Golden Gate Photo

Aa Lava Volcanic rock. A Hawaiian term for lava flows typified by a rough, jagged, clinkery surface. Here is an example from Newberry National Volcanic Monument in Oregon.

Accreted Terrane The landmass from one tectonic plate that was added to another one where the plates converged (see Plate Tectonics).

Agate A variety of quartz composed of cryptocrystalline (too small to be seen through an ordinary microscope) silica. Agate is a variety of chalcedony, often mixed with or alternating with opal, and typically characterized by alternating color bands. It is found in vugs in volcanic rocks or limestone. Here is an example of this colorful rock from Cedar Breaks.

Age An interval of geologic time shorter than an Epoch.

Agglomerate A pyroclastic deposit composed of volcanic fragments and bombs in a loose aggregation or welded together.

Alluvial Fan Topographic feature. A sloping mass of alluvium deposited by a stream, particularly in an arid or semiarid region where a stream issues from a narrow canyon onto a plain or valley floor. An example of a particularly steep alluvial fan from Death Valley can be seen here.

Amphibole A mineral group with the general formula A2B5(Si,Al)8O22(OH)2, where A is mainly the metals Mg, Fe, Ca, or Na, and B is mainly Mg, Fe+2, Al, and Fe+3. This group includes some common minerals in igneous and metamorphic rocks.

Andesite A dark colored, fine-grained volcanic rock that, when porphyritic (contains visible crystals that grew out of the molten lava), contains phenocrysts (those crystals) composed mostly of the feldspar mineral Sodic Plagioclase.

Anticrepuscular Rays of sunlight that occur during twilight (before sunrise or after sunset) which appear to converge at the point directly opposite of the sun (antisolar point). Not to be confused with rays that filter through breaks in, or around, clouds, these shafts of light lie in the stratosphere, high above the clouds, as demonstrated in this photo from the Grand Canyon.

Aręte Topographic feature. A rocky sharp-edged ridge or spur, commonly present above the snowline in rugged mountains sculptured by glaciers.

Argillite A compact rock, derived from mudstone or shale, but more highly indurated and lacks the fissility of shale or the cleavage of slate. It is the product of weak metamorphism.

Basalt Typically a volcanic rock, although may also be the result of relatively shallow magma intrusions. Dark colored, fine-grained, and composed primarily of the feldspar mineral Calcic Plagioclase and the amphibole mineral Pyroxene. Basalt is most often associated with ocean crust volcanics like Hawai'i, but also often occurs on continents, sometimes as great layered flows like the Columbia River Basalts.

Batholith Geologic structure. A large plutonic mass that has over 40 square miles (100 km2) of surface exposure and no known floor. Its formation is believed by most investigators to involve the upwelling of molten rock.

Butte Topographic feature. An isolated flat-topped hill with steep slopes or cliffs, typically capped with a resistant layer of rock and bordered by talus (rocky debris). It represents an erosional remnant carved from flat-lying rocks. The summit is smaller than a Mesa.

Caldera Volcanic feature. A large basin-shaped volcanic depression, somewhat circular, with a diameter that is many times greater than the diameter of the vent or vents that fed it from below. Crater Lake in Oregon is a classic example.

Cambrian The first of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Precambrian Era (about 570 million years ago) to the beginning of the Ordovician Period (about 500 million years ago).

Carboniferous The fifth of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Devonian Period (about 345 million years ago) to the beginning of the Permian Period (about 280 million years ago). In North America, it is further divided into the Mississippian and Pennsylvanian ages.

Cenozoic The latest of the four Geologic Eras in which geologic time is divided. It extends from the end of the Mesozoic Era (about 65 million years ago) to the present. The Cenozoic Era is subdivided into the Tertiary and Quaternary periods (on a different basis, it is subdivided into the Paleogene and Neogene periods).

Chert Sedimentary rock. Hard, dense, and chiefly composed of microcrystalline (crystals that can only be seen through a microscope) or cryptocrystalline (crystals too small to be seen with a microscope) quartz. Chert may be the result or organic or inorganic precipitation. Most often, it occurs as nodules in other sedimentary rocks. But it may form in layers, typically referred to as Bedded or Banded Chert, which are commonly formed from the deep-sea deposition of the siliceous skeletons of the microscopic organism Radiolaria.

