Tectonic process

To a first approximation, fast tectonic processes occur at plate boundaries (Fig. 9-6). The... 

Given a sufficient period of time, in the absence of tectonic processes, continents would presumably be eroded flat to about sea level. It is not surprising, therefore, that most tectonically quiet areas on continents tend to have low elevations and are often flat, whereas tectonically active areas, mostly mountain belts, have high elevations and steep slopes (Figs 9-2 and 9-4). 

The topography and structure of the ocean floor are highly variable from place to place and reflect tectonic processes within the Earth s... 

Tectonic processes control the terms on the right-hand side, as already discussed. This also defines the sum of terms on the left-hand side. The control of the relative proportions of elements on the left-hand side is uncertain. The most plausible controls based on present data are ... 

The Fe and Mn that diffuse downward are subject to precipitation as carbonate and sulfide minerals in which the metals are present in reduced form. These minerals do not undergo any further chemical changes unless tectonic processes (uplift) cause them to come into contact with O2. As with the oxide phase, other metals tend to coprecipitate into the sulfide minerals, such as cadmium, silver, molybdenum, zinc, vanadium, copper, nickel, and uranium. 

Life evolved soon after Earth s formation, befiare any continents were present, during a time when the oceans were chemically and thermally controlled by tectonic processes. Thus, it has been proposed that the life evolved in hot anaerobic submarine environments similar to present-day hydrothermal vent systems. This hypothesis is supported by the observation of structures, thought to be the remains of protocells, in rocks formed by hydrothermal processes 3.5 to 3.8 billion years ago. Thus, the first organisms on Earth were probably anaerobic hyperthermophiles. Hydrothermal vent habitats probably offered an additional benefit by providing a stable environment relatively isolated from the catastrophic effects of bolide impacts. In other words, submarine hydrothermal vents coifld have acted as refugia enabling survival of early life forms. 

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