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Geological process of glacier

已有 6746 次阅读 2008-2-4 15:40 |个人分类:地球科学

  

Geological process of glacier

Liao Yongyan

(Guangdong Ocean University, Zhanjiang, 524025, China)

ABSTRACT

  By comparing and analysing the data about glaciology, geology, geophysics, and paleo-climatology, it turns out that geological evolution results from glacial isostasy mainly. As the glaciers formation, the enormous volume of seawater move to the polar region and become ice cap, the sea level goes down. With the effect of gravity and glacial isostasy, these huge glaciers make powerful pressure to the earth that have got hydrostatic equilibrium before, then the glaciers and the lithosphere beneath the glaciers sink vertically. With the sinking of glacier and the lithosphere together, under the powerful pressure, the earth expansion, the earth would make a pressure vertical to the earth surface in an outward direction. With lasting influences of the powerful pressure, the lithosphere fault occurs and follows the formation of mid-oceanic ridge that leads to thalattogenesis. As the glaciers ablation, the ablation water from polar region move to the oceans, the sea level rises, and the polar region’s lithosphere rises from the previous falling state. The earth contraction occurs resulted from the gravity of the lithosphere, earth mantle and seawater. Once the earth contraction appears, the seafloor shrinks, the oceanic lithosphere is squeezed, and brings out the epeirogenesis. Since the first glacier appearance in the earth, glacial isostasy has been affecting geologic evolution. The present plate structure of the earth is the result of glacial isostasy influences in geologic history.

Key words:  Glacier, Geological process, Earth evolution, Geotectonics, Tectonics, Volcano, Earthquake.

INTRODUCTION

  With the further acquaintance about the earth, many theories have been put forward on geotectonics, as famous as the Geosyncline-platform theory, the Continental drift theory, the Sea floor spreading theory and the Plate tectonics theory (Morgan, 1968; Isachs et al., 1968; Mckenzie and Morgan, 1969). But all these theories don’t solve the problems of geodynamics (Smith and Lewis, 1999; Orvos, 1999). The Plate tectonics theory are supported by variety of scientific basis and measured data from palaeomagnetism, seismology and paleobiology, so, the plate tectonics theory was considered as the great achievement of geology in 20th century (Fu and Huang, 2001; Luhr, 2003). The plate tectonics theory made a moderate explanation on the geological problems of ocean and oceanic lithosphere from the two hundred million years ago to now, but many problems still suspend, such as the geological problems of ocean and oceanic lithosphere before the two hundred million years, the continental geological problems, especially the dynamics problems of plate movement (Smith and Lewis, 1999; Orvos, 1999; Fu and Huang, 2001; Stacey, 1992). To solve the problems, such as evolution process of continental lithosphere, the growth mechanism of the lithosphere, geodynamics of earth evolution, we made the research of the systematical comparison and analysis on the data of geology, paleobiology, geophysics, geochemistry and paleoclimatology.

THE THEORY OF “EQUILIBRIUM OF MANTLE BUOYANCY INTERFACE”

First, a small experiment is done with few pieces of wood block in a water basin. The wood block is in different shapes, dimensions and specific gravity (Fig.1). As we know from Archimedes’ principle, the weight of the wood is equal to the buoyancy affected by the block part soaking in the water. So, on the assumption that there is no external force, no matter how big the dimension of the wood block is (smaller than the water in the basin), no matter how much the specific gravity of the block is (less than the specific gravity of the water), no matter what the shape of the block is, and no matter how depth the block locate in the water of the basin, the wood block would floats on the surface of water by the buoyancy of water. We call the water surface the “Buoyancy interface”(Fig.1, e),

Fig.1 Wood block buoyancy experiment. A, the wood block of big specific gravity; B, the wood block of common specific gravity; C, the wood block of small specific gravity; D, wood block; E, the small wood block that put on the upper part of the wood block; F, the small wood block that put together with the immerged part of the wood block.

  Despite the shape and dimension of the wood block, the more specific gravity the wood block has, the more deeper the block immerge, and vice versa.

  Regardless of shape, size or specific gravity (less than the specific gravity of water), if a small wood block (immersed in water) is put together with the immerged part of wood block (Fig.1, f), the whole block would rise up relative to the “Buoyancy interface”, and regains balance at the position of this interface (Fig.1, g), because of the added buoyancy is more than the added weight. If a small wood block is put on the upper part of wood block (Fig.1, c), it would, only adding its weight rather than changing its buoyancy, go down relative to the “buoyancy interface”, and then regains the balance at this interface again (Fig.1, d). The phenomenon of changing the buoyancy and weights on the both sides of the “buoyancy interface” and then regaining the balance is called the “Equilibrium of buoyancy interface”(Fig.1).

