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GLOSSARY OF TECHNICAL TERMS - M THROUGH Z
GLOSSARY A through L
Maar Volcano A low broad volcanic crater formed by multiple shallow explosive eruptions. Ubehebe Crater in Death Valley National Park, California, and Malheur Maar in Diamond Craters Outstanding Natural Area, Oregon, are examples of a Maar Volcano.
Magma Naturally occurring molten rock material, generated within the earth and capable of intrusion and extrusion, from which igneous rocks have been derived through solidification and related processes.
Marble A metamorphic rock consisting predominantly of recrystallized calcite and/or dolomite.
Marine Deposits Consolidated sediments deposited in a marine (oceanic) environment.
Massif A massive topographic and structural feature, especially in an orogenic belt, commonly formed of rocks that are more rigid than those of its surroundings.
Mesa A flat-topped mountain or plateau bounded on at least one side by a steep cliff. Spanish for Table.
Mesozoic The third of the four Geologic Eras in which geologic time is divided. It extends from the end of the Paleozoic Era (about 230 million years ago) to the beginning of the Cenozoic Era (about 65 million years ago). The Mesozoic Era is subdivided into the Triassic, Jurassic, and Cretaceous periods.
Metasediments A sediment or sedimentary rock that shows evidence of having been subjected to metamorphism.
Metavolcanics An informal term for volcanic rocks that show evidence of having been subjected to metamorphism.
Miocene The fourth of the five Geologic Epochs of the Tertiary Period. It extends from the end of the Oligocene Epoch (about 22.5 million years ago) to the beginning of the Pliocene Epoch (about 5 million years ago).
Mississippian The first of the two Geologic Ages of the Carboniferous Period extending from about 345 to 310 million years ago.
Monocline A local steepening in an otherwise uniform gentle dip. This view of Split Mountain in Dinosaur National Monument in Utah shows the eroded beds that have been subjected to monoclinal folding.
Monolith A large solitary block or pillar of stone. Beacon Rock along the Columbia River Gorge in Washington is an example of a monolith.
Moraine Usually refers to a mound or ridge of unstratified glacial drift (glacial debris, till) deposited by the direct action of glacier ice. Moraines left along the sides of a receded glacier like the ones along the Nisqually Glacier at Mt. Rainier, Washington, are called Lateral Moraines. Moraines built at the toe of the receded glacier, like the one left over from the Pleistocene Epoch at Bloody Mountain in Long Valley Caldera, California, are called Terminal Moraines.
Obsidian A black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal (curved) fractures.
Oligocene The third of the five Geologic Epochs of the Tertiary Period. It extends from the end of the Eocene Epoch (about 37 million years ago) to the beginning of the Miocene Epoch (about 22.5 million years ago).
Ophiolite An assemblage of mafic (ferro-magnesium mineral-rich rock type) and ultramafic igneous rocks, including their metamorphic equivalents (like serpentinite) formed from later metamorphism of the mafic and ultramafic rocks. The origin of this assemblage is associated with the early development of a geosyncline (a large downwarped basin or trough in the Earth's crust into which large amounts of sediment and volcanics accumulate).
Ordovician The second of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Cambrian Period (about 500 million years ago) to the beginning of the Silurian Period (about 435 million years ago).
Orogeny The process of formation of mountains, including thrusting, folding, and faulting in the outer and higher layers, and plastic folding, metamorphism, and plutonism in the inner and deeper layers.
Pahoehoe A Hawaiian term for basaltic lava flows typified by a smooth, billowy or ropy surface.
Paleocene The first of the five Geologic Epochs of the Tertiary Period. It extends from the end of the Mesozoic Era (about 65 million years ago) to the beginning of the Eocene Epoch (about 55 million years ago).
Paleozoic The second of the four Geologic Eras in which geologic time is divided. It extends from the end of the Precambrian Era (about 570 million years ago) to the beginning of the Mesozoic Era (about 230 million years ago). The Paleozoic Era is subdivided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
Pennsylvanian The second of the two Geologic Ages of the Carboniferous Period extending from about 310 to 280 million years ago.
Period An interval of geologic time longer than an Epoch and shorter than an Era.
Permian The last of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Carboniferous Period (about 280 million years ago) to the beginning of the Mesozoic Era (about 230 million years ago).
Pillar In Cave Studies, a geologic deposit in which a stalactite growing down from the roof connects with a stalagmite growing up from the floor to make a continuous column.
