphere: The outer, rigid shell of the Earth, situated above the asthenosphere and containing the crust, continents, and plates.

M

Magma: Molten rock material that forms igneous rocks upon cooling. Magma that reaches the surface is referred to as lava.

Magma chamber: A magma-filled cavity within the lithosphere.

Magnitude: A measure of earthquake size, determined by taking the common logarithm base 10) of the largest ground motion observed during the arrival of a P-wave or seismic surface wave and applying a standard correction for distance to the epicenter.

Mantle: The main bulk of the Earth, between the crust and core, ranging from depths of about 40 to 3480 kilometers. It is composed of dense mafic silicates and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.

Mesophere: The lower mantle.

Mohorovic discontinuity: The boundary between crust and mantle, marked by a rapid increase in seismic wave velocity to more than 8 kilometers per second. Depth: 5 to 45 kilometers. Abbreviated "Moho" or "M-discontinuity."

Monocline: The S-shaped fold connecting two horizontal parts of the same stratum at different elevations. Its central limb is usually not overturned.

N

Normal fault: A dip-slip fault in which the block above the fault has moved downward relative to the block below.

O

Oblique-slip fault: A fault that combines some strike slip motion with some dip-slip motion.

Outgassing: The release of juvenile gases to the atmosphere and oceans by volcanism.

P

Pahoehoe: A basaltic lava flow with a glassy, smooth, and undulating, or ropy, surface.

P-wave: The primary or fastest wave traveling away from a seismic event through the solid rock, and consisting of a train of compressions and dilations of the material.

R

Right-lateral fault: A strike-slip fault on which the displacement of the far block is to the right when viewed from either side.

S

Sea-floor spreading: The mechanism by which new sea floor crust is created at ridges in divergence zones and adjacent plates are moved apart to make room. This process may continue at 0.5 to 10 centimeters/year through many geologic periods.

Seismic refraction: A mode of seismic prospecting in which the seismic profile is examined for waves that have been refracted upward from seismic discontinuities below the profile. Greater depths may be reached than through seismic reflection.

Seismic surface wave: A seismic wave that follows the earth's surface only, with a speed less than that of S-waves. There are Raleigh waves (forward and vertical vibrations) and Love waves (transverse vibrations).

Stratovolcano: A volcanic cone consisting of both lava and pyroclastic rocks, often conical.

Stress: A quantity describing the forces acting on each part of a body in units of force per unit area

Strike-slip fault: A fault whose relative displacement is purely horizontal.

Subduction zone: A dipping planar zone descending away from a trench and defined by high seismicity, interpreted as the shear zone between a sinking oceanic plate and an overriding plate.

S-wave: The secondary seismic wave, traveling slower than the P-wave, and consisting of elastic vibrations transverse to the direction of travel. It cannot penetrate a liquid.

Syncline: A large fold whose limbs are higher than its center; a fold with the youngest strata in the center.

T

Thrust fault: A dip-slip fault in which the upper block above the fault plane moves up and over the lower block, so that older strata are placed over younger.

Transform fault: A strike-slip fault connecting the ends of an offset in a mid-ocean ridge. Some pairs of plates slide past each other along transform faults.

Tsunami: A large destructive wave caused by sea-floor movements in an earthquake.

V

Volcanic ash: A volcanic sediment of rock fragments, usually glass, less than 2 millimeters in diameter that is formed when escaping gases force out a fine spray of magma.

Volcanic block: A pyroclastic rock fragment ranging from about fist- to car-sized.

Volcanic bomb: A pyroclastic rock fragment that shows the effects of cooling in flight in its streamlined or "bread-crust" surface.

Volcanic cone: The deposit of lava and pyroclastic materials that has settled close to the volcano's central vent.

Volcanic dome: A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava.

Volcanic ejecta blanket: A collective term for all the pyroclastic rocks deposited around a volcano, especially by a volcanic explosion.

Volcanic emanations: Gases, especially steam, emitted from a vent or released from lava.

Volcanic glass: Can refer to either Obsidian or Pumice.

Volcanic pipe: The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano.

Volcano: Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent.