Glossary
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Note that words in italics are defined elsewhere in this glossary.
Alluvial fan. An outspread, gently sloping mass of sediment deposited by a stream where it issues out of the mouth of a narrow canyon draining from and upland area. Viewed from above, an alluvial fan typically has the shape of an open fan with the apex being at the mouth of the canyon. Alluvial fans are common in arid to semi-arid regions, but can be covered with forests in the California Coast Ranges. Alluvial fans may merge together to form an apron-like slope along the base of a mountain front.
Alluvium. A general term for unconsolidated sediments deposited by flowing water on stream channel beds, flood plains, and alluvial fans. The term applies to stream deposits of recent times and it does not include subaqueous deposits, such as in lakes or undersea.
Anaglyphic image. A type of photographic image or drawing that can create a three-dimensional view when viewed through colored filter glasses—red-and-cyan colored lenses are most common (standard). Anaglyphic images are created using two standard photographs taken in parallel position a short distance apart, then colors are subtracted from the two images (blue and green from the left image, and red from the right image) before the two images are merged into a reconstructed image (called an anaglyph). The red colored lens filters out green and blue from the anaglyphic image whereas the cyan colored lens filters out the red. With red-and-cyan 3D viewing glasses on, the brain reconstructs a 3D view from the original stereo pair of images used to make the anaglyphic image. Standard anaglyphic 3D glasses are worn with the red lens over the left eye.
Basalt. A dark-colored igneous rock, commonly extrusive (from volcanic eruptions) and composed primarily of the minerals of calcic plagioclase and pyroxene, and sometimes olivine. Basalt is the fine-grained equivalent of gabbro.
Beheaded stream. Streams draining across an active strike-slip fault trace may be captured by an adjacent stream. With loss of its water supply or a source of sediments, the older channel will remain as a beheaded stream channel as fault motion continues.
Cenozoic. The era of time spanning about 65 million years ago to the present. The term applies to rocks that formed or accumulated in that time period. The Cenozoic Era is subdivided into the Tertiary and Quaternary periods.
Chert. A hard, dense sedimentary rock, consisting chiefly of interlocking microscopic crystals of quartz and may contain opal. It has a conchoidal fracture and may occur in a variety of colors.
Coast Range Ophiolite. An assemblage of mafic and ultramafic igneous rocks of Jurassic to possibly Cretaceous age and whose origin is associated with the upper mantle and the lower oceanic crust of the ancient Farallon Plate. The Farallon Plate predates the development of the San Andreas Fault system, and rocks of the Farallon Plate were either subducted or partially accreted (merged) into the crust that now makes up the Coast Ranges. The Coast Range Ophiolite is associated with serpentinite terranes throughout much of coastal central and northern California. The modern Juan de Fuca Plate offshore of the Cascades volcanic range in Oregon and Washington is a remnant of what was once the greater Farallon Plate.
Colluvium. A general term applied to loose and incoherent surficial deposits, usually at the base of a slope and brought their chiefly by gravity.
Conglomerate. A coarse-grained sedimentary rock composed of rounded to subangular fragments (larger than 2 mm in diameter) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay; the consolidated equivalent to gravel.
Creep. In earthquake terminology, creep is the slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. In landslide terminology, creep is slow, more or less continuous downslope movement of surface materials (mineral, rock, and soil particles) under gravitational stresses.
Cretaceous. The final period of the Mesozoic Era (after the Jurassic Period and before the Tertiary Period of the Cenozoic Era). The Cretaceous Period began about 144 million years ago and ended about 65 million years ago.
Debris flow. A moving mass of rock fragments, soil, and mud in which more than half of the particles being larger than sand size (otherwise it would be a mudflow) and with 70 to 90 percent of the material consisting of sediment (the rest is water and trapped gasses). Slow debris flows may only move a few feet per year, whereas rapid ones can reach speeds greater than 100 miles per hour. Debris flows can display either turbulent or laminar flow characteristics.
