Geography 140
Introduction to Physical Geography

Lecture: Introduction to Tectonic Uplift

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 IV. Processes of uplift in the earth's crust are the subject of this lecture, 
     now that we've covered the structure of the planet and the materials 
     making up the earth's crust.  
     A. We can conceptualize the levels of landform relief at different 
        scales.
        1. First order relief would be global scale contrasts between 
           continents and ocean basins, between, say, Africa and the Indian 
           Ocean or North America and the Pacific Basin.
        2. Second order relief would be regional scale contrasts, such as 
           between great regional mountain systems (such as the Rockies or the 
           Himalayas) and lower, flatter features, such as plains (e.g., the 
           Great Plains) and plateaux (e.g., the Tibetan Plateau).
        3. Third order relief would be local scale contrasts, such as between 
           local mountains and hills or mountain ranges and local valleys and 
           plains. Locally, think of Signal Hill versus the Long Beach 
           portion of the Los Angeles Coastal Plain.  Similarly, you could 
           think of the Santa Monica Mountains and the San Fernando Valley.
     B. The uplift of the earth's crust is very evident at all orders of 
        relief: As you look at the map of the world, you first notice it's 
        divided into oceans (71%) and continents (29%). You then notice other 
        features, such as the great mountain arcs, lesser mountains, plains, 
        islands. If you had a map of ocean basin topography, you'd also notice 
        abyssal plains, continental shelves, and, most conspicuous, the mid-
        Atlantic ridges and western Pacific trenches. If you had a more 
        thematic global map, you'd also be struck by the distribution of 
        volcanoes and earthquakes. In this lecture, I'll summarize a theory 
        which accounts for all these features of our planet in one fell swoop.
        1. This is the plate tectonics theory, also sometimes known by its 
           older name: continental drift theory. This was first put forward as 
           a serious theory by Alfred Wegener (1880-1930).  Some geotrivia for 
           you:  He was Wladimir Köppen's son-in-law, Köppen being 
           the guy who came up with the descriptive climate classification you 
           learned about in the lecture on climate types (which I used to 
           situate the tree-dominated, shrub-dominated, and grass-dominated 
           biomes).  THAT Köppen.
        2. Wegener's life story is rather sad -- and informative. He earned 
           his doctorate in the field of planetary astronomy back in 1905.  He 
           somehow became more interested in meteorology (the study of weather 
           and climate, which is probably how he wound up dating Köppen's 
           daughter?) and left the field of his degree. He got sent off to 
           fight in the German army in WWI and was seriously injured in 1914.  
           While recuperating, he started thinking about how weird it was that 
           South America's east coast seems to fit so tidily into the west 
           coast of Africa, and the Arabian Peninsula seems to fit into the 
           notched northeast coast of Africa.  
           a. Unlike everyone else who'd noticed it before, he started putting 
              together a theory that maybe, in fact, these different land 
              masses DID once upon a time fit right into one another and 
              drifted away subsequently. 
           b. By 1915, he'd put together more evidence that this could have 
              happened:  
                i. The agreement in thicknesses and compositions of 
                   sedimentary rock beds in Africa and South America older 
                   than about 135 million years ago
               ii. Biogeographical evidence involving animals whose closest 
                   relatives were on the other continent, and absolutely 
                   identical plant and animal fossils on the two sides in the 
                   135 million b.p. time frame 

                   [ fossil biogeography of Gondwana, USGS ]