The earth consists of core, mantle and crust (lithosphere) from the inner to the outer surface (Fig.2). There is hydrosphere outside of the lithosphere, and atmosphere is outside the hydrosphere. The mantle part is may regarded as fluid (especially the asthenosphere). Supposing the crust (lithosphere here) was made of many pieces of small block, every block of lithosphere would float on the mantle fluid that specific gravity is bigger than the lithosphere. No matter which height (the distance to the earth’s core) of the lithosphere block before is, since the gravity of the lithosphere block equals the buoyancy that the mantle fluid produces to the immerged part of the lithosphere block, the lithosphere block and the mantle fluid would at least maintain a hydrostatic equilibrium at an interface that is as the “Buoyancy interface” of the above wood block buoyancy experiment. So the equilibrium interface is called the “Mantle Buoyancy interface”(Fig.2, F). The “Mantle Buoyancy interface” is an imaginary interface, and in fact, cannot be seen because it is surrounded by the lithosphere.

 

Fig.2 The structure of the Earth’s interior. A, the lithosphere; B, the fluid mantle ( asthenosphere ); C, the part of higher mantle; D, the lower mantle; E, the Earth’s core; .F, Mantle Buoyancy interface.

 

    If without consider the effect of the earth is attracted by the moon and other celestial bodies, and the effect of the rotation of the earth, the “Mantle Buoyancy interface ” would be a spherical interface. If the rotation of the earth is considered, the “Mantle Buoyancy interface ”would be an ellipsoid that take the equator as major axis and the two poles as minor axis. So locating beneath the earth’s lithosphere, the “Mantle Buoyancy interface ” is similar to the Geoid and smaller than the “Geoid”.

  Provided there is no effect of outside force on the lithosphere and the mantle, the lithosphere and mantle should get along with the hydrostatic equilibrium with the effect of buoyancy and gravity. According to the above experiment, if a small matter of lithosphere (immersed in the Earth’s mantle) is put together with the immerged part of the lithosphere, this part of lithosphere(the small matter of lithosphere and immerged part of the lithosphere together) rises up relative to the “Mantle Buoyancy interface”, and then regains a new balance at this interface, and vice versa. The formation of this kind of balance after changing the buoyancy and weight on both sides of the “Mantle Buoyancy interface ” is called the “Equilibrium of Mantle Buoyancy interface ”.

  In accordance with this equilibrium principle, if additional “roots” is put together with the part of the lithosphere underneath the “Mantle Buoyancy interface ” somewhere, the lithosphere rise up compared with the interface. That is to say, the lithosphere would produce an appropriate “branches”. Conversely, if a certain part of the lithosphere produced definite weight of additional “branches”, the lithosphere should fall down compared with the interface. That is to say, the lithosphere would come out appropriate “roots”.

THALATTOGENESIS RESULTED FROM THE GLACIER FORMATION

Glacier-induced thalattogenesis resulted from transfer of huge masses (Kivioja, 1967). During the formation of the Antarctic ice cap (1.4×107km2 in area, about 2.64×1019 kg in weight, 2000m in average height with maximum height of 4267m) (Hambrey and Alean, 2004; Qin and Ren, 2001), it was only natural that the continental lithosphere beneath the glacier began to fall caused by glacial isostasy (Marquart, 1989; Davis et.al.,1999; Makinen and Saaranen, 1998; Davis and Mitrovica,1996; Boulton et.al., 1982; Clark et.al.,1994). With the glacier and lithosphere beneath the glacier together fell, the earth was expanded. Then, according to the hydromechanics principle, the breach of lithosphere in oceanic or land (in which the lithosphere is most easy breach) would occur.

What happened with the Antarctic ice cap is a good case in point for analysis of the thalattogenesis in glacier formation.

Before the formation of the ice cap, the lithosphere and mantle were in hydrostatic equilibrium (Fig.3, a). After the formation of Antarctic glacier, the seawater of 2.64×1019kg was shifted from the ocean onto Antarctica causing oceanic level of the earth descent. The mass on the oceanic lithosphere decreased and the mass on the Antarctica lithosphere increased. Since the transfer of mass from the oceanic lithosphere to the Antarctic lithosphere was so gradual and slow that the earth can show its particular plasticity (Luhr, 2003; Lambeck, et. al., 1998; Peltier and Jiang, 1996)(Fig.3, a-d). Ultimately, the lithosphere beneath the ice cap of the Antarctica was to sink substantially in accordance with the “Equilibrium of mantle buoyancy interface ” or the glacial isostasy (Marquart, 1989; Davis et.al.,1999; Makinen and Saaranen, 1998; Davis and Mitrovica,1996; Boulton et.al., 1982; Clark et.al.,1994)(Fig.3, c-f).