Debris flood. A typically disastrous flood, intermediate between the turbid flood of a mountain stream and a debris flow, ranging in sediment load between 40 to 70 percent (the rest is water and trapped gasses).
Deflected drainage. A stream that displays offset by relatively recent movement along a strike-slip fault. Fault motion and characteristics of the bedrock adjacent to and within a fault zone can influence erosion patterns and diversion of stream drainages over time.
Dip. The angle that a rock layer or any planar feature makes with the horizontal, measured perpendicular to the strike and in a vertical plane.
Dip-slip faults. Inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A reverse fault in which the fault plane is inclined at an angle equal to or less than 45° is called a thrust fault.
Earthquake. Ground shaking caused by a sudden movement on a fault or by volcanic disturbance.
Earthquake fault. An active fault that has a history of producing earthquakes or is considered to have a potential of producing damaging earthquakes on the basis of observable evidence. Not all faults are active or are considered earthquake faults.
Epicenter. The point on the Earth’s surface above the point at depth in the Earth’s crust where an earthquake begins.
Escarpment. A long, more or less continuous cliff or relatively steep slope facing in one general direction, separating two level or gently sloping surfaces, and produced by faulting or erosion.
Extrusive. Igneous rock that forms from the eruption of molten material at the surface. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash.
Fault. A fracture or crack along which two blocks of rock slide past one another. This movement may occur rapidly, in the form of an earthquake, or slowly, in the form of creep. Types of faults include strike-slip fault, normal fault, reverse fault, and thrust fault.
Fault line. The trace of a fault plane on the ground surface or other surface, such as on a sea cliff, road cut, or in a mine shaft or tunnel. A fault line is the same as fault trace. Faults lines can often be difficult to resolve from general surface observation due to cover by younger sediments, vegetation, and human-induced landscape modifications.
Fault zone. A fault or set of related faults that is expressed as a zone of numerous small fractures or of “breccia” or “fault gouge.” A fault zone may be hundreds of feet wide and may locally have a complex structure.
Fault system. A collection of parallel or interconnected faults that display a related pattern of relative offset and activity across an entire region (for example, the San Andreas Fault system).
Fault scarp. An escarpment or cliff formed by a fault that reaches the Earth’s surface. Most fault scarps have been modified by erosion since the faulting occurred.
Franciscan Formation. An assemblage of rocks exposed throughout the Coast Ranges of California that consists of a mix of volcanic rocks, chert, shale, greywacke sandstone, limestone, basalt, and other oceanic crustal rocks that have been partially metamorphosed during their migration from place of origin in a deep ocean basin to being accreted by plate tectonic forces onto the west coast of North America. The name Franciscan was first applied to bedrock of Jurassic and Cretaceous age in the San Francisco region, but the name is commonly used throughout much of coastal central and northern California.
Gabbro. A group of dark-colored, basic intrusive igneous rocks composed principally of calcic-plagioclase minerals (labrodorite or bytonite) and augite, and with or without olivine and orthopyroxene. It is the approximate intrusive equivalent of basalt.
Geologic time scale. Geologists have subdivided periods in Earth's history is measured periods spanning millions of years (Ma). Segments of time periods have been named to help define the chronology of events (such as mountain range formation), the formation of rock units (such as the age of a lava flow), the age of fossils, organizing geologic map units, and other purposes. Below is a standard geologic time scale listing names of major time periods with time span information.
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Geomorphology - the study of the earth's surface including classification, description, nature, origin, and development of landforms and their relationships to underlying structures and the history of geologic changes as recorded by these surface features. Examples of geomorphic features associated with faults are illustrated below.
Graben. An elongate, structurally depressed crustal area or block of crust that is bounded by faults on its long sides. A graben may be geomorphically expressed as a rift valley or pull-apart basin.
Great Valley Sequence. A thick sequence of late Mesozoic age sedimentary rocks (150 to 65 million years old). These rocks consist mostly shale, sandstone, conglomerate and are exposed throughout parts of California’s Coast Ranges and underlies much of the Great Valley west of the Sierra Nevada Range. The Great Valley Sequence represents sedimentary material deposited in shallow shelf to deep-sea environments along the western continental margin mostly before the development of the modern San Andreas Fault System.
Holocene. The name applied to the time span that corresponds with the post-glacial warming period in which we now live. The Holocene Epoch began about 11,000 years ago (at the end of the Pleistocene Epoch of the Quaternary Period), about the time that human population growth and distribution expanded worldwide.
Igneous. A rock or mineral that solidified from molten or partly molten material (referring to magma underground or lava on the surface). The word igneous also applies to the processes related to the formation of such rocks. Examples of igneous rocks include granite, gabbro, and basalt.
Intensity. A measure of ground shaking describing the local severity of an earthquake in terms of its effects on the Earth’s surface and on humans and their structures. The Modified Mercalli Intensity (MMI) scale, which uses Roman numerals, is one way scientists measure intensity.
Intrusive. Igneous rocks that forms from the process of emplacement of magma in pre-existing rock. Intrusive igneous rocks typically cool slowly compared to extrusive igneous rocks formed on the Earth’s surface and therefore commonly have a coarse crystalline texture (like granite or gabbro). The word intrusive applies to both the intrusion process and the rock so formed.
Jurassic. The second period of the Mesozoic Era (after Triassic Period and before Cretaceous Period) and spans the period of time between about 206 and 144 million years ago.
Landslide. A general term covering a wide variety of mass-movement landforms and processes involving the down slope transport of soil and rock under the influence of gravity. Usually the displaced material moves over a relatively confined zone or surface of shear. Landslides have a great range of morphologies, rates, patterns of movement, and scale. Their occurrence reflects bedrock and soil characteristics and material properties affecting resistance to shear. Landslides are usually preceded, accompanied, and followed by perceptible creep along the surface of sliding and (or) within the slide mass. Slumps, debris flows, rockfalls, avalanches, and mudflows are all forms of landslides.
Linear trough. A straight valley that may be bounded by linear fault scarps. A linear trough may be a graben or a rift valley and may be modified by erosion.
Linear drainage. A stream drainage that follows the trace of a fault. Stream alignment may be a result of strike-slip fault motion or the erosion of sheared and pulverized rock along a fault zone.
Linear ridge. A long hill or crest of land that stretches in a straight line. It may indicate the presence of a fault or a fold (such as an anticline or syncline). If it is found along a strike-slip fault it may be a shutter ridge or a pressure ridge.
Linear scarp. A straight escarpment where there is a vertical component of offset along a
fault (either normal or reverse). Linear scarps may also form when preferential erosion removes softer bedrock or soil along one side of a fault.
Mafic. A mnemonic term combining and “Ma” (for magnesium) and “Fe” (for ferric iron). The term is used to describe dark-colored igneous minerals rich in iron and magnesium, as well as the rocks that bear those minerals. See also ultramafic.
Magnitude (M). A numeric measure that represents the size or strength of an earthquake, as determined from seismographic observations.
Mesozoic. The era of geologic time spanning about 248 to 65 million years ago. The Mesozoic Era follows the Paleozoic Era and precedes the Cenozoic Era. The Mesozoic Era is subdivided into the Triassic, Jurassic, and Cretaceous Periods. The term also applies to rocks that formed and accumulated in that time period.
Metamorphic. Pertaining to the process of metamorphism or to its results. Metamorphism is the mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions imposed at depth below the surface and below surficial zones where processes of sedimentation, compaction, and cementation take place. Examples of metamorphic rocks include slate, marble, quartzite, greenstone, gneiss, and schist.
Miocene. An epoch of the late Tertiary Period, after the Oligocene Epoch and before the Pliocene Epoch, representing the time span between about 23.8 and 5.3 million years ago.
Normal fault. A fault in which the hanging wall appears to have moved downward relative to the foot wall. The dip angle of the slip surface is between 45 and 90 degrees. Many normal faults in mountainous regions form from gravitational pull along mountainsides and may be associated with the headwall escarpment of slumps.