Pillow Basalt A particular type of the volcanic rock basalt that forms pillow-shaped masses, indicative of their underwater formation.
Plate Tectonics The theory of global tectonics in which the lithosphere (rigid part of the Earth's crust) is divided into a number of plates whose pattern of horizontal movement is that of rigid bodies that interact at their boundaries, resulting in earthquakes and volcanic activity (such as the well-known Pacific Ring of Fire).
Playa A term used in the southwestern U.S. for a dry, barren area in the lowest part of an undrained desert basin, underlain by clay, silt, or sand.
Pleistocene The first of the two Geologic Epochs of the Quaternary Period. It extends from the end of the Tertiary Period (about 1.8 million years ago) to the beginning of the Holocene (Recent) Epoch about 11,000 years ago.
Pliocene The last of the five Geologic Epochs of the Tertiary Period. It extends from the end of the Miocene Epoch (about 5 million years ago) to the beginning of the Quaternary Period (and Pleistocene Epoch) about 1.8 million years ago.
Plug Dome Volcano A volcanic dome characterized by an upheaved, consolidated mass filling the conduit.
Pluton An igneous (rock formed from molten or partially molten material) intrusion.
Porphyry An igneous rock of any composition containing conspicuous phenocrysts (large well-formed crystals) in a fine-grained groundmass.
Precambrian The first of the four Geologic Eras in which geologic time is divided. It extends from the beginning of Earth history (over 4 billion years ago) to the beginning of the Paleozoic Era (about 570 million years ago).
Pumice A light-colored cellular glassy rock commonly having the composition of rhyolite.
Pyroclastic Deposit A deposit of fragmentary debris from a volcanic explosion. It may consist of volcanic ash and/or breccia.
Pyroclastic Flow A volcanic deposit formed by the action of a lahar or blast or landslide of hot volcanic debris. The resulting consolidated rock is called an ash-flow tuff (if predominantly ash) or ash-flow breccia (if predominantly larger blocks).
Quartz Crystalline mineral composed of silicon dioxide (SiO2).
Quartz Diorite A plutonic rock that is intermediate in composition between acidic and basic and in which quartz comprises 5% to 20 % of the felsic (light mineral) constituents.
Quartzite A granoblastic metamorphic rock consisting mainly of quartz, formed by recrystallization of sandstone by regional or thermal metamorphism (metaquartzite). Also refers to a sandstone consisting of quartz grains cemented by secondary silica (orthoquartzite).
Quartz Monzonite A granitic rock in which quartz comprises 10% to 50 % of the felsic (light mineral) constituents and in which the alkali feldspar to total feldspar ratio is between 35% and 65%.
Quaternary The second of the two Geologic Periods of the Cenozoic Era. It extends from the end of the Tertiary Period (about 1.8 million years ago) to the present. It is further divided into the Pleistocene and Holocene Epochs.
Rhyolite A group of extrusive (volcanic) igneous rocks with phenocrysts (visible crystals that grew out of the molten lava) of quartz and alkali feldspar in a glassy to cryptocrystalline (too small to be seen through an ordinary microscope) groundmass (matrix).
Richter Scale Scale for the measurement of the magnitude of an earthquake. Each increase of one on the scale represents a ten-fold increase in the amount of ground motion on a seismograph (however, the amount of energy released is about 30 times greater).
Roof Pendant A downward projection of country rock into an igneous intrusion. Often found as an erosional remnant of the overlying deposit intruded by the igneous rock. This overlying deposit is typically metamorphosed by the igneous intrusion.
Scoria A pyroclastic rock composed vesicular cinders, typically of basaltic composition. Its cellular nature is due to the escape of volcanic gases before solidification.
Schist A metamorphic rock, formed under regional metamorphic conditions, in which most of the minerals exhibit a parallel crystal orientation, resulting in a foliated fabric.
Serpentine A group of minerals with the formula (Mg,Fe)3Si2O5(OH)4. They are typically green or greenish gray and have a silky or greasy luster. They are formed from the metamorphic alteration of magnesium-rich silicate minerals. A rock consisting almost entirely of serpentine minerals is called serpentinite.
Shale A fine-grained sedimentary rock formed by the compaction of clay, silt, or mud. It has a finely laminated structure, which gives it a fissility along which the rock splits readily.
