The Geology of Bear Territory
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Glossary

Here are some of the more technical terms used in this tour. If you would like to see explanations of these terms in styles or formats different from those offered here, or if you want to learn more geological (and related) terms, some recommended sources include:

blue dotUCMP's Glossary of Natural History Terms: Geological Terms
blue dotUSGS NEIC's Common Terms in Seismology
blue dotUSGS Dictionary of Earthquake Terms - intended for kids, but likely more useful to adults.
blue dotIllustrated Glossary of Geologic Terms - from Steven Richardson at Iowa State University
Term Say what?!
aseismic Occurring without earthquakes. See creep.
basalt One of several kinds of extrusive igneous rock, and generally contains the minerals feldspar, pyroxene, and olivine. This rock type makes up most of the ocean floor.
beheaded
channel
A river or creek bed cut off from its primary water source by movement along a fault and changes in topography. This process usually occurs gradually, with channels first being offset before they are disconnected.
borehole A cylindrical hole drilled into the ground, into which instrumentation, such as seismometers, can be lowered. Since they can be many meters deep, boreholes allow instruments to operate with less interference from weather, passing cars, unstable soils, and other phenomena that occur at the surface.
craton A portion of a continent that has remained relatively stable and undeformed over geologic time.
creep Gradual, relatively slow movement along a fault that releases stress without causing earthquakes (aseismically). In some regions, creep is thought to occur only in the uppermost several kilometers of the crust. In others, however, it appears that creep occurs at greater depths. The town of Hollister is well-known for its location along the creeping portion of the San Andreas Fault.
crust The outermost layer of the solid earth, spanning depths of about 0-10km under oceans and 0-40km under continents.
earthquake Sudden rupture along a fault.
evolution The process of changing over time, on many scales, by which novel lifeforms come to exist and to diverge from previously-existing lifeforms. A more in-depth exploration of this vast biological subject can be found in the Evolution Wing of the UCMP's Online Exhibit Halls.
extrusive Extrusive rocks, such as basalts, are formed when molten material from within the earth is expelled onto the surface. Because of the difference in temperature between the hot interior and relatively cool exterior of the earth, the molten materials cool rapidly. Very fine crystals, characteristic of this type of rock, form as a result of the sudden cooling.
fault
(zone)
A planar break in or between brittle rocks. Faults can range in length from a few centimeters to many kilometers. While faulting can have many causes, much of it occurs at places of weakness in the crust, such as plate boundaries. Faults often occur in groups or branched systems, with forces and motion distributed among them. For this reason, it is often useful to refer to a fault zone, a region in which faulting is present, instead of thinking of faults in isolation. Depending upon the orientation of motion along a fault, it can be classified as one of three main types: strike-slip, dip-slip (including thrust/reverse and normal), and oblique-slip (a combination of dip-slip and strike-slip). The IRIS Consortium website has good animations of these three types of faults and the motion that occurs along them.
fossils The remains of ancient plants or animals preserved over time by their natural surroundings.
geologic time A really, really long time... The time that has elapsed since the earth formed some 4.51 (give or take 0.06) billion years ago (Allègre, 1995). For a look at how this vast temporal expanse has been approached by scientists and others, see the UCMP's online exhibit on the history of the geologic timescale.
headwater The major source of a stream's flow.
Holocene The name given to the division of geologic time spanning approximately the past 10,000 years.
igneous Formed from originally-molten materials. Two main types of rocks generated this way are extrusive (volcanic) and intrusive (plutonic).
intrusive Formed when molten materials cool below the surface of the earth. Rocks formed this way, including granite, tend to have larger crystal grains than extrusive rocks, due to their slower cooling. They are also called plutonic rocks.
liquefy Behave as a fluid. During the strong shaking that can be caused by an earthquake, sands, soils, and landfill (especially if unconsolidated or water-saturated) may liquefy. This process, which can cause considerable damage, is called liquefaction.
metamorphic Formed by the alteration of a previously-existing material. Rocks of this type may have originated as either sedimentary or igneous rocks that were later altered by high temperature or high pressure. These are often found in subduction zones and other areas where the crust is being deformed and/or heated.