Fig.3 The thalattogenesis with the Antarctic glacier formation. A, the magma flows out of the mid-oceanic ridges; B, the Antarctic glacier; C, the old lithosphere; D, the fluid mantle; E, the new lithosphere formed from magma.

 

The earth is a closed fluid spheroid, and the earth’s crust (or lithosphere) is the container of this fluid (Fig.2). According to the Hydromechanics principle, external force cannot or can hardly cause the closed fluid to compress. And according to the Baske principle, the pressure on the fluid inside a closed container transfers without change through all parts of the fluid to the inner wall of the container (Zhao and Wu, 1981). So the sinking of the lithosphere in Antarctic caused by the enormous glacial mass would produce huge pressure with the asthenosphere, which, then, spread throughout the fluid mantle (asthenosphere) to any place of the earth’s asthenosphere, and to any direction at the right angle with the earth crust (Fig.3, b), and during the transmission the pressure remained unchanged and directed outward (Fig.3, b). Thus, the earth expanded under such huge pressure (Fig.3, c-f).

If the earth expansion, the surface area of the earth would increases. Since the flexibility of the earth crust is not very strong, when the earth expansion, the earth lithosphere would break (Fig.3, c; Fig.4, b). That is to say, the force created by the formation of the glacier would break a certain part of the earth lithosphere (Fig.3, c-e). For releasing the pressure that resulted from the earth expansion, magma flowed out of the earth crust via the break gap (Fig.3, c-e). The break gap usually is the mid-oceanic ridges. A lot of magma gradually flowed out of the mid-oceanic ridges, formed the oceanic floor (Fig.4, c-e), causing the oceanic floor to spread (Fig.4). The continuous magma flows out of the mid-oceanic ridges until the pressure releases completely. This is called thalattogenesis (Fig.4).

Fig.4 The mid-oceanic ridges formation, and causing the oceanic floor to spread. A, the outside of the Earth lithosphere; B, the inside of the Earth lithosphere; C, the direction of magma to flow; D, the direction of oceanic floor to spread; E, the old oceanic floor; F, the new oceanic floor formed early; G, the new oceanic floor formed later; H, the oceanic floor formed last.

 

In the oceanic area, causing by the pressure of fluid of mantle, the magma flowed out of the mid-oceanic ridges and formed the new oceanic lithosphere. Actually, the area under the land crust also has magma surging upward, but the magma cannot break though the thick continental lithosphere. It only intruded into the crack in the continental lithosphere, became the intrusive rock, which is mostly formed of neutral or acidic rock, mainly is high SiO2 rock such as granite. The formation of the glacier is a gradual process, so the eruption of magma from the mid-oceanic ridge is a gradual process, too. One eruption after another of basalt magma made the lithosphere of oceanic floor spreading (Fig.3, c-f; Fig.4, c-e).

With the Arctic being an ocean, the Antarctic ice cap has only monopolar effect on the earth, so the glacier squeezed the earth from one pole. While there are bipolar glaciers formations in the two poles of the earth, it would squeeze the earth from both poles.

Once the glacier formed, glacier flows and produces enormous force. So, when monopolar or bipolar glacier forms, and when glacial flowing force overcomes the tension of the lithosphere beneath glacier, lithosphere fracture would happen. Once there is a fracture in the continental plate, like the mid-oceanic ridges in the ocean, a new thalattogenesis would appear. The disintegration of the continental plates follows after the formation of the new oceans.

Under the effect of different tension of the different lithosphere, it is easier in some places of the oceanic lithosphere to produce the formation of the mid-oceanic ridges. Some lithosphere can form the mid-oceanic ridges only when the pressure is great enough. So it can be said that not all mid-oceanic ridges erupt magma and lead to thalattogenesis at the same time with the same intensity of the earth expansion. While some mid-oceanic ridges erupt magma early, some mid-oceanic ridges erupt magma later. While some is quick, some is slow. Since the spread in oceanic floor follows the formation of mid-oceanic ridges, the mid-oceanic ridge forms early, the oceanic floor spreads early, the mid-oceanic ridge forms later, the oceanic floor spreads later, the mid-oceanic ridges do not forms, the oceanic floor do not spreads, and the stronger the mid-oceanic ridge erupts, the faster the oceanic floor spreads. The formation of the mid-oceanic ridges between the newly disintegrated plates follows the same pattern.

