To map folded or tilted rock, geologists describe each layer's orientation with two measurements, strike and dip. With right-triangle trigonometry they then predict where the layer goes underground: how deep to drill for a coal seam or an aquifer.
- Strike: the compass direction of a horizontal line on the tilted layer. Imagine the layer meeting a perfectly flat lake: the shoreline runs along the strike.
- Dip: how steeply the layer slopes down from horizontal, measured at a right angle to the strike, and the direction it slopes. Water poured on the layer would run straight down the dip.
Why it works
On a tilted plane, every direction slopes a different amount, but only one is horizontal (the strike) and only one is steepest (the dip direction), and the two are always at right angles. In the vertical slice that points down the dip, the layer is a straight line at the dip angle $\delta$. Walk a horizontal distance $x$ from where it crops out, and the layer lies a depth $d$ below you, the opposite side of a right triangle: $d = x\tan\delta$. A layer cropping out in a band $w$ wide on level ground has true thickness $t = w\sin\delta$.
Key idea
Strike is the compass direction of a level line on a tilted layer; dip is its downward slope, at right angles to the strike. Down the dip, depth $= x\tan(\text{dip})$.
Worked examples
Worked example: How deep to drill?
A coal seam crops out on level ground, strikes north–south and dips $25^\circ$ east. How deep is it under a point 300 m east of the outcrop?
- East is straight down the dip, so use $d = x\tan\delta$.
- $d = 300 \tan 25^\circ = 300 \times 0.466 \approx 140\,\text{m}$.
Answer: About $140\,\text{m}$.
Worked example: Which way does it dip?
A layer strikes N40°E (azimuth $40^\circ$) and dips to the southeast. What is its dip direction as an azimuth?
- The dip direction is $90^\circ$ from the strike: $40^\circ + 90^\circ = 130^\circ$ or $40^\circ - 90^\circ = 310^\circ$.
- Southeast is $130^\circ$ (S50°E).
Answer: $130^\circ$.
Common mistake
It is easy to draw the dip direction along the strike, or at some random angle. Dip always points straight down the slope, at a right angle to the strike. Check by picturing water on the layer: it runs downhill perpendicular to the level shoreline, never along it.