Glossary

a method of determining the numerical age of rocks. Absolute dating uses isotopic measurements to calculate the time elapsed between a rock-forming event and the present.

observational error that indicates how close measurements are to their true value

a ridge-shaped fold in which the bedding planes slope downward from the hinge with the oldest layers in the center of the fold

the Earth's mantle, below the lithosphere, in which there is plastic flow and convection

the plane or surface that divides the fold symmetrically. The axial plane may be vertical, horizontal, or inclined at any angle

a layer of sediment, sedimentary rock, or volcanic rock "bounded" above and below by more or less well-defined bedding surfaces

a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine.

a type of chemical sedimentary rock that have a biologic component to their origin.

when rocks fail as rigid blocks or solids

a variety of chemical and detrital sediments such as limestone, dolostone, or marl and are largely composed of calcium carbonate

a white or colorless mineral composed of calcium carbonate. Often found in sedimentary rocks such as limestone and is deposited in caves to form stalactites and stalagmites

binding together of rock particles by cement

sedimentary rock formed by chemical and organic reprecipitation of the dissolved products of chemical weathering.

a cone formed around a volcanic vent by fragments of lava ejected during an eruption. They typically only erupt once.

sedimentary rocks composed of fragments, or clasts, of pre-existing minerals and rock. These are typically the result of physical weathering.

cleavage is a type of foliation that forms within fine grained rocks due to deformation and metamorphism

exertion of force on something so that it becomes denser

an instrument with a magnetized pointer that shows the direction of magnetic north and bearings from it.

a type of fracture that has a smoothly curving surface of fine-grained materials. Some say this is shell-like. This forms in material which have no planar surfaces of internal weakness or planes of separation (no cleavage). Such a curving fracture surface is characteristic of glass and other brittle materials with no crystal structure.

a line on a map joining points of equal height above or below sea level

the innermost portion of the Earth, lying below the mantle and made of iron and nickel. It is divided into a liquid outer core, which begins at a depth of 2,898 km (1,800 mi), and a solid inner core, which begins at a depth of 4,983 km (3,090 mi)

extremely hard aluminum oxide, used as an abrasive. Ruby and sapphire are varieties of corundum.

a side-on view or diagram showing geologic features in a vertical view to illustrate structure and stratigraphy that is hidden underground. Features can include rock units, faults, topography, and more. These often accompany geologic maps, which are an overhead view, which can help to visualize the three-dimensional structure of the region.

for organic matter, decay is rot or decomposition by the action of bacteria, insects and fungi

decrease in either elemental or isotopic abundance; can be lower in SiO2 or U235 as this depends on the geochemistry you are investigating.

the process by which fertile land becomes desert, typically as a result of drought, deforestation, or inappropriate agriculture.

faults which move along a tilted plane

A volcano that is potentially active and not erupting now, but that is considered “active” because it could erupt in the future. Mount Hood is currently dormant.

indicates shape change of a material through bending or flowing during which chemical bonds may become broken but subsequently reformed into new bonds

increase in either elemental or isotopic abundance; can be higher SiO2 or U235 as this depends on the geochemistry you are investigating.

volcanoes that will not erupt again

an observation that's been confirmed many times so scientists can accept it as "true." But everything in science has some uncertainty; nothing is ever scientifically "true" beyond a shadow of a doubt.

Felsic is an igneous rock that is rich in light-colored silicate minerals such as feldspar and quartz (silica).

repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness.

the block of rock that lies on the beneath an inclined fault or a mineral deposit

sometimes called forams; a single-celled planktonic animal with a chalky shell through which slender protrusions of protoplasm extend. Most are marine organisms, and when they die, their shells form thick ocean-floor sediments.

the increase in temperature with depth in the Earth

the science of measuring the pressure and temperature history of a metamorphic or intrusive igneous rocks. This term is a combination of geobarometry, where a pressure of mineral formation is resolved, and geothermometry where a temperature of formation is resolved.

a metamorphic rock made up of bands that differ in color and composition; generally there are light-colored bands rich in feldspar and quartz and dark-colored bands rich in mafic minerals such as biotite or amphibole

an elongated block of the Earth's crust lying between two normal faults that has been displaced downward

a coarse-grained, intrusive igneous rock typically composed of quartz, feldspars, and amphibole/biotite.

equigranular texture formed in metamorphic rocks

the block of rock that lies on the above of an inclined fault or of a mineral deposit.

an imaginary line where the limbs of the fold meet. It is also the line of maximum curvature.

hydrothermal metamorphism occurs when hot, chemically active, mineral laden waters interact with a surrounding preexisting rock (protolith or country rock)

an idea or proposition that can be tested by observations or experiments, about the Earth

a mineral that forms under specific pressure and temperature conditions, and provides information about the rock's metamorphic history. Examples include kyanite, garnet, etc.