Shear Zone A tabular zone of rock that has been crushed by many parallel fractures, typical of a fault zone, due to shear strain.
Shield Volcano A broad, gently sloping volcanic cone of flat domical shape, usually several tens or hundreds of square miles in extent, built chiefly of overlapping and interfingering lava flows.
Silurian The third of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Ordovician Period (about 435 million years ago) to the beginning of the Devonian Period (about 395 million years ago).
Slate A fine-grained metamorphic rock, mostly formed from shale, which possesses a slaty cleavage (thin, parallel fracture pattern).
Slot Canyon An informal term for a very narrow, often sinuous canyon, typically incised by the action of flash floods. The canyon walls are only a few feet to a few tens of feet across, yet the canyon may be tens to hundreds of feet deep.
Solar Rays A general term for any rays of sunlight radiating away from the sun (the solar point). They may include rays shining through holes in the clouds (also known as "God Rays"), or rays of sunlight radiating from the sun's position below the horizon before sunrise or after sunset (also known as crepuscular rays). Also see Anticrepuscular.
Spatter Cone A low, steep-sided cone composed of spatter, very fluid pyroclastic material, built by lava fountains along a fissure or around a vent. Craters of the Moon in Idaho has classic examples of Spatter Cones.
Spider Lightning Lightning bolts which travel horizontally, typically along the well-defined base of a thundercloud. The name reflects the crawling action of the lightning branches, like the movement of a spider on a ceiling.
Spire An informal term for a narrow, pointy column of rock formed by erosion of the strata around it. It is similar to a Hoodoo.
Stalactite A cylindrical or conical deposit of mineral matter, usually calcite, which hangs down from the ceiling of a cave, deposited by drops of water.
Stalagmite A conical deposit of mineral matter, usually calcite, which is developed upward from the floor of a cave by the action of dripping water.
Stratovolcano A volcano that is constructed of alternating layers of lava and pyroclastic deposits, along with intrusions of magma.
Stromatolites The distinct pattern left by Precambrian blue-green algae on the host rock.
Subalpine Refers to the growing or living conditions in mountainous regions just below the timberline.
Subduction Zone A long, narrow belt in which subduction (the process of one lithospheric plate descends beneath another) takes place.
Tarn A lake formed within a depression of a glacial moraine.
Terrestrial Deposits Consolidated sediments deposited in a terrestrial (non-marine) environment. Also referred to as Continental Deposits.
Tertiary The first of the two Geologic Periods of the Cenozoic Era. It extends from the end of the Mesozoic Era (about 65 million years ago) to the beginning of the Quaternary Period (about 1.8 million years ago). It is further divided into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs.
Thrust Fault Earthquake fault formed by the movement of one rock mass up and over another rock mass at a low angle (45o or less) over most of its extent due to compressional forces.
Triassic The first of the three Geologic Periods of the Mesozoic Era. It extends from the end of the Paleozoic Era (about 230 million years ago) to the beginning of the Jurassic Period (about 195 million years ago).
Tufa A chemical sedimentary rock composed of calcium carbonate, formed by evaporation as an incrustation around the mouth of a spring or along a stream. It can also form a thick concretionary deposit in a lake or along its shore, as is the case of the Mono Lake Tufa.
Tuff A general term for all consolidated pyroclastic deposits. If the ash builds up on the surface and is still very hot, it may fuse together in the heat and become flattened under its own weight forming welded tuff, like this example from Chiricahua National Monument in Arizona.
Ultramafic Igneous (rock formed from molten or partially molten material) rock types composed chiefly of mafic (magnesium-iron) minerals.
Ventifact A general term for any stone or pebble shaped, worn, faceted, cut, or polished by the abrasive or sandblast action of windblown sand, generally under desert conditions. An example from Death Valley can be seen here.
Volcanic Ash Fine unconsolidated pyroclastic matter (under 2mm in diameter). When consolidated, this material is called tuff.
Volcanic Breccia A pyroclastic rock consisting of angular volcanic fragments that are larger than 64mm in diameter and may or may not have a matrix.
Volcanic Bomb A blob of lava that was ejected while viscous and obtained a rounded or aerodynamic shape while in flight.
Volcaniclastic Pertaining to a clastic rock containing volcanic material in various proportions, without regard to its origin or environment.
GLOSSARY A through L
Many of the geological terms above are adapted from the American Geological Institute Dictionary of Geological Terms, Third Edition.
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