mid-ocean
ridge
An area where new seafloor is created as basalts are extruded from within the earth. As this new material pushes its way up and out and cools, it causes the material around it to move further out. This phenomenon is called seafloor spreading, and it is the way oceanic basins open up and expand. They don't expand indefinitely, however. Where the edges of oceanic plates meet the edges of continental plates, the denser oceanic plates are often forced downward in a process called subduction, which is ultimately responsible for "recycling" the rock. See illustrations, and even make your own model of this process!
mineral A naturally-occurring, solid, crystalline substance, usually inorganic, with a specific chemical composition that can vary within defined limits. Different rock types are often formed of different characteristic sets of minerals.
offset channel A river or streambed crossed by a fault that is deformed by fault motion. The amount of offset is generally related to, and can be helpful in determining the slip rate of the fault.
paleontology The study of ancient life through fossils.
plutonic Related to the cooling of molten materials within the earth. A pluton is a body of hot material that rises within the earth because it is less dense that the material around it. When this material cools before breaking the surface, it forms plutonic, or intrusive, rocks.
retrofit Modification of an existing structure, in this case to strengthen against potential damage by earthquakes. This may consist of simply bolting a house to its foundation, or of installing a state-of-the-art shock-absorbing system. For more information on retrofitting homes, buildings, and bridges, see Determining the Safety of Your Home or School from the USGS' The Next Big Earthquake and the FAQ page on earthquake engineering from the Berkeley Seismological Laboratory.
scarp Abbreviated form of "escarpment". The surface created by vertical displacement along a fault.
sedimentary Related to, or formed by, the transport and deposition of materials. Sandstone, which forms when small quartz particles form a cohesive rock, and limestone, which forms when the remains of aquatic organisms settle to the bottom and harden together, are examples of sedimentary rocks.
seismogram A record of ground motion, generally produced by specialized instrumentation. Historically, seismograms have been recorded in diverse ways: by projecting a beam of light onto photographic film, by scratching the surface of a metal sheet, by suspending a pen above a sheet of paper, and by digitizing information for display on a computer screen.
seismology The study of earthquakes and seismic waves. The work of this branch of science is applicable not only to understanding earthquakes themselves, but also to subjects as diverse as plate tectonics, Earth's interior [PDF file], structural engineering, volcano monitoring, natural resource prospecting, and monitoring the international ban on nuclear testing (CTBT).
seismometer An instrument designed to measure the motion of the ground.
slickensides Rock faces that have been smoothed and scraped by rubbing against other rocks along a fault.
slip
(rate)
Movement along a fault. This may be gradual, as with aseismic creep, or sudden, as in an earthquake. The average amount of movement per year is called the slip rate.
strike-slip A type of fault along which horizontal motion predominates. The direction of motion is characterized by the relative motion of a block to an observer on the opposite block, and is either described as "right-lateral" or "left-lateral".
stylolite An irregularly-shaped band, usually in limestones, formed where rock layers have come together due to the dissolution of intervening material.
subduction
(zone)
The plunging of one plate beneath another due to convergence. Commonly, oceanic plates will subduct beneath less-dense continental plates. In regions where this is occurring, known as subduction zones, numerous features are commonly observed: volcanoes, earthquakes, island arcs, ocean trenches, and mountain belts.
topography The elevation(s) and relief of land surfaces in a region.
trenching A method of studying geology, and particularly faults, that lie beneath the surface. It involves digging one or more long holes and carefully studying the sides to see the composition and spatial relationships of the exposed layers of rock and soil. From that information, geologists can infer some of the geological processes at work in an area.
unconsolidated Not in the most compact form. This term is generally applied to soils and other layers of loose materials that have not hardened into rock. Unconsolidated sediments pose a threat during large earthquakes due to their tendency to amplify ground shaking and to liquefy.
vesicular Full of holes. The holes, or vesicles, in some rocks are caused by gases bubbling out of molten material.
volcanic Related to, or formed by, the eruption or extrusion of molten materials onto the surface of the earth (on land or under water). Volcanic, or extrusive, rocks are one type of igneous rock.
xenolith The Greek root-word "xeno" means strange or foreign, and "lith" means rock. A xenolith, then, is a "strange rock" - one whose origin is unlike that of the rock in which it is found. For example, volcanic rocks can contain pieces of unrelated rocks usually found deeper in the earth that were ripped out during an eruption. Those pieces are called xenoliths.