Cinder Cone Volcanic feature. A conical hill formed by the accumulation of cinders and other pyroclastic debris around a volcanic vent. Wizard Island, in Crater Lake, Oregon, is an example of a Cinder Cone.

Cirque A deep steep-walled recess or hollow, horseshoe-shaped or semicircular in plan view, situated high on the side of a mountain and produced by the erosive activity of a mountain glacier.

Clastic Pertaining to a rock or sediment composed primarily of fragments derived from pre-existing rocks or minerals and transported from their place of origin.

Conglomerate Sedimentary rock. Composed of coarse-grained, rounded to subangular fragments greater than 2mm in diameter (includes granules, pebbles, cobbles, boulders) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Crepuscular Rays of sunlight that occur during twilight (before sunrise or after sunset) which appear to converge at the sun (below the horizon).

Cretaceous The last of the three Geologic Periods of the Mesozoic Era. It extends from the end of the Jurassic Period (about 135 million years ago) to the end of the Mesozoic Era at about 65 million years ago.

Cross-Bedding Sedimentary structure created by layers of silt, sand, and/or gravel (greater than 1 cm thick) that are deposited in progressively building layers inclined at an angle to the main bedding. Checkerboard Mesa is a good example of preserved cross beds from ancient sand dunes.

Dacite A fine-grained volcanic rock with the same general composition as andesite, but having less of the feldspar mineral Calcic Plagioclase and more quartz.

Debris Avalanche Geologic deposit and event related to a Landslide, but containing abundant snow and/or ice. As the debris tumbles downslope, the snow and ice break apart giving the slide more mobility and speed.

Devonian The fourth of the six Geologic Periods of the Paleozoic Era. It extends from the end of the Silurian Period (about 395 million years ago) to the beginning of the Carboniferous Period (about 345 million years ago).

Dike A tabular body of igneous rock that cuts across the structure of adjacent rocks or cuts through massive rocks. Chasm View in the Black Canyon of the Gunnison, Colorado, exposes some good examples of dikes.

Dripstone A general term for calcite or other mineral deposit formed in caves by dripping water. It includes stalactites and stalagmites, as well as similar deposits formed by flowing water.

El Niņo Refers to the warming of the eastern Pacific Ocean waters near the equator, which typically results in dramatic alterations in the weather patterns in many parts of the Americas. El Niņo is Spanish for a male child and is a reference to the "Christ Child". The term originated from Peruvian fisherman who noted changes in fishing catches (caused by changes in the water temperature) occurring around Christmas time.

Eocene The second of the five Geologic Epochs of the Tertiary Period. It extends from the end of the Paleocene Epoch (about 55 million years ago) to the beginning of the Oligocene Epoch (about 37 million years ago).

Epoch An interval of geologic time longer than an Age and shorter than a Period.

Era An interval of geologic time longer than a Period and shorter than an Eon. Geologic time can be summarized, in chronological order, as the Precambrian, Paleozoic, Mesozoic, and Cenozoic Eras.

Exfoliation The process by which concentric scales, plates, or shells of rock are successively spalled or stripped from bare surfaces of a large rock mass. It's caused by physical or chemical forces producing differential stresses within the rock.

Feldspar A group of rock-forming minerals with the general formula MAl(Al,Si)3O8, where M can be the metals K, Na, Ca, Ba, Rb, Sr, or Fe. Feldspars are the most widespread mineral group, occurring in all types of rock and comprising about 60% of the Earth's crust.

Fin A general term for a thin ridgeline of rock resulting from erosional forces. The Organ in Arches National Park, Utah, is a fin.

Franciscan Complex Also referred to as the Franciscan Assemblage, this includes the suite of sedimentary rocks that were deposited in an offshore trench during the Cretaceous Period when the Pacific Plate was subducting beneath the North American Plate. Also added to the mix are slivers of oceanic crust (Pacific Plate) volcanic and metavolcanic rocks as the assemblage was accreted to the continent (North American Plate). Franciscan rocks are found scattered throughout the central coast of California, including Mt. Diablo.

Fulgurite The glassy tubes formed when lightning strikes the ground, particularly sandy deposits, and fuses the particles together.

Fumarole A hole or vent from which volcanic fumes or vapors issue.

