Ini tulisan saya 12 tahun yang lalu, sudah dipublikasi dalam beberapa jurnal.
FROM GEOSYNCLINAL TO SUPERPLUME :
THE RISES AND FALLS OF TECTONIC THEORIES
The Beginning : Origin of Mountains
The greatest ranges of mountains encircling the globe such as the Alps-Himalayan, Appalachians, Urals, and the Rockies, have caused admiration and questions among the early geologists or earth scientists. Backed by the geological knowledge that had developed since the 18th century, they know that the mountains were formed by the crumpling of the Earth’s crust, but how ?
Begun by the speculations of the origin of the mountains, the great tectonic theories in geology were born. Now, we witness that the tectonic theories are so well developed that they not only can explain the origin of the mountains, but also the origin of other large-scale structural features on Earth : continents, ocean basins, oceanic ridges, oceanic trenches, island arcs, high plateaus, and others. Geologists now also use the tectonic theories to search for oil and gas and mineral deposits.
Let us now refresh our minds with the great tectonic theories in geology : geosynclinal theory, continental drift, undation theory, plate tectonics, and superplume tectonics. There are other tectonic theories, but they can be included in such a way into the mentioned theories.
The geosynclinal idea came from James Hall (1859), the great American geologist in his age, who noted that the folded and locally metamorphosed Paleozoic strata in the Appalachians are much thicker than correlative but less deformed strata beneath the Allegheny Plateau to the west. As both sequences bear fossil evidence of deposition in shallow waters, Hall concluded that the site of the folded mountain range, whose fold axes parallel its length, had been first an elongate belt in which subsidence and coordinate sedimentation had been more rapid than in adjoining tracts. This place of sedimentation was called as geosyncline.
The theory holds that elongate belts of deep subsidence and related thick sedimentation called geosynclines are the precursors of later mountain ranges in which the exceptionally thick geosynclinal strata are exposed by grand uplift following or accompanying thorough folding and metamorphism.
The Hall’s idea were corrected and developed by James Dana (1873), a great American geologist, who noted that the crumpling of the geosynclinal prism postdated the downwarping, not contemporaneously as initially noted by Hall. Dana distinguished three successive phases of geosynclinal cycle : sedimentation, tectogenesis, and orogenesis. Metamorphism and magmatism was incidental to the scheme and dependent on local conditions during downbuckling.
The geosyncinal theory were continually developed up to the mid of 20th century (Kay, 1951). The provinciality of American ideas based on the Appalachian region was challenged by European ideas based on the Alpine (Haug, 1900; Stille, 1924, Haarmann, 1930). A reconciliation between the two viewpoints resulted in terminologies of miogeosynclinal and eugeosynclinal sequences were entered into the single body of geosynclinal concepts. So, the geosynclinal theory lasted for more than 100 years since the time of Hall until the new global tectonic theories came into being in 1960’s. Before that, most syntheses of geologic history rely heavily upon the geosynclinal theory.
The fixistic view of geosynclinal gained a fierce challenge when Alfred Wegener published his theory in 1912 about the movement (mobilistic view) of continents on the globe. The movements of continents have caused collision that eventually formed mountains.
Continental Drift Theory
Soon after the first reliable world maps were made, scientists noted that the continents, particularly Africa and South America, would fit together like a jigzaw puzzle if they could be moved. One of the first men to give the idea serious study was a Frenchman named Snider-Pellegrini in 1858 who showed how the continents looked before separation. The concept was not considered seriously until 1908 when an American geologist Frank Taylor pointed out a number of geologic facts that could be explained by a continental drift.
The ideas were best explained by Alfred Wegener, an Austrian meteorologist but likes geology as a hobby. Wegener based his theory not only on the shape of continents but also on geologic evidence such as similarities in the fossils found in Brazil and Africa. He drew a series of maps showing three stages in the drift process and called the original large landmass Pangaea (meaning all lands). Wegener believed that the continents, composed of light granitic rock, somehow plowed through the denser basalts of the ocean floor, driven by forces related to the Earth’s centrifugal rotation and tidal attraction of the Sun and Moon.