Intermediate igneous rocks are moderate amounts of silica (52-63 wt.%) and have nearly equal amounts of felsic and mafic minerals.

lines on a map connecting points where metamorphism of rocks occurred under the same pressure and temperature conditions. These can indicate when an index mineral or metamorphic assemblage occurs

a gemstone made of either pyroxene (jadeite) or amphibole (nephrite)

laws of science are statements, based on repeated experiments or observations, that describe or predict a range of natural phenomena

the areas on either side of the axial plane that stick out like arms or legs

a complex process in which loose grains of sediment are converted into rock. Lithification may occur when a sediment is deposited or later.

physical characteristics of a rock or stratigraphic unit

rigid outer layer of the Earth, consisting of crust and upper mantle

Mafic rock is silicate mineral or igneous rock rich in magnesium and iron. Mafic minerals include olivine, pyroxene, amphibole and biotite mica.

the bulk of Earth's interior. It lies between Earth's core and its thin outer layer, the Earth's crust. The mantle is about 2,900 kilometers (1,802 miles) thick.

a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures

a term for describing the relative temperature and pressure conditions under which metamorphic rocks form

change in the composition of a rock as a result of the introduction or removal of chemical elements

a naturally occurring, inorganic, solid that can be defined by a chemical formula and a crystal structure.

a dip-slip fault in which the block above the fault has moved down relative to the block below

sedimentary rock formed by the accumulation and lithification of organic debris, such as leaves, roots, and other plant or animal (shells or skeletal) material.

leaves, roots, and other plant or animal material

rock formations that are visible on the surface, usually in a cliff or man-made exposure along a road

a metamorphic rock with a shale protolith

a geological phase diagram that connects the stability ranges of metamorphic minerals or mineral assemblages using metamorphic reactions

a type of optical microscope used to identify rocks and minerals in thin sections.

the vertical angle between a horizontal plane and the axis of a feature. Plunge is measured along the axis of a fold, whereas dip is measured along the limbs

having minute spaces or holes through which liquid or air may travel

observational error that indicates how close measurements are to each other.

a rock that existed before it gets metamorphosed, also called parent rock

a shiny yellow mineral consisting of iron disulfide and typically occurring as intersecting cubic crystals. Also called fool's gold.

a hard white or colorless mineral consisting of SiO2, silicon dioxide, found in all rock types, the stable polymorph at the Earth's surface. It is often colored by impurities as in amethyst and citrine.

a method to determine the order of past events by comparing the ages of different geological events

a fault one in which one side of the fault, the hanging wall, moves up and over the other side, the foot wall

the splitting apart of a region into two or more regions separated by normal faults. Also when a tectonic plate is split into two or more tectonic plates separated by divergent plate boundaries

A series of processes which relate rocks in Earth, including igneous intrusion or extrusion, weathering, erosion, transport, deposition as sediment, which lithifies into sedimentary rock, metamorphism, remelting, and leading again to igneous activity

the degree of smoothing from interaction of sedimentary particles.

a clastic sedimentary rock composed of sand-sized minerals, rock fragments, or organic material

a type of foliation developed in medium grained metamorphic rocks, often in mica-rich rocks

an elastic wave in the earth produced by an earthquake or other means

the number of digits needed to accurately describe a measurement, starting from the first nonzero digit.

distribution of grain sizes in sedimentary rocks

a measurement to describe the orientation of a planar geologic feature. Strike is the direction of an imagined horizontal line across the plane. Dip is the angle of the plane measured downward from horizontal.

a fault in which rock strata are moved in a horizontal direction, parallel to the line of the fault

the sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate

found as a native mineral that is always lemon yellow and consists only of sulfur (S). It is found near volcanic vents and in the cap rock of salt domes.

a fold with a downward arc or curve with the youngest layers in the center of the fold

a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can reach up to thousands of kilometers such as the Pacific and Antarctic plates t. Plate thickness ranges from less than 10 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interiors of North and South America).

size, shape, and arrangement (or fabric) of the mineral grains and crystals

explains known facts and also allows scientists to make predictions of what they should observe

a reverse fault that is at an incline of less than 45 degrees

map showing the arrangement of the natural and artificial physical features of an area

Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals.

a type of geologic contact-a boundary between rocks usually caused by a period of erosion. It can also by a significant pause in sediment deposition

the process of being worn by exposure to the atmosphere