1. List three lines of evidence in support of the crustal uplift concept.
Marine sedimentary rocks with fossils form parts of the highest mountains on Earth. Less spectacular, but equally convincing, are sedimentary rocks deposited in marine and beach environments now at elevations well above any possible, past sea level elevations. The Cretaceous marine shales, shoreline sandstones, coastal swamp sediments, and coals of Utah, Colorado, New Mexico, and northeastern Arizona are good examples.
Convincing evidence for geologically recent uplift can be seen in wave-cut platforms and cliffs now above sea level along the west coasts of North and South America. Since sea level has been rising for at least the last 30,000 years, these features indicate crustal uplift (not falling sea level). The famous Temple of Serapis near Naples, Italy (Fig. 20.3), was built above sea level, dropped below sea level for a period of time and is now just about at sea level. Marine organisms bored the marble columns up to 20 feet above the floor. These down-and-up crustal movements occurred during the last 3000 years.
During the 1964 Alaskan earthquake, coastal areas near the epicenter were uplifted as much as five meters; along many tectonically active coastlines, Quaternary marine deposits, such as coral reef rock and beach sands, are at elevations well above today's sea level.
2. What evidence initially led geologists to conclude that mountains have deep crustal roots?
Evidence for lower density, crustal roots in association with strongly elevated, mountainous regions first came to prominence in the mid-nineteenth century. Astronomical and transit surveys, undertaken by the British Government in India, gave differing results. The discrepancies were too large to be accounted for by experimental error; and furthermore, they seemed to be largest in northern India, close to the Himalayan Mountains. Transits are oriented vertically with a hanging plumb bob; the bob is supposed to point directly to the center of Earth. In the Indian surveys, lateral variations in the masses of rock at depth along the survey route caused slight errors in the orientations of the survey instruments (Fig. 20.5), but astronomical surveys run at the same time were not affected. To explain the discrepancies evident in the transit data, Airy proposed that lower density rock displaced higher density rock at depth beneath high standing, mountainous areas, giving rise to the concept of deep crustal roots.
3. What happens to a floating object when weight is added? Subtracted? How does this principle apply to changes in the elevation of mountains? What term is applied to the adjustment that causes crustal uplift of this type?
The fundamental idea involved is the Archimedes principle. Objects floating in a more dense fluid displace a volume of fluid equal in weight to the weight of the floating object. Thus when weight is added, the object will sink until a new equilibrium level is achieved. If weight is subtracted, the floating object will rise for the same reason. The same idea applies to mountains, blocks of unusually thick, low density, continental rock whose basal portion is floating in denser, mantle rock. As a mountain range is eroded, it will slowly rise to adjust for the loss of weight. Isostasy is the concept and the resulting uplift or subsidence is termed an isostatic movement.
4. List one line of evidence in support of the idea that the lithosphere tries to remain in isostatic balance.
Glacial rebound is a good example. Areas in Canada and Scandinavia have risen measurably during historic time and are still rising slowly. Due to the added weight (load) of the ice, the crust-mantle boundary was locally lowered as higher density, mantle material flowed away from the volume occupied by the depressed, basal part of the crust. When the ice melted, mantle material flowed back into the region formerly occupied by the lower density, crustal material, causing the crust-mantle boundary and the land surface to rise to their preglacial, equilibrium positions.
5. What do we call the site where sediments are deposited along the margin of the continent where they have a good chance of being squeezed into a mountain range.
Such a site would be any marine depositional basin bounded by continents, continental fragments, island arcs, or any two of these features facing each other across a subduction zone. Continental shelf and accretionary wedge deposits both can be involved. As the basin closes, the sediments are squeezed and compressed as if they were caught between the jaws of a closing vise. As shown in Figure 20.14, continental margin sediments from the subducting Indian plate are detached during the collision and backthrust to form the southern frontal hills of the Himalayas. Ocean basin and accretionary wedge sediments that were deposited along the southern margin of the Eurasian plate have been crumpled and uplifted to form parts of the central Himalayas.
6. Which type of plate boundary is most directly associated with mountain building?
Mountain building is most directly associated with convergent plate boundaries. The edge of the upper plate evolves as a continental margin or volcanic (mountain) arc. Sediments on the subducting plate are sheared off and accumulate on the upper-plate margin as an accretionary complex. Sediments deposited along an upper-plate margin are squeezed, crumpled and uplifted as nonsubducting, low-density blocks such as seamounts, an island arc, continental fragments, or an entire continent approach the subduction zone and close the marginal basin. Compressive stresses generated by the collision cause the rocks of the upper plate to be shortened horizontally and thickened by numerous thrust faults and reverse faults.
