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Plate Tectonics and the Global Climate
Life on Earth reels under the Supercontinental cycles. Every Supercontinent forms in a "Tectonic Pause" with a global ice age. Massive ice sheets covers whole or part of the Supercontinent. Supercontinental breakup creates the new plates with a severe global warming and rapidly melts the ice sheets, exiting the global ice age period. Breakup and joining of the plates in between two Supercontinental periods causes small scale global warming and ice ages. Mass extinction occurs in the ice age and global warming periods. Evolution takes place in the post global warming period. Life thrives in the steady moving of the plates or in the period of oceanic plate subduction.
Continents collided and separated many times in the history of the Earth. Collision of the continents creates the folded mountains at the subduction zone and this mountain buildup slows the subduction process, ultimately stopping the subduction when all the forces acting at the subduction zone nullifies each other in a state of equilibrium. When all the subduction zones in the Supercontinental collision reaches the state of equilibrium, plates stops moving and closes all the ridges causing a "Tectonic Pause". In this period Earth's crust forms as a single plate and triggers the global ice age. When ice covers most part of the crust it is a snowball Earth event. After a brief period, the stationary and heavier Supercontinent breaks up by creating new ridges. Oceanic crust at the continental shelves breaks to form the trenches. These newly created ridges and trenches, reverses the movement of the plates. The open ridges on the continent spews the green house gases and ejects the magma onto the ice sheets, melting them rapidly in a severe global warming. With the raising sea level, water seeps into the open ridges further heating and circulating the oceanic waters. Accumulation of green house gases and the heat from the new ridges melts the ice exiting the global ice age.
Ridges are the major source of heat supply into the oceans. Speed of the Tectonic plates dynamically changes as they grow or shrink in size. Total heat supply or the total magma outflow into the ocean depends upon the combined length of the ridges and the speed of the plates. Increase in the speed of the plates or formation of a new ridge adds more magma into the ocean waters and causes a global warming. Hot ocean currents melts the ice caps at the poles and increases the sea level. Steady subduction of the oceanic plate maintains a constant temperature on the planet. Continental collision reduces the speed of the subducting plate and releases less magma into the oceans, cooling the ocean waters. Cold ocean water starts to freeze from the poles and reduces the sea level in an ice age. Subsequent global warming and ice age together forms a distinct sedimentary layer on the continental crust.
Plate Tectonics and the Tectonic Pause
Plate Tectonics can be described in terms of the Statics (a study of the rigid body mechanics), as the resultant force of all the forces acting at a subduction point (SP). SP is any point on the line of subduction. In a newly created trench, oceanic plate submerges because of its weight and the continental plate slides on to the lowered oceanic plate with an inclination. On this trench only two forces will be working at any subduction point as shown in Fig. 1. Force of the continental or overriding plate (O) acts towards the SP and the buoyancy of the mantle (B) also acts towards the SP, in a perpendicular line to the curvature of the oceanic plate. The resultant force (R) of the O and B acts towards the ocean and slides the continental plate on the oceanic plate as observed by the Zhong and Gurnis. This moves the SP towards the ocean and increases the dipped portion of the oceanic plate under the continental plate. This ocean ward movement of the SP also called as the trench migration. In this period the oceanic plate remains stationary. Younger and soft magma within the ridge always splits in half at the center of the ridge while the plates are in motion. Ridge builds both the plates equally at half the rate of subduction, even if a plate is stationary at the ridge. This buildup of the plate makes the ridge appear like moving away from the stationary plate with a ridge migration. Initially the dip angle (A) of the subducted slab will be very high. The weight of the slab will be less compared to the buoyancy of the liquid mantle on the slab, as a result the slab floats on the surface of the liquid mantle. Ocean ward movement of the SP keeps the slab flat or parallel to the surface. As the slab increases in length acquires the weight and slowly sinks into the mantle, decreasing the dip angle. Interaction between the trench migration and the slab weight keeps the slab suspended in the liquid mantle. While the continental plate continues to slide on the oceanic plate, the dipped part of the oceanic plate increases and bends at the SP. This change in the curvature at the SP shifts the B towards the ocean. Ocean ward movement of the B acts as