- The time necessary for half of the atoms of a parent isotope to decay into the daughter isotope.
- The degree of resistance of a given mineral to scratching, indicating the strength of the bonds that hold the mineral's atoms together. The hardness of a mineral is measured by rubbing it with substances of known hardness.
- A cliff that projects out from a coast into deep water.
- Holocene Epoch
- The second epoch of the Quaternary Period, beginning approximately 10,000 years ago and continuing to the pres-ent time. See also Pleistocene Epoch.
- A spit that curves sharply at its coastal end.
- A high mountain peak that forms when the walls of three or more cirques intersect.
- A hard, dark-colored, dense metamorphic rock that forms from the intrusion of magma into shale or basalt.
- A block of rock that lies between two faults and has moved upward relative to the two adjacent fault blocks. See also graben.
- hot spot
- An area in the upper mantle, ranging from 100 to 200 kilometers in width, from which magma rises in a plume to form volcanoes. A hot spot may endure for 10 million years or more.
- hydraulic conductivity
- The extent to which a given substance allows water to flow through it, determined by such factors as sorting and grain size and shape.
- hydraulic gradient
- The difference in potential between two points, divided by the lateral distance between the points.
- hydraulic lifting
- The erosion of a stream bed by water pressure.
- A molecule that is entirely made up of hydrogen and carbon.
- hydrogen bond
- An intermolecular bond formed with hydrogen.
- hydrologic cycle
- The perpetual movement of water among the mantle, oceans, land, and atmosphere of the Earth.
- A form of chemical weathering in which ions from water replace equivalently charged ions from a mineral, especially a silicate.
- hydrothermal deposit
- A mineral deposit formed by the precipitation of metallic ions from water ranging in temperature from 50º to 700ºC.
- A tentative explanation of a given set of data that is expected to remain valid after future observation and experimentation. See also theory.