B : Orientation of Structures

Question 1

Provide 4 examples of planar features

Answer 1

1. tabular igneous intrusive bodies such as dykes and sills;
2. bedding planes in sedimentary rocks;
3. the fabric or foliation produced by alignment of sheet silicate minerals such as mica in metamorphic rocks, which reveals the direction of flattening during deformation;
4. joints and faults produced by the failure of rocks in response to stress (and which therefore reveal the orientation of stress at some time in the past).

Question 1

Provide 3 examples of linear features

Answer 1

1. glacial striae (which reveal the direction of ice movement);
2. the fabric or lineation produced by alignment of amphiboles seen in metamorphic rocks (which reveal the direction of stretching acquired during deformation);
3. alignment of elongate clasts or fossil shells in sedimentary rocks (which reveals current direction).

Question 2

What does orientation mean?

Answer 2

The angles between a structure and a frame of reference (typically defined by the north and vertical directions). aka attitude

Question 3

What does bearing mean?

Answer 3

A direction relative to north.

Question 4

What are azimuths?

Answer 4

A bearing measured clockwise from north from 0 - 360°. Use 3 digits

Question 5

How do you convert azimuths to US quadrant method?

Answer 5

Azimuth uses 3 digits measured from 0. The quadrant method uses 2 digits plus compass directions. For example, 087° N87E

Question 6

What does magnetic declination mean?

Answer 6

The azimuth of the Earth’s magnetic field.

Question 7

What does inclination mean?

Answer 7

An angle of slope measured downward relative to horizontal.

Question 8

What is the plunge of a line?

Answer 8

Value of inclination of a line, measured downward relative to horizontal. Use 2 digits

Question 9

What is the trend of a line?

Answer 9

The azimuth of a line, measured in the direction of downward plunge. Use 3 digits

Question 10

What is dip?

Answer 10

The value of the inclination of a plane. Represents the inclination of steepest line that can be drawn on the plane.

Question 11

What is dip direction?

Answer 11

the azimuth of the steepest line in the plane.

Question 12

What is strike?

Answer 12

Azimuth of a horizontal line that lies in a plane. Is 90 degrees to the dip direction

Question 13

What is the right hand rule?

Answer 13

An orientation-measuring convention for planes. When facing the strike direction, the plane dips your right. (Or, dip direction is 90° clockwise from the right-hand-rule strike direction.) strike is always 90° counter-clockwise from the dip direction.

Question 14

What is the UK method of writing strike and dip?

Answer 14

dip direction is counterclockwise from the strike,

Question 15

What does surface trace mean?

Answer 15

The line along which a geological surface intersects the topographic surface.

Question 16

What is a fault cut-off?

Answer 16

Intersection line between a fault plane and an older planar rock unit.

Question 17

What is a hinge?

Answer 17

A line or point of maximum (tightest) curvature on a folded surface.

Question 18

What is a subcrop?

Answer 18

A feature on an unconformity surface formed where the unconformity cuts off (intersects) a older rock unit or surface.

Question 19

What does rake or pitch mean?

Answer 19

Orientation of a line that lies in a plane, measured from the strike direction, within the plane. On a vertical plane the rake of a line is the same as its plunge. On all other planes, rake ≥ plunge.

Question 20

What is a pole?

Answer 20

A line that is perpendicular to a given plane.

Question 21

What is a contour?

Answer 21

A curving line on a map that separates higher values of some quantity from lower values.

Question 22

What is a structure contour?

Answer 22

A contour based on the elevation of a geological surface, separating higher and lower parts of the surface; a line joining points of equal elevation on a geological surface.