 As a result of spread of the oceanic floor, the sizes and distribution of ocean in the earth would be changed. The old ocean may enlarge or diminish, and some new oceans may appear in some places, too, where there is originally none.

EPEIROGENESIS RESULTED FROM THE GLACIER ABLATION

Epeirogenesis caused by glaciers ablation is to be analyzed with the present types of ablated Arctic pole glaciers.

When the ice cap in the north polar stopped formation, and began disappearance, large quantities of water poured into the ocean, caused oceanic level to rise. The polar lithosphere stopped sinking and began to rebound, so the earth’s inner pressure decreased. With the effect of the oceanic level rise and the earth’s inner pressure decrease together, the lithosphere contraction occurred. The lithosphere contraction brought about the epeirogenesis (Fig.5).

Fig.5 The epeirogenesis caused by glaciers ablation. A, the Arctic glacier; B, old continental lithosphere; C, the new oceanic lithosphere formed from magma; D, the old oceanic lithosphere; E, the most new continental lithosphere; F, the new continental lithosphere.

Since the continental lithosphere is much thicker and more rigid than oceanic lithosphere (Luhr, 2003), it is far more likely for oceanic lithosphere to contract than continental lithosphere. So, epeirogenesis mainly happens in oceanic lithosphere (Fig.5, b-c).

Squeezed by the two sides, the mid-oceanic ridges close up. If they have the same specific gravity and acting force, the oceanic lithosphere on both sides of the ridge would rise together. If the two sides have unequal density or produce unequal forces, the heavier side should thrust under the lighter one, the epeirogenesis occurs according to the “equilibrium of mantle buoyancy interface ”.

Squeezed by lithosphere on both sides, once the width of the oceanic lithosphere is too much for the rigidity of oceanic lithosphere to support, it should deform, either rising to form anticline (sea mount) or sinking to form syncline (sea basin)(Fig.6, b). Because its relatively bigger density of oceanic lithosphere than continental lithosphere, and the increasing amount of seawater from glacier ablation, and the reducing inner pressure of the earth, the area of oceanic lithosphere in sinking should be much larger than in rising, that is to say, areas of sea basin is much large than that of sea mounds (Fig.6, b).

Fig.6 Geosyncline formation and epeirogenesis. A, the oceanic lithosphere; B, the sediment formed early; C, the negative pressure antrum under the geoanticline; D, the sediment formed later; E, the pile of volcano; F, the asthenosphere; G, the mantle; H, the basalt; “”, volcano eruption.

 

Once a sea basin has formed, sediment would appear in it (Fig.6, b). If the two side of the basin were continental lithosphere, since the weathering of continental lithosphere is very stronger than the oceanic lithosphere, the continental lithospheres can produce a lot of sediment to the basin, to form thousands meter to hundred of thousands meter of sediment. With glacier continually ablation, the lithosphere shrinks greatly, and the rim of the sea basin is seriously squeezed from two sides by the force of the earth contraction. Since the rim of the sea basin is squeezed, with the effect of rigidity of lithosphere, the bottom of the sea basin would continually sink (Fig.6, b-c). The thousands meter of sediment that have deposited in the sea basin intensifies the sea basin further sinking. The sunken sea basin would deposits much more sediment in it again. Continual sinking makes the sea basin deeper and its mouth narrower and narrower. When the sea basin sinking to a certain depth, the sea basin (syncline) becomes geosyncline (Fig.6, c). Since the continental lithosphere can produce much more sediment to basin, when the two side of the basin were continental lithosphere, the sea basin (syncline) can easily to become geosyncline. So, the geosyncline always forms between the two continental lithospheres. If the geosyncline forms between the two continental mountains, the sediment is very thick, and the geosyncline sinking is very deep, for example the Himalayan geosyncline.

Further sinking causes the bottom to bend more, finally it breaks when the bending becomes too much for the lithosphere to hold itself (Fig.6, d). With the followed formation of volcanoes, large amount of basalt magma erupts owing to the effect of negative pressure (Fig.6, e).

Based on the “equilibrium of mantle buoyancy interface ”, with the sinkage of geosyncline, the geoanticline would rise. Via the rise of geoanticline, the geosyncline and the geoanticline attain the equilibrium between the buoyancy and gravity (Fig.6, b-c).

With the geoanticline further rising and the geosyncline further sinking, the bending of the lithosphere between geosyncline and geoanticline is aggravated. When the oceanic lithosphere, connecting the geosyncline and the geoanticline together, cannot bear this kind of bending, the bending lithosphere breaks. The earthquake occurs and the geosyncline and geoanticline separate (Fig.6, g).

