Lab # 11: Geomorphology (Name) GEO101 Phil Degginger 1 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 In this lab, we will be looking at geomorphology, which is the study of changing landscapes, their forms, and causes of landscape changes. There are several different components or categories that create different geologic landforms. These components are as follows: Fluvial (river-driven), Aeolian (wind-driven), Glacial (glacier-driven), Tectonic (earthquake fault-driven), and Mass Movements (gravity-driven). Geologists study these varied 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), southwest (transform faults, and variable precipitation), and west (coastal=wave and tide action, heavy precipitation and river input). We will examine now the various components of geomorphic processes that create the landforms which geologists study. Keep in mind that a geomorphic landform is essentially a response of the land (sediments) to the forces operating on it. These forces as stated above include, but may not be limited to, a single force. The forces are: gravity, water, ice, wind, and tectonics. Fig 1. Sue Flood-Getty Images 2 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 Fluvial (River) Landforms There are two styles of fluvial landforms that we will look at in this lab. They 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 (a quick change in elevation). The river 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. Fig.2 Phil Degginger 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 a given point in time. Meandering rivers have several very distinct features that can be observed. The sinuosity of these rivers creates sharp curves. As the river flows through these curves, water erodes at the land on the outside of the curve. This feature on the river is called the cut bank. On the inside of the curve, deposition of sediments is occurring. This is known as a 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. 3 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 A C B Fig. 3 www.geocaching.net Fig. 4 www.limpopo.riverawarenesskit.org 4 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 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 this diagram to help you to answer questions on Aeolian landforms in the question part of this lab. Fig. 5 design.epfl.ch Glacial landforms 5 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 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 gases 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, sub-glacial 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 forms 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! Examine the below diagram and use it to answer questions in the question portion of the lab on glacial landforms. 6 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 Fig. 6 http://www.indiana.edu/~sierra/papers/2012/klapperich.html 7 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 Mass Movements/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. Fig. 7 www.geologyin.com 8 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 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 my show what is called “crossbedding” or “planar bedding”, indicative of braided river flow. Fig. 8 www.geologyblogspot.com Fig. 9 earthsci.org 9 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 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 landscapes responds differently. In either case, when the fault “steps over” to the right or left, Fig. 10 bwww.webpages.uidaho.edu Fig. 11 quakeinfo.ucsd.edu 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. The San Andreas Fault is a right lateral fault, see the diversion in the stream below and also study the response of landscapes in the diagram below (as mentioned above with sag ponds and pressure ridges). 10 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 Lab 11 Question section: 1. On the image for Figure 2, there are many islands present in the braided river. Answer the following questions. a. What are the islands named? b. Are these islands permanent or temporary features of the river? c. Imagine that this river is located at the outlet for meltwater from a glacier in Alaska. Would the velocity and volume of the water be high or low at what times of the day, and at what times of the year? 2. Answer the following questions regarding Figure 3. a. What feature does arrow A point to? b. In what direction is arrow A’s feature migrating towards? c. The feature indicated by arrow B is a backswamp. What could this feature have been before it became a backswamp? d. What feature could develop in the area that arrow C points to (over time)? 11 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 3. Answer the following questions regarding Figure 5. a. What are the 3 main factors that lead to dune formation? b. What are the two main environments where dunes may be found? c. What are the differences between a Barchan dune and a Parabolic dune? 4. Answer the following questions regarding Figure 6. a. How are moraines formed? b. What is the difference between a valley that is formed by a river, and one that has been formed by a glacier? c. How do you think a hanging valley has been formed? 5. Use figures 7-9 to answer the following questions. a. What are the main factors that contribute to the occurrence of mass movements? b. For the following mass movement types, characterize the speed, nature of motion, material type, and water content of each: i. Debris Avalanches ii. Mudflows iii. Earthcreep iv. Rock Avalanche c. Characterize the sediment sorting of a debris flow deposit compared to a river (fluvial) deposit. 12 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16 6. Use figures 10 and 11 and the text above them to answer the following questions: a. What is a piercing point, and how can one use offset streams to measure the offset of a fault? b. How are sag ponds formed? c. How are pressure ridges formed? 13 Written by K. Sean Daniels, MS candidate, Geology, San Diego State University, 10/16/16
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