Joints are fractures that represent rock sprung apart (usually fractions of a centimeter unless/until widened by weathering) but next to which the rock on either side has not moved laterally. Most rock units, when seen in the field, show extensive breaking without any displacement. Every outcrop seems to be replete with cracks. These two photos show jointing as seen under exceptional viewing conditions:
In space imagery, major joints can be conspicuous under certains conditions, where they are long, continuous, wide-spaced and enlarged by erosion. They can be conspicuous in Landsat imagery. This is well shown by the joint system cutting a basalt flow in Zambia over which flows the Zambesi River (Victoria Falls).
Several sets of long, intersecting joints are evident in this Landsat subscene that center on a plateau underlain by Precambrian metasedimentary rocks southwest of Windhoek, Namibia:
Megajoints also control the topography in this Landsat image of the Nigerian Shield (southern Nigeria), a region of metasedimentary and basaltic rocks. At least 6 fracture sets are discernible. Surface and ground waters have opened up the fractures so that they now are narrow valleys in some places. Shadows cast by valley walls help to enhance fracture expression. Part of the upland erosion surface is vegetated (red).
We turn now to lineaments. This subject has been a leading point of contention over much of the history of use of space imagery for geologic studies. In the early days of Landsat, perhaps the most commonly cited use of space imagery in Geology was to detect linear features (the terms "linears" or "photolinears" are also used instead of lineaments, but 'linear' is almost a slang word) that appeared as tonal discontinuities. Almost anything that showed as a roughly straight line in an image was suspected to be geological. Most of these lineaments were attributed either to faults or to fracture systems that were controlled by joints (fractures without relative offsets). Lineaments are well-known phenomena in the Earth's crust. Rocks exposed as surfaces or in road cuts or stream outcrops typically show innumerable fractures in different orientations, commonly spaced fractions of a meter to a few meters apart. These lineaments tend to disappear locally as individual structures, but fracture trends persist. The orientations are often systematic meaning, that in a region, joint planes may lie in spatial positions having several limited directions relative to north and to horizontal. For example, 60% of the joint planes might fall into a cluster of fractures with azimuths between N40W and N60W and inclinations between 30 and 40 degrees).
Where continuous subsurface fracture planes that extend over large distances and intersect the land surface produce linear traces (lineaments). A linear feature in general can show up in an aerial photo or a space images as discontinuity that is either darker (lighter in the image) in the middle and lighter (darker in the images) on both sides; or, is lighter on one side and darker on the other side. Obviously, some of these features are not geological. Instead, these could be fence lines between crop fields, roads, or variations in land use. Others may be geo-topographical, such as ridge crests, set off by shadowing. But those that are structural (joints and faults) are visible in several ways. They commonly are opened up and enlarged by erosion. Some may even become small valleys. Being zones of weak structure, they may be scoured out by glacial action and then filled by water to become elongated lakes (the Great Lakes are the prime example). Ground water may invade and gouge the fragmented rock or seep into the joints, causing periodic dampness that we can detect optically, thermally, or by radar. Vegetation can then develop in this moisture-rich soil, so that at certain times of year linear features are enhanced. We can detect all of these conditions in aerial or space imagery.
When we illuminate a space image, particularly a transparency on a light table, we can often spot numerous linear or boundary discontinuities that we can record on tracing paper. The resulting lineaments map may have dozens, even hundreds of straight or slightly curved lines. But, scientific skepticism can raise key questions: Are they real? Do they all correspond to the same feature or phenomenon? If not, what is the proper identity of each one? And, finally, how do we verify their existence and determine their identities? These are important issues since we know lineaments (particularly faults) play key roles in location minerals and some oil or /gas deposits. They may also give clues to structural activities that bear on earthquakes. We save responses to these queries until the end of this section, after we cover how lineaments appear in images. Near the end of Section 5, we will again consider how human operator variations affect the correctness and consistency in picking valid lineaments amidst phony lines.
Within a month of the Landsat launch, images of the central Rocky Mountains were delivered to a team of investigators at the University of Wyoming. On this team, Professor Ronald Parker, a structural geologist, had been field mapping lineaments and other deformation features in the Wind River Mountains in the west-central part of the state. This great block of metamorphic and igneous basement rocks, flanked and patchily-covered by Paleozoic sedimentary rocks, is part of a segment of the Rockies where sections of the crust had been uplifted six km or more above other segments. Other segments stayed put or had downdropped to form deep (up to six km below the present surface) basins now back-filled by erosional debris from the surrounding mountain groups. As shown here by this Landsat image, the Wind River Mountains (center right) lie between the Wind River Basin (upper) and Green River Basin (lower), with the Gros Ventre and Hoback Ranges to the west. The Wind River Mountains rise to more than 3940 m (13,000 feet) above sea-level and have been strongly dissected by rivers and glaciers that exposed and carved into many major faults and lineaments.