Thus, Geosyncline separated and was freed from geoanticline. Without the drag of geoanticline, according to the “equilibrium of mantle buoyancy interface ” and isostasy, the geosyncline would uplift by the buoyancy resulting from the asthenosphere (Fig.6, g-h). Without the supporting of geosyncline, the geoanticline would falls, and then impacts the magma below it and lead to volcanic eruption (mainly of neutral or acidic magma eruption) (Fig.6, g-h). At the same time, geoanticline lithosphere (only consist of oceanic lithosphere), which is heavier than the geosyncline lithosphere (consist of oceanic lithosphere and a lot of light sediment), can thrust under the lighter geosyncline lithosphere. So the geosyncline would rise further to form a central swell, ending in a new geological structure formation.

When the new geological structure of lithosphere reaches to a certain thickness, according to the “equilibrium of mantle buoyancy interface ” and the isostasy, it should rise above the sea level to form a part of continent or a new land (Fig.5, b and c). This is called epeirogenesis. The process is as follows:

 

        

Thus, since the partial ocean plates turning into the continent plates, the ocean plates reduces and continent plates enlarge (Fig.5).

Sea basins or geosynclines can also form between the continental lithosphere and the oceanic lithosphere, or between the thick lithosphere and the thin lithosphere, or between the weighty lithosphere and the light lithosphere. Under this condition, squeezed by the continental lithosphere (or the thick or the light one) and the oceanic lithosphere (or the thin or the weighty one) on both sides, the geosyncline forms (Fig.7, a-c). Since the specific gravity of continental lithosphere is small than the oceanic lithosphere, the two sides have unequal density and produce unequal forces. The point of action force at oceanic lithosphere side is low, and the point of action force at continental lithosphere side is high. So, when the geosyncline sinks to proper depth, the heavier oceanic lithosphere should thrusts under the lighter continental lithosphere (Fig.7, d). Thus, the geosyncline bottom leans to one side then folds under the continental lithosphere (Fig.7, e).  According to the “equilibrium of mantle buoyancy interface ” and the isostasy, the folded lithospheres would uplift (Fig.7, f). When the thickened lithosphere, the folded three layers lithospheres (one layer oceanic lithosphere and two layers continental lithosphere), uplifts, it drags the single layer oceanic lithosphere uplift. When the drag force overcomes the tension of the single layer lithosphere, lithosphere fracture would happen (Fig.7, g). Then, the earthquake and volcanic eruption occur (Fig.7, g). The volcanic eruption forms the volcanic pile (Fig.7, h). The volcanic pile and the uplift lithosphere together form the island arc. Before and behind the island arc, the trench and basin form (Fig.7, h).

 

Fig. 7 The formation of island arc and trench between continent and ocean. A, oceanic lithosphere; B, continental lithosphere; C, fluid mantle; D, trench; E, negative pressure antrum; F, the pile of volcano; “”, volcanic eruption.

 

THE FORMATION OF PALEOCRUST AND EVOLUTION OF THE CONTINENTS

The earth formed 4.5-4.6 billion years ago (Wood, 1968; Hanks and Anderson, 1969; Ringwood, 1960). At that time, the kinetic energy of impact resulted from the earth formation, and the radioactive energy made the temperature of the earth’s surface rising (Ringwood, 1979; Anders, 1968; Hanks and Anderson, 1969; Taylor, 1993; Lyons and Vasavada, 1999). When the temperature of the earth’s surface reached the melting point of high SiO2 rock (such as granite), the granite melted, the magma forms.

Before the granite lithosphere form, the earth was so hot that it was only covered with magma. With H2O in the form of vapor, there was no ocean. 3.8-4.0 billion years ago, the temperature began to drop because of continuous heat radiation (Nutman et. al., 2001). When the surface temperature of magma layer went down below the solidifying point of the magma, the most primitive earth lithosphere (paleocrust) began to form (McClendon, 1999; Nutman et.al., 2001).

Though the paleocrust has formed on the surface of magma layer of earth surface, inside the earth, with the more weighty matter sank and the lighter matter rose in the magma, a lot of heat energy occurred resulted from the geopotential (Ringwood, 1960; Tolstikhin and Hofmann, 2005; Anders, 1968). Under the effects of the heat energy resulted from the radioactive energy and geopotential together, the earth’s interior melted gradually, then the core of iron and nickel, the mantle formed.

Before the formation of the paleocrust, the magma layer of the earth was in liquid form and the materials vary in their density from the interior to the surface: the closer to the surface, the lighter, and the closer to the interior, the more weighty. So the paleocrust first to solidify was the relatively light, high SiO2 rock that close to the granite, the followed downwards to solidify was relatively heavier, low SiO2 rock that close to basalt. The paleocrust was mainly made of high SiO2 rock (such as granite), so, the paleocrust was also called the granite lithosphere. Because the granite is lighter than basalt and other rocks, the older a continent plate is, the lighter it is.

