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GEOLOGY
Selected terms used in
the study of volcanoes and Vulcanology.

The following is a partial list of terms used in Vulcanology, the study of volcanos and associated events. The terms are provided strictly as a study aids and references extracted from a much larger glossary of terms used by professionals in these fields.
aa
A basaltic lava which has a rough, blocky surface.
Andesite line
The petrographic boundary between andesites and basalts. It marks the boundary between explosive (felsic) and oceanic (mafic) volcanoes in the Pacific region.
ash fall (air fall)
The precipitation of airborne volcanic ash.
basalt
Dark colored, fine grained volcanic rock which is commonly called lava. Basaltic lavas are generally very fluid.
caldera
A topo-volcanographical feature, usually greater that 1.5 miles in diameter, which is formed when the top of a volcanic cone collapses downward into the volcanic cone. Caldera will usually fill with water, creating fourmations known as caldera lakes, often featuring a small island which is theremains of a subsequent lava dome. An example of such a lake is Crater Lake in the state of Oregon (USA).
cinder cone volcano
A topo-volcanographical feature which is generally pyramidial in appearance. Cinder cone eruptions are generally considered to be single events where eruptions throw pyroclastic materials into the air where they cool to cinders and fall back to the ground, forming the characteristic cone shape.
composite cone volcano
A topo-volcanographic feature which is generally slightly concave upward and consists of alternating layers of of ash and dust. Composite cones are usually the highest and most dangerous of the the volcanic cones.
columnar jointing
A geologic formation formed as hot volcanic rocks cool and contract forming polygonal columns which are usually vertical to the surrounding lava flow terrain but may be curved. Devil's Tower in Wyoming (USA) as a spectacular example of this kind of formation.
felsic
A lava type comprised primarily of feldspar (fel) and silica (s) usually found at continental boundry subduction zones. These are generally thick, pasty lavas with high silicon dioxide levels.
lahar
A flow of mud or debris consisting of volcanic material.
lapilli
Molten or solid ejected pyroclastic materials ranging from 4 to 32 mm in size.
lava plateau
A large mesa (tableland), which may have an area reaching thousands of square miles, which is underlain by a thick sequence of lava flows.
maar
A broad volcanic crater formed by multiple shallow explosive eruptions. Maars will commonly contain a lake and are surrounded by a low ring of ejected materials. (Also see phreatic eruption)
mafic
A lava type comprised primarily of magnesium (ma) and iron (fe) usually found at the mid-ocean ranges and hot spots. These yeail fluid lavas which retain very little gasses and generally do not erupt as violently.
nuee ardente (ash flow)
A very hot, dense cloud of volcanic ash and other pyroclastic material which is produced by explosive deisintegration of viscous lava in a vent and travels very rapidly down the slope of a volcano.
pahoehoe
A lava flow which has smooth, billowy, or ropy surfaces.
phreatic eruption
Volcanic eruption, consisting mostly of steam, which is caused by the interaction of magma and steam.
pyroclastic
A descriptive term used to identify fragmented material--ash, cinders, blocks, and bombs--which is ejected from a volcano.
shield cone volcano
The largest types of volcano (areawise) composed of piles of usually basaltic lava in a usually-gentle convex upward slope. These volcanoes are most commonly identified with Hawaii and Iceland.
tephra
The name given to pyroclastic materials of all shapes and sizes.
tsunami
A large sea wave which is produced by a submarine earthquake, landslide, or volcanic eruption. Commonly called a tidal wave.
volcanic ash
A very fine, gritty volcanic dust whose particles are less than 4mm in diameter.
volcanic dome (lava dome volcano)
Steep-sided protrusions of viscous, glassy lava. Often found within the crater of a large volcano or caldera but may be free-standing.
VEI (Volcanic Explosivity Index)
A scale for rating the volcanic eruptions according to the volume of material ejected, the height to which the material rises, and the length of time over which the eruption continues.
volcanic hazardst
Ash flows and ash falls, debris flows (lahars), gases, peripheral landslides, lava flows, tsunami and weather changes. Of these, lava flows are the most potentially destructive however ash falls and release of gasses can cause serious health and respiratory problems including death by asphyxiation.
veil index (dust veil index)
A measure of the effect of volcanic dust on incoming solar radiation and resulting effects on regional or global climate.
welded tuff
A glassy pyroclastic material which has been made hard by the weldingtogether of its particles by the action of retained heat.

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