69) Folds, Boudinage, and Joints

69) Folds, Boudinage, and Joints


Welcome back geology fans! In our last
episode, we covered the part of this chart that covers the structures formed
when rocks are stressed in various ways and respond with brittle strain. Today we
look at the ductile strain structures, and add one more low stress but brittle
strain structure at the end. We start with compressive stress and ductile
strain which results in folding. Take a piece of paper and push its ends
together and it will bend rather than break and thus make a fold in the paper
not a reverse fault. We will break folds into more subtypes in a moment, but to
start with there are two main types: an upward fold with the crest at the top
which is called an anticline, and a downward fold which we call a syncline.
Now, one way to remember the difference between synclines and anticlines is
that the anticline looks a bit like a capital letter A. Synclines look more
like the downward pointing V-shape in the Y of syncline. But generally these
forms are below ground, and so we look to their surface expressions to tell a
syncline from an anticline. Recall back to our relative age dating episodes in
which we discussed the law of superposition which says that layers on
the bottom are older than layers on top. If we take a bunch of layers and bend
them upward into an anticline, and then shave this feature down, at the surface
we will see a repeating sequence of beds with the oldest material in the center.
To the left of the central oldest bed the younger flanking layers all dip to
the west and to the right of the center of symmetry all beds dip to the east. So
an anticline has beds dipping away, outward from, the central bed which is
oldest in the center. As you might expect things are reversed in a down-warped
syncline. The center bed is the youngest as it was formerly on top, and the beds
on either side dip inward towards the central youngest bed. In a syncline, beds
get older as you move away from the center of the fold. With an anticline,
beds get younger away from the center. But nature is messy, so what if a fold, be it a syncline or anticline, has a plunge
into the earth? In other words, what will it look like at the surface if the fold
ridge is not horizontal, but tilted? Imagine taking a bunch of layers and
making them into an anticline, and then dipping that fold into the ground and
shaving it off level. Instead of symmetrical layers and stripes on either
side of the axis of symmetry we see a V-shape form at the surface. With a
plunging anticline we will see the oldest layers in the center still, and
youngest layers on the flanks, and the layers all dip away from the center of
symmetry just like with any other anticline. The V-shape at the surface
also tells us which direction the fold is plunging into the earth. With a
plunging anticline, the V points in the direction the fold is plunging into
the earth. With a plunging syncline, the V points in the opposite direction of
plunge, but as with any other syncline the youngest beds are in the center, and
all flanking beds dip inward towards the center of symmetry.
Besides level and plunging synclines and anticlines, we can further
classify folds as being symmetrical or asymmetrical, isoclinal when the limbs
parallel each other, overturned when one limb ends up upside down,
similarly overturned recumbent when the fold axis is near horizontal, and chevron
when they zigzag. Regardless of specifics, almost all folding is the result of
compressive stress and ductile strain, but shear can come in during folding to
create what are known as drag folds within larger compressive folds. Also
note that we get one other type of fold from ductile strain and shear stress; the
monoclinal fold. This last fold type looks like a layer of rock was bent in
its middle without disturbing the orientation of the beds on either side.
If one considers this fold in formation, it is easy to see that if it continues
the stress will eventually cause brittle strain and make a fault with signs of
fault drag, bent rock, on either side. What type of fault the monocline may
finally become depends on what type of stress is causing it, as we discussed in
our last episode, but we put monoclonal folds in the shear stress portion of our
chart. If it is pure shear stress, then this would make the monoclonal fold
eventually break into a strike-slip fault. But we can imagine any incipient fault type as starting as a monoclonal fold
possibly, and thus we must be careful assigning stress to a monoclonal fold
unless we have more contextual evidence such as surrounding faults of the
compressive reverse, tensional normal, or shear strike slip types. If everything
around you says compression, you might lean towards compression in less certain
cases. As we get further into field geology you will hear me say more and
more that nature doesn’t like to be pigeonholed, and that we must always keep
context in mind. We have one last square to examine on our stress strain
