Geology Glossary

Ammonite: an extinct cephalopod that had a carbonate shell that was often similar in shape to the present-day chambered nautilus. Other present-day cephalopods include octopus and squid. Fossilized ammonites are very useful for biostratigraphic dating of rocks because new species evolved rapidly.

Biostratigraphy: the differentiation of rock units through the study of the fossils they contain, thus allowing the rocks units to be dated relative to each other. Biostratigraphy is based on the evolution of species through time.

Carbonate Compensation Depth (CCD): the ocean depth below which the all calcium carbonate dissolves in the water before it becomes incorporated in the seafloor sediment.

Carnelian: a translucent orange-red form of chalcedony, a cryptocrystalline variety of quartz.

Cryptocrystalline: containing crystals too small to be recognized with an ordinary microscope.

Diagenesis: the chemical and physical changes to sediments as they are buried and compacted into sedimentary rock.

Dike: a tabular body of igneous rock that cuts across the structure of adjacent rocks.

Earth’s orbital cycles: cyclic changes in the Earth’s orbital parameters that can affect climate and oceanography. Primary cycles are the approximate 100,000 year eccentricity cycle (how round or oval the orbit is), 42,000 year obliquity cycle (the amount the axis of rotation is tilted) and the approximate 21,000 year procession cycle (the place in the orbital cycle in which one pole or the other is pointed towards the sun).

Equatorial upwelling zone: an area of ocean near the equator where winds create currents that divert surface water away to the north and south, allowing cold, deep, nutrient-rich waters to rise to the surface and replace the diverted surface waters.

Foraminifera: a group of single-celled organisms characterized by a shell or test of one to many chambers that often resemble the shell of a tiny snail. Foraminifers that float in the water have calcite tests, whereas those that live on the seafloor may have shells composed on tiny sedimentary particles glued together. Foraminifers are useful for biostratigraphic dating of rocks.

Greenstone: dark green metamorphosed basalt altered during interaction with hot, mineral-rich seawater. Chlorite and other green minerals resulting from the alteration account for the green color.

Hot spot: the surface expression of a rising plume of hot mantle material. Hawaii and Yellowstone are examples.

Hydration: the chemical combination of water with another substance.

Mid-ocean ridge: A major elevated linear feature of the seafloor where two tectonic plates are being pulled apart and new ocean crust is formed. It is a type of divergent plate boundary.

Ocean trench: a long, narrow, deep trough in the seafloor which marks the line along which a tectonic plate bends down into a subduction zone. Ocean trenches form the deepest parts of the ocean floor.

Oxidize: the union of oxygen with a substance (in this case a mineral).

Phenocryst: a large crystal surrounded by a finer matrix in an igneous rock.

Radiolarian: microscopic or near microscopic single-celled marine zooplankton with ornate skeletons made of silica.

Radiometric dating: Determining the age of rocks by measuring the ratio of certain radioactive elements in them, relative to the amount of decay products from those radioactive elements. The age can be determined by multiplying the ratio by a known decay constant.

Ribbon chert: a form of bedded chert that has many thin, continuous chert beds separated by small shale partings, resulting in an outcrop that looks like a series of chert ribbons.

Sill: a tabular igneous intrusion that parallels the bedding of the surrounding rock.

Subduction zone: the zone between a sinking oceanic plate and an overriding plate. It is a type of convergent tectonic plate boundary.

Turbidites: sediments deposited from bottom-flowing, density-driven underwater landslides that flow down an underwater slope. Turbidites are characterized by moderately sorted sediments ranging from cobbles to coarse sand at their bases to fine sand and silt at their tops.

Vesicles: small spherical cavities in a volcanic rock produced by air or gas bubbles in the molten rock. Vesicles may sometimes become filled with quartz, calcite or other minerals.

Upwelling zone: an area where nutrient-rich, deep ocean waters flow upward to reach the surface. Upwelling zones result from wind-driven surface currents that move water away from a coast, or cause water to diverge from an area, like the equatorial upwelling zone.