The paleocrust, as a poor conductor of heat, prevents the heat from getting out of the earth. So, once the paleocrust formed, the surface of the earth kept cooling down because of the heat radialization. , 3.8 billion years ago, when the surface temperature of the earth dropped below condensation point of water, large quantities of vapor condense to water, the initial ocean appeared (Nutman et. al., 2001).

There was no thalattogesis or epeirogenesis on the earth until the first glacier appeared. Before the glacier appears, the earth was almost completely covered with water. The level surface, unlike the earth we have now, irregular and uneven, was generally even with only small swells, like the craters of the moon, caused by the impact of other celestial bodies against the earth Fig.8, a.

Then the first polar ice cap appeared in Precambrian (Kaufman et al., 1997; Donnadieu, 2004 Hambrey and Alean, 2004).

During the formation of the glaciers, some places of the paleocrust had torn open, creating mid-oceanic ridges, from which evolved the secondary oceanic lithosphere (called oceanic lithosphere for short). The secondary ocean, short for the ‘ocean’, covered the oceanic lithosphere (the ocean which covered the paleocrust, called ‘paleo-ocean’ for short). Oceanic lithosphere was only a little weightier than paleocrust, so, there was not much difference between oceanic lithosphere and paleocrust in their thickness. So, the secondary ocean and paleo-ocean have the similar depth. And the paleo-ocean was connected together with the secondary ocean.

Since their had a little difference between their thickness, both the paleocrust and the oceanic lithosphere had the same way to form the continental plates under the influence of epeirogenesis, when glaciers ablated causing the earth to contract (Fig.8, a-b).

Fig.8 The formation of continental nucleus and the evolution of the continents. A, the paleocrust; B, the secondary continental lithosphere; C, the continental nucleus; D, new continental lithosphere; E, the most new continental lithosphere; F, the oceanic lithosphere.

 

Both the paleocrust and the oceanic lithosphere were thin, so the sea basin (syncline) formed from them weren’t too large under the influence of epeirogenesis of glaciation (Fig.9). The sea basins (syncline) and seamounts (anticline) were numerous and small in size and are distributed in alternation with each other (Fig.9, c). When further ablation of the glaciers caused the earth to further contract, syncline evolved into geosyncline (Fig.9, d) which became central swells finally (Fig.9, e). The process continued until swells of various sizes and geoanticlines pieced together to become continental nucleus(Fig.8, b; Fig.9, e-f). These continental nucleus developed either from paleocrust or oceanic lithosphere or both (Fig8, b-e), and they were distributed in paleo-ocean or secondary ocean. The earliest continental nucleus were numerous and small in size (Fig.8; Fig.9).

 

Fig.9 The formation process of continental nucleus. A, the syncline (sea basin); B, the anticline; C, oceanic lithosphere; D, continental nucleus.

 

In the Precambrian, there were more than three major formations and ablations of glaciers (Kaufman et al., 1997; Donnadieu et al., 2004; Zhang et al., 2002). Repeated glaciation caused the continental nucleus to become larger and larger until they evolved into paleo-platforms such as Siberia Platform, Canadian Platform and African Platform (Luhr, 2003) (Fig.8, b-e). These platforms were mainly made of granite with high SiO2, with small amount of deuterogene such as basalt with low SiO2.

In the following post-Ordovician, Carboniferous and Quarternary ice age, these platforms underwent more piecing together and breaking up, with new continental lithosphere constantly joining in, and finally evolved into what is now called geo-plates.

These paleo-platforms were not thick enough for deep and large geosynclines to form on, hence no high mountains developed. But when the plateforms on both sides of the sea basin became thick enough and deep and large geosynclines formed, high mountains like those of today appeared.

In the period of formation of glacier, the formation of glaciers was a gradual process. That is to say, in the process of formation of glacier, there was temporary secondary ablation of glacier. The glaciation of the glacial formation mainly made the oceanic lithosphere spreading at mid-oceanic ridges, but meantime, there were occasional epeirogenesis like the formation of sea basins and the maturing of geosynclines.

Similarly, in the period of the ablation of glacier, the ablation of glaciers was slow, too. That is to say, in the process of ablation of glacier, there was temporary secondary formation of glaciers. So the ablation of glacier mainly made the formation of sea basins and the development of geosyncline, but meantime, there were occasional thalattogenesis (mid-oceanic ridges form the oceanic lithosphere) as continuous replenishment of material to epeirogenesis.

