Introduction
There are a number of scientific works done on super continental cycles and have mainly focused on the amalgamation, break-up and the drifting away of supercontinents. Researches on evolution in the other hand focus mainly on the gradual change of the earth’s continents and the organisms (both flora and fauna) over successive generations (Beck 1995, p.799). Super continental cycle and evolution have been addressed separately for many years. However, it has been established that during any two super-continental cycles, tectonic plates join and break-up causing global warming and ice-age periods. During such processes there are many changes that occur as a result of post-global warming (Volonteri 2010, p.1). In general terms whenever tectonic plates are apart, live thrives and evolution takes place when plates combine to form a single crust. It can therefore be observed that super continental cycles happen as a result of motion of the tectonic plates above which the earth’s continents rest. For that reason, it is believed that the beginning of tectonic motions marked the beginning of the first super continental cycle hence and therefore one cannot be studied without the other.
From time immemorial, many theories have come up in relation to continental movement on the Earth’s surface. Since the development of the theory of plate tectonics, more and more studies have been done regarding the effects and relationship of plate tectonics to the sequential super continental cycles. The gradual developments of the globe is also said to have been attributed to the super continental cycles. These cycles have substantially left great effects some of which are beneficial to ecosystem like the diversification of species and other destructive ones like the ice-age which causes extinction of species (Thompson, 2003 p 355). This paper focuses mainly on super continental cycles and how they happen in relation to plate tectonic theory and the relationship of these cycles to the evolution of the earth.
Super Continental cycles and plate tectonics
A Supercontinent can be defined as a mass of land from which continents form and it is believed that one of the largest continental masses broke into several parts that drifted apart to form the present six continents. The land masses included Pangea, and its successors Gondwanaland and Laurasia. A Super continental cycle is a progressive geological sequence where the continents of the earth alternatively amalgamate into one, split into several continents, and then assemble successively together (Zhao et al, 2004 p 101). Early studies dated the first super continental cycle about 2800 million years ago.
Scientists believe that a complete continent cycle lasts 300-800 million years. It results from random movement of plate tectonics upon which a third of the earth’s continent rests. These tectonic plates continuously disperse and collide as part of the Earth history. It therefore makes it hard to take in the plate tectonics without the super continental cycle.
The earth is made of three major layers namely Crust, Mantle and the Core as shown in figure 3 below. Supercontinents do not permit heat flow from the interior part of the earth. This then leads to overheating which in turn causes deformation of the upper part of the Earth’s mantle which is a weak zone. The deformation causes the Crust to vault upwards eventually making it crack, allowing magma to rise. The continental drifts later leads to joining of fragments that had been forced to move away from the crust and the cycle is repeated (Storey, 1999 p 621).
Supercontinents do not permit heat flow from the interior part of the earth. This then leads to overheating which in turn causes deformation of the upper part of the Earth’s mantle. This upper part is a weak zone. The deformation causes the Crust to vault upwards eventually making it crack, allowing magma to rise. The continental drifts later leads to joining of fragments that had been forced to move away from the crust and the cycle is repeated (Stephane, 2001 p 121).
There are features that subsequently recur after 425 million years. Some of the notable features are the deposits of global ice, impacts of mountain ranges and changes in sea levels. They are foundations that clearly illustrate the realization of Supercontinents. Though it is very hard to tell the precise time of the former super continental movements, its duration is continually stable. The recurring nature and the continually stable duration of the super continental movements play a great role in explaining the Plate tectonics concepts. The old and heavy oceanic plate submerges causing movement of the plate tectonics and a rise in oceanic basin due to disintegration of super continental lithosphere.