Geyser A type of hot spring that intermittently erupts jets of hot water and steam, the result of groundwater coming into contact with rock hot enough to create steam under conditions preventing free circulation. Many of the best examples are found in Yellowstone National Park in Wyoming.

Glacial Erratics Rock fragments ranging from pebbles to house-size blocks carried by glacial ice, deposited at some distance from the original outcrop, and typically lying on bedrock of a different rock type. They are indicators of ancient glacial activity. Examples in California can be found in the Tuolumne Meadows portion of Yosemite National Park and at Lassen Volcanic National Park.

Glacial Striations A series of fine parallel grooves cut into the bedrock by rock fragments embedded at the base of a moving glacier, or cut into the rock fragments themselves. A good example can be seen in Lassen Volcanic National Park.

Gneiss A metamorphic rock that is distinguished by alternating bands or lenses of granular minerals (like quartz) and bands or lenses of flaky or elongate prismatic minerals (like mica or amphibole). Gneiss is formed under the high pressure and temperature of regional metamorphism.

Grand Staircase The Grand Staircase refers to the stair-stepping of various-colored cliffs though much of northern Arizona and southern Utah. In ascending order, they include the Triassic-Age Vermilion Cliffs, the Jurassic-Age White Cliffs, the Cretaceous-Age Gray Cliffs, and the Eocene-Age Pink Cliffs.

Granodiorite A group of coarse-grained plutonic rocks intermediate in composition between quartz diorite and quartz monzonite (U.S.). It typically contains quartz, feldspars, the mica biotite, and amphiboles.

Graywacke A dark gray, consolidated, coarse-grained sandstone consisting of poorly sorted angular to subangular grains of quartz, feldspar, and dark rock and mineral fragments embedded in a matrix of compact clay.

Hanging Valley A topographic feature where the mouth of a tributary valley (formed by a stream or a glacier) is notably higher than the floor of the main valley it empties into due to the more rapid deepening of the main valley. In glacial valleys, this is due to the greater erosive power of the truck glacier. Bridalveil Falls cascades down a hanging valley created during Pleistocene glaciations.

Hematite A common mineral; a form of iron oxide with the formula Fe2O3.

Holocene The second of the two Geologic Epochs of the Quaternary Period. It extends from the end of the Pleistocene Epoch (about 11,000 years ago) to the present. It is also referred to as the Recent Epoch.

Hoodoo A column or pillar of rock produced in a region of sporadic heavy rainfall by differential weathering or erosion of horizontal strata, and facilitated by joints and layers of varying hardness. Examples are found in some parts of Canyonlands National Park in Utah.

Horn A high pyramidal peak with steep sides formed by the intersecting walls of three or more cirques. One of the more recognized examples of a horn in North America is Grand Teton in Grand Teton National Park, Wyoming.

Incised Meander A meander in a stream carved downward into the surface of the valley in which it originally formed, suggesting the rejuvenation of a meandering stream due to rapid vertical uplift or a lowering of base level. Also called an entrenched meander. A textbook example is the Goosenecks of the San Juan in southeastern Utah.

Island Arc A curved chain of islands, like the present Aleutians in Alaska, rising from the deep-sea floor and near to the continents. Its curve is generally convex toward the open ocean.

Jurassic The middle of the three Geologic Periods of the Mesozoic Era. It extends from the end of the Triassic Period (about 195 million years ago) to the beginning of the Cretaceous Period (about 135 million years ago).

Lahar A landslide or mudflow of pyroclastic material on the flank of a volcano. Also the name of the deposit it produces. Here is a view of a lahar deposit from Mount St. Helen along the Toutle River.

Landslide A general term for a wide variety of processes and landforms involving the downslope movement, under gravity, of masses of soil and rock material.

Lava Tube A hollow space beneath the surface of a solidified lava flow, formed by the withdrawal of molten lava after the formation of the surficial crust. Lava Beds National Monument has many excellent and accessible examples.

Loma Prieta Earthquake This was the 7.1 Magnitude earthquake that struck the San Francisco Bay vicinity at 5:04PM (Pacific Time) on October 17, 1989. The epicenter was actually much closer to the town of Santa Cruz than to San Francisco.

GLOSSARY M through Z

Many of the geological terms above are adapted from the American Geological Institute Dictionary of Geological Terms, Third Edition.