It had not proved easy to map these lineaments in the field or from the limited aerial photos. As part of the project, NASA had previously flown a U-2 on a single pass, taking pictures of terrains that included this next image. The photo shows an eroded granitic surface with numerous lineaments, exposed in a rugged glaciated high mountain surface, with almost no tree cover on its higher elevations.
Dr. Parker, during five field seasons (camping in the high country, and supported by pack mules) had completed ground mapping faults, shear zones, and filled dikes in about 20% of the Wind River uplifted block. He supplemented his work by photointerpretating the U-2 strip. His map prior to receipt of the Landsat imagery is shown below on the left.
After recciving a Band 5 Landsat (ERTS) image as a transparency, using a light table he produced the lineaments map on the right in just three hours (including a break). Some of the newly plotted lineaments had been discovered earlier by other geologists, but most were heretofore unknown, and some of those have since been verified in the field. This great breakthrough effort now meant that we could map the synoptic overview of a large section of fractured continental crust with acceptable reliability in less than a day rather than months of difficult field work in poorly accessed terrain. This new method also meant great savings in time and money.
But, this mapping is of necessity incomplete and somewhat misleading. The maps below compare the ERTS and Skylab (Mission S-190B) Lineaments maps from the central Wind River Mountains. We plotted the orientation (azimuth or compass bearing) of the dominant direction relative to north, of each linear feature in a rose diagram (at the bottom of the map).
This diagram shows directions for all linear sets that we grouped in intervals (here 10 degrees), such that the length of the tapering bar in each interval is proportional to its frequency distribution (relative proportion) among all lineaments in the intervals distributed over the 180 degrees; that encompass west to east trends. Note that the dominant directions for ERTS lineaments are NE, whereas those for Skylab are NNW.
The cause of this difference between the two observations is simply time-of-day. The ERTS image was acquired around 10:30 A.M., local time, when the Sun's rays came from a SE position at a moderate elevation angle. Fractures occupying depressions that trend NE are shadowed on their NW side, and hence, stand out in the image as shadow relief. Whereas, those trending NW are equally illuminated on both sides and hence largely invisible and easily missed. On the other hand, the Skylab photo was taken in mid-afternoon, when the Sun was shining towards the NE at a higher elevation, so that shadows would maximize along NW trending lineaments. This illumination bias is a well-known effect (see page 8-4) and forces us to decide cautiously about structural trends when only data sets taken at one time of day are available. The obvious solution is to use multiple data sets, obtained at different times. Unfortunately this is not an option with Landsat. However, a geostationary satellite that can image at any time would circumvent this problem, but no high resolution systems [superior to meteorological ones] are operational yet.
Even as Dr. Parker and his cohorts were testing ERTS-1 as a means of speeding up their mapping of the Wind River Mountains, another geologist, Dr. Yngvar Isaachsen of the New York State Geological Survey, was examining his test site (he was one of the original Principal Investigators in the ERTS evaluation program in Geology). His mission: to test his hypothesis that ERTS imagery would significantly improve his long-standing studies of fracturing of the Precambrian massif known as the Adirondacks Mountains. (This large uplands in upstate New York is a surface expression of the Canadian Shield exposed after uplift and erosion south of the edge of the Shield near Montreal.) Here is what he saw in the first imagery he received:
Within days, Isaachsen had drawn this provisional map of lineaments in the Adirondacks:
Isaachsen is a true geologist - a field man. He proceeded to spend a half year field-verifying ("ground truthing") the existence and nature of the lineaments he had drawn on an improved map. Some of these proved to be shear zones, suggesting faults. Most could only be construed as fracture zones with no evident displacements (the majority of lineaments were hidden, many covered by glacial deposits). Some he discovered were artifacts - fence lines that kept trees on one side, farmers fields, etc. But after this verification study, the tectonic map of New York was judged to have been improved in the Adirondacks by at least 50%.
With some trepidation, the writer has added this map which plots the major lineations across the 48 U.S. states. The map was made by Mr. Douglas Carter of the U.S. Geological Survey, a leader in their remote sensing program and a respected structural geologist. He used both the Soil Conservation Service U.S. MSS Band 5 mosaic and individual transparencies of nearly all individual Landsat frames (he observed using a light table). The reason for caution: In the early days of ERTS/Landsat-1 geologists were finding lineaments everywhere. Nearly all were never field-checked, so their veracity remains unknown. In the writer's (NMS) opinion, some of Carter's lineaments are valid; others ???
Other studies on the next page speak to both the efficacy and the shortcomings of using space imagery to map lineaments.