Welcome back structural geology fans!
Today is a big day as we hit on one of
the most important skills for a field
geologist; the ability to take
measurements on structures and put these
measurements on a map that lets other
geologists know exactly what you saw out
there. To begin, layers of rock are
basically planes of rock, and faults and
joints are basically planar breaks in
rock, and even with a fold we can imagine
a plane of symmetry, and thus measuring
planar features is key to our science. A
place on a plane can be described in
just two intersecting lines, but we go a
bit farther to not only say where we are
on the map, for which you can go back to
the beginning of this series to review
latitude and longitude or the USPLS
system to get your location, but today we
are going to learn how to represent the
orientation of any plane in 3d space at
your location on the map. To do this, we
use the concepts of strike and dip. A
strike is defined as the direction of
the line made by an intersection of an
imaginary horizontal plane with the
plane of the structure in question. To
simplify this discussion, let's focus on
a layer of rock which has been taken out
of its original horizontality and tilted
in some direction. If we were to submerge
our tilted layer in water, we would see
the horizontal water surface
intersecting the dipping layer in a line.
We use our compass to measure and report
the direction of that line, and at the
Colorado School of Mines, we teach our
students to prefer use of the azimuth
system. This is the system that goes from
0 to 360 degrees around the compass rose.
North is 0 degrees, East is 90, South is
180, West is 270 degrees. But wait, there
is a problem here. Looking at our water
intersecting our rock layer we see that
the strike line is just that, a line not
a directional arrow. So in this case with
the strike line going exactly
North-South we could call it 0 degrees
for North or 180 degrees for South.
Again, at the Colorado School of Mines, we
teach our students a consistent method,
and honestly all geologists should start
following suit for consistency and ease.
We use what is known as the "right-hand-
rule". First, you need to figure out which
is your right hand, then put the fingers
together and the thumb out at a right
angle to the fingers. Now think that your
fingers are heavy and want to go down
the slope you were measuring. With the
fingers of your right hand going down
the slope of the plane of interest, your
thumb will point in the one direction
out of the two possible to use for the
strike. So with our example with a strike
line that is going North-South, we see
that a right hand rule would tell us to
measure the direction to the South. In
other words, we put the strike here as
180 degrees, not zero.
Besides this strike azimuth number using
the right hand rule, we need to measure
one other geometric aspect of the bed;
its dip. The dip of a planar feature is
the maximum angle between the plane and
our imaginary horizontal surface. I have
to say the maximum angle of dip as any
angle measured that is not perpendicular
to the strike will be less than the true
dip of the bed. One way to quickly assess
this dip direction is to actually, or
imagine, pouring water on the planar
surface and seeing or imagining which
way the water would run. That is
basically the dip direction as water
will tend to take the steepest angle
down the face. For our previous example, the
strike was 180 degrees, and the dip is 45
degrees. In our notebook, we can quickly
write down our location and 180/45.
With that we have the orientation of the
plane in question. Note that I could say
that the bed is dipping 45 degrees to
the west, but I don't need to as long as
I use the right hand rule. To repeat, we
use the fingers of the right hand to
point down dip, and our right thumb at a
right angle above the fingers points us
in the correct strike direction.
Put your right thumb pointing in the
direction of strike, and your fingers
will be at a right angle in the
direction of dip! So with a 180/45
we know the bed is dipping 45 degrees
from vertical to the west. If the strike
and dip were 0/45, that plane would be
dipping 45 degrees to the east. Now that
we have the concept of strike and dip
using azimuth coordinates and the right
hand rule, let's see how it is measured
in the field. We will cover basic field
equipment in a future episode, but here
we introduce you to the use of the
compass in field geology. Near the
beginning of this series, we discussed
the use of compasses and orienteering, so
we already saw that a map and compass
are essential tools of the geologist. But
the compass is also how we measure
strike and dip. Professional geologists
prefer the Brunton field compass as it
has the ability to freeze the compass
needle, has more precise gradations, has a
bubble level, has a dip indicator, and all
the extras needed for sighting and
surveying. However these are usually over
$300 US, so may I recommend that
any beginning geologists just go out and
get a relatively cheap compass, but be
sure that it has a dip indicator on it.
The Silva Ranger compass does many of
my beginning students very well. The only
thing I do to adapt the Silva Ranger is
glue a small bubble level on its face, as
so. With a circular bubble level, we can
make our compass as close to horizontal
as possible.
Recall that strike is the line formed
between the plane of interest and a
horizontal surface. With the bubble level
we can make our compass the horizontal
surface, and when placed with the long
edge against the plane of interest we
can take the strike in the right hand
thumb position. Once the strike is
recorded, we can put the long edge of the
compass along the steepest dip direction,
and with a Brunton we adjust the dip
dial with the bubble level, and then we
can read the dip angle. With a cheaper
compass like our Silva Ranger, we need
to put the east and west markers in line
with the phosphorescent markers used for
orienteering, and the free hanging dip
indicator now gives us the angle of dip;
zero, 45,
90. Let's practice this on a nice flat
face of this statue. You first
determine which direction to take strike
with the right hand, so step one is to
remember which hand is your right hand.
