Geology glossary (G to K)
This page of the glossary covers G to K. You can navigate to the next section from the links at the bottom of this page.
Gabbro a dark, coarse-grained intrusive igneous rock, rich in calcium, magnesium and iron; the chemical equivalent of basalt
Geological time scale The division of all of Earth history into blocks of time distinguished by geological and evolutionary events, ordered sequentially and arranged in a hierarchy of eons, eras, periods and epochs.
Geology The scientific study of the Earth, its origins and evolution, the materials that make it up, and the processes that act upon it.
Geomorphology The study of the classification, description, nature, origin, and development of landforms and their relationship to underlying structures, and of the history of geologic changes as recorded by these surface features.
Glacial lake A lake that derives much or all of its water from the melting of glacier ice, fed by meltwater, and lying beyond the glacier margin.
Glacial abrasion The process by which a glacier erodes the underlying bedrock through contact between the bedrock and rock fragments embedded in the base of the glacier.
Glacial till Drift that is deposited directly from glacial ice and therefore not sorted. Also called till. See also glacial drift and boulder clay.
Glacier A moving body of ice that forms on land from the accumulation and compaction of snow, and that flows downslope or outward due to gravity and the pressure of its own weight.
Glaciation The formation, movement, and recession of glaciers or ice sheets.
Gneiss A coarse-grained, foliated metamorphic rock marked by bands of light-coloured minerals such as quartz and feldspar that alternate with bands of dark-coloured minerals.
Granite A commonly light coloured, coarse grained, igneous (plutonic) rock that contains the minerals quartz, orthoclase and plagioclase. Granite is commonly found on continents but is virtually absent in the ocean basins.
Graptolite The graptolites are an extinct group of animals, prevalent during the Palaeozoic. They lived as colonies, protected within a tough organic 'exoskeleleton'. The majority of these colonies were not attached to the sea floor, but floated in the water column.
Head The name given to the mixture of soil and scree that forms in valleys and on valley slopes as a result of the continual freezing and thawing of ground under cold climatic conditions (often tundra-like) such as those associated with periods of glaciation.
Holocene The last 10,000 years of the Earth’s history, corresponding to the time since the final retreat of the last great ice sheets. The Holocene has been characterised by relatively warm temperatures.
Ice age An interval of time during which the Earth is substantially cooler than usual and a significant portion of its land surface is covered by glaciers. Ice ages generally last several million years.
Ichthyosaur Name given to a member of a group of extinct dolphin-shaped reptiles that were one of the top predators inhabiting the Mesozoic (Triassic, Jurassic and Cretaceous) seas.
Igneous rock A rock made from molten (melted) or partly molten material that has cooled and solidified.
Inlier An area or formation of older rocks completely surrounded by younger rocks.
Intrusive rock An igneous rock formed by the forcing of magma into pre-existing rocks (see also dyke and sill).
Joint A fracture in rock along which no movement has occurred.
Jurassic A Period of geological time, dating from about 205 to 142 million years ago.
Kame An irregular mound or ridge of sediment deposited against an ice front.
Kettle hole A hollow in sand and gravel caused by subsidence where an underlying body of ice has melted.
Karst A topography characterised by caves, sinkholes, disappearing streams, and underground drainage systems. Karst forms when groundwater dissolves rock dominantly composed of limestone, dolomite, or gypsum.