a break to the continental slide and finally stops the continental slide when the two forces acts opposite to each other, nullifying the resultant force in an equilibrium state. Buoyancy on the slab decreases with the decreasing dip angle and when the dip angle decreases further, the weight of the slab exceeds the buoyancy on it and as a result exerts the force (W) away from the SP into the mantle. Subducting or the oceanic plate also exerts the force (S) away from the SP, towards the ocean, as shown in the Fig. 2. Dip angle decreases until the S exceeds the W and moves whole of the oceanic plate into the trench from the adjoining ridge or breaks the oceanic plate by creating a new ridge and moves the part associated with the SP into the trench. This breakup can happen in the oceanic crust or on the continental crust beyond the ocean. Ridge associated with the oceanic plate also moves towards the trench. When the slab pulled further down, it melts and sinks into the hot liquid mantle and creates the stratovolcanoes on the overriding plate. Fast moving oceanic crust increases the W as the length of the slab increases in the mantle. Continuous melting of the slab keeps the W constant and decreases the S as it approaches the trench. Pull of the oceanic plate under the continental plate exerts the force on the continental plate and lifts it at the SP. Subduction of the oceanic plate continues until the ridge joins with the trench creating a fault. All the subduction points having the same resultant force moves the oceanic plate uniformly. Difference in the resultant force causes the plate to move in different speeds. This causes the ridge to split and creates different plates and moves them in different speeds towards the trench depending upon the local subduction rates.
If there is another continent on the subducting plate, the two continents collides rising a folded mountain chain after the oceanic crust on the subducting plate was consumed. The continental crust of the subducting plate gets folded at the surface of the subduction zone. The weight of these newly created folded mountains (M) acts upward at the SP as shown in the Fig. 3. M acts as a break to the subduction process and as it builds up slows the subduction. Decrease in the subduction rate and the continuous melting of the slab may reduce the length of the slab considerably. W decreases with the decreasing length of the slab. Subduction process, mountain buildup and the melting of the slab continues until all the forces acting at the SP reaches the state of equilibrium. At this point the absorption of the slab stops and ceases the mountain buildup. In turn the two continental plates glued together by the mountain chain and moves united towards the existing subduction zones. At this time only two forces will be acting at the SP, buoyancy lifts upward and the weight of the crust including mountains acts downward in an equilibrium state. Depending upon the relative motion of the surrounding plates and the final motion of the new plate, any of the adjoining ridges closes completely and creates a new ridge on the ocean floor. Ridges on the ocean floor continuously rearranges in different directions while the continents were heading towards the Supercontinental formation. When all the continents were collided to form the Supercontinent, folded mountain chains and the magma within the ridges glued the plates together as a single plate causing the Tectonic pause.
Supercontinent and the global Ice age
In the continental collisions oceanic ridges were releasing lesser and lesser magma and green house gases into the oceans. This slow loss of heat source in oceanic waters and the subsequent decrease in the green house gases caused the oceans to cool. Closed ridges on the Supercontinent caused the oceans to freeze and the frozen ice sheets spread to the Supercontinent causing the global ice age, sometimes as the snowball Earth event. Unlike the intermediate ice ages, Supercontinental ice ages were global and were part of the land and ocean. This removal of water from the oceans and depositing on the continental crust as the ice sheets drastically reduces the sea level and exposes the continental shelves. Frozen ocean and the exposed continental shelves causes a major mass extinction of the marine life. Supercontinent remains stationary for a brief period until its breakup. Hot Spots are the only vent for the dissipation of heat within the mantle and will be continuously active in the Supercontinental period. Hot spots on the oceanic crust emerges onto the surface on the lowered sea level.
Hot spots on the continental and the oceanic crust harbor the life in the Supercontinental period. In every global ice age or Supercontinental period all the life forms on different continents comes together to one place. Species have to survive the encroaching ice sheets and also have to safeguard themselves from the new predators joined from the other continents. Pre Supercontinental period will face a major mass extinction in the global ice age and the post Supercontinental period will have an explosion in the evolution of the surviving species.
