Google Earth: a new geological resource

FEATURE Feature Google Earth: a new geological resource Since its release in June 2005, Google Earth has been bringing satellite images of our planet into our homes, or at least to those homes with broadband connections. Computer users, excited by seeing their own houses from on high, or even their cars parked in the drive, have been raving about this impressive piece of software which can be downloaded to your PC (but not, as yet, your Mac) free of charge. After a weekend playing around with Google Earth, I can confirm that there is a whole range of potential applications for this software in teaching Earth science. Google Earth (http://earth.google.com) is a programme that constructs pictures of the surface of our planet by downloading satellite data from a remote server. The location and size of the region in the picture is fully under the control of the user. View Earth as a globe, spin it with a drag of the mouse, and marvel at its large-scale physiographical features such as the Tarim Basin in central Asia, oceanic trenches and transform faults. To explore in more detail, one can zoom in to reveal rivers, lakes, cities and roads. Unlike with traditional remote sensing images where it is easy not to see the wood for the trees, this zoom facility allows small-scale topographic and geological features to be instantly placed in a broader regional context. Inevitably, resolution imposes a limit to the zoom facility; resolution varies from one region to another and is higher in some urban areas. In other respects too, Google Earth offers advantages over other images, such as aerial photographs. First, the picture can be rotated about a vertical axis allowing us to take a look at things from the other side. This helps enormously with the visualization of the three-dimensional form of the features in question. Secondly, the tilt of the line of sight can be adjusted by the user. Vertical viewing delivers something resembling a map, whereas a sideon ‘car-window’ view is obtained by reducing the tilt of the line of sight. Again it is the change in appearance of the object under these rotations which helps convey the 3D geometry of the structure to the observer. Other teachers will know that threedimensional visualization poses significant problems for some students and should welcome this new tool as an aid to teaching maps and their interpretation. A third feature that Google Earth offers is the ability to move the location of the observer; something that simulates a journey across the region. This, coupled with rotation and tilt, gives the impression of a flight of exploration to follow geological structures across the ground, to dive in to see them a close-up, and to turn to be able to see them from another vantage point. This is an exciting experience that can be one of real scientific discovery. As data transmission speeds increase, so as to allow the rapid downloading of higher resolution images, the scientific potential of this tool will increase still further. The programme allows interesting locations to be remembered as placemarks, which does away with the need to record the location’s latitude and longitude for its later retrieval. In fact Google Earth comes supplied with some placemarks of city views and of some spectacular scenery. An example is Mount St Helens; one mouse click flies you directly to this volcano. Once there, using the navigation tools provided, the user can closely inspect the crater, the dome and deposits resulting from the 1980 eruption. Another supplied placemark of geological interest is the Grand Canyon. This conveys the dramatic geomorphology of the canyon but naturally cannot reconstruct the geological contacts because the latter crop out on the steep canyon walls and are therefore invisible to the satellite’s camera. A few hours of geological globetrotting will unearth other sites of interest. A few examples are discussed below. Richard J. Lisle School of Earth Sciences, Cardiff University, Cardiff, CF10 3YE, UK [email protected] Landforms Cuestas are erosional landforms consisting of asymmetrical ridges, the asymmetry arising from the dip of the underlying strata. Examples as beautiful as those depicted in the textbooks are to be seen in © Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 1, January–February 2006 29 FEATURE Horizontal beds Map patterns consisting of sedimentary rock units following the ground contours are characteristic of a simple structure consisting of horizontal strata. An example is to be found in southern Namibia (25°47′S, 17°07′E) where a layer-cake structure of Palaeozoic sedimentary rocks is dissected by valleys (Fig. 2). An oblique view of the valley sides will convince the student that the beds are indeed flatlying. Faults, fractures and shear zones Fig. 1. Alluvial fan in Death Valley, California. Morocco. Move the mouse and see the latitude and longitude of the cursor, displayed on the screen, change. Take a trip to latitude 28°15′N; longitude 10°4′W to view cuestas with lengths of 270 kilometres. It is no sweat at all to