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Unit One - Geological Field
Methods
2. Measuring Rock Structures
Objectives - The
student will be able to:
- Explain the importance of
measurement in geology.
- Describe the difference
between magnetic north and true north.
- Describe the difference
between the azimuth and quadrant system of measurement.
- Explain how to determine
the strike and dip.
Explanation
Rock outcrops are places
where bedrock is exposed at the surface. Rock structures are features
which can be observed from an outcrop. It can be things like the
bedding of sedimentary rocks or the foliation of metamorphic rocks.
An inclined plane intersects with a flat, or horizontal plane to form
a line. By definition, this is the “strike” of the
inclined plane. The “dip” of the inclined plane is
defined as the angle below the horizontal measured at 90 degrees to
the strike. Linear features are defined by measuring the direction
(or bearing) of the axis and its angle below horizontal (or “plunge”).
By recording the strike and dip, or the bearing and plunge at locations
in the field it is possible to create a map of an area showing geometrical
relationships of geological features in an area. This is important
in mineral and petroleum exploration because it allows more confidence
in the prediction of subsurface resources.
Directional Measuring Systems
The
reference system for direction is geographic north and south poles which
are located on the spin axis of the earth. In the northern hemisphere,
the north pole is used for reference. The earth’s magnetic
north pole does not correspond to the geographic north pole. The
magnetic north pole is located in northern Greenland. The magnetic
north pole slowly moves over long periods of time.
The
most common method of measuring a rock structure is to use a compass.
Since a magnetic compass needle points to the magnetic north pole, the
compass needs to be adjusted so that the directions can be understood
as relative to true north. This is done by setting the “declination”
on the compass, which is the angular measurement between true north
and magnetic north. In north America the declination is east declination,
which means that when facing true north, the magnetic north pole is
to the right. You can check your compass quickly by aligning it
to read 0 degrees (true north) and see if the compass needle points
to the right, or east, of true north.
Two main
systems are in use to measure direction, including the “azimuth”
and the “quadrant” systems:
The
Azimuth System
The
azimuth system uses true north set to 0 (or 360) degrees. The
complete circle is divided into 360 degrees. In other words, true north
is 0 or 360 degrees, east is 90 degrees, south is 180 degrees, and west
is 270 degrees. The azimuth system is the system most commonly
used today.
The
Quadrant System
The
quadrant system is also set with reference to both true north and true
south. In this system directions (or angles) are measured in degrees,
but are measured with reference to the closer of the true north or true
south directions. In other words, bearing directions (angles)
do not exceed 90 degrees. For example, a bearing direction which
is 30 degrees south of due east is described as “South 60 East”.
| Azimuth |
Quadrant |
Azimuth to Quadrant |
Quadrant to Azimuth |
| 75 degrees |
north 75
degrees east |
same numeric
value |
same numeric
value |
| 135 degrees |
south 45 degrees east |
subtract from 180 |
subtract from 180 |
| 215 degrees |
south 35 degrees west |
subtract 180 |
add 180 |
| 315 degrees |
north 45
degrees west |
subtract
from 360 |
subtract
from 360 |
Information
from http://www.fes.uwaterloo.ca/crs/geog165/mapcoords.htm.
Visit the site for more information on mapping.
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Quadrant
system
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Azimuth
System
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Dip Measuring System
Inclination
is measured using an inclinometer to measure the dip. An inclinometer
is a device which measures the dip or plunge. There are two main
types: 1) bubble level type, and 2) pendulum type. The bubble level
type works the same way a carpenters level works: a bubble trapped
in liquid in a curved tube seeks the uppermost portion of the tube.
This type of inclinometer is quite accurate and is generally preferred
for measuring rock structures. The other type of inclinometer, the
pendulum type, has a weighted needle suspended from a point. The
needles vertical alignment with the earth’s gravity field allows
it to be used as a reference.
Measuring Planar Geological Features
First, the
observer must use an inclinometer to find a horizontal line within the
planar feature of interest. By definition, the bearing of this line
is the strike direction of the feature. The most common method is
to use the inclinometer which is built into the compass, if available.
One can usually get pretty close to a horizontal line projection without
the aid of an inclinometer by moving to a position where the eye is in
the plane of the feature. Aligning your arm within the plane, and then
follow the plane with your arm until it seems to be horizontal.
Once the direction of the horizontal line in the plane is established,
line up the compass with the direction of this horizontal line.
The azimuth of this line will be the bearing of the strike direction.
Once the bearing
of the strike is established, the direction at which the dip should be
measured can be calculated by subtracting 90 degrees from the bearing.
The inclinometer is then aligned with this new direction, which is at
90 degrees to the strike. Then use the inclinometer to measure the
angle below the horizontal. In other words, a dip of 0 degrees is horizontal,
and a dip of 90 degrees indicates something vertical.
 The
general direction of dip must be specified. If only the dip angle
is noted, and not the general dip direction, there would be two possibilities.
This can be done by noting the general dip direction at the time of
measurement, or by using the right hand rule. This rule states
that the general dip direction, if not otherwise specified, is to the
right when facing the azimuth direction of the strike. For example,
if the azimuth is between 0 and 90 degrees, then the dip is to the southeast.
Many types of planar geological
features exist, the entire list being beyond the scope here.
In sedimentary or volcanic rocks the bedding is of particular interest.
In metamorphic rocks, the bedding is said to be “relict”,
but still is an important feature to measure. Fault surfaces
are extremely important to measure so that outward projections can
be extrapolated and the fault can be traced.
In metamorphic
rocks, there is the alignment of flakey or platy minerals which causes
a “foliation”. We measure each foliation in an outcrop
and note which minerals are associated with each. By looking at
cross-cutting relations, the metamorphic history can be interpreted.
It is also important to measure brittle fracture patterns in rocks, especially
with regards to mining. The fracturing effects the strength of the
rock and the manner in which it will behave in blasting or mining conditions.
Measuring Linear Geological Features
Measuring a linear geological
feature requires measuring an inclined line which is aligned with the
feature. The standard is to project the inclined line to a horizontal
plane, and then measure the bearing of this line. The angle created
between the inclined and horizontal lines is called the plunge.
The plunge is measured using an inclinometer, in a similar manner when
measuring the dip as described above.
There are
many types of linear geological features which are important to measure.
Some are real objects (such as aligned minerals) and others have to be
created using the imagination (such as the axis of a fold). Lineations
measured together with strikes and dips at the same observation point
are particularly useful. At these points on the map the strike and
dip is shown together with the lineation measurement.
Plotting Structural Data on a Map
Structural
data is generally recorded in a field notebook on site, but at some point
should be plotted on a map. For this reason the location data also
needs to be collected on site. The location data consists of X and
Y coordinates which are used to pinpoint the site on a grid type map.
The grid map is essentially the same as an X – Y graph, except that
the Y values are called “northings” and the X values are called
“eastings”. Northings increase in value going north,
and Eastings increase in value going east. The northings and eastings
can be whatever units are desired, such as feet, meters, miles, etc...,
depending on what map scale is desired. For example, on a map with a scale
of 1 inch = 1000 feet, a site with coordinates of 4000 East, 4000
North will plot 4 inches to the east and 4 inches to the north of the
origin.
Mapping images courtesy
Matthew A. d'Alessio, UC Berkeley
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