Except in few elevated areas and around the Hot spots, massive ice sheets covers the planet and makes the Supercontinent heavier. This addition of new ice sheets to the already balanced mountains or to the crust around the mountains disturbs the equilibrium. Hot spots heats the stationary and heavier Supercontinent and weakens the crust. Build up of the pressure within the mantle and the weakened crust breaks to form the new ridges to evenly distribute the weight on the crust. At the same time oceanic and continental margins breaks to form the new trenches. This will be a violent and explosive period in the history of the Earth. Open ridges ejects the magma onto the massive ice sheets melting them rapidly. Ridges also spews the green house gases into the atmosphere heating the environment in a severe global warming. Further melting of the ice sheets increases the sea level and the raised water seeps into the new ridges forming the oceanic ridges. Hot spots will diminish in the intensity after the ridges were formed. Hot spots on the oceanic crust, a base for the survival of the life in the Supercontinental period submerges in the increased sea level.
In the early history of the Earth, floating landmasses were freely moved on the hot molten surface of the Earth. They were freely colliding, merging and separating on the liquid surface. When the Earth started to cool all the landmasses were clubbed together as a super landmass. This super landmass remained together while the Earth cooled off further and acquired the water. Water cooled the surface and solidified it as a rigid crust. Further cooling of the crust frozen the water and spread the ice sheets to the Super landmass by creating the first ice age on the Earth. In this period Sun was faint and the continental landmass was less causing the ice sheets to cover all the landmass and continued for longer periods even after the breakup. Increase in the landmass and the brighter sun shortened the later global ice ages and covered less area on the globe. Ice sheet coverage may also depend upon the position of the Supercontinent on the Earth. If the Supercontinent forms around the equator it will have less coverage of ice sheets and on the poles more coverage. In the initial global ice ages it could be possible that the ice on continents melted just around the continental shelves and on the equator. Oceans might have definitely freed from the ice sheets at least at the equator. We may not find the traces of these ice ages in the continuously recycling ocean floors, but may find these traces on the previous Supercontinental folded mountains.
Global warming and the Ice ages
Rate of sea floor spreading is a main element in controlling the global temperatures. Higher the rate of sea floor spreading adds more heat to the oceanic waters and also increases the content of green house gases in the atmosphere, causing a global warming. Global warming and the hot ocean water together melts the ice sheets on the poles and raises the sea level. Raising sea floods the continents and creates a new sedimentary layer with the fresh deposit of sediments. Formation of a new ridge or increase in the subduction rate causes the global warming. Severity of the global warming depends on the speed and size of the new plates and the place of the break on the crust. Continental breakup will cause more severe global warming than the break in the oceanic crust.
Lower the rate of sea floor spreading releases less magma into the oceanic waters. Cold ocean water starts to freeze from the poles and decreases the sea level. Exposed continental crust solidifies the deposits and forms as a distinct sedimentary layer. Merger of a ridge with the trench forming a fault, continental collision and the state of equilibrium in the subduction causes the ice ages. Sedimentary layers starts to form in the global warming period and ends with the ice age or with another global warming period and resembles the tree ring development in the summer and winter periods of a year. Mass extinction occurs in the global warming and ice age periods. Whichever species adopts the gradual climate change will survive and others face the extinction. Evolution in the surviving species occurs in the post global warming period. Constant rate of sea floor spreading emanates steady flow of magma and green house gases and keeps the environment in a steady phase.
Ice age on the Gondwana land around 200 million years ago (ma) is the recent Supercontinental global ice age. Snowball Earth events around 600-800 ma and around 2400 ma were also happened when all the continents were together as a Supercontinent. Sedimentary layers on the first Supercontinent around 2700-2800 ma may be the result of a global ice age. If the Supercontinental cycle happens after every 425 million years (my) and the last Supercontinent broke apart around 200 million years ago (ma), the sequence of the previous Supercontinental periods falls around 200, 625, 1050, 1475, 1900, 2325, 2750 ma. Supercontinental formations around 1050, 1500 and 1950 ma might have also caused the snowball Earth events.
