Aerial Photos and Mass Movement Lab 7
In this week's assignment you will not be using your lab manual. We will be looking at air photos and mass movement, two topics not covered in your lab manual. If you were in class on Friday, February 18, you would have received a hard-copy of the lab material. If you were not, the material below is the same as that contained in the hardcopy with the exception of one figure that is not available online. To be prepared for the lab, study the material below and the associate material from your textbook.
This assignment will introduce you to the use of aerial photos, stereoscopic viewing, and concepts of mass movement (also referred to as mass wasting). Note that much of the material below is taken from Hamblin and Howard, Exercises in Physical Geology, Prentice-Hall, 1999. Some of the definitions are derived from Christopherson, Elemental Geosystems, Prentice-Hall, 1998.
Step 1: Aerial Photographs and Stereoscopic Viewing
Aerial photographs are photographs that are taken from an airplane, usually with specialized equipment. Typically these photographs are taken in sequences containing large numbers of photographs of adjacent areas. Aerial photographs have long been a fundamental tool in geologic studies because they show Earth's surface features in remarkable detail from a vertical perspective. To beginning students, however, a vertical view of Earth's surface is new and unnatural. Many features at first glance may appear simply as unfamiliar or exotic patterns, quite unlike the features we are used to seeing from the ground. Roads, buildings, farmlands, and lakes may be recognizable, but some experience is needed to identify various types of terrain, rock bodies, and other geologic features.
Stereoscopic Viewing: One of the great advantages of aerial photographs is that they can be viewed stereoscopically - in which an image of the landscape appears in three dimensions. This is possible because aerial photographs are taken in sequence, along a flight line with approximately 60% overlap of any two adjacent photos. When two adjacent photos along a flight line are viewed through a stereoscope, your brain combines the images on the photographs to produce one three-dimensional image of the surface. Hills and valleys appear to stand out in bold relief. Stereoscopic viewing of the landscape is like having a personal view of the landscape from directly above. There is an added advantage because photographs can be used to make notations and for mapping.
Some of the aerial photographs in this lab are printed in pairs called stereograms. These are pairs of photographs, shot from slightly different angles, placed on the page approximately 2 1/4 inches apart (about the same distance as between your eyes). To view a stereogram, simply place a lens stereoscope directly over the stereogram so that your nose is directly above the line separating the two adjacent photos (the TA will demonstrate; see Figure 1). The axis of the stereoscope should be perpendicular to the line separating the photos. If stereo-vision is not attained immediately, rotate the stereoscope slightly, or adjust the lens separation on the stereoscope to fit your eyes better.
Step 2: Mass Movement
Mass movement (also referred to as mass wasting) is all movements of materials propelled by gravity. Mass movement can includes all types of slope failure, whether fast or slow, and can occur in materials that are wet or dry. Because of its potential for destruction, mass movement has been studied by engineers as well as geologists. As a result, it has been classified in various ways. There are several different types of mass wasting (see the textbook, p. 325-332). Some of these types will now be discussed briefly.
Rock fall: Rock falls include the free fall of fragments, ranging from a single grain to huge blocks weighting millions of tons. Small-to-moderately sized fragments are commonly broken loose by ice wedging and accumulate at the base of a cliff as a talus cone.
Debris flows: A debris flow is a mixture of rock fragments, mud, and water that flows downslope as a viscous fluid. Different sub-types of debris flows contain different ratios of these materials. Movement can range from a flow that is similar to that of freshly mixed concrete to that of a fluid stream of mud with a velocity nearly equal to that of running water. Debris flows include mudflows composed mostly of fine-grained material, avalanches (extremely fast moving flows composed of dry rock or other debris), and lahars (volcanic mud flows).
Landslides: Although the vague term landslide has been applied to almost any kind of slope failure, a true landslide involves movement along a well-defined slippage plane. A landslide block moves as a unit along a definite fracture, with much of the material moving as a large slump block. The detached block leaves behind a distinct curved incision, or scar. The slippage plane is typically spoon shaped. As the block moves downward and outward, it commonly rotates in such a way that bedding or other identifiable surfaces are tilted backward toward the slippage plane. In the lower part of the slump block, part of the displaced material may move as a debris flow. The characteristic scar, tilting of bedding or other surfaces, and jumbled, poorly drained hillocks formed by previous slides serve to identify terrains that have been modified by landslides.
Creep: The almost imperceptibly slow movement of material in response to gravity is called creep. Individual soil particles are moved downslope by the expansion of soil moisture as it freezes, by cycles of moistness and dryness, by daily temperature variations, or by the activity of livestock or other animals. Creep typically results in features such as bent trees, leaning fences and power poles, and damage to buildings or other structures.
Rock glaciers: Rock glaciers are long, tongue-like masses of angular rock debris with pore spaces filled with ice. They resemble a glacier in general outline and form. The surface of a rock glacier is typically furrowed by a series of parallel flow ridges, similar to those in an advancing lava flow. Evidence of movement includes concentric wrinkle ridges, a lobate form, and a steep front. Measurements indicate that rock glaciers move as a body downslope at rates ranging from 2 in/day to 3 ft/yr. Rock glaciers commonly occur at the heads of glaciated valleys and are fed by a continuous supply of rock fragments produced by ice wedging on walls above. Ice in the pore spaces between the rock fragments presumably is responsible for much of the flow movement. With a continuous supply of rock fragments from above, the weight of the mass increases and causes the mass of ice and rock to flow. Favorable conditions for the development of rock glaciers thus include steep cliffs and a cold climate.
Last updated February 19, 2000