60.4: Orientation of structures
- Page ID
- 22860
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Knowing the position of geologic structures is important for geologic mapping. These structures include sedimentary features such as beds, metamorphic features such as foliation, and igneous features such as dikes. Their positions in space can be measured in the field and recorded accurately on the map using symbols and numbers.
There are several approaches to expressing this orientation, but the basics are that we want to (a) describe the position of the structure relative to the compass (north, south, east, west, etc.) — we call this ‘declination’ — and (b) we want to be able to describe how inclined that structure is relative to horizontal (i.e., in reference to the Earth’s surface) — this is its ‘inclination’ or ‘dip.’ We measure the declination using either ‘dip-direction’ or ‘strike.’
Most students find it easier to think about measuring bed orientation in “dip and dip-direction,” but many professors have been trained with “strike and dip.” Let’s briefly examine both.
Dip & dip-direction (vs. strike & dip)
This block diagram shows the relationship of a plane-shaped feature of interest (in this case, a bed of yellowish rock, perhaps a sandstone) with two planes that intersect it: one is a horizontal plane and one is vertical. The line of strike is the intersection between the bedding plane and the horizontal plane. The dip-direction is perpendicular to the strike: it is a line at the intersection of a vertical plane and the horizontal plane. It’s the direction in which the rock dips: if you were to tilt a bottle of water over the outcrop surface, the little stream of water would run downhill in the direction of dip. But which way is that?
Both strike and dip-direction are compass directions, measured with reference to north. They are most efficiently expressed as a three-digit number that varies between 000\(^{\circ}\) and 360\(^{\circ}\). This is “azimuth notation.” Some geologists prefer “quadrant notation;” to measure east or west of north or south by the appropriate number of degrees and then express that as something like “N40\(^{\circ}\)W,” which would mean “40\(^{\circ}\) west (counterclockwise) from north.” That same orientation could be described in azimuth as “320\(^{\circ}\)” (that is the 360\(^{\circ}\) of true north minus 40\(^{\circ}\).)
If that sounds confusing, don’t worry: On geologic maps none of those numbers appear. Instead, the orientation of the line of strike is simply shown graphically. Usually “north” is at the top of the map page, so a line running from the top of the page toward the bottom would be a northerly strike. From a north-south strike, the beds could dip either to the east, or to the west. Those are the only two directions that are perpendicular to north. The dip-direction is always perpendicular to the strike. The little dip-tick line (the “stem” of the T shape, jutting out from the strike line at a right angle) shows dip-direction.
The dip itself is the angle of inclination, measured from horizontal down to the bedding plane. It is most efficiently expressed as a two-digit number that varies between horizontal (00\(^{\circ}\)) and vertical (90\(^{\circ}\)). This number is written at the end of the dip-tick line, as shown here in the two examples of strike and dip symbols as they appear on geologic maps. Note that nowhere is the dip-direction or the strike direction written down as numbers. Instead, they are depicted graphically with the orientation of the T shape of the symbol. The long line (the top of the “T”) is the line of strike. The short line that comes out of it (the stem of the “T”) is the dip-direction. The number at its end is the dip angle in degrees from horizontal.