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14.1: Introduction

  • Page ID
    6106
  • m14_glacier_full.jpg
    Figure 1. Meltwater from Grinnell, Gem, and Salamander Glaciers feeds the teal lakes of Grinnell Valley in the Many Glacier are of Glacier National Park, Montana.

    Introduction

    Landslides

    Landslides constitute a major geologic hazard because they are widespread, occur in all 50 states and U.S. territories, and cause $1-2 billion in damages and more than 25 fatalities on average each year. Expansion of urban and recreational developments into hillside areas leads to more people that are threatened by landslides each year. Landslides commonly occur in connection with other major natural disasters such as earthquakes, volcanoes, wildfires, and floods.

    The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over-steepened slope is the primary reason for a landslide, there are other contributing factors:

    • Erosion by rivers, glaciers, or ocean waves create over-steepened slopes.
    • Rock and soil slopes are weakened through saturation by snowmelt or heavy rains.
    • Earthquakes create stresses that make weak slopes fail.
    • Earthquakes of magnitude 4.0 and greater have been known to trigger landslides.
    • Volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
    • Excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure.

    Slope material that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

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    Figure 2. Debris flow scars formed in 1968-1969 in greater Los Angeles. The image taken months after the debris flows occurred.

    Landslide Locations

    Landslides occur in every state and U.S. territory. The Appalachian Mountains, the Rocky Mountains and the Pacific Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems. Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides.

    Although the physical cause of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.

    – Excerpts from “Landslides 101,” Landslide Hazards Program, U.S. Geological Survey

    Glaciers in Glacier National Park. Montana, USA

    A glacier is a body of snow and ice of sufficient size and mass to move under its own weight. Glacier movement may be detected by the presence of crevasses, or cracks that form in the ice as the glacier moves. In Glacier National Park (GNP), USGS scientists define glaciers according to the commonly accepted guideline in which a body of ice has an area of at least 0 .1 km2 (100,000 m2), or about 25 acres. Below this size, the ice is generally stagnant and does not move, unless it is on a steep slope. Glaciers are dynamic, changing in response to temperature and precipitation. A glacier forms when winter snowfall exceeds summer melting. Glaciers retreat when melting outpaces accumulation of snow.

    While the glaciers that carved GNP’s majestic peaks were part of a glaciation that ended about 12,000 years ago, the relatively small alpine glaciers that cling to mountainsides today are considered geologically new, having formed about 7,000 thousand years ago. These glaciers grew substantially during the Little Ice Age (LIA) that began around 1400 AD and reached their maximum size at the end of the LIA around 1850 AD. Their maximum sizes can be inferred from the mounds of rock and soil left behind, known as moraines. Aerial photography reveals moraines of over 150 glaciers that existed in Glacier National park at the end of the LIA. In 2015, only 26 active named glaciers remain.

    Early park visitors and scientists noted that glaciers were retreating as early as 1914. The climate was already warming and glaciers were responding, but the industrial revolution added more carbon dioxide to the atmosphere, furthering glacier retreat. In Glacier National Park, as well as around the globe, glacier mass and extent has decreased during the 20th century in response to warmer temperatures.

    – Excerpts from “Retreat of Glaciers in Glacier National Park,” Northern Rocky Mountain Science Center, U.S. Geological Survey

    Select an image to view larger

     

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    Figure 3. A view from a Washington Army National Guard helicopter shows the bypass (left) that has been created to facilitate emergency workers’ efforts after a mile-long stretch of Highway 530 was affected by the landslide.

     

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    Figure 4. This photo shows one of the houses damaged by the 2005 Laguna Beach Landslide. The house experienced an undermining of the landslide but remained intact.

     

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    Figure 5. Talus cones on the north shore of Isfjorden, Svalbard, Norway.
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    Figure 6. Some people think these terraces were created by sheep! This is a splendid example of soil creep, or solifluction. When it is frosty, the moisture in the soil freezes and the ground expands because ice expands outwards at right angles to the slope. Then, when it thaws, the soil subsides vertically, creating these little terraces on grassy slopes.

     

    m14_Oberaargletscher_150.jpg
    Figure 7. This is not a glacier from Glacier National Park, but rather it is “Oberaargletscher,” as seen from Oberaar, Bern, Switzerland in July 2010.

     

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    Figure 8. Exit Glacier in Kanai Fords National Park, Alaska
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    Figure 9. The Fox Glacier in New Zealand

    Module Objectives

    At the completion of this module you will be able to:

    1. Explain how slope stability is related to slope angle, and how various factors influence the strength of materials on slopes.
    2. Explain what types of events can trigger mass wasting, and the types of motion that can happen during mass wasting.
    3. Describe the main types of mass wasting, the steps we can take to delay it, and some of the measures that can be taken to mitigate the risks associated with mass wasting.
    4. Describe the timing and extent of Earth’s past glaciations and the important geological events that led up to the Pleistocene glaciations.
    5. Explain the differences between continental and alpine glaciation.
    6. Identify the various landforms related to alpine glacial erosion, and various types of glacial lakes.

    Activities Overview

    See the Schedule of Work for dates of availability and due dates.

    Be sure to read through the directions for all of this module’s activities before getting started so that you can plan your time accordingly. You are expected to work on this course throughout the week.

    Read

    Physical Geology by Steven Earle

    • Chapter 15 (Mass Wasting)
    • Chapter 16 (Glaciation)

    Module 14 Assignment: A Virtual Field Trip Guide to Exploring the Volcanic History and Evidence of Mass Wasting at Sentinel Peak, Tucson, Arizona

    20 points

    After you complete the reading, you can start working on Module 14 Assignment – A Virtual Field Trip Guide to Exploring the Volcanic History and Evidence of Mass Wasting at Sentinel Peak, Tucson, Arizona

    Discussion 5 – Climate Change

    30 points (+5 points extra credit)

    This week’s discussion guidelines are different from the previous discussions. We will be having a conversation about climate change and the ways in it which it might personally affect you or your family.

    Discussion 5 Instructions

    Check the Schedule of Work for Due Date.

    Module 14 Quiz

    10 points

    Module 14 Quiz has 10 multiple-choice questions and is based on the content of the Module 14 readings and assignment.

    The quiz is worth a total of 10 points (1 points per question). You will have only 10 minutes to complete the quiz, and you may take this quiz only once. Note: that is not enough time to look up the answers!

    Make sure that you fully understand all of the concepts presented and study for this quiz as though it were going to be proctored in a classroom, or you will likely find yourself running out of time.

    Keep track of the time, and be sure to look over your full quiz results after you have submitted it for a grade.

    Your Questions and Concerns…

    Please contact me if you have any questions or concerns.

    General course questions: If your question is of a general nature such that other students would benefit from the answer, then go to the discussions area and post it as a question thread in the “General course questions” discussion area.

    Personal questions: If your question is personal, (e.g. regarding my comments to you specifically), then send me an email from within this course.