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Glossary of Geological Terms | Introduction | |
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Glossary of Geological Terms | Introduction | |
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Below is a glossary of geological terms used in this website. For other volcanic terms and photographs of many of the features listed below, visit the United States Geological Survey photo-glossary. To return to the page you were on before, simply click the ‘back’ button on your browser window. | ||
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A’a: Hawaiian term, pronounced "ah-ah", describing a type of lava flow characterised by a very rough, jagged surface. A'a lava is much more viscous than pahoehoe lava and also flows downslope at a much slower pace. The slopes of Rangitoto Island show great examples of blocky a'a flows. Basalt: a dark-coloured, fine-grained volcanic rock having 45-52% silica content. The main minerals in basalt are plagioclase feldspar (>50%) and pyroxene, with minor iron oxides. Olivine is sometimes present. Breadcrust(ed) bomb: a variety of volcanic bomb that has a cracked surface resembling a bread crust. Cracks form when the interior of a bomb expands after a solid crust has formed. Explosion crater (or maar): a broad, shallow-rimmed depression produced by sub-surface explosions. Explosions may occur when rising magma interacts with groundwater, producing steam. The associated large increase in gas volume causes an upward blast, which blows out the overlying rocks. Some of the debris forms a rim, known as a tuff ring. Lava: molten magma that has reached the earth’s surface. If enough lava is emitted from a vent, it may flow outward under the force of gravity to cover surrounding land. The extent of a lava flow depends on the topography, the volume of lava emitted, and the viscosity of the lava. Lava fountain (or fire fountain): a plume of incandescent lava sprayed into the air as gases rapidly escape from magma when it reaches the surface. Fountaining is usually limited to very liquid magmas (e.g. basalt) with high gas contents. Hawaiian volcanoes erupt in this way, with fountains sometimes spraying hundreds of metres into the air. Magma: molten rock material consisting of silicates and dissolved gases such as steam and CO2. It may also contain suspended crystals, or rock fragments (xenoliths) from the surrounding rocks. Magma is generated deep in the earth’s crust where temperatures are high, and is the source of igneous rocks. Pahoehoe (or ropy lava): Hawaiian term describing a type of lava flow surface that has a smooth or ropy appearance. This occurs when very fluid lava continues to flow beneath a flexible crust, which begins to twist and wrinkle. Compare with a'a. Pressure ridge: an elongate, crumpled, uplifted part of the solid crust of a lava flow, resulting from the pushing of liquid lava beneath. Rafting: occurs when a block of rock material (commonly scoria) is carried (rafted) downslope upon or within a lava flow, e.g., material that has been dislodged from a crater wall where it has been breached by a lava flow. Scoria: an igneous rock that contains abundant vesicles. Scoria is formed when magma rich in dissolved gases is erupted at the earth's surface. At the surface the magma is under less pressure and the gases escape to form bubbles, which are trapped as the magma cools and solidifies. Scoria cone: a volcanic cone composed of scoria that has been erupted from a vent. There is usually a crater in the centre of the cone. Tuff: a fine-grained rock made mainly of volcanic ash and fragmented country rock that has fallen from the air during an eruption or explosion. Tuff is therefore comprised of ‘pyroclastic’ material. Tuff ring: a raised rim made of tuff occurring around an explosion crater. Vent: an opening or fissure at the earth's surface through which volcanic materials are extruded or erupted. Vesicles: small holes formed in a volcanic rock by gas bubbles present when the lava cooled and solidified. Viscosity: the tendency within a liquid to resist flow. The more viscous a material is, the more force that is required to cause it to flow. For example, basalt lavas are less viscous than rhyolite lavas, and therefore flow more easily. This is the reason that basalt forms extensive flows, whereas rhyolite forms domes. The viscosity of a lava depends on the silica content, and the water and gas contents. Volcanic ash: fine, dust-sized particles (<2 mm) erupted from a volcanic vent. Volcanic bomb: a clump of molten lava, ejected from a volcanic vent while still partly molten. Bombs assume rounded, elongated (spindle), or ribbon-like shapes as they spin and twist through the air. If they land before becoming completely solid, ‘cow-pat’ shapes may form on impact. Bombs may be vesicular or solid. |
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