Eventually, both the Continental plate and the oceanic plate get older resulting to an increase in the lithosphere’s density. It takes approximately 200 million years after which tensions in the Earth’s crust which can produce earthquakes or volcanic eruptions are felt. All these are caused by the oceanic plate which starts to slide under adjoining tectonic plate into the mantle and congregate the margins of the continent which are divided as a supercontinent (Barbara and David, 2002 p. 107)
The plate tectonic theory can be viewed in a different manner. The theory describes the plate on the basis of it being a rigid body. The interior folded mountains establishment after the collision of continents relaxes the adjoining tectonic plate from sliding under. This gradual plate movement discharges magma at the crest which in turn chills the ocean. A single plate is thus formed when the discharged magma from the crests fills the earth’s surface. When the earth is fully covered and no more heat from the core is able to make its way to the earth’s surface, a global ice age is experienced and at times it can lead to a snowball earth (Brasier, 1980 p. 699).
With time, the supercontinent disintegrates arising to new plate. This then results to magma being discharged once again and hastily melting the existing ice sheets and this leads to a harsh global warming. (Beck R.A.et al. 1995 p.290)
The relationship between the Super Continental cycles and Evolution
Subsequent breaking and fusion of the earth’s continents in the super continental cycle that is believed to take place every 300 to 500 million years, contribute immensely to evolution on the earth. When continents collide, land is squeezed together and the sea level falls and when they separate or break apart, a new sea floor is created hence a rise in the sea level. Worldwide climatic conditions are highly influenced by these cycles. The development of mountain ranges worldwide and the stable rotation of the earth on its axis have all come about due to the development of supercontinents and the effects it has on the mantle of the earth. Volcanic activity is in most cases correlated with plate tectonic motions.
Plate tectonic movement has a great impact on how organisms are distributed in different continents and climatic zones of the earth. The distribution is checked mainly by climatic and physical obstacles. Plants and animals in the same region which have similar climatic conditions are in most cases similar in characteristics and survival requirements. It is therefore believed that regions are divided by natural climatic barriers which are as a result of tectonic movement (Monroe and Wicander, 2009 p.2).
The most remarkable climatic effects or changes that happen during the super continental cycle are global warming and Ice age periods. When continents collide and stop moving, they merge into one large mass called a super continent and during this period, the earth’s crust becomes a single crust triggering a global ice age. This brings about a time of no diversification of species as well as extinction for both plants and animals due to the extreme harsh conditions. Consequently, when the continents are apart, climatic conditions are favorable in most continents and life thrives as well as evolution of new species. Between one super continental cycle and another, global warming also may occur and cause extinction of species as well (Navarro, 2002 p. 162)
Continents distribution and topography influence wind and ocean water currents in a big way. In turn, wind and currents impact on global climate which has a strong influence on how plants and animals are distributed in different regions of the continents (Monroe and Wicander, 2009 p. 1). When barriers like an ocean separates once similar set of plants and animals, species change. If conditions on the other side are remarkably different, then species have to adapt to the new environment. Adaptations in most cases may be so intense to the extent of evolving a totally new and different species.
Other than the fauna and the Flora kingdom, the super continental cycle has had a great impact on the galaxy as far as evolution is concerned. This considers the processes and the relative transformation of cosmos from a homogenous to a heterogeneous state. It also involves aspects of how galaxies change through time and the various processes that have led to the generation of the diverse structures that are seen today in the galaxies. Initially, the geologists said that the universe was uniform as seen in the cosmic microwave background and there was no significant structure hence no galaxies (Navarro 2002, p.155). However the theory of Einstein-de-sitter and Friedmann on galaxy formation ascertains that these structures developed as an effect of augmentation of primordial oscillations. These are the tiny changes in the cosmos mass in a restricted area. The process of galaxy formation began with formation of dark matter plants colonizing a salty area which are ascertained to be more plentiful than any other matter hence dominating the development of the total density oscillations as the baryons fell into the prospective wells of dark matter (Thompson 2003, p.354). The salty area is believed to be the oceanic waters that form during the super continental cycles. As the universe chilled, dark matter began condensing together with the gas in them. The result of these, supposedly led to accumulation of dark matters and gas into denser areas where they gravitated to form structures making the first galaxies. During this epoch, the universe was mainly composed of hydrogen, helium and the dark matter but after the formation of the first proto galaxies, these gases condensed to form stars. With time most massive stars ceased to be because they blazed away more quickly than the tiny ones leading to the formation of black holes. The black holes further collected in the middle of the proto galaxy and integrated to form super massive black holes that were seen as quasars once they accreted gas (Navarro, 2002 p. 163).