trek across Death Valley, California to view a spectacular alluvial fan at 37°19′N; 117°47′W. Place the cursor at its apex (Fig. 1) and look at the foot of the screen to discover that its altitude is 4567 feet above sea level. Comparing this with the altitude of the base tells us that the fan has a height of about 1400 feet (427 metres). A still larger alluvial fan with an area in plan view of 1200 square kilometres is to be found in the Shulehe river valley, northern China (40°14′N, 96°40′E). Sip a cool drink and examine sand dunes in a variety of forms. Those in the Namib Desert (25°55′S, 15°00′E) are aesthetically beautiful. Note their variation in shape and orientation around rocky windbreaks. Similarly impressive are those in the Takla Makan Desert in Turkestan (38°48′N, 80°03′E) that could be investigated further as a student project. Fig. 2. Horizontal beds cropping out on valley sides, southern Namibia. 30 Faults with dextral and sinistral separations are visible in Western Algeria (25°36′N, 4°48′W). Do they form conjugate sets of faults and, if so, what direction of maximum compressive stress is suggested? These images invite further thought. Fracture patterns in Pilbara Craton, NW Australia (21°11′S, 119°33′E) involve several sets. This area could be used for a project involving measurement of fracture lineaments, the construction of rose diagrams and statistical analysis. An example of the relationship between faulting and topography is seen in the Precambrian basement of the Sierra de Ambato, NW Argentina (28°12′S, 65°50′W). Compare the positions of north trending escarpments with the thrust faults shown on the map published by de Urreiztieta and his colleagues in 1996. A topographic expression of extensional faults is seen in Fig. 3, where elongate depressions in Utah (38°07′N, 109°53′W) correspond to the numerous graben structures. In the Arabian Shield at (23°48′N, 42°20′E), major lateral movements occur across ESE trending zones. These zones resemble major faults, involving kilometres of sinistral movement. However if you Fig. 3. Graben structures in the Canyonlands National Park, Utah. The E-W width of the image is 3.3 km. © Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 1, January–February 2006 FEATURE 131°58′E) and the folded layers have a whaleback form (e.g. the folded Neoproterozoic rocks of the Northern Flinders Ranges (29°48′S, 137°49′E). Other outcrop patterns produced by multiple folding, including the Type 2 pattern described by Ramsay and Huber in 1987, can be discerned on images of northern Namibia (20°40′S, 15°20′S). Beautiful Type 3 patterns occur at (20°58′S, 13°47′E). Fig. 4. Plunging folds, western Morocco. zoom in on the zones where the offset is concentrated it becomes apparent that the displaced units are dragged round in a ductile fashion. These are major shear zones. Folds Folding produces dramatic geological map patterns, and satellite images of folds in the Appalachian Valley-and-Ridge Province (40°35′N, 76°44′W) are well known. Google Earth allows you to view these from different angles and thereby to gain an appreciation of the plunges of the folds’ hinge lines. The view from directly overhead, however, is a distortion of the real shape of these structures. To get a fairer impression of their cross-sectional shape use the tilt facility that enables you to view them looking down the hinge lines. It would be difficult to find better illustrations of plunging folds than those in Western Morocco (28°17′N, 10°54′W) (Fig. 4). Further east (28°52′N, 9°42′W) this down-plunge viewing trick can be used to good effect. Look down the north-easterly fold plunge and you get an immediate picture of a structural cross-section; it reveals that the greenish unit overlies the brown coloured one and that the folding within these two units is harmonic. In Southern Algeria (26°10′N, 2°04′E), erosion has carved a flat topographic surface though a sequence of folded units (Fig. 5). The closed loop shapes cropping out on this horizontal surface must be the expression of a dome-and-basin geometry of the folding. Take a close look at these images and try to distinguish the domes from the basins. Such ‘eggtray’ folds are termed non-cylindrical; cylindrical folds having a simpler roofing sheet form. Noncylindrical folds can form by the superimposition of two folding trains with different trends. If one of these directions of folding dominates, the circular ring map pattern is replaced with elliptical shapes (24°21′S, Fig. 5. Erosion has carved a horizontal section through these dome-and-basin folds, southern Algeria. © Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 1, January–February 2006 After scouring the pictures for detail, try zooming out to see larger scale structural features, such as the regional arrangement of folds. An en echelon fold pattern is seen in Algeria at (33°48′N, 2°38′E). Such a staggered arrangement of a fold train is common in deformation zones with strike-slip movements. Salt domes and salt glaciers Evaporite rocks such as rock salt have a much lower density than typical