Lab 11 Geomorphology.docx
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GEOL101 Dynamics of the Earth – Fall 2023
Name: Emily Thomson
Laboratory 11: Geomorphology
Section:
Introduction
In this lab, we will explore geomorphology, which is the study of how landscapes form and evolve
over time. Common landscapes that may be characterized as fluvial (river-driven), aeolian
(wind-driven), glacial (glacier-driven), tectonic (earthquake fault-driven), and mass movement
(gravity-driven). Geologists study these driving processes to understand, and sometimes predict the
behavior of geomorphic processes. This is important for Californians because we have a quite varied
landscape from the northwest (forested, many rivers), to the east (dry, snow accumulation in the
mountains, and basin and range faulting), to the southwest (transform faults, and variable
precipitation), and to the west (coastal wave and tide action, heavy precipitation and river input).
In this lab, we will “tour” these landscapes and explore the driving forces that produce them. As with
all of geology, the landscape of a given location will evolve over time in response to one or more of
these driving forces related to gravity, water, ice, wind, and tectonics.
Fluvial (River) Landforms
Two general styles of fluvial landforms commonly found in nature are braided rivers and meandering
rivers.
Braided rivers
are rivers that are essentially choked with an abundance of sediment or have a high
degree of slope. The river's response to not being able to move the abundant sediment is to break
into many, many channels. Braided rivers occur commonly in glacial regions, arid environments
(alluvial fan), and deltas (due to a very low river velocity, and therefore, a more limited ability to
transport sediment). Due to the abundance of channels in a braided river system, islands are formed
between the channels as they wind through their environment. These islands are called
braid bars
.
This generally is a response to abrupt changes in river velocity, which can be driven by seasonal and
daily changes to water input to the system, or a response to tectonic events.
Figure 1. Braided river system in Alberta, Canada. Photo by Will Buckley
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Meandering rivers
are rivers that “meander” or wander over time across a valley or floodplain.
These rivers are different from braided rivers because they are generally
confined to a single
channel
at any given time. The sinuosity of these rivers can form sharp curves as water flows
moves quickly along the outer edge of a curve and actively erodes the land forming a steep
cut
bank
. On the inner edge of the curve, water moves relatively slowly and sediments tend to settle out
and be deposited as a gently sloping
point bar
. Point bars and cut banks will migrate laterally over
time. In addition, in some cases, a cut bank may cut through the channel wall of the river, finding a
shorter path for the river since water will always take the easiest way down slope. On the following
page is a diagram illustrating what these forms look like. Please use this as a guide to answer
questions regarding fluvial landforms later in the question portion of the lab.
Figure 2. Aerial photo of a meandering river system in Alaska. Photo by Will Buckley
Figure 3. Diagram of meandering river features. (www.schoolworkhelper.net)
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Figure 4. Additional diagram of meandering river showing various descriptive features.
(www.internetgeography.net)
Aeolian (Wind-driven) Landforms
If you have been to the beach, then you are familiar with the transport of sediment by wind because
you have seen the dunes that form there. These dunes are created by winds that move grains of
sand. The formation of a dune is a direct response of sand grains to the direction of wind, whether it
is in constant or changing direction. Lastly, the formation of a dune may depend on whether or not
there is vegetation present to provide an anchor around which a dune may form. The beach is not
the sole environment where dunes can form. As you may have seen in some movies, if you have not
been to the desert, sand dunes may form in a desert environment as well! Below are multiple styles
of dunes that geologists’ study, please use the above text and Figure 6 on the next page to help you
to answer questions on Aeolian landforms in the lab.
Figure 5. Sand Dunes in Death Valley. (Photo by Michael Shainblum)
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Figure 6. Different types of sand dunes and their relation to wind and sand availability.
Glacial landforms
Throughout earth’s history, glaciers have had a varied past. They have at times covered the entire
earth (snowball earth!), cycled between advancing and retreating or been completely absent from
the earth’s surface. Glaciers consist of snow that has turned to ice, also known as
firn
. When
glaciers advance, they grind up the rocks and sediments over which they move.