              iii. The finding of tropical and subtropical fossils in 
                   Antarctican coal deposits
               iv. The finding of glacial deposits in South Africa.
                v. In other words, a lot more than just the oddball line-up of 
                   the coasts.  
           c. His book, The Origin of Continents and Oceans, came out 
              in 1915, but geologists and physical geographers thought he was 
              a meddler untrained and unqualified in their area of interest, 
              and they dismissed his theory and evidence as a crackpot idea.  
              The poor schmo never lived to see his theory vindicated, and he 
              never got a university professorship until a couple years before 
              he died (and pretty heroically, too) in 1930.  Ironically, by 
              the early 1970s, it was pretty evident that Wegener was right 
              and the discovery of the mechanisms behind continental drift 
              gave rise to the plate tectonic theory, which today dominates 
              geology and physical geography. You can read a short and rather 
              sad summary of this unfortunate fellow's life by clicking here.
        3. Plate tectonics is important not just because it explains the 
           observations above: It also has other implications.
           a. By severing land masses and altering their climates, it has 
              allowed new species of living beings to evolve, separate from 
              their relatives (vicariance biogeography, produced by allopatric 
              speciation).  An example is provided by the 25 species of 
              eucalyptus in one genus, Monocalyptus, that 
              differentiated in southern Australia because of climate changes 
              that isolated various populations from one another so that they 
              couldn't exchange genes with one another anymore).
           b. By reconnecting land masses with land bridges, plate tectonics 
              has allowed the dispersal of various life forms. Horses provide 
              an example of this.  They first evolved in North America and 
              then crossed the Bering Land Bridge between Alaska and Siberia 
              (which had been exposed as dry land when the Pleistocene ice 
              ages caused ocean levels to drop).  Horses dispersed into Asia, 
              Africa, and Europe but they went extinct in North America.  
              Horses survived in the Old World until the Spanish brought them 
              back to the New World as a rather temporary military advantage 
              against the Indians.  Some Native American nations on the 
              periphery of the Spanish Empire quickly took to this new 
              technology of horseback riding (and guns) and utterly 
              transformed their traditional livelihoods and military 
              activities -- as a result, they were able to challenge Spanish 
              and US hegemony for a long while.
           c. Plate tectonic activity has changed the earth's climates through 
              moving landmasses around, through mountain building (orogeny, 
              which leads to orographic precipitation, rainshadow deserts, and 
              higher, cooler climates on mountains and plateaux), and through 
              diverting warm ocean currents.
           d. It's determined the distribution of mineral resources (gold, 
              diamonds, uranium, coal, oil...).
           e. It now produces events of great significance to human societies: 
              volcanic eruptions, earthquakes, tsunamis.
     C. Plate tectonics holds that the lithosphere is broken into several 
        rafts or "plates" of various sizes:

        [ plates, This Dynamic Planet, USGS ]

