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Continental reconstructions must take continental shelves into account because sea level varies over time. Causes of sea level change include climate changes that alter the amount of polar ice, and variations in rates of seafloor spreading that alter ocean basin volumes. Increased carbon dioxide in the atmosphere from combustion of fossil fuels may result in the greenhouse effect and cause sea level to rise in the future.
Before continents actually separate, forces stretch them apart, faulting occurs, and rift valleys form. The rifting mechanism produces uplift at the edges from crustal thinning. If stretching continues, the asthenosphere eventually reaches the surface and forms volcanoes along the rift. When the volcanoes become continuous, ocean floor is created. Though initially it may be sub-aerial, normal seafloor spreading then occurs and oceans form.
Following rifting the processes of erosion, deposition, and subsidence shape the continental margin. The uplifted sides of the rift valley are eroded, and the sediments are deposited in the rift valley below sea level. The continental margins become sedimentary basins because the rift valley floor continues to subside long after the continents have separated. The stretched and heated continent conductively cools and contracts from its original depth beneath sea level to even greater depths. Sediments act as a load on this crust and cause even more subsidence.
Salt is often deposited in beginning ocean basins. Rift valleys may be repeatedly flooded with seawater, and if conditions are right, the seawater evaporates between floods, leaving behind salt layers. If the salt is buried by denser sediments, it may rise toward the surface as diapirs, or salt domes. Buoyant oil and gas which might have formed above the salt layer then migrates to the edges of the salt. .
Hydrocarbons form from undecomposed organic matter, which requires rapid burial after the death of micro-organisms in anoxic environments such as river deltas, continental margins, flood plains, or the deep sea. Maturation of hydrocarbons occurs after deposition of the organic matter: pressure and temperature must chemically alter the organic material into complex organic molecules. Gas results from higher burial temperatures than oil. After oil matures, it usually migrates away from its source beds to be trapped by impermeable caprock. Otherwise the buoyant oil and gas escapes to the surface.
Mantle convection is the fundamental driving force that propels the lithospheric plates. Convection is caused by density and temperature anomalies in the asthenosphere. Geophysical data describes the mantle processes that include the state of stress within the stable plate, heat flow and depth deviations from those predicted by the plate model, deep seismic velocity anomalies, and long-wavelength gravity anomalies.
Absolute plate motions are derived from hotspot trails. A hotspot is a volcanic center fixed relative to the Earth's mantle. As a plate moves over a hotspot, an island chain (e.g. the Hawaiian-Emperor islands) forms. All volcanoes away from plate boundaries, and areas of excessive volcanism on plate boundaries, are believed to be hotspots. These define where plates are “blow-tourched” apart and where “continental drift” occurs.