When they retreat, they leave glacial deposits of sediments behind. These glacial deposits leave
clues for geologists to study both the past behavior of glaciers and the type of climate that existed in
the geologic past. We are currently in a period of glacial decline that is unprecedented in the speed
in which it is occurring. 99% of geologic scientists agree that human contributions of carbon dioxide
and other greenhouse gasses are a major factor in this current event.
In this section, we will examine the landforms that can be observed where glaciers are the agents of
geomorphic change. When a glacier moves downhill under the influence of gravity, it carries rocks,
boulders, and sediments (the amalgamation of this material is known as glacial till). The glacier
deposits glacial till in front of it (
terminal moraine
), along the sides (
lateral moraine
), and
sometimes, in the middle of two glaciers that are merging into one from separate valleys (
medial
moraine
). When the glacier retreats or melts, these forms are left behind for geologists to observe.
As a glacier grinds away at the rock underneath of it, it creates a very distinct shape of valley called
a
U-shaped Valley
. River valleys are different from glacial valleys since they cut valleys with steeper
sides, known as V-shaped valleys. During times of melting, subglacial streams flow under the glacier
and create stream deposits of cobbles, gravel, and sediment. When the glacier retreats, these
deposits are exposed and known as
eskers
. There are multiple other types of glacial landforms:
horns (think the Matterhorn in Switzerland), arêtes (sharp ridges), cirques (like an amphitheater
carved out of the side of a mountain), and tarns (glacial lakes from meltwater, that reside in cirques).
For these features to have been formed, think of how much ice must have been present to have
worn down the underlying bedrock, miles thick in some cases! If you were to find yourself in a
situation when crossing a glacial-fed river is necessary,
it is important to consider how the glacier
contributes to river flow before crossing
. When a glacier is the primary source of water for rivers, the
4
volume and velocity of river flows can fluctuate at different times of the day and year. Generally, the
volume and velocity would be highest during warmer temperatures when glaciers melt more
frequently and would make crossing a river during these times more difficult. Examine the below
diagram and use it to answer questions in the question portion of the lab on glacial landforms.
Figure 7. Diagram of features associated with alpine glaciation.
(www.indiana.edu/~sierra/papers/2012/klapperich.htm)
Mass Movements and Mass Wasting
A mass movement in geomorphology refers to the downslope travel of rocks and sediments under
the influence of gravity. In California, and the western United States in general, mass movements are
a particularly potent hazard to life and property. Therefore, geologists and engineers work together to
design strategies that may help to prevent, or mitigate, the effects of these events. Mass movements
may be triggered by any or all of the following factors:
excessive rainfall, excessive snowmelt,
tectonic events, and generally weak materials
. In the diagram below, are multiple examples of
these hazards, and what drives them to occur. Use this diagram to answer questions in the question
portion of the lab.
When geologists study geologic deposits which are a result of mass movements, they look for clues
that illustrate what deposit is a debris flow compared to a river deposit. Therefore, in the field it is
useful to know that a debris flow is typically what is called
“matrix-supported
”. This means that the
rocks, or “clasts”, in the deposit do not touch and are separated by mud. A river deposit on the other
hand is typically
“clast-supported”
. This means the clasts or grains touch and in the case of a
braided river, the sand and silt in the river may show what is called
“cross-bedding” or “planar
bedding
”, indicative of braided river flow.
5
Figure 8. Different types of Mass Movements classified according to the dominant material,
water/air content, and velocity of the movement. (
www.geologyin.com
)
Tectonic Landforms
Tectonic landforms are created by earthquakes, and in California, we have a significant number of
active, large faults. Because of this proximity, we are able to observe these landforms fairly easily
when we are on or near an active fault. Common tectonic landforms include
sag ponds, diverted
streams,
and
pressure ridges
. The style of landforms most commonly observed are on transform
faults, like the San Andreas Fault system. However, these can also be seen in normal faults, which
are common in the desert east of San Diego and east of the Sierra Nevada mountain range. We will
focus on the bold terms above.
