Shofi,
Kita pernah diskusikan plume tectonics di IAGI-net sekitar April tahun
2000. Saya melanjutkannya dengan menulis artikel di bawah ini (waktu itu
atas permintaan Pak Herman Darman) dan ini telah dimuat di Berita IAGI pada
tahun yang sama.
Barangkali ada gunanya, sebagai tambahan alamat website dari Pak Rovicky,
terutama untuk menyoroti perbedaan/persamaannya dengan teori2 tektonik
sebelumnya.
Salam,
awang
(maaf saya kirim ke milis, siapa tahu ada yang memerlukannya juga)
"From Geosynclinal to Superplume :
The Rises and Falls of Tectonic Theories"
By
Awang H. Satyana
(Exploration BPMIGAS)
A month ago, during the second week of April 2000, a technical discussion
on some aspects of tectonics enlivened the IAGI-net. Commenced by a
comparison between plume tectonics and undation theory, the discussion
widened to eventually covered the great tectonic theories ever born in the
science of geology. This brief writing summarizes the discussion and is
enriched by some reviews of the tectonic theories, their rises and falls,
and the last status we have today.
We will see that the rises and falls of the tectonic theories actually
show the battle between the two schools of thought : fixistic vs.
mobilistic. This debate-of-centuries has been started since the middle of
the 19th century, during the 20th century, and interestingly shows a
tranquility by the end of the 20th century. Let us begin with the reviews.
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.
Geosynclinal Theory
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.
Undation Theory
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.
Superplume Tectonics
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.
Epilogue
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.
Basic Readings
W.R. Dickinson (1971) : Plate Tectonics in Geologic History
A. Cox (1973) : Plate Tectonics and Geomagnetic Reversals
R.W. van Bemmelen (1977) : Undation Theory
W.K. Hamblin (1978) : The Earth's Dynamic Systems
S. Maruyama (1999) : Global-Scale Material Circulation in the Earth's
Interior