Oblique-slip faults. Faults that display significant components of both horizontal (strike-slip) and vertical (dip-slip) motion.
Offset drainage. A stream that displays offset by relatively recent movement along a strike-slip fault. A better term is deflected drainage.
Ophiolite. An assemblage of mafic and ultramafic igneous rocks ranging from basalt to gabbro and peridotite, including rocks derived from them by later metamorphism (such as serpentinite), and whose origin is associated with the upper mantle and the formation of oceanic crust at spreading centers in deep ocean basin settings.
Paleozoic. The era of geologic time spanning about 543 to 248 million years ago. The Paleozoic Era follows the Precambrian Era and precedes the Mesozoic Era. The term also applies to rocks that formed and accumulated in that time period.
Plate tectonics. The scientific theory that the Earth’s outer shell is composed of several large, thin, relatively strong “plates” that move relative to one another. Movements on the faults that define plate boundaries produce most earthquakes.
Pleistocene. The Quaternary Period is subdivided into the Pleistocene Epoch and the Holocene Epoch. The Pleistocene Epoch represents the time span of about 1.8 million to about 11,000 years ago. Many episodes of continental glaciation and intervening ice-free periods occurred within the Pleistocene Epoch. The Holocene Epoch began about 11,000 years ago, about the time that human population growth and distribution expanded worldwide.
Pliocene. An epoch of the late Tertiary Period following the Miocene Epoch and proceeding the Quaternary Period (or Pleistocene Epoch) and representing the time span from about 5.3 to 1.8 million years ago. The cycles of ice-age glaciations and intervening warming periods began in Pliocene time.
Plutonic rock. A rock formed at considerable depth by crystallization of magma and/or by chemical alteration. It is usually medium- to coarse-grained with a granitic texture.
Pressure ridge. A pressure ridge is a topographic ridge produced by compressional forces along a strike-slip fault zone. Pressure ridges typically are located where there are bends along a fault or where faults intersect or stepover. Pressure ridges can be shutter ridges and can occur on one or both sides of a fault or within a fault zone.
Pull-apart basin. A surface depression will form along a fault where down warping of the surface occurs, such as from a developing fold or a fault-bounded graben. Closed depressions can form where extensional bends or stepovers occur along a strike-slip fault zone.
Quaternary Period. The period of time spanning about 1.8 million years ago to the present. The Quaternary Period is subdivide into two unequal epochs–the Pleistocene Epoch extends from about 1.8 million years ago to about 11,000 years ago, and the Holocene Epoch that extends from about 11,000 years ago to the present. The Quaternary Period encompassed many cycles of ice-age continental glaciations and intervening warming periods. The Holocene Epoch corresponds with the last warming period in which we now live.
Reverse fault. A fault in which the hanging wall has moved up relative to the foot wall.
Rift valley. A valley that has formed along a tectonic rift. Rift valleys may be grabens or pull-apart basins, may be structurally complex, and are typically modified by erosion.
Rockfall. The relatively free falling or precipitous movement of a newly detached segment of bedrock of any size from a cliff or very steep slope; it is most frequent in mountainous areas during spring when there is repeated freezing and thawing of water in cracks in rock. Movement may be straight down or in a series of leaps and bounds down the slope; it is not guided by an underlying slip surface (like a slump).
Rupture zone. The area of the Earth through which fault movement occurred during an earthquake. For large earthquakes, the section of the fault that ruptured may be several hundred miles in length. Ruptures may or may not extend to the ground surface.
Sag pond. If a natural depression associated with a fault or associated with a pull-apart basin along a fault system can hold water, even temporarily, it is called a sag pond.
Scarp. A line of cliffs produced by faulting, slumping, or erosion. Scarp is an abbreviation for the word escarpment—meaning essentially the same thing.
