The main assignment is in the pdf. Pictures should help to complete assignments.
Goal: Students will develop an understanding of the Earth’s crustal motion from the global to local scale. Learning Objectives: 1. Become familiar with the various types of plate movements that occur as a result of Earth’s shifting crustal plates 2. Understand the character and causes of rock folding and faulted landscapes to identify the resulting landforms 3. Contrast composite volcanoes of the continental crust and shield volcanoes of the oceanic crust Materials: Rulers, colored pencils and pens, calculator, TASA physiographic map of the ocean floor, laminated Andersonburg and Landisburg USGS topographic maps, laminated Grenville Dome topographic map, laminated Modoc Point and Wocus USGS topographic maps Tectonic Processes and Landforms 227 Figure 11.1 Earth’s lithospheric plates. The seven largest plates (North American, South American, Pacific, African, Austra- lian, Antarctic, Eurasian) account for about 94% of Earth’s surface. Even though the remaining plates can be relatively small, many are responsible for major regional tectonic activity, such as the Juan de Fuca plate that produces mountain building, volcanoes and earthquakes in the Pacific Northwest of the United States and Canada. (USGS) Tectonic Processes and Landforms 11 228 Tectonic Processes and Landforms Plate Tectonics—Global Scale It was a physical geographer, Alfred Wegener, who proposed the theory of continental drift in 1915. The theory developed over time to be known as plate tectonics, which was not widely accepted until the 1970s. Simply put, plate tectonics describes two parts of the Earth’s outer layer - the lithosphere, or earth’s outer crust, and the asthenosphere, or the soft part of the mantle. The asthenosphere flows like a liquid even though it is a solid. The lithosphere is broken into seven major plates, which float atop the plastic-like asthenosphere (Figure 11.2). These plates move, creating different types of boundaries between the plates: convergent (colliding), divergent (spreading), and transform (moving laterally). The result Introduction Physical geographers are concerned with landforms and their geologic structure because these characteristics influence atmospheric processes (e.g., orographic uplift, continentality), soil formation (e.g., parent material, slope), and geomorphology, to name a few. If you hear or witness earthquakes and volcanic activity, you are reminded of the fact that Earth’s outer crust, or lithosphere, is extremely dynamic. In fact, the Earth’s crust is broken into a handful of slowly drifting plates (Figure 11.1). Figure 11.3 (a) An illustration of the lateral movement occurring at transform plate boundaries. Examples of this type of plate boundary are the Dead Sea trans- form fault, which runs north-south along the east side of Israel and into the Gulf of Aqaba, and the San Andreas fault zone in southern California; (b) Wallace Creek (California) flowing from the left (east) drops off the North American Plate onto the Pacific Plate and make right angle turns to accommodate the opposing movement of the two plates. (USGS) Figure 11.2 The sequence of tectonic events converging during the process of subduction. This type of plate contact is exemplified in the Pacific Northwest of the United States, but occurs at several locations such as the Nazca-South American, Cocos-Caribbean, and Caribbean-North American plate contacts. The mafic magma and felsic magma at subduction zones will mix resulting in the production of andesite and diorite, not just granite and rhyolite. Tectonic Processes and Landforms 229 of movement at these boundaries includes volcanism, mountain building, and ocean ridge and trench formation. Where plates collide (convergent boundary), and one part of the crust slides below the other, it is called a subduction zone. Oceanic crust is denser than continental crust, so it slides beneath the continent in these zones. Notice in Figure 11.2 that subduction results in volcanism such as in the Pacific Northwest of the United States. Contacts at Plate Margins There are four basic types of plate contacts, that is, the manner in which two or more plates contact one another. Passive plate margins are those where little tectonic activ- ity occurs. Typically these passive margins were once active but are now stable. An example of such a margin is near the east coast of North America, where it joins the western edge of the Atlantic Ocean basin. Geologists know that this plate margin has not always been passive, because the Appalachian Mountains are a product of tectonic activity at this margin long ago. Transform plate margins are characterized by horizontal rather than vertical move- ment at the contact. Examples of this type of plate margin include the well-known San Andres Fault zone of Califor- nia (Figure 11.3b), and the Dead Sea fault zone along the border of Jordan and Israel. Divergent plate margins are, as the name implies, where two or more plates are moving away from each other in a process called rifting. Gaps cre- ated in the rift zone by the divergence are slowly or some- times rapidly filled with magma and