Orientation of strata

Strata are layers of rock , whether of sedimentary (e.g., sandstone or limestone ) or of extrusive igneous (e.g., lava flow) origin. Sedimentary strata are formed when Earth's gravity acts upon particles being transported by wind, water , or ice and pulls them down to the earth's surface, where they form a layer. Sedimentary strata also may form from debris flows and viscous mud flows that move according to gravity. Extrusive igneous strata are formed when Earth's gravity acts upon particles within viscous molten rock and pulls them into a sheet-like or tabular mass called a lava flow. Extrusive igneous strata can also form when pyroclastic material is blown out of a volcano and falls to Earth, forming a layer of volcanic debris. All such layers obey the laws of superposition , original horizontality, and lateral continuity. Of these laws, original horizontality is most pertinent to this discussion because this law predicts the original orientation of rock strata (horizontal). Horizontal is the original orientation of essentially all rock strata.

For this reason, if rock strata is found with some orientation other than horizontal, a force has acted upon the rock strata to re-orient it (change it from its original state). Re-orientation of rock strata occurs principally due to tectonic forces acting within Earth's crust .

Orientation of rock strata is defined as the attitude of layers of rock in three-dimensional space . In order to measure the orientation of rock strata, geologists use a system of measure consisting of two different compass bearings and an angular measurement. The first compass bearing is that of strike . Strike is defined as the compass bearing, relative to north, of the line of intersection between an imaginary horizontal plane and a dipping rock stratum. The second compass bearing is that of dip direction, which is the direction of maximum inclination down from strike (dip direction is always perpendicular to strike). The angular measurement is called dip magnitude, which is the smaller of two angles formed by the intersection of an imaginary horizontal plane and a dipping rock stratum. The compass bearings and angular measurement are made in the field by using a hand-held device called a Brunton compass. In subsurface strata, e.g., layers drilled during petroleum exploration, orientation of strata (i.e., strike and dip measurements) are made by electrical sensor devices lowered into the drill hole on a cable.

Studies of the orientation of rock strata are useful for helping understand the origin of crustal deformation in general and mountain building in particular. Orientation of rock strata can help geologists deduce the direction and type of stresses in Earth's crust that produced the observed deformation. Orientation of rock strata is studied also for purely practical purposes, e.g., in predicting the distribution of hydrocarbons , mineral deposits, or groundwater within dipping strata. For example, groundwater will flow down the dip direction within an inclined aquifer stratum.

Orientation of strata has a strong effect upon surficial landforms developed by weathering of stratified rocks. For example, flat-topped hills and mountains (e.g., butte, mesa, and pinnacle) have a flat-lying (i.e., horizontally oriented) layer of relatively insoluble rock on top, which protects weaker, underlying strata from erosion . Another example of a landform created by rock strata is the flatiron, which is an asymmetrical hill formed by the upturned edges of a dipping layer (rock stratum).

Orientation of rock strata can pose problems for engineering geologists because contacts between steeply dipping strata (i.e., strata with high dip-magnitude angles) can act as planes of weakness. Such planes can promote rockslides and rock falls, both of which can take lives and damage property due to sudden movement of rock material along detachments formed by weak contacts between rock strata.

The orientation of rock strata can be found on most geological maps. The standard symbol for orientation of rock strata is the strike-dip symbol, which consists of a long bar, parallel to strike, and a short spike, parallel to dip direction. This symbol is two to three millimeters in size on most maps and is printed directly upon the mapped layer or stratum possessing the measured dip and strike.

See also Folds; Plate tectonics