Definitions and images to illustrate geological terms, links to images and website articles


A geological map provides a graphic representation of selected geological features within a desired surface topographic or subsurface area. The size and relative position of each feature on the map corresponds to its correct geographic situation according to an established scale and projection. When mapping out a region, standardized terms are employed to describe the spatial orientation (attitude) of planar and linear elements of folds, faults, and other geological structures (diagram):

. The strike of a plane is the compass direction of the line formed by the intersection of the horizontal plane with the inclined plane under consideration. So, strike marks the geographical direction perpendicular to dip. ├ Strike and dip are depicted on geological maps by a long line (strike) with a short perpendicular line (dip) and a number indicating the angle of dip (degrees).

In North Amercia, strike is often expressed as the angle E or W of true North (0º-90º). In the European system, compass directions are expressed as azimuths. The azimuth is measured clockwise along an horizontal plane from the true North direction to the strike line (0º-359º). N=0º, E=90º, S=180º, W=270º.

A strike tending 26º east of true north would be expressed as N26ºE in Canada and the US, and as 026º in the European system. Similarly 74º west of true north would be expressed as N74ºW in the North American system, and as (360-74=) 286º in the European system.

.. The dip of a plane is analogous to plunge, and is the angle in degrees measured from an horizontal plane down to the inclined plane under consideration. That is, dip is the angle between the inclined plane and the horizontal plane, and is measured along a vertical plane perpendicular to the strike line of the plane.

Strike and dip directions of a fold are always mutually perpendicular, though two planes could have the same numerical strike (direction) and dip (angle). That is, a plane inclined at 45º to the horizontal (dip) that is facing SSE (135º) could have a strike (direction) of 45º East (o45º).

Strike and dip are differentiated in North America by expressing the direction according to the geographic quadrant faced by the planes. By European convention, strike is expressed as a three digit azimuthal number, and dip as a two digit angular number. Thus, a plane striking 25º and dipping 45º toward the southwest would be noted: 025-45SW.

Labels: , , , , , , , , ,


<< Home

. . . evolving since 01/07/07