Hydrology

Tectonic Water Cycle

In terms of a rough calculation of the total water of the Earth, the surface of the planet is considered to have one ocean mass (1 OM). Earth originally had about 4.2 OM locked up inside it when it first accreted from space about 4.5 billion yrs. (years) ago. The 1 OM now on the surface had escaped since accretion to form the modern oceans, leaving about 0.2 OM behind in the upper mantle. The lower mantle still has about 3 OM locked up inside it. The molten core of the Earth beneath has no water at all in it. The tectonic water cycle has water constantly escaping from inside the Earth through volcanic eruptions on land and beneath the sea. In fact, more than 50% of all volcanic gas comes out as steam in new or juvenile water, to be added to the surface supplies. Some water is also returned to the interior of the Earth as it goes down subduction zones or is absorbed directly into the undersea floor.

Yearly fluxes of water through the Earth's upper crust include (1) shallow hydrologic flow on the continents, amounting to about 37,400 km3 (cubic kilometers) per year, and (2) free hydrostatic water movement to depths of possibly as much as 15 km in sedimentary basins on continents, where the sediments compact and emit water like a sponge under their own weight at compactive water expulsion rates of ∼3 km3/yr. Beneath the oceans, in the seafloor plates to depths of ∼5 to 7 km, there is total flank convection or free convection of water at rates of >240 km3/yr. Close to the volcanic rift zones in the oceans, forced convection of water in the rocks of the sea bottom amounts to ∼40 km3/yr.

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