Glossary Starting with the letter: H


The time required for half of the initial number of atoms of a radioactive parent element (such as 14carbon ) to change into atoms of its daughter element (14 nitrogen).

Headward Erosion

The process by which the higher, originating end of a river wears away the rock around it, thereby lengthening tributary streams.

Holocene Epoch

The most recent geologic epoch of the Quaternary Period extending from the end of the Pleistocene (11,000 years ago) to the present.


A fixed point on the Earth’s surface defined by long-lived volcanism.


A hydrosphere is the total amount of water on a planet. The hydrosphere includes water that is on the surface of the planet, underground, and in the air. A planet's hydrosphere can be liquid, vapor, or ice.

Hydrothermal Fluids

Hot brines either given off by cooling magmas, or produced by reactions between hot rock and circulating water, that concentrate minerals in solutions.