lava. The mid-oceanic ridge in the center of the Atlantic Ocean basin is an exam- ple of this process; the rift zone is the focus of submarine eruptions (Figure 11.4). Recent eruptions on Iceland are a manifestation of this process—Iceland is oceanic crust extended above sea level. An example of continental rift- ing is the East African Rift Zone, where the rift valleys contain lakes (e.g., Lake Victoria, Lake Tanganyika, and Lake Albert) (Figure 11.5). The fourth type of plate margin is that of plate convergence, which occurs in three differ- ent situations: (1) oceanic plates colliding with continental plates, (2) oceanic plates colliding with one another, and (3) continental plates colliding with one another. When oceanic and continental plates come into contact tectoni- cally, the more dense oceanic plate will subduct, or move under the less dense continental plate in a process called subduction (Figure 11.2). This is the most violent type of contact, resulting in some of the most powerful earth- quakes (9.0 +) and violent volcanic eruptions. An exam- ple is found in the Pacific Northwest of the United States, where the relatively small Juan de Fuca plate is subduct- ing beneath the North American Plate, resulting in earth- quakes and volcanoes such as Mt. St. Helens, Mt. Rainer, and Mt. Shasta. The second type of plate convergence is Figure 11.5 The East Africa Rift system (John Wiley and Sons, Inc.). Figure 11.4 Plate rifting or fractures in plates, illustrated here by sea-floor spreading and divergence of the Atlantic Ocean bottom. Rifting also occurs in continental plates. 230 Tectonic Processes and Landforms that of ocean plate to ocean plate, an example of which is the Philippine Plate colliding with adjacent oceanic plates, resulting in earthquakes and submarine eruptions, some of which have been large enough to rise above the Pacific Ocean to become islands (e.g., the Marianas Islands). Third is the continent colliding with con- tinent, which is exemplified by the collision between the Indian Plate and the Eurasian Plate, creating the Himalayas. Tec ton ic s — Regiona l Scale Compressional forces can operate at all spatial scales, but when they occur at the regional scale, large folds develop within the rock layers to the extent they can create signifi- cant topography, the Appalachians being a good example of this. A second expression of regional- and even local-scale tectonics is faulting, which can be produced by either ten- sional or compressional forces. Folds Our understanding of geologic time in concert with stratigraphy helps us understand the evolution of landforms. Consider two land- forms, the syncline and the anticline (Figure 11.6). A syncline is a down- ward-folding or U-shaped curve while an anticline is the opposite or upward-folding curve. If these land- forms existed long ago, and have been subsequently eroding for mil- lions of years, they likely have not maintained their original shape. By understanding stratigraphic corre- lation, or the relationship between the stratigraphy and the landform evolution, scientists can identify synclines or anticlines after erosion has removed the original relief. If younger strata (layers) are found at the center and older strata are found outward, it was likely a syn- cline. If the older strata are found at the center and the younger strata are found outward, it was likely an eroded anticline. The Appalachian Figure 11.7 Erosional model for the Appalachian Mountains. Figure 11.6 The sequence and types of folds possible in sedimentary rocks. Tectonic Processes and Landforms 231 Figure 11.8 The place that gave hogbacks their name—here the Chaco River cuts through Hogback Mountain about 20 miles west of Farmington, NM. (L. Maher) Figure 11.9 Ridges (forested) and valleys (cities and agriculture) of central Pennsylvania—a topography produced by the erosion of anticlines and synclines during the development of the Appalachian Mountains. Note the river cutting through the rock layers in the binds of the folds; this is called an antecedent river. (NASA) 232 Tectonic Processes and Landforms Mountains region is one of the best examples of folding. The Allegheny orogeny (mountain building episode) occurred about 325-260 million years ago, and given the geologic antiquity of this event, a great deal of erosion has occurred since that time. The result has been the etching out of the less resistant rock strata to produce ridges consisting of more resistant rock strata, which are often referred to as cuestas or hogbacks (hogback ridges) (Figures 11.7 and 11.8). If a river system exists within the land- scape (rock layers) above the folds, the channel will be superimposed on the more resistant rock layers of the fold limbs as they weather out to create hogbacks. As a result, the rivers cuts through the hogback to create a water gap. The river is referred to as an antecedent river (Figure 11.9), in that it existed prior to the emergence of the hogbacks. If a river developed in the valleys resulting from the devel- opment of the hogbacks, then it is a subsequent river. Faults Faults are breaks that occur in rocks, be they sedimentary, igneous or metamorphic types. Four basic