GeologyRocks

Fulgurite

A hollow tube of glass (predominantly lechatelierite, which is naturally fused silica dioxide or Quartz) formed when lightning strikes quartz-rich sand. Often referred to as "petrified lighting".

Gangue

The unwanted material around ore minerals

Grykes

The gaps between clints in limestone pavement. They are formed from dissolution of limestone, probably along joints. They are also known kluftkarren.

Hoa

A channel or pass connecting an atoll lagoon with the open ocean, e.g. at Mururoa Atoll (French Polynesia)

Hydrocarbon

A solid, liquid or gas composed of hydrogen and carbon. Examples include coal, oil and natural gas.

Ice Age

An extended period of time where there is extensive glaciation covering the Earth.

Isostacy

The study of the responses of the Earth to loads (mountain belts, glaciers) placed on it. Isostatic theory states that at the depth of compensation all pressures exerted by rocks above are equal. Northern Europe is currently re-bounding from glaciers that were there during the Last Glacial Maximum, such that Scotland is raising and Kent is sinking.

Isostatic Sea Level Change

Sea level change on a local scale. Can be caused by glaciers making continents sink or melting glaciers making the continents rise again (isostatic rebound) or local tectonic activity. Combined with the eustatic changes make a relative sea level change.

Isotropic

An isotropic mineral appears opaque in thin section in polarised light.

Other meanings:

• Materials: In the study of mechanical properties of materials, "isotropic" means having identical values of a property in all crystallographic directions.

• Optics: Optical isotropy means having the same optical properties in all directions.

Jurassic

The middle period of the Mesozoic from 199Ma to 145Ma.