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Subduction is the slanting and downward movement of the edge of a crustal plate into the mantle beneath another plate.
It occurs when an incredibly dense ocean crust meets a deep ocean trench.
The different half-lives involved cause the signals to decay over different timescales.
"A" stands for "activity", and "o" stands for "initial".
The temporal variability of crustal and mantle processes occurring at the Mid-Ocean Ridge (MOR) system is a key component for understanding the dynamics of these globally important volcanic structures.
Time series observations of the MOR have demonstrated that eruptive and tectonic events can take place on very short time scales and they can cause dramatic hydrologic, biologic and geologic changes at these undersea volcanoes. (1998) Geochronology and Geochemistry of lavas from the 1996 North Gorda Ridge eruption, Deep Sea Research II, 45, 2751-2599.
Seafloor spreading leads to the renewal of the ocean floor in every 200 million years, a period of time for building a mid-ocean ridge, moving away across the ocean and subduction into a trench.
Such timescales can be assessed with natural Actinide-series decay chains because chemical disruption to secular equilibrium systems initiates parent-daughter disequilibria, which re-equilibrate by the shorter half-life (t½) in a pair (see Fig. My group's work in this area invloves the application of such nuclide disequilibria in volcanic rocks from a variety of tectonic provinces to determine as precisely as possible the age of an individual eruption and/or the relative timing between eruptive events.
To this end, I and colleagues continue to develop radiometric tracers to obtain quantitative age information over time scales ranging from weeks to hundreds of years. (1998) Petrology and geochronology of basalt breccia from the 1996 earthquake swarm of Loihi Seamount, Hawaii: Magmatic history of its 1996 eruption, Bulletin of Volcanology, vol 59, 577-592.
He also found out that the temperature near to the mid-Atlantic ridge was warmer than the surface away from it.
He believed that the high temperature was due to the magma that leaked out from the ridge.
When the molten magma reaches the oceanic crust, it cools and pushes away the existing rocks from the ridge equally in both directions.