Place your right hand with the fingers
going downslope, and your thumb at a
right angle points us in the proper
strike direction. Now take your compass
and make it as level as possible, then
place the long edge right up against the
slope you're measuring. Turn the dial to
put the red in the shed, that is the
large orienteering arrow on the dial is
moved under the magnetic north arrow. Now
read the azimuth degree number that is
aligned with the tick mark in the
direction of strike we figured out with
the right hand rule. In this case, the
strike of this surface is 190 degrees, a
bit west of due South. To get the dip,
which is going to have to be towards the
west, we align the east and west sides of
the dial with our opposing orienteering
tick marks, determine the steepest angle
of the slope (if needed you could pour
some water on the surface but you should
start to get this down without using
that trick as water is of such value in
actual fieldwork). We place the long edge
of the compass against the slope in the
direction of this steepest dip, and now
you can use the free hanging dip
indicator to read the dip. In this case,
the dip is about 56 degrees to the west.
With the right hand rule, we don't need
to note the dip direction as it will
always be 90 degrees to the right of the
strike. Why do geologists shell out
more money for a Brunton style
compass? Well, first it comes with a level
already. We have more precise and easier
to read dial gradations, dampers to
reduce the swaying motion of the needle,
a hold button to freeze the needle in
place to make it easier to read, and the
dip indicator is controlled with a
bubble level that can be put into proper
measuring position and then removed from
the surface to read the dip. With a
Silva-style free-hanging dip needle, you
have to crane your head to read the
measurement while the compass is still
against the planer surface. Now, both
varieties have a mirror which can come
in handy when you have to measure above
your head and still read the compass, or
in other odd angles. But those Brunton
extras are just really nice to have, but
not absolutely necessary. So whichever
route you take will get you your data.
But how do we represent this data which
has location and orientation components.
We place a symbol on a map at the
location where the measurement was taken,
so clearly it is important to know how
to read a topographic map which was
covered at the very beginning of this
series. Assuming all viewers have gotten
up to speed with locating themselves on
a map, we can move on to how to draw the
symbol for strike and dip at that
location. The basic symbol is a T-bar
with the long bar representing the strike,
and the short bar the dip. If as usual
North is at the top of your map, then a
North strike, that is 0 degrees azimuth,
would be drawn as a vertical line
straight up and down on the page of your
map. But we know that line is really
pointing up to 0, so which side of the
line will the dip go to? We could look at
our surface and note the direction of
dip, or you can just remember that you're
using the right hand rule and thus if
you put your right thumb in the actual
strike direction of 0 degrees, your
fingers are pointing in the direction of
dip; to the East in this case. Draw the
small line perpendicular to the strike
line in the direction of dip determined.
At the end of this dip line, write the
dip degree angle from horizontal, and
you're done! With that symbol on the map,
every geologist or knowing individual
will understand the exact orientation of
the feature you measure to at that
location. If your right hand rule strike
is due south, 180 degree azimuth, the
strike line is, well, a vertical line. But
we put our right thumb down towards the
South direction, and now our fingers
point to the West, and our symbol is a
mirror image of the zero degree strike.
Giving some examples assuming all have
45-degree dip: this is the symbol for 30
degrees strike, 60 degrees, 90 degrees, 120
degrees, 150 degrees, 180 degrees, 210, 240,
270, 300, 330, and 360,
which we note is the same as zero,
and so use zero as the strike and not 360
to simplify our situation. Clearly in
nature we don't always have such
perfectly flat and smooth surfaces as
our statue example. We often carry map
boards or field notebooks with us, and
these planar objects can be placed
against a more rough irregular plane
to give a consistent surface to measure.
But what if the only place you can
measure a bed is on the bottom of the
layer? Right hand rule works by placing
your right hand on top of the surface
you are measuring, so if you have to
measure a bed from below you need to
place the back of your right hand on the
underside of the bed with the fingers
downslope, and your thumb again points in
the strike direction. As you can see, this
will give you the same strike and dip
direction as if you'd measured it on top.
And if your bed has been overturned, that
is, has been tilted so much from original
horizontal position that the top is now
under the bottom, well, you measure the
bed as if it wasn't overturned, placing
the palm of your right hand on the upper
side of the overturned bed or the back
of your right hand on the lower side as
you would a non-overturned bed. However,
we let other geologists know this bed is
overturned by giving a special symbol on
the map, which looks like our previous
strike and dip symbol but with a J curve
for the dip, and the curved part of the J
is on the backside of the symbol and the
straight part crossing the strike line
is pointing in the actual direction of
dip. But two more symbols are needed for
the two extremes of dip angle: 0 and 90;
that is, perfectly horizontal planes or
perfectly vertical. A horizontal surface
such as this floor of the statue has no
strike, as the horizontal plane matches
the surface, and by definition of being
horizontal it has no dip angle. In such
cases, we make a circle with the plus
inside like so, and tell any other
geologists the orientation of the plane
here is horizontal. Perfectly vertical
planes, like this face of our statue, are
the other extreme, and in this case there
is a definite strike direction but no
definite
dip direction. In that case we make a
long bar for the strike like we did with
any other dip between 0 and 90, but the
dip line becomes a shorter bar bisected
by and through the middle of the strike
line. You've now told the world the plane
you measured here is vertical. I don't
have to write the dip is zero when using
this symbol, and I don't have to write the
dip is 90 degrees using this symbol. To
emphasize again, measuring strike and dip
is one of the most fundamental tools in
a field geologist's kit, and it is
essential if you want to go further in
the upcoming episodes of this series
that you go out and practice this skill.
But the Devil's in the details, as usual,
of what exactly can be measured with
strike and dip, and how we might alter
our map symbols to communicate these
variously measured features. When out in
the field with students, and I see them
with compass to rock, I often ask them
what are they measuring. What are you
measuring strike and dip on? They better
be able to tell me, is it a bedding
surface, or a fault or joint surface, or
cross-bedding surface, or whatever. When
we come back next time, we look into the
devilish strike and dip details, here on
Earth Explorations.