7. What is an accretionary wedge? Briefly explain its formation.
As a subducting plate bends and begins its descent into the mantle, most sediments are scraped from basaltic, oceanic crust and piled onto the leading edge of the upper plate, much as snow piles up in front of a moving snowplow. These sediments accumulate (accrete) as a wedge-shaped stack of reverse-fault slices with the tapered edge of the wedge pointing toward the subduction zone. This complexly deformed and faulted, sediment pile is the accretionary wedge. The Barbados Ridge and its only emerged area, Barbados Island, lie about 150 km east of the Lesser Antilles island arc. Exposed graywackes, mudstones, and minor pelagic "shales" (muds) show that the ridge and island are part of an accretionary complex accumulating against the eastern margin of the Caribbean plate where a segment of the North American plate is subducting beneath the Caribbean plate.
8. What is a passive margin? Give an example. Give an example of an active continental margin.
Continental margins are characterized by their tectonic activity. Passive margins (Fig. 20.13A), the east coasts of North and South America for example, exhibit subdued, "quiet", tectonic movements such as slow uplift and subsidence punctuated by occasional, localized faulting. They have wide continental shelves and their continental slopes merge seaward into abyssal plains. Passive margins form originally by continental rifting and are modified by erosion and deposition as they move away from a mid-ocean ridge. For this reason, they are also known as "trailing" margins.
Active continental margins occupy areas of plate convergence, subduction, and local, transform faulting (Figs. 20.13B-C & 20.19). Tectonism, intrusion, and volcanism are active and long-lived. The western margins of North and South America are good examples (Fig. 20.19).
9. In what way are the Sierra Nevada and the western Andes similar?
Both represent compressive, continental-margin orogens and magmatic arcs driven by subduction of oceanic lithosphere. The Sierra Nevada owe their present-day, lofty elevations to late Cenozoic normal faulting, but most of the rocks comprising the range are mid- to late-Mesozoic batholithic granites and granodiorites. These represent solidified, mid-crustal chambers that once fed magma upward to a contemporaneous, continental-margin volcanic arc. The arc was situated on the western margin of North America above a subducting part of the since disappeared, Pacific ocean floor that lay east of the East Pacific Rise and east of the present-day trace of the San Andreas fault. Thus rocks of both ranges originated in similar, convergent, continental-margin settings (Figs. 20.13B-C & 20.19), but the major episode of subduction associated with emplacement of the Sierran granites ended in early Cenozoic time, whereas subduction along the western margin of South America has been more or less continuous since early to middle Mesozoic time.
10. Suppose a sliver of oceanic crust was discovered 1000 kilometers into the interior of a continent. Would this tend to support or refute the theory of plate tectonics? Why?
An oceanic-crust sliver in the interior part of a continent does raise a substantive problem as to its origin. As part of the overall plate tectonic theory, slices and slivers of oceanic crust are known to be accreted to continental margins at subduction zones. Oceanic crust terranes accreted in pre-Mesozoic time could be "positioned" more toward a continental interior if a sizable continental terrane was accreted during a later collisional event, leaving the oceanic-crust sliver surrounded by continental crust. Thus a continental interior, oceanic crust sliver can be explained in terms of continental accretion and the plate tectonic theory. Older ideas on the origin of exotic, continental interior terranes are contrived and unconvincing.
11. How does the plate tectonics theory help explain the existence of fossil marine life in rocks atop the Ural Mountains?
The Ural Mountains, a north-south range in west-central Russia, mark the closure site of an ancient, marine basin that once existed between the European and Siberian parts of the Eurasian plate. As the two continents converged and joined, the sediments in the former marine basin were lithified, crumpled, and uplifted into a mountain range that includes fossiliferous, marine, sedimentary rocks.
12. In your own words, briefly enumerate the steps involved in the formation of a major mountain system according to the plate tectonics model.
The student should concentrate on continent-continent, collisional ranges like the Himalayas or continent-ocean, collisional ranges like the Andes. Both involve compressive stresses with horizontal shortening, thickening, and uplift of the continental crust. In addition, subduction-related stratovolcanoes are built on top of the uplifted, Andean-type margins. Reverse faults and folds would be evident. Older, metamorphic rocks (schists and gneisses) that formed deep in the crust may be uplifted and uncovered by tectonic denudation and prolonged erosion. The association of volcanic rocks and stratovolcanoes should be evident.
13. Define the term terrane. How is it different from the term terrain?
Terrain is used in a physiographic or geomorphic sense to describe the form and nature of a landscape. For example, the terrain of southeastern Pennsylvania is characterized by gently rolling, forested hills interspersed with cultivated fields and open pasture lands in the valleys.
