Plate Tectonics and the Global Climate

KARUNAKAR MARASAKATLA
email-id: marasakk@bis.adp.com
Date: 08/01/2000.

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

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

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

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.

Case Studies:

Snowball Earth

Permian extinction

Tertiary period

Conclusion

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.

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References and Notes

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