Strike and dip

Geologists use a prescribed method of determining the attitude (or orientation in three-dimensional space ) of rock layers or any other planar geological feature (e.g., metamorphic foliation, fractures, faults, and tops of tabular units like formations). The method involves measurement of strike and dip of the rock layers or planar features. Strike is defined as the compass direction, relative to north, of the line formed by the intersection of a rock layer or other planar feature with an imaginary horizontal plane. The intersection of two flat planes is a straight line, and in this instance, the line is geologic strike. According to convention, the compass direction (or bearing) of this line is always measured and referred to relative to north. A typical bearing is given, for example, as N 45° E, which is a shorthand notation for a bearing that is 45 degrees east of north (or half way between due north and due east). The only exception to this north rule occurs where strike is exactly east-west. Then, and only then, is a strike direction written that is not relative to north.

Dip, as a part of the measurement of the attitude of a layer or planar feature, has two components: dip direction and dip magnitude. Dip direction is the compass direction (bearing) of maximum inclination of the layer or planar feature down from the horizontal. If a marble is held anywhere along the strike line on a layer or planar feature and then is released, thus allowing it to roll down the layer or planar feature, the marble would roll along a line showing true dip direction. This true dip direction is always perpendicular (i.e., at a 90 degree angle) to strike. Dip magnitude is the smaller of the two angles formed by the intersection of the dipping layer or planar feature and the imaginary horizontal plane. However, dip magnitude can also be equal to either zero or 90 degrees, where the layer or planar feature is horizontal or vertical, respectively.

A specially designed instrument, called a Brunton pocket transit, is used by geologists to measure strike, dip direction, and dip magnitude in the field. A Brunton pocket transit has a compass, bubble level (for finding horizontal), and a dip-angle measuring device (clinometer) built into it. Information on strike and dip obtained using the Brunton is then conveyed to a geologic map and plotted there using a strike and dip symbol. This symbol, about the length of the word the on this page, consists of a long bar oriented parallel to strike and a short spike perpendicular to the long bar showing the bearing of true dip direction. A small number printed by the symbol indicates the actual dip magnitude in degrees. On some maps, this number is not printed or is not printed by all such symbols.

Measurement of strike and dip (i.e., the attitude of rock layers or other planar geologic features) helps geologists construct accurate geologic maps and geologic cross-sections. For example, data on rock attitudes helps delineate fold structures in layered rocks. Attitude of other geologic structures like faults can be understood using strike and dip as well. It is especially critical to understanding geologic relations among rock bodies in the subsurface realm that surficial (relating to the surface) strike and dip of rocks is well known.

For entirely subsurface studies of strike and dip, devices called dip-meter tools can be lowered into drill holes. These tools, which use electrical properties of rocks in the well wall to sense attitude, help delineate subsurface orientations of rock layers and other planar features. Information from such sub-surface studies is critical in areas where surficial attitude measurements are not adequate to understand subsurface structures. All surface and subsurface information on attitudes of rocks is important in helping geologists understand the structure and origin of Earth's crust .

See also Faults and fractures; Folds; Orientation of strata