It is true that the platform, or continental plates, were generally very stable. But, when the both sides of the continental platforms or continental plates uplift resulted from the epeirogenesis, relatively, the middle part would sink to form graben. If this happens in the course of the formation of glaciers, or in the course of the formation of the secondary glaciers during the ablation of glacier, it may produce the formation of new mid-oceanic ridges. In these instances, a continental plate may break into two or more new plates.

If mid-oceanic ridges were mostly east-west direction (such as the mid-oceanic ridge of Alps formation period), it would mainly make the oceanic lithosphere spreading at north-south direction. The plates would drift at north-south direction. Finally, via the epeirogenesis, the mountains would form at east-west direction. If mid-oceanic ridges were mainly north-south direction (such as the mid-oceanic ridge of Andes and Rockies formation period), it would mainly make the oceanic lithosphere spreading at east-west direction. The plates would drift at east-west direction. Finally, via the epeirogenesis, the mountains would form at north-south direction. When the plates drifted at north-south direction, during the process of drift, the plate’s latitudes changed, the geomagnetic pole shifted. The existing earth plates are the results of geological changes including the continuous expansion and contraction of the earth and separation, combination of the plates. Therefore, finding out about the changing pattern of geomagnetic pole may provide information about the drift, separation, combination of various plates. And this information, in turn, may help us determine the distribution and size of the plates in all historical ages.

THE EVIDENCE OF THE ISOSTASY AND THE LITHOSPHERE HAS PLASTICITY

The Arctic glacier has existed in the area of the Scandinavia and the Hudson Bay of Canada by the time of Quaternary period. After the ablation of the glacier, since 15000 years ago, the area of the Hudson Bay has uplifted 300m, and this area still holds a rate of 2cm/year to rise. After calculated, if it wants to return the altitude before the glacier formation and rebuilt the estate of isostasy, this area would still needs to raise 80m (Tao, 1999; Stacey, 1992). Beginning from 10 thousand years ago, after the ablation of the glacier, the area of Scandinavia had uplifted 250m, and now still is rising as a rate of 1cm/year (Tao, 1999; Stacey, 1992). Other reports show: postglacial doming of Fennoscandia is estimated to have reached a maximum uplift value of 850 m (Gudmundsson, 1999). This shows that the earth, the fluid spheroid, which be closed by the solid lithosphere has proper plasticity certainly. When the formation of the glacier, both the glacier and the lithosphere beneath the glacier together sink as a result of the enormous glacial isostasy (Marquart, 1989; Davis et.al.,1999; Makinen and Saaranen, 1998; Davis and Mitrovica,1996; Boulton et.al., 1982; Clark et.al.,1994). In the ablation period of glacier, with the enormous glacial weight gradually vanishes, the lithosphere rebounds upPeltier and Jiang,1996; Lambert et. al.,2001; Lambeck et.al., 1998; Gudmundsson,1999.

 

REFERENCES

Anders E., 1968,Chemical processes in the early solar system, as inferred from meteorites. Accounts of Chemical Research, v. 1, p. 289-298.

Boulton G. S., Baldwin C. T., Peacock J. D., McCabe A. M., Miller G., Jarvis J., Horsefield B., Worsley P., Eyles N., Chroston P. N., Day T. E., Gibbard P., Hare P. E., von Brunn V., 1982, A glacio-isostatic facies model and amino acid stratigraphy for late Quaternary events in Spitsbergen and the Arctic. Nature, v. 298, p. 437-441.

Clark J. A., Hendriks M., Timmermans T. J., Struck C., Hilverda K. J., 1994, Glacial isostatic deformation of the Great Lakes region; with Suppl. Data 9409. Geological Society of America Bulletin, v. 106, p. 19-31.

Davis J. L., Mitrovica J. X., 1996, Glacial isostatic adjustment and the anomalous tide gauge record of eastern North America. Nature, v. 379, p. 331-333.

Davis J. L., Mitrovica J. X., Scherneck H. G., Fan H., 1999, Investigations of Fennoscandian glacial isostatic adjustment using modern sea level records. Journal of Geophysical Research, B, Solid Earth and Planets, v. 104, p. 2733-2747.

Donnadieu Y., Goddéris Y., Ramstein G., Nédélec A. and  Meert J., 2004, A 'snowball Earth' climate triggered by continental break-up through changes in runoff: Nature, v. 428, p. 303-306.

Fu R. S. and Huang J. H., 2001, Geodynamics: Beijing, Higher Education Press, p. 1-320.

Gudmundsson A., 1999, Postglacial crustal doming, stresses and fracture formation with application to Norway, Tectonophysics, v. 307, p. 407-419.

