Glossary

Glossary
This is a "descriptive" glossary and may not contain the precise definitions of all terms. It is meant to provide additional information for the Field Trip webpage narrative. Please consult a geology textbook or dictionary for precise definitions.

Aa: Pronounced "ah ah". A rough, rocky textured lava flow. Usually associated with cooler lava (about to solidify) or a high silica content, viscous lava. An Hawaiian term.
Alluvial: Relating to alluvium: clastics deposited by streams, rivers, or floodwaters. In general, clay, silt, sand, and gravel transported and deposited by running water; the primary mechanism of erosion on the earth's surface. Back
Andesite: A fine grained, extrusive igneous rock with more silica content than basalt/gabbro, but less silica than rhyolite/granite. The coarse grained equivalent of andesite is called "diorite". Andesite is named after the igneous rock erupted by the many stratovolcanoes of the Andes mountain range in South America. Back
Ash (volcanic): Volcanic ash is created when erupting magma is literally blown into very small particles by the violent expansion of gasses dissolved in the magma. Unlike the ash resulting from combustion, volcanic ash is not a result of anything "burning". Back
Basalt: A dense, silica poor, iron and magnesium rich, usually dark colored igneous rock which constitutes the majority of the outer mantle of the earth's interior and forms the oceanic crust. The dark gray or black lava erupted by volcanoes is usually a type of basalt. Back
Dike: A dike is a long and narrow (tabular, like a table top) "discordant" magmatic intrusion; dikes cut across local sediment and rock layers. Back
Erosion: Erosion is the transport (movement) of weathered material from it's place of origin and its deposition somewhere else. Most erosion on earth is caused by running water. The wind is also a significant means of erosion, as are landslides and ocean currents. Back
Fault: A fracture in the earth's crust where the materials on one side have moved in relation to the materials on the other side. Faults may provide an easy path for magmatic intrusion. The sudden movement of material along either side of a fault causes earthquakes. Back
Igneous: A type of rock resulting from the solidification of magma. Many of the characteristics of igneous rocks are a result of their having been in a liquid (molten) state at one time. Back
Intrusion: A magmatic intrusion occurs when molten magma forces its way into local sediment layers or rock. Intrusive features are below ground; we only see them on the surface after erosion, or man, removes overlying material and makes them visible. Back
Joint: A rock fracture that does not show any movement. Like a crack in a window.
Lava: Magma which has been erupted onto the surface of the earth, or rock formed by the solidification of that magma. Back
Lithify: Relating to lithification; the process by which loose, unconsolidated material is turned into rock. Lithification can be accomplished by high temperatures, high pressures, or the chemical deposition of "cements" (minerals) within spaces between particles. Back
Magma: Molten igneous rock below ground. Magma erupted onto the surface of the earth is called lava. Back
Metamorphic (rock): A metamorphic (changed) rock is produced by subjecting the original material to one or more of the following; high pressure, high temperature, and/or chemical rearrangement. Metamorphic rock may be heated to high temperatures, but never melted (else it becomes an igneous rock). Normally, metamorphism is produced by the weight (pressure) of thick overlying deposits, magmatic heating, tectonic processes (subduction), and/or the presence of water.
Mineral: In geology, a mineral is a specific, naturally ocurring, crystalline substance. One or more minerals combine to form most rocks found on the earth's surface. Back
Neck: The solidified magma tube that fed a volcanic vent, exposed by the erosion of the volcano and/or surrounding surfaces. Back
Pahoehoe: Pronounced "pa hoi hoi". A smooth, ropey lava flow. Usually associated with hot, fluid lava. An Hawaiian term.
Porphrytic: Containing larger than normal mineral crystals in a fine-grained matrix. A porphrytic igneous rock usually indicates that the parent magma experienced a period of slow, underground cooling, then the magma solidified relatively quickly. The slow cooling promotes the larger crystal growth while the rapid final solidification produces the fine-grained (small crystal) matrix. Back
Rhyolite: A fine-grained, extrusive igneous rock of the same composition as granite. Rhyolite is formed when the parent magma solidifies rapidly and large mineral crystals do not have time to grow. Granite, a coarse grained or large crystal rock, is formed when the parent magma solidifies underground after a long cooling period which promotes the larger mineral crystal growth. Back
Scoria: Erupted fragments of high gas content magmas. The resulting lava is vesicular; full of gas holes and bubbly looking. Also called volcanic "cinders". A Cinder Cone volcano is formed by the eruption of large amounts of scoria. Back
Silica: The most common substance on the surface of the earth and a common component of rocks and soils. Pure silica is the mineral known as quartz and is composed of two atoms of the element oxygen (the most abundant element in the earth's crust) and one atom of the element silicon (the second most abundant element). Various forms of silica combine with other elements (aluminum, sodium, iron, etc) to create silicic minerals which are constituents of most rocks of the earth's crust. Common rocks contain from about 45% to over 75% silica in one form or another. Back
Sill: A sill is a relatively thin and wide (tabular, like a table top) "concordant" magmatic intrusion; sills intrude between or along (paralell to) local sediment and rock layers. Back
Weathering: In geology, weathering is the breakdown of rock into smaller pieces, or into different substances, by mechanical and chemical processes. Rocks can weather into soil, for instance. Weathering speeds erosion. Back
Zeolite: A class of minerals produced by low-level metamorphism, or an instance of those minerals. Zeolites typically form in voids or cavities of rocks buried under thick layers of mineral enriched deposits. Layers enriched with Calcite tend to form Carbonate zeolites and areas enriched with Gypsum tend to form Sulphate zeolites. Back

Last modified: 1/15/2004