Do the global geodynamic cycles of the Phanerozoic represent a feedback system of the Earth and is the Moon involved as an acting external force?
published: Jan 1, 2006
ArtNo. ESP171015701005, Price: 29.00 €
The Phanerozoic time span is characterised by two 300 million year long geodynamic cycles of different geological, geophysical, and biological processes. These processes are linked together in one way or the other, which conceptionally leads to the definition of a geodynamic feedback system. Sea level variations, variations in magmatic/volcanic activity, deposition cycles of oil and gas source rocks, changes in the magnetic field, and global climate changes testify to this development. The variations in shelf areas, among other things linked to the variations in sea level, the tidal dissipation influenced by these and the Moon's recession and rotational history of the Earth connected therewith bring the Moon into the system as an active participant, which as an external force can cause a feedback between the geodynamic processes and force these into cyclic dependencies. If the shelf areas on the Earth increase due to preceding geological processes the tidal dissipation increases depending on the distance between the Earth and the Moon and the Earth's rotation is decelerated to a greater extent. Assuming a slip between the mantle and the core of the Earth, this deceleration effect should first be predominantly taken over without any time delay by the Earth's mantle. Differences in rotation may have an effect on processes at the core-mantle boundary, e.g. the stabilisation of the Earth's magnetic field, and on ascending magma within the convection cells of the mantle. An increase in magmatic activity allows the observed volumes of the mid-ocean ridges to increase, causing the sea level to be pushed upwards and leading to an increase in the size of the shelf areas and consequently strengthening the tidal dissipation. The carbon dioxide content of the Earth's atmosphere increases due to the mantle degassing connected with the magmatism, to which the biosphere reacts with increased production of organic biomass and carbonate, predominantly sedimented on the shelf areas, the main habitat of marine flora and fauna. On the other hand, the increase in the spreading rate of the newly generated sea bottom accompanying the increased magmatic activity accelerates the subduction process at the active plate margins, leading to a more intense mountain formation in the accretionary wedge together with the emplacement of magmatic intrusions, increased volcanic activity, and the development of a mountain root. This process is followed by intensified erosion of the mountain ranges, which in turn results in a higher sedimentation rate on the continental shelves, and fills these – in addition to the carbonates – with clastic sediments, resulting in the shelf areas becoming smaller. This process allows the tidal dissipation to decrease, which should make it possible for the Earth's core to make up for the slowness of the Earth's mantle. Now is the time for an inverse phase of the geodynamic processes. Only when the shelf areas grow again due to sea level rise subsequent to the metamorphism and dehydration of the mountain roots and when the surface of the deposits in sedimentary basins falls below sea level – due to the unbalance between a decreasing sedimentation rate and ongoing compaction and subsidence – does the tidal dissipation increase again and can introduce the next cycle. This cycle will now experience a decrease in amplitude due to the Moon's recession, which has in the meantime taken place and due to the associated reduction in the available convertible energy. Taking into account the evolution of the shallow seas during the Phanerozoic, the continental growth back to the beginning of the Archean, conditions for the filling of the Earth's oceans in the Hadean as derived from zircon crystals, and the widely accepted theory on the origin of the Moon, the Moon's recession rates back in time as well as the early rotation history of the Earth can be estimated.
phanerozoicum • geodynamic cycles • sea level variation • moon's recession • earth's rotation • hadean • continental growth • ocean filling • orogeny • magnetic field • magmatic activity • sedimentation rate • climate