Two types of galaxies arose after the formation and evolution process. They are namely, the blue and the star galaxies which are more of the spiral form and the red non-star forming ones which are the elliptical galaxies. The elliptical galaxies contained very huge black holes whose function was to prevent any other hot gas from cooling onto the galaxy. Then was formation of the elliptical galaxy which mostly occurred in huge dark matter halos, which corresponded to the available group of galaxies. Therefore, no more stars could form but the galaxy kept on growing by coalescing with other big galaxies which happened to fall in the cluster (Thompson 2003, p.353).
A number of galaxies have black-holes at the centre that have differences in their masses ranging from a few millions to billions of solar masses. These black holes can emit large amounts of energy during their growth process that in most cases power galactic nuclei and quasars. A small amount of this energy could easily stop the formation of stars by heating and casting out the ambient gas if it is absorbed by the host galaxy. There are two types of galaxies; the football shaped elliptical and the pancake shaped spirals. It’s the spirals that structurally contain central bulges and each bulge consists of a central black hole. The mass of the bulges and the black holes are directly proportional because the two are formed at the same epoch (Zhao et al, 2004 p. 102). This clearly indicates that the formation of black holes and that of bulges occur at the same time or are associated. Hence black holes are essential in the galaxy formation process.
It was believed that the universe was formed through the Big Bang with small haloes in homogeneities that developed into lumps called haloes with time. Galaxies grew through accretion of gas that fell to the centre in cold flows in low-mass haloes. On the other hand, high-mass haloes did not form galaxies. This is because they were dominated by heat hence the gas did not accrete onto galaxies. Combination of tiny haloes formed big ones which contained a huge number of galaxies that are referred to as clusters or groups. As a result, the merging of galaxies within haloes led to the transformation of discs into bulges which provided a chance for the development of galaxies when they ceased to accrete gas. In cases where there was merging of galaxies that were still accreting, the gas fell into the centre, triggered stars to burst out and fed the rapid growth of black holes. In response, the black holes released energy into the surrounding gas and produced winds which compressed the gas that accelerated the rate at which stars were formed. In galaxies which fail to accrete gas, there is lack of star formation and black hole accretion (Beck et al, 1995). Research has also revealed that massive black holes dwell in most local galaxies and other previous studies have also established that a number of relations exist between the super massive black holes masses and the properties of host galaxies (Volonteri, 2010 p.1). Therefore all these findings imply that black holes play a very fundamental role in the formation of galaxies.
On the other hand, black holes have been known to be closely associated with the stellar mass and its speed distribution within the host bulges. This shows an informal relationship between the formation of black holes and that of bulges though they can be interpreted in two ways. First, the formation of stars and black holes occurs simultaneously because both of them feed from a similar gas and they get to the centre by disc instabilities. In addition, the accretion of black hole ceases when all the gases have been used up by star formation hence the two are interdependent on each other. Secondly, the formation of stars normally ceases when the black holes blow away all the gas outside the host galaxies (Volonteri 2010, p.1).
Conclusion
Theories associated with super continental cycle lead to a distinct way of learning about the Earths evolution and geological history. It is clear that the habitual cyclic routines or super continental cycles are fully controlled by the plate tectonic processes. In addition, the evolution of life on earth can be well understood by keenly following the super continental cycle and the changes that occur as a result throughout the history. This can be attributed to the diverse biological changes resulting from climatic and topographical changes that have great significance to the evolution. These include the circulation of the ocean, formation of the global ice-age and the global warming. Certainly it is with the throbbing of the super continental cycle that the fauna, flora and the physical features such as the rocks, mountain belts and the structure of the earth are improved and reintroduced.
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