In the Permian age all the continents were collided reducing the subduction rate and releasing less magma into the oceans. This caused the ocean to cool and started to freeze from the poles. When all the ridges were closed in the Supercontinental "Tectonic Pause", frozen ice sheets spread to most part of the Supercontinent reducing the sea level. Frozen ocean and the exposed continental shelves with the global ice age caused the major mass extinction in the history of the Earth. Breakup of the Pangaea thawed the environment with a global warming and started the Mesozoic era.
Breakup and the subsequent increase in the speed of the Indian plate from the African plate around 65 million years ago added more magma to the oceans and heated the oceanic waters causing a global warming. Global warming caused the ice caps to melt at the poles and increased the sea level. Increased sea level flooded the continental regions, starting the Tertiary sedimentary layer in the flooded regions of the continents. Extinction of the dinosaurs coincides with this period. Collision of the Indian plate with Asia reduced the rate of subduction and released less magma into the oceans and caused the recent ice age. Decreased sea level solidified the sedimentary layer and created a new boundary for the new deposits. In between the breakup and collision of the Indian plate, there was a steady phase or constant rate of sea floor spreading for about 60 million years in the whole Tertiary period while the oceanic crust was steadily subducted under the Asia.
This hypothesis not only proposes a Supercontinental "Tectonic pause" with a global ice age but also combines the ice ages, global warming, sea level changes and formation of sedimentary layers with the Plate Tectonics and creates a unified view of this dynamic Earth.
Plate Tectonics is the result of all the dynamic forces acting at the subduction point. In the process plates break apart and joins together causing the small scale global warming and ice ages. When plates stop moving in the Supercontinental formation, younger magma solidifies and thickens at the ridges and eventually joins all the plates and covers the planet with massive ice sheets in a global ice age. In the subsequent Supercontinental breakup, new ridges will be created and the planet gets a new life. Plate Tectonics cools the Earth in a controlled manner. For the survival of Plate Tectonics on a planet, it needs to have water on its surface to cool and solidify the out coming magma at the ridges and huge lighter continental crust to get split into different plates. It also needs to have the hot convecting fluid mantle to melt the subducted slab and to split the Supercontinental crust.
Global warming and the ice ages are the natural phenomena on the Earth. Present thawing may be the result of increase in subduction rate at any of the existing trenches or formation of any new ridge in recent times. Intensity of the Sun couldn't able to stop the Supercontinental freeze in the snowball Earth period. It means just alone the Plate Tectonics and the Hot spots can support a minimum life forms on a planet. Life will thrive around the ridges and clings to the Hot spots during the global ice age period. Increase in the star's intensity may help in the evolution of more complex life forms. Planet or a satellite covered with ice and Hot spots may be building the Plate Tectonics for the first time or Tectonically pausing in a Supercontinent.
References and Notes
1. P. F. Hoffman, D. P. Schrag, Snowball Earth, Sci. Am., 282, 1, p68-75.
2. P.F. Hoffman, A.J. Kaufman, G.P. Halverson, D.P. Schrag, A Neoproterozoic Snowball Earth, Science, 281, p1342-1346.
3. D.A. Evans, N.J. Beukes, J.L. Kirschvink, Low-latitude glaciation in the Paleoproterozoic, Nature, 386, p262-266.
4. C. Zimmer, Ancient Continent opens Window on the Early Earth, Science, 286, p2254-2256.
5. S. Zhong, M. Gurnis, Dynamic interaction between tectonic plates, subducting slabs, and the mantle,Earth Interactions, 1, 1-003. [Available online at http://EarthInteractions.org/]
Zhong and Gurnis derived the plate kinematics of continental slide and the slab subduction using the mathematical simulation of the mantle convection. They didn't simulate the continental collision.
6. D. H. Erwin, The Mother of Mass Extinctions, Sci. Am., 275, 1, p72-78.
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