Lab 7: Geologic Structure - Strike and Dip

 

Geologists need a way to describe the structural attitude of rock layers and bodies. Structural attitude refers to the way a stratum is oriented in 3D, relative to the surface of the Earth, which locally may be treated as a planar, horizontal surface. If a stratum is horizontal, there is no dip angle, but with any degree of tilting, a dip angle, down from the horizontal, can be measured. A dip of 0 degrees means the stratum is horizontal. A dip of 90 degrees means the stratum is perfectly vertical. Angles between 0 and 90 degrees indicate an amount of tilt between horizontal and vertical.

 

Strike is the direction perpendicular to dip (perpendicular means "at right angles to" and can also be called orthogonal). Strike is the longer line in the strike and dip symbol posted on those illustrations, sort of like a squat "T" symbol.

 

The following animation shows dip angles ranging from 10 to 90 degrees. If you think of due North as extending directly away from you, as the viewer, the dip direction here is toward the southwest, and strike is northwest-southeast. A 3D version of the strike and dip symbol is plotted on the surface of the dipping stratum:

 

 

On maps below, the strike and dip symbol is posted to show the structural attitude of strata at that location. Along the Texas coast, late Cenozoic age strata wrap around the outcrop -- the sweeping arc of the Texas coastline is a result of this structure of the Gulf Coastal Plain. First, take a look at the geologic map of this region of Texas:

 

 

Observe the thin stratum colored dark red that is easy to follow from the Mexican border at bottom left, around the coast and parallel to it, over to the border with Louisiana on the right. Along the whole stretch, strata dip toward the coast line, but the strike changes (the strata are bent). Strike and dip symbols show this change in structural attitude of the beds:

 

 

Here are strike and dip symbols on map snippets for three areas along this outcrop:

 

Strike and dip in South Texas: Strike, shown by the longer line, runs from a north-northeast direction to a south-southeast direction (For a directional refresher: north is at the top, at the 12 o'clock position, east on the right at 3 o'clock, south on the bottom at 6 o'clock, and west on the left at 9 o'clock).

 

Strike of coastal strata near Houston is turned a bit from South Texas and runs northeast to southwest (you could say it southwest to northeast, same thing).

 

Strike of coastal strata just north of Beaumont, along the Louisiana state line, has become almost east-to-west.