Hambrey M., Alean J. 2004, Glaciers: Cambridge, Cambridge University Press, ed., 2, p. 1-376,

Hanks T. C., Anderson D. L., 1969, The early thermal history of the Earth. Physics of the Earth and Planetary Interiors, v. 2, p. 19-29.

Isachs B., Pliver J. and Sykes L. R., 1968, Seismology and new global tectonics: J Geophys Res., v. 73, 5855 p.

Kaufman A. L., Knoll A. H. and Narbonne G. M., 1997, Isotopes, ice ages, and terminal Proterrozoic earth history: Proc. Natl. Acad. Sci., v. 95, p. 6600-6605.

Kivioja L. A. 1967, Effects of mass transfers between land-supported ice caps and oceans on the shape of the earth and on the observed mean sea level. Bulletin Geodesique, v. 85, p. 28-288.

Lambeck K., Smither C., Johnston P., 1998, Sea-level change, glacial rebound and mantle viscosity for northern Europe. Geophysical Journal International, v. 134, p. 102-144.

Lambert A., Courtier N., Sasagawa G. S., Klopping F., Winester D., James T S., Liard J. O., 2001, New constraints on Laurentide postglacial rebound from absolute gravity measurements. Geophysical Research Letters, v. 28, p. 2109-2112.

Luhr J. F., 2003, Earth: New York, Dorling Kindersley Publishing, p. 1-520.

Lyons J. R., Vasavada A. R., 1999, Flash heating on the early Earth. Origins of Life and Evolution of the Biosphere, v. 29, p. 123-138.

Marquart G., 1989, Isostatic topography and crustal depth corrections for the Fennoscandian geoid. Tectonophysics, v. 169, p. 67-77.

Makinen J., Saaranen V., 1998, Determination of post-glacial land uplift from the three precise levellings in Finland. Journal of Geodesy, v. 72, p. 516-529.

McClendon J. H., 1999, The origin of life. Earth-Science Reviews, v. 47, p. 71-93.

Mckenzie D. P. and Morgan W. J., 1969, Evolution of triple junctions: Nature, v. 224, 125 p.

Morgan J. M., 1968, Rise trenches, great faults and crustal blocks: J Geophys Res., v. 73, p. 1959-1982.

Nutman A. P., Friend C. R. L., Bennett V. C., 2001, Review of the oldest (4400-3600 Ma) geological and mineralogical record; glimpses of the beginning. Episodes, v. 24, p. 93-101.

Orvos P., 1999, Origin of continents on planetary bodies. Acta Geologica Universitatis Comemianae, v. 54, p. 85-91.

Peltier W. R., Jiang X. H., 1996, Mantle viscosity from simultaneous inversion of multiple data sets pertaining to postglacial rebound. Geophysical Research Letters, v. 23, p. 503-506.

Qin D. H. and Ren J. W., 2001, Antarctica Glaciology: Beijing, Science Publishing House, p. 1-220.

Ringwood, A. E., 1979, Composition and origin of the earth, in McElhinny, M. W., ed., The earth: Its Origin, Structure, and Evolution: London, Academic Press, p. 1-58.

Ringwood A. E., 1960, Some aspects of the thermal evolution of the earth. Geochimica et Cosmochimica Acta, v. 20, p. 241-259.

Smith A. D., Lewis C., 1999, The evolution of scientific thinking in the mantle plume concept; criticism and non-plume synthesis. Journal of the Geological Society of China, v. 42, p. 1-40.

Stacey F. D., 1992, Physics of the Earth: Brisbane, Brookfiel Press, ed., 3.

Tao S. L., 1999, Earth science conspectus: Beijing, Geology press, p.12-22.

Taylor S. R., 1993, Early accretional history of the Earth and the Moon-forming event.In: Campbell I. H., Maruyama S., McCulloch M. T. ed. “The evolving Earth”. Lithos, v. 30, p. 207-221.

Tolstikhin I., Hofmann A. W., 2005, Early crust on top of the Earth's core. Physics of the Earth and Planetary Interiors, v. 148, p. 109-130.

Wood J. A., 1968, Meteorites and the origin of planets: New York, McGraw-Hill Book Co., p. 1-117

Zhao J. Y. and Wu S. X., 1981, Mechanics: Beijing, People’s Education Publishing House, p. 342-428.

 

______This paper comes from the book, Geoscience principle (Liao, Y. Y. Geoscience principle. Beijing: China Ocean press 2007, 1-245), wrote by yongyan liao. The e-mail of the writer of this book is: rock6783@126.com or liaoyy@gdou.edu.cn

 

 

 

 

 

 

 

 

 





 





 

















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