Activity 1: Strike and Dip | |||||||||||||

Geology exists in
three dimensions. Yet
when we display
it on a map, we
see only two dimensions. To convey accurately the nature of geologic objects, we must use a shorthand notation to specify orientation in three
dimensions. For a
planar feature, its three-dimensional orientation is described by its strike and dip. Strike is the line formed by the intersection of a horizontal plane with the plane of the
feature (Fig. 1).
The strike of a
rock layer fixes its
orientation with respect
to compass direction. The angle the layer dips into the ground is its dip. It
is always perpendicular to strike. To visualize strike and dip, consider a tilted board placed in a tub of
water. The
board’s strike corresponds to the line traced on the board by the surface of the water. The dip is the direction water slowly
poured on the
board would flow down. | |||||||||||||

| Fig. 2: The strike-and dip-symbol and the determination of strike. | ||||||||||||

The strike-and-dip symbol is T-shaped (insert of
Fig. 2). The long
line is parallel to
the strike, and the cross member indicates the direction of dip. These two lines are always at right angles to each
other. A number
next to the strike-and-dip symbol indicates the number of degrees the
bed dips from the
horizontal (Fig. 2). The quantitative value of the strike is not shown explicitly with the strike-and-dip symbol.
It is obtained by
measuring the
angle between the
strike line of
the symbol and a line
pointing north (Fig. 2). Strike is expressed in terms of quantrant, S45ºE, or as an
azimuth, 135º. In addition to the general strike-and-dip symbol, three unique orientations of rock layers require special strike-and-dip symbols (Fig. 3). | |||||||||||||

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Before doing this activity, you can practice with strike and/or strike-and-dip using two interactive practice exercises. | |||||||||||||

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