Terrane is a much more comprehensive, geologic term describing specific areas or regions with certain, common elements of lithology and geologic history. A terrane can vary from a single, truncated seamount enclosed in an accretionary wedge complex to distinctive rock masses with much larger dimensions. Terranes may have oceanic, island arc, or continental affinities. Some may have rifted apart from the parent continent, then recombined in a subsequent compressive event. Others, such as some along the western margin of North America (Fig. 20.16), formed elsewhere and migrated to their present-day locations. Thus some terranes are identified as exotic or far-traveled. Larger, subcontinent-sized terranes (Fig. 20.19) represent portions of the crust and lithosphere joined together in the geologic past as the continent grew and enlarged. In general, adjacent terranes share common geologic histories after joining; prior to that time, their geologic histories may be quite different.
14. On the basis of current knowledge, describe the major difference between the evolution of the Appalachian Mountains and the North American Cordillera.
The final stage in the evolutionary history of the Appalachian orogen was energized by the continental collision between North Africa (then part of Gondwanaland) and North America in Pennsylvanian time. The northern Cordillera evolved over a long lasting interval of plate convergence starting in early- to mid-Mesozoic time. Along most of the North American Pacific margin, convergence was replaced by transform faulting beginning in mid-Cenozoic time. Terrane accretion, volcanism and plutonism, folding, thrust faulting, and horizontal shortening and crustal thickening characterized its evolution, but compressive deformation occurred without continental collision. This was the major difference in the tectonic evolution of the two orogens. However, prior to closure of the proto-Atlantic ocean basin, the evolutionary history of the Appalachian orogen was very similar to that of the northern Cordillera where the active continental margin faced a subducting oceanic plate carrying an island arc or two, numerous seamounts and other small terrane blocks, but no piggy-backed, continental mass.
15. Briefly describe the formation of fault-block mountains. (Box 20.1)
The forces or stresses are tensional. The crustal rocks are stretched and eventually fail by cracking or rupture. The faulted blocks are often tilted. The downtilted or downdropped blocks forms the valleys and the uptilted or uplifted blocks makes the mountains. Fault-block mountains develop when tensional stresses accompany or follow regional uplift (Box 20.1).
16. Compare the forces of deformation associated with fault-block mountains to those of most other major mountain belts.
Fault-block mountains develop in areas where widespread uplift causes fragmentation of the brittle, upper crust. Most active, block-bounding faults have steep dips, but subhorizontal dips characterize associated, deformational zones at mid-crustal levels (Fig. 15.21). Stresses are tensional; deformation involves horizontal stretching and crustal thinning. The impulse for uplift may come from below; but internal, gravitational stresses drive the deformation (Box 20.1).
In contrast, most major mountain belts include two, fundamental, geologic components in their development: 1) volcanism and batholithic intrusion, and 2) horizontal shortening accompanied by crustal thickening. The compressive stresses are generated laterally due to plate convergence and subduction, and folding and thrust faulting are common structural characteristics (Fig. 15.26; Box 20.2).
17. Contrast the opposing views on the origin of the continental crust.
The contrasting views relate to the rates at which continental materials have been recycled and to what proportion of today's continental lithosphere was formed and already in place early in Earth's history. We know that the continents grow by accretion (Figs. 20.16 & 20.19), but the history of Pangaea also tells us that large continents can break apart every two hundred million years or so. How has the volume of continental crust and lithosphere changed over geologic time? Has continental crust slowly and continuously been added or has it mostly been recycled from crust that formed early in geologic time?
Continental crust is too low in density to be subducted, and the small mass pulled down with denser, oceanic lithosphere should quickly melt and be returned to the surface by volcanism. Pelagic sediments, composed partly of continental materials, are subducted but their volumes are very small. Partial melting of rising mantle plumes that may originate near the core-mantle boundary contributes "new" basalt to the oceanic lithosphere; and occasionally, slabs of oceanic lithosphere (ophiolite complexes) are shoved onto a continental margin for "permanent" storage.
The fates of deeply subducted, oceanic slabs hold the key to the extent of lithospheric recycling. If the slab material remains at depth indefinitely, very little of the subducted, oceanic lithosphere would ever be added to the continents. On the other hand, a slab may sink to the core-mantle boundary zone and contribute remobilized, old oceanic crust to a rising mantle plumes. This process would involve extensive recycling of oceanic crust. If a slab "parked" beneath a continent partially melts, fusible components of the slab could be added to the continent through intrusion and volcanism. For example, kimberlites and other magmas that originate at great depths might include components derived from ancient, subducted slabs "parked" more or less permanently at depth in the mantle. Basaltic underplating and flood basalt effusion add large volumes of mafic rock to the continents, but as noted previously, whether or not this basaltic magma is "pristine" or includes recycled oceanic crust is a matter for considerable debate. Newer isotopic studies based on the samarium-neodymium and rhenium-osmium decay schemes may help to track the premagma geologic history of basalt magma constituents.
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