Wegener thought that the Alpine-Himalayan chain was formed by collision of the Eurasia with Africa and India and that the Andes and Rockies were piled up by friction as the Americas were dragged through the viscous substratum. This idea seemed so bizarre to the fixistic geosynclinal theory during Wegener’s days. Most geologists and geophysicists rejected Wegener’s theories, although many scientific observation supporting continental drift were known in Wegener’s time. However, a few noted scholars, considered seriously the theory, especially Arthur Holmes of England who considered hypothetical mechanism of driving forces for continental drift in his textbook Principles of Physical Geology (1944), and a South African, Alex Du Toit, who compared the landforms and fossils of Africa and South America and further expounded the theory in his book Our Wandering Continents (1937).
The continental drift theory provided the base where the more global and integrated theory called as plate tectonics theory developed in 1960’s. Before that, we will see the advent of other tectonic theory called as undation theory, a theory that was initially inspired by fixistic model of geosynclinal theory, but actually also accommodated the mobilistic view of the continental drift theory and also this theory relates with the recent tectonic theory of superplume.
Inspired by geodynamic views of Stille (1924) and Haarmann (1930) in geosynclinal view, Reinout van Bemmelen, a Dutch geologist ever working for a long time in Indonesia, developed his theory called as undation theory (1931). This theory was mainly based on and supported by van Bemmelen’s works on the geology of Indonesia. It can be stated that there are two periods of the development of this theory. The first period was from 1930’s to 1950’s, when the fixistic view was more predominating, and the second period was in 1970’s when van Bemmelen modified his theory to make a reconciliation between fixistic and mobilistic views. The following is van Bemmelen’s modified undation theory.
The undation theory gives a synthesis of the terrestrial evolution, uniting geological, geophysical, and geochemical data into a comprehensive model. It accepts the major views of the new global tectonics (plate tectonics) and gives full attention to the geochemical evolution of our planet. Note that this is fully integrated in superplume tectonic theory developed in 1990’s. The undation theory started with the periodic convection circuits of masses in the solid earth which will cause differential vertical movements at the surface called as undations. The vertical movements of undations produce potential gravitative energy with fields of internal stresses. This leads eventually to lateral movements of masses, called gravity tectonics.
Based on the tectonic responses, van Bemmelen divided the undations into five classes. Mega-undations of global dimension result from ascending currents of lower mantle which produce upwarps of the outer spheres. Their ascent is volumetrically compensated by subsidence of adjacent geosynclinal zones. The related stress fields may result in continental drift and the sea-floor spreading. Geo-undations of a smaller scale result from upwellings of upper-mantle matter and give rise centres of diastrophisms. Compression of marginal trenches into nappes such as of the Pennine and the Alpine are responses of the geo-undation. Meso-undations are represented by the mountain and island-arcs which are pushed up from the foredeep-trenches. Minor-undations are connected with still more restricted diapirism of gneissic domes and batholiths. The resulting gravity tectonics may lead to intra-crustal mushrooming. Local-undations are caused by laccolithic pockets of magma and their gravity tectonic responses are such as mushrooming, outflows of mud, salt, lavas, and melanges or collapses of volcanoes.
The latest development of the undation theory is the insight that there is a causative relation between the major gravity anomalies of the globe and the global geodynamic processes. This leads to a more rheological interpretation of geodynamic processes. However, this theory has never been as widely exposed as the plate tectonics theory, so that not much people know about the undation theory.
Plate Tectonics Theory
“A revolution in earth sciences” has been advocated by most earth scientists when the plate tectonics theory developed during the 1960s and gained its integrated concept in early 1970’s. The plate tectonic theory is a comprehensive descriptive model for the kinematic pattern of current tectonic movements on the globes. The theory braids the concept of continental drift, sea-floor spreading, and oceanic transform faults.
Although continental drift was proposed 50 years earlier, the theory of plate tectonics was not developed until the early 1960s when marine geology surveys had provided enough data of the topography of the ocean floors and their magnetic and seismic characteristics. Many earth scientists were involved to the birth of this global tectonic theory. Their works can be grouped as works within the fields of ocean floor topographic mapping, geomagnetics, seismology, and general geology (mountain-building, magmatic evolution, heat flow, and others). Among leading scientists are : Bill Menard, Bob Dietz, Bruce Heezen, Maurice Ewing, and Mary Tharp (topography of ocean floor); Harry Hess and Xavier Le Pichon (sea-floor spreading); Tuzo Wilson (transform fault); Runcorn, Allan Cox and Brent Dalrymple (geomagnetic polarity); Fred Vine and Drummond Matthews (magnetic anomaly on oceanic ridges), Dan McKenzie and Jason Morgan (geometry of plate); Bryan Isacks, Jack Oliver, and Lynn Sykes (seismology); John Dewey and John Bird (mountain building); and James Gilluly (magmatic evolution). The classical papers leading to plate tectonic theories were compiled and edited by Allan Cox under the title of Plate Tectonics and Geomagnetic Reversals.