        1. There are seven huge plates:
           a. North American, including much of the North Atlantic and Arctic 
              sea floors
           b. South American, including much of the South Atlantic sea floor
           c. Eurasian, including much of the North Atlantic and Arctic sea 
              floors
           d. African, including much of the South Atlantic and some of the 
              Indian Ocean sea floors. 
           e. Indo-Australian, comprising the Indian Subcontinent and 
              Australia and including much of the Indian Ocean and a part of 
              the southwesternmost Pacific Ocean sea floor
                i. It should be noted that credible evidence of a geologically 
                   new split between the Indian section and the Australian 
                   section has been presented by scientists at Columbia 
                   University in 1995, which suggests that the Indo-Australian 
                   plate began a process of splitting into two distinct plates 
                   with slightly different relative motions about 8 million 
                   years ago (that would be geologically new)!
               ii. The notion is not universally accepted yet, but support is 
                   growing for differentiating the Australian major plate from 
                   the Indian minor plate, so this is a new idea that hasn't 
                   yet had time to percolate into the textbooks!
           f. Antarctican, including a wide swath of the southern ocean sea 
              floors
           g. Pacific, comprising most of the Pacific Ocean and not including 
              any continental-scale landmasses.
        2. There are also six minor plates:
           a. Nazca, a part of the southeastern Pacific Ocean immediately west 
              of South America
           b. Cocos, in the eastern Pacific just north of the Nazca Plate and 
              west of Central America
           c. Caribbean, most of the Caribbian Sea
           d. Arabian, including most of Saudi Arabia, Jordan, Syria, Iraq, 
              and part of Iran.
           e. Philippines, consisting of a diamond-shaped area in the western 
              Pacific northeast of the Philippines and south of Japan.
           f. Scotia, in the South Atlantic just north of Antarctica and east 
              of the southern tip of South America.
        3. There are also a number of smaller platelets, which are located 
           along the boundaries of larger plates in a sort of crush zone 
           between them. It turns out that some of these may not really be 
           small plates but the remnants of much larger plates swallowed uder 
           other plates.
           a. Rivera Platelet, off the southern tip of Baja California.
           b. Juan de Fuca, off the coast of northernmost California, Oregon, 
              Washington, and British Columbia in the northeastern Pacific 
                i. This platelet is particularly interesting, as it's quite 
                   possibly broken into three smaller platelets:
                   a. The Explorer Platelet on the north by BC
                   b. The Juan de Fuca Platelet proper in the center, west of 
                      Washington and Oregon.
                   c. The Gorda Platelet to the south, just west of 
                      northernmost California.
               ii. This group of associated platelets is now believed to be 
                   what's left of a major plate, the Farallon Plate, that is 
                   now buried under western North America and sticks out as 
                   the Juan de Fuca system AND the Cocos Minor Plate and the 
                   Rivera Platelet!
           c. Adriatic, in the Adriatic Sea east of Italy           
           d. Hellenic, in Greece
           e. Anatolian, much of Turkey 
           f. Iranian, most of Iran            
           g. Fijian, islands of Fiji, north of New Zealand and northeast of 
              Australia             
           h. Solomonic, west of the Fijian Platelet, northeast of Papua New 
              Guinea          
           i. Bismarckian, west of the Solomonic Platelet, north of New Guinea
        4. There are even dinkier plates, called terranes.  These are small 
           bits of lithosphere believed to have been shorn off plates as they 
           subduct (or get buried under other plates, about which more later), 
           which subsequently are accreted (or stuck) onto other plate edges, 
           sometimes in great number.  As a result, their pattern of rock 
           types and chemical composition does not resemble the geology of 
           surrounding countryside (those that apparetly originated far away 
           are called "suspect terranes," and those that seem to come from 
           nearby are called "native terranes"). There are many of them in the 
           American West, including many in Southern California in the 
           Transverse Ranges (e.g., the San Gabriels).
     D. These move around over the æsthenosphere.
        1. The motion is not straightforward in one direction, though, because 
           plates on a sphere don't move as a mass in a straight line.  
           Rather, they rotate in a kind of windshield wiper-like motion, 
           around a rotational axis that can be imagined as a line extending 
           from the center of the earth to its surface (the intersection of 
           the rotational axis with the surface could be thought of as the 
           rotational pole).  There is a small amount of absolute motion in a 
           tight small circle at the plate's rotational pole and larger 
           amounts of motion in wider circles farther away from the rotational 
           pole.  The theory describing this rotational kind of motion on a 
           spheroidal surface is called the "Euler Principle" (pronounced 
           "oiler").  Like I said, it creates a windshield wiper effect, the 
           upshot of which is it's hard to characterize an entire plate's 
           direction of drift.
        2. That said, here's an attempt to describe several plates' motion, 
           anyway:
           a. The North American plate is moving west-northwest (northwest off 
              the east coast and actually southeast by the time you look at 
              California!).
           b. The South American plate is moving generally to the northwest.
           c. The Nazca plate just offshore is moving east-northeast against 
              the Andes Mountains).
           d. The Cocos plate is moving northeast against Central America.
           e. The Pacific plate is moving northwest.
           f. The Eurasian plate is moving eastbound (to the northeast in 
              Europe and to the southeast in China).
           g. The African plate is moving northeast against the Mediterranean.
           h. The Indo-Australian plate is moving to the northeast, smacking 
              into Eurasia and the Pacific.  The Australian section (or 
              possibly separate plate) may be moving counterclockwise to the 
              Indian section, a little bit north of northeast).
           i. The Antarctican plate is moving eastbound, rotating around a 
              point to the east-southeast of Southern Africa.
           j. The Philippines plate is moving westbound, right into Eurasia.
           k. The Caribbean plate is moving northeast.
           l. The Juan de Fuca system of platelets is moving southeast against 
              the northwest coast of North America.

           [ relative plate motions determined by space geodesy, 
NASA ]

In the next lecture, I'll talk about how the RELATIVE motions of these plates 
create very different geomorphological circumstances.  For now, remember 
orders of relief; Wegener's argument and evidence he used; the importance of 
plate tectonics; the four size classifications of plates; the names, 
locations, and general motions of the major plates, minor plates, and the Juan 
de Fuca platelet system; how the Juan de Fuca platelet system and the Cocos 
plate may relate to the buried Farallon plate; and how the Euler Principle 
describes the motion of plates at the surface of the earth.

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Document and © maintained by Dr. Rodrigue
First placed on web: 11/23/00
Last revised: 07/02/07

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