combinations; tensional stress with ductile strain. This results in a form
known as boudinage, which comes from the French “boudin”, which means “sausages”
as this rock structure looks like a series of sausage links either joining
or disconnected from each other. If we take silly putty and stretch it, that is
apply a tensional stress slowly enough that it deforms with ductility, we see
that it tends to thin in one area while allowing other areas to stay thick. Rock
can do this too, and if you have a problem with the idea that rocks can
flow like silly putty, recall our old friend the metaconglomerate with
smooshed pebbles which flowed under the right conditions of heat and pressure.
So if we see boudinage we know the rock was pulled apart with tension while under
enough heat and pressure at a slow enough rate that the rock deformed with
ductile strain into the classic sausage shapes of boudinage.
Before leaving the basic structures, I have to dip back into the area of this
chart with brittle strain. The faults we described last episode are all due to
brittle strain, and have large and rapid enough stress to cause movement across
the broken surface. So a fault is defined as a break in the rock with movement on
either side. But anyone who has spent enough time around rocks knows that
rocks can break but have no relative movement across the break. In that case,
we call the break a joint, not a fault, and since a joint can be made by weaker
stress of any kind we will overlay joints across all stress
types. Though with knowledge on how various joints form, we can usually say
what type of stress produced it. Lava flows cool from the top down, and
since cooling rock is contracting rock we can get randomized centers of
contraction which cause tension in the rock, popping it apart in columns that
migrate from the top of the contracting lava to the bottom where cooling and
contracting occurs last. The result is these vaguely political columns commonly
seen in lava flows called columnar jointing. The twisted pattern seen in
columnar jointing can be due to thermal properties of the lava causing the
cooling to develop in a nonlinear, non- vertical manner. But columnar jointing
can, like our combination faults, have a bit of shear stress as well as tension
or compression causing them. Joints which have a bit of shear stress on them will
often produce what we call feather structure. These feathery patterns in the
rock often originate from a point or small area of initial failure, and blow
out from there. Looking in the direction the feather structure opens out tells us
the direction of the shear stress that helped make the joint. In this case, the
rock the feather structure was in was pushed down and the face that it was
against, but is now gone, was pushed up. That motion will cause the resulting
fracture pattern seen here. The joint pattern seen around Arches National Park
near Moab Utah is caused by salt below the sandstone and other rock units
dissolving and flowing out from underneath. The arch is seen in
Arches are there because of the jointing in the area. Faults, folds, and
joints are very important not only for telling the history of an area
geologically, but in controlling surface processes involved in erosion and
deposition, and so a knowledge of geologic structures is very important in
understanding landforms that we see around us in nature. There’s a lot more
to structural geology than knowing the basic faults, folds, joints, and boudinage patterns, and I will touch on these in later episodes, but I hope in the next
few episodes to get my viewers up and running with basic field geology so we
will save more complicated structural geology concepts for later and plunge
into the basic method used to measure and report structural data in the field.
Rock beds, faults, lava flows, fold axes are all planar features, and if you have
any familiarity with geometry you know you can define the position of a plane
with two intersecting lines in the plane. When we come back next time, we will
cover the very important geologic concepts known as “strike and dip”, here on
Earth Explorations.

8 thoughts on “69) Folds, Boudinage, and Joints

  1. Hello Prof.
    Thank you for your videos and podcast.
    They have been very helpful since I started my geology major in NY.
    Helping me study for structural geology exam next week 😮

  2. Folds are an anomaly that can’t be explained by uniformitarianistic thinking. You just can’t bend rock to the extent visualized . Have you considered a single global catastrophic flood?

  3. Your content has helped me a great deal . Without which it would have been difficult to understand a few concepts. I wish there were more. I am doing geology from books, research papers and YouTube . Please share more videos.

  4. This really helped with studying for the NES for my teacher certification. The video format is really beneficial since even pictures in the textbooks are sometimes hard to visualize.

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