Plate tectonics theory offers a unified explanation for most features of the Earth. The Earth’s surface is segmented into intact, semirigid slabs or plates of lithosphere, move about with respect to one another by riding upon a less rigid undermass called the asthenosphere. Junctures between plates coincide with the world’s active seismic belts. The three types of junctures are : divergent-in which plates move away from one another, convergent-in which plates move toward one another, and simple shear (transform)-in which plates slide past one another. Major Earth features can be explained by plate tectonics. For example, mountain ranges take place at convergent boundaries of plates.
Plate tectonic theory has been so widely exposed, even to public people through educational films in TV. Today, plate tectonic theory is so developed and has been applied for the sake of both science and economy. Terrane concept, developed in late 1980s, is still in mobilistic view of plate tectonic theory. Three reference books on the geology of Indonesia (Warren Hamilton, 1979 : Tectonics of the Indonesian Region; Charles Hutchison, 1989 : Geological Evolution of South-east Asia, and Hall and Blundell (eds), 1996 : Tectonic Evolution of Southeast Asia) were written in plate tectonic view.
The last theory discussed in this writing is superplume tectonic theory which integrates all tectonic theories and is applied to planetary scale. This theory is still in its development stage.
Plume tectonics developed mainly by Japanese earth scientists since 1994 (mainly S. Maruyama, M. Kumazawa, S. Kawakami, and Y. Fukao) was attempted to reach an understanding of the principle of the dynamic Earth’s behavior, specifically the relationship between surface environmental change and the episodic-dynamic output of heat and materials from the Earth’s interior.
The circulation of matter in mantle (plume) has been known since the days of Arthur Holmes wrote his textbook in 1944. He proposed that the circulation was a driving force for continental drift. During the development of plate tectonic theory, Earth’s mantle plume was also considered to relate with a possible origin of the Hawaiian islands and hotspots (Wilson, 1963) and that the deep mantle convection was related with plate motions (Morgan, 1972). In addition to this, during the 1980s, material evolution in mantle were much discussed by A. E. Ringwood in Australia and Don Anderson in America based on experimental petrology. Based on these, and backed by new technologies on seismic tomography of Earth interior, ultra-high pressure experiments, computer simulation, and comparative planetary sciences, Maruyama et al. developed their theory of superplume tectonics or simply called plume tectonics since 1994.
Plume tectonics based its theory on the global-scale material circulation. Oceanic plates subducts into the deeper mantle from the trench, and becomes stagnant at the 660 km thermal boundary layer. The successive accumulation of cold material on the boundary layer eventually collapse to cause a sudden avalanche and downflow (downwelling coldplume) into the lower mantle. These mass undergoes dissolution of metalic iron and partial melting within the lower mantle. With time, this would be differentiated to form the structure which finally uprises to become a rising hot superplume. New mantle materials exit from the oceanic ridges or continental rifts are expression of mantle plumes. A hotspot is the surface manifestation of an upwelling mantle plume below lithospheric plate. The mass circulation in the mantle affects the structure of the Earth’s core through downgoing and upgoing material circulation in whirlpool movement. Hierarchy of activity of superplume has been established : normal period, pulse period, break-up period of supercontinent, and mantle overturn. Maruyama et al (1994) has mapped at a global-scale presences of upwelling or downwelling plumes.
Plume tectonics is still in development stages. However, major structural features such as sedimentary basin, continental plateau, trench, and ridges are now known related with underlying mantle plume. Plume tectonics has also been known to relate with the formation of diamond-bearing kimberlitic pipes.
We have seen the rises and falls, debates and reconciliations of major tectonic theories in the science of geology for the last 150 years. Geosynclinal theory has been ceased by most earth scientists, undation theory has not gained much responses due to lack of publication, plate tectonics has been accepted by most earth scientists today, and plume tectonics is actually beyond the ground of the debate since this theory is complementary to other theories. Man’s quest on his own planet is actually a never-ending story.