GLOSSARY – G

GLOSSARY – G

by: Charles O’Dale

GEOLOGY
Science of the study of the accessible parts of the Earth’s crust, their rocks, their structures,their fossils and their resources with the aim to get a picture of the development of the planet and the life on it. So, geology is partly a historical science and partly a natural science. Although “geo” means earth, the term geology is used also with the study of other planetary bodies, e.g., planetary geology, geology of the Moon.

GEOMORPHOLOGY
The scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth’s surface.


The following examples illustrate the shortcoming of using geomorphology in crater identification:

Alsever Lake compared to Brent impact crater

Brent Crater – IMPACT
Alsever Lake – NON IMPACT

Alsever Lake (image LEFT) is located at the southern boundary of Algonquin Park. It is similar in appearance to the Brent impact crater (image RIGHT) at the northern boundary of Algonquin Park. Both structures have two distinct bodies of water forming a circular patterns.


Skootamatta  Lake compared to Presqu’ile impact crater

Presqu’Ile Impact structure – IMPACT
Lake Skootamatta – NON IMPACT

Scootamatta Lake (image LEFT) is located in Southern Ontario. It is similar in appearance to the Presqu’ile impact crater (image RIGHT) located in north central Quebec. Both structures have distinct bodies of water forming circular patterns.


GEOLOGIC DATING
Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth’s surface to determine the relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon there are many horizontal layers, which are called strata. The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
[see – DATING– geologic]

GEOPHYSICS
Physics of the Earth related with all aspects of physical properties, structures, and processes of the Earth (and other planetary bodies). Geophysics of impact structures comprises gravity, magnetic, seismic, geoelectric, and other measurements. Several buried impact structures have been detected by geophysical studies

GPa
Gigapascal, 1 GPa = 1,000 MPa (Megapascal) = 109 Pascal, the SI unit of pressure. GPa is commonly used in the high-pressure range of shock deformation, 1 GPa = 10 kbar.

GRADY-KIPP FRAGMENTS
In an impact cratering model, the rock fragments resulting from tensile stress related with therarefaction wave.

GRANOPHYRE
Igneous rock with granitic composition. Also local term for an impact melt rock in the Vredefort impact structure, South Africa.

GRAVIMETRY – Gravity anomaly 
Geophysical method to measure variations of the gravity field related with subsurface density variations. Impact structures commonly show pronounced gravity negative anomalies due to the occurrence of low-density breccias, rock fracturing, and replacement of ejected material by post-impact young sediments. In very large impact structures, relative positive anomalies may be produced by the uplift (see; central uplift) of high-density material from the Earth’s lower crust and upper mantle.
[see – GRAVITY ANOMALY ]

GRIES (= gravel, grit),
German term especially used to describe a heavymonomictic grit brecciation in rocks of the Ries impact structure.

GRIT BRECCIATION
Cataclastic deformation by shearing and granulation of hard (competent) rocks typically found in impact structures. Also see monomictic movement breccias.