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Aa
Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.
Basalt
Volcanic rock (or lava) that characteristically is dark in color, contains 45 to 54 percent silica, and generally is rich in iron and magnesium
Breccia
Term used to describe a conclomerate comprising angular or subangular fragments.
Caldera
The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes
Dyke
A tabular igneous intrusion that cuts across the bedding of the country rock.
Felsic
A igneous rock rich in light-coloured minerals such as feldspar and quartz. Opposite of mafic.
Fumarole
A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.
Lapilli
Literally, "little stones"; round to angular rock fragments measuring 2mm to 64mm in diameter, which may be ejected in either a solid or molten state.
Lava
Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.
Lava Flow
An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet like body formed by outpouring lava.
Lava tube
A tunnel formed when the surface of a lava flow cools and solidifies, while the still-molten interior flows through and drains away.
Mafic
An igneous rock composed chiefly of one or more dark-colored minerals.
Magma
Molten rock beneath the surface of the earth.
Magma chamber
The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
Magmatic Differentiation
A term used to describe the chemical evolution of magma as it rises, cools and heavy crystals precipitate out of solution and so changing its chemical composition. As the process continues heavy minerals are lost and the magma becomes progressively more siliceous, changing in composition from picrite through basalt towards rhyolite. The incorporation of silica-rich crustal materials accelerates this process.
Miocene
A epoch in Earth history from about 24 to 5 million years ago. Also refers to the rocks that formed in that epoch.
Obsidian
A black or dark-colored volcanic glass, usually composed of rhyolite.
Pahoehoe
A Hawaiian term for lava with a smooth, billowy, or ropy surface.
Phenocryst
A conspicuous, usually large, crystal embedded in porphyritic igneous rock.
Phonolite
A felsic igneous rock which contains too little silca to precipitate silicate minerals such as felspar and pyroxene.
Plinian eruption
An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.
Pumice
Light-colored, frothy volcanic rock, often formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles (e.g., Monta Blanca eruption).
Pyroclastic
Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
Pycroclastic fall Deposit
Pyroclastic material deposited by gravitational settling through the air. Such deposits are characterized by being relatively well sorted by poorly bedded.
Pyroclastic flow
Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.)
Scoria
Dark coloured (usually basaltic) lava clasts > 64mm.
Scoria cone
A volcanic cone built entirely of loose clasts of scoria pyroclastic material
Shield volcano
A gently sloping volcano in the shape of a flattened dome, built almost exclusively of lava flows.
Strato volcano
A volcano composed of successive lava flows and pyroclastic material.
Tephra
Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.
Trachybasalt
An extrusive rock intermediate in composition between trachyte and basalt.
Trachyte
A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase.
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