Depending on the style of transform fault (left lateral-Garlock fault, or right lateral-San Andreas) the
landscape responds differently. In either case, when the fault “steps over” to the right or left, there is
a side that experiences an extension in the landscape, which creates a depression for a sag pond to
form. On the other side, a pressure ridge is created due to compression of the Earth. Streams that
cross a fault in a straight line prior to an earthquake will be diverted in the direction of slip of the
opposing side to the source of the stream. The offset stream channels provide what are called
“piercing points”
, areas where a formerly continuous feature has been offset by faulting. These
piercing points can be realigned to help to determine how much the landscape has been displaced
by a fault. The San Andreas Fault is a right lateral fault, see the diversion in the stream below and
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also study the response of landscapes in the diagram below (as mentioned above with sag ponds
and pressure ridges).
Figure 9. Aerial photo of an offset stream (center-ish of photo) along the San Andreas Fault.
(Photo by Will Buckley)
Figure 10. Diagram of responses of landscapes as a result of faulting. (www.quakeinfo.ucsd.edu)
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Lab 11 Questions
Question 1. Answer the following questions regarding Figure 1.
Notice the ‘islands’ between channels of the braided river. These are called point bars. What are
they typically a result of?
When the velocity decreases and the sediment amount is high, the sediment gets deposited creating
the island-like landforms.
Are braided rivers associated with a high or low amount of sediment?
High sediment
Braided rivers typically transport and deposit coarse sand and gravel-sized sediment. What does this
tell you about the energy of a braided river?
It tells you that braided rivers have high energy with abundant sediment
The river is located at the outlet for meltwater from a glacier in Alberta, Canada. Imagine having to
cross the river while you’re on a backpacking trip in the summer; however, it must be done when the
velocity and volume of the water is low. When would be the best time of the day to cross the river
and why? Keep in mind that a glacier largely controls the water load.
It would be best to cross it in the morning when it is cooler and the volume of the water is
lower, making the current slower
Question 2. Answer the following questions regarding Figure 2 (use Figure 3 to help
determine features).
What feature does arrow A point to?
undercut bank
What direction is arrow A’s feature migrating towards (north, south, east, west)?
W
What feature does arrow B point to?
Oxbow lake
What feature does arrow C point to? Would this feature be associated with deposition or erosion?
Point bar, associated with deposition
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Question 3. Answer the following questions regarding Figure 6:
Sand dunes are typical features in desert environments. What other environment might sand dunes
be found?
They are found in arid regions, but also can be found on most coasts like along beaches. The wind
affects the mounds of loose sand that creates the dunes.
Dune formation is dependent on what three main factors?
wind, water, and vegetation
What can a Barchan dune turn into if the sand supply increases?
transverse dunes
Question 3. Answer the following questions regarding Figure 7.
How can you tell if a valley has been formed by a glacier, as opposed to a river?
You can tell if a valley is a glacier formed by the shape of the valley. Glacier formed valleys are
straighter, U -shaped as opposed to V shaped valleys carved by rivers.
How do hanging valleys form?
They form when glacier ice deeply erodes a main valley and then leaves tributary valleys hanging
above the main valley floor.
How do eskers form?
They are made of sand and gravel. Eskers form when the sediment is deposited by glacial meltwater
into subglacial tunnels that are within and underneath glaciers, or through meltwater channels on top
of glaciers.
Question 4. Use Figure 8 to answer the following questions.
For the following mass movement types, characterize the speed, nature of motion, material type,
and water/air content of each:
Slump
unconsolidated material, nature of motion is slide of fall, slow velocity, low water content
Rock Fall
fast velocity, high air content, rock material, with slide or fall nature of motion
Debris Slide
Moderate velocity with high water content, unconsolidated material, slide or fall nature of
motion
Earth Flow
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slow velocity, low water content, unconsolidated material, flow nature of motion
What are the main factors that contribute to the occurrence of mass movements? Must name them
all.
excessive rainfall, excessive snowmelt, tectonic events, and weak materials
What feature(s) do braided rivers show to indicate braided river flow?
cross bedded multi threaded channels that branch and merge to create a braided pattern
Question 5. Use Figures 9 and 10 and the text above them to answer the following questions:
What are tectonic landforms created by?
earthquakes
What features do offset streams provide that help to measure the offset of a fault, and how might
one use these features to measure the offset?
piercing points and these features measure the offset by determining how much the landscape has
been displaced by a fault
What feature forms when a depression in the landscape is created from a step-over in the fault?
Sag pond.
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