When we want to record the orientation in space of a geological surface (e.g. a stratigraphic body), we have two different ways of doing it:
a) Dip direction and dip angle: Dip direction is just the direction toward which the plane is inclined. Dip angle is the angle of inclination of the plane. In the example included in this article, we would write it down as 090/45; the plane is inclined 45º towards the east (090º). Easy.
b) Strike and dip angle: Strike is the direction of the imaginary line which would represent the intersection between the plane and a horizontal surface. But here it comes the problem: the strike, as we know, can be represented by two different conjugated angles, and we need to follow some kind of convention; typically, the strike is expressed as the acute angle between the intersection line and the north direction. In our example, it would be 0ºN. But how which convention we take for indicating towards which direction is dipping the bed? For that we use the right hand rule.
There are two different right hand rules... Did you know that? Oh, that is not very convenient...
- American right hand rule: Looking to the strike direction, the bed dips to the right. In our example, that would fit with 000/45. So, looking to the north, the bed dips to the right, to 090 (east).
- British right hand rule: The thumb of the right hand indicates the dip direction, and the heal points to the strike direction. So in our example, we would record the orientation as 180/45. That is: Strike towards 180 (perfect south bearing), and dip of 45º towards 090.
180/45E
Which would mean, no doubt, a strike of 180 and a dip direction towards the east, with a magnitude (dip angle) of 45. If you use strike, please remember to write always as a companion of the dip angle the quadrant where the dip direction would be. For example, 045/56SE. In this case we are talking of 120/56 (dip direction/dip angle). 045/56NW; now we would be talking of 315/56 (dip direction/dip angle).
But what happens with measures like:
45W/34
What does that mean? I have seen it. Does it mean 45º from north towards west, so 315? or does it mean strike of 045 and an a dip angle of 34 towards W? Is that? Uh, I don't think so.
The truth is that, besides right-hand rules, many geologists use other conventions, sometimes personal ones, and maybe in a wrong way. Perhaps using only NW and NE quadrants for the strike. Perhaps not.
It is clear that using strike values when representing the orientation of a bed we are introducing a risk factor if we don't state clearly which right hand rule we are using. Of course, for some people in some situation the usage of strike/dip angle may be very useful:
- When we speak of general trends in a big area (e.g. an orogen, in order to show that a trend is parallel or not to the structure).
- In a mine, in order to visualise quickly if the strike direction is the same as certain mineralisation or lineaments.
- In engineering geology, for the same reason, in order to know if the bedding/joints have the same orientation than a well, a tunnel, etc.
I would suggest another right hand rule... If you use strike for describing the attitude of a plane, your right hand should be cut!.
But perhaps that is very extreme... so far.