Sedimentary. Materials consisting of sediments or formed by deposition. The word sedimentary applies to both the processes and the products of deposition. Examples of sedimentary rocks include shale, sandstone, conglomerate, limestone, and chert.
Seismic hazard. The potential for damaging effects caused by earthquakes. The level of hazard depends on the magnitude of likely quakes, the distance from the fault that could cause quakes, and the type of ground materials at a site.
Seismicity. The likelihood of an area being subject to earthquakes, or the phenomenon of earth movements.
Serpentinite. An ultramafic rock consisting almost wholly of serpentine-group minerals (such as antigorite and chrysotile) derived from the alteration of peridotite. Accessory chlorite, magnetite, and talc may be present.
Shutter ridges. A shutter ridge is a ridge formed by vertical, lateral, or oblique displacement on a fault that crosses an area having ridge and valley topography, with the displaced part of the ridge “shutting in” the valley. Shutter ridges typically are found in association with offset drainages.
Sidehill benches. A step-like surface on the side of a hill or mountain. Both recent fault activity or erosional differences of bedrock lithology across a fault may produce sidehill benches and associated linear scarps. Sidehill benches may also form from slumping that may or may not be associated with faulting.
Slickensides. A polished and striated rock surface produced by friction along a fault.
Slip. The relative displacement of formerly adjacent points on opposite sides of a fault, measured along the fault surface.
Slump. A type of landslide where the downward slipping mass of unconsolidated material or rock moves as a unit. A slump block usually displays backward rotation and on a more or less horizontal axis parallel to the slope or cliff from which it descends. Slumps typically form a fault-like escarpment and may occur at the head of a landslide.
Spreading center. A linear area where new crust forms where two crustal plates are moving apart, such as along a mid-oceanic ridge. Spreading centers are typically seismically active regions in ocean basins and may be regions of active or frequent volcanism.
Stepover. Closely spaced strike-slip faults within a greater fault zone over which the total displacement is distributed.
Strike. The direction taken by a structural surface, such as a layer of rock or a fault plane, as it intersects the horizontal.
Strike-slip fault. A generally vertical fault along which the two sides move horizontally past each other. If the block opposite an observer looking across the fault moves to the right, the slip style is termed “right lateral.” If the block moves to the left, the motion is termed “left lateral.” California’s San Andreas Fault is the most famous example of a right-lateral strike-slip fault. Strike-slip faults produce produce a variety of landforms including shutter ridges, pull-apart basins, sag ponds, and deflected streams.
Subduction zone. A boundary along which one plate of the Earth’s outer shell descends (subducts) at an angle beneath another. A subduction zone is usually marked by a deep trench on the sea floor. An example is the Cascadia Subduction Zone offshore of Washington, Oregon, and northern California. Most tsunamis are generated by subduction-zone-related earthquakes.
Terrace. A relatively level bench or step-like surface breaking the continuity of a slope. Natural bench-like terrace features include elevated-marine terraces (along rising sea coasts), stream terraces (along incising streams), or structural terraces (such as along a fault).
Tertiary. The first period of the Cenozoic Era (after the Cretaceous Period of the Mesozoic Era). The Tertiary Period spans the time of about 65 to 1.8 million years ago. The Tertiary Period is subdivided into 5 epochs—Paleocene, Eocene, Oligocene, Miocene, and Pliocene). It is followed by the Pleistocene Epoch of the Quaternary Period.
Thrust fault. A fault with a dip angle of 45º or less over its extent on which the hanging wall appears to have moved upward relative to the foot wall. Horizontal compression or rotational shear is responsible for displacement. (See also reverse fault and oblique-slip fault.)
Transform fault. A special variety of strike-slip fault along which the displacement suddenly stops or changes form. Many transform faults are associated with mid-oceanic ridges and plate boundaries that show pure strike-slip displacement, like the San Andreas Fault.
Ultramafic. A rock composed chiefly of mafic minerals (rich in iron and magnesium, and less than about 45 percent silica, such as olivine, augite, or hypersthene. Pyroxene and serpentinite are ultramafic rocks. |