sedimentary rocks. For this reason their buoyancy can force them upwards, arching up the overlying sedimentary rocks and even piercing through them. The resulting structure is a salt dome. Such domes may have a topographic expression like those in Iran (26°40′N, 54°32′E). In some cases, the salt rises as far as the Earth’s surface. In humid climates such surface salt would be removed promptly by solution but the in arid climate of Iran this salt survives to flow slowly down surface slopes as salt glaciers. Those seen on images at (27°30′N, 54°34′E) have features similar to those observed on ice glaciers. The downhill creep of these glaciers testifies to the very low viscosity of rock salt. Note the surface features of these glaciers with those described elsewhere in Iran. Large-scale tectonic features Large-scale structural patterns, that would normally only become apparent after a lengthy regional 31 FEATURE synthesis based on traditional maps, are often strikingly displayed on Google Earth. Examples are fold belts (e.g. the Zagros Mountains), regional unconformities (e.g. those east of the Arabian Shield), and greenstone belts (Pilbara Craton, NW Australia). Didactic examples of inliers are observed in the Atlas Mountains (29°25′N, 8°34′W) and patterns of heterogeneous deformation around resistant massifs in the Hoggar, Algeria (22°59′N, 3°53′E). Using the programme a student could undertake a simple mapping exercise. For example, using the line draw tool (found in Tools > Measure > Path), the task could be to draw the contact of the Peruvian coastal batholith. Fig. 6. Volcano, SP Mountain, Arizona. Volcanoes and sheet intrusions If you are interested in recent volcanism take a Google flight over Arizona. In the image of the SP Mountain at (35°34′N, 111°37′W) one can clearly see a 250-metre tall volcano with a near perfect conical form (Fig. 6). Its steeply sloping flanks and crater walls suggest a young age for the cone. A dark lava flow has clearly erupted from the volcano, but one can deduce from the image that this must have occurred before the conical edifice was formed. This lava flow extends northwards for 7 km and has draped over the topography and filled valleys in its path. An eroded volcano is seen at Ship Rock, New Mexico (36°41′N, 108°50′W). A sharply pointed hill composed of igneous rock is an erosional remnant of the central feeder pipe of a volcano. Radiating from this volcanic neck are vertical dykes. It is possible that these dykes filled a set of radial fractures induced by the magma pressures in the central pipe. A swarm of volcanoes is to be found in Algeria (22°44′N, 4°45′E) Steep sheet intrusions are easy to recognize in several places around the world. In Zimbabwe (19°53′S, 29°59′E), the 6 km thick Great Dyke can be traced across almost the entire length of Zimbabwe for a distance 480 km. In contrast, discontinuous dyke segments can be picked up on the pictures from the Precambrian Pilbara Craton, Western Australia (21°47′S, 119°23′E). Meteorite craters A very photogenic hole in the ground is seen in Arizona at (35°02′N, 111°01′W). It is the aptly named Meteor Crater. To find out the size of any feature on an image, go to the ‘Tools’ menu and select ‘Measure’. You simply draw a line over the image and Google Earth tells you how long the line is. If you do this you find that the crater’s diameter is 1.2 km. Meteor Crater is calculated to be the result of 32 impact by a iron meteorite of diameter 30 metres with a estimated weight of 63 000 tonnes (see Marshak 2002 for discussion). An Australian example is the Wolfe Creek Meteorite Crater (19°18′N, 127°46′W). Conclusion This software will not be universally greeted; it will do to the atlas and globe businesses what the Internet has done for encyclopaedia salesmen. On the other hand, it will be welcomed by students, teachers and researchers in Earth sciences. It will be particularly useful in the teaching of geomorphology, structural geology and geological map interpretation, and will provide a valuable source of data for student projects. In addition, it’s all for free! Well, almost. I suppose it is financed by the catering outlets that advertise on the site. In this case I am so impressed with this product that I now feel morally obliged to buy a beer at the OK Bar next time I’m in Flagstaff Arizona. Further reading De Urreiztieta, M., Gapais, D., Le Corre, C., Cobbold, P.R. & Rossello, E. 1996. Cenozoic dextral transpression and basin development at the southern edge of the Puna Plateau, northwestern Argentina. Tectonophysics, v.254, pp.17–39. Google Earth Website: http://earth.google.com Lisle, R.J. 2004 Geological Structures and Maps. Butterworth-Heinemann, UK. Marshak, S. 2005. Earth: Portrait of a Planet, 2nd edn. Norton, New York. Ramsay, J.G. & Huber, M.I. 1987. The Techniques of Modern Structural Geology. Vol. 2, Folds and Fractures. Academic Press, New York. Talbot, C.J. 1979. Fold trains in a glacier of salt in southern Iran. Journal of Structural Geology, v.1, pp.5–18. © Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 1, January–February 2006