Exploring the Dynamic Nature of Gradients on Earth
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A gradient indicates a variation of any parameter, such as the degree of inclination of a plane; in physics, the rate at which, for example, pressure or temperature vary in space or time; in chemistry, a gradient can be identified as the variation of electro-chemical potential differences between two bodies; in biology, a gradient can be considered as the progressively increasing or decreasing differences in the growth rate, metabolism, physiological activity of a cell, organ or organism. Several gradients of parameters acting together with different times and rates on systems with a large number of degree of freedom can determine non-linearity, chaos and self-organization that can ultimately evolve into catastrophic collective behaviors.
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A gradient generates forces producing or being the expression of a difference of a certain physical observable in time or space. Earth hosts Life because of the mobility of its mantle, thermal gradients inducing volcanic eruptions and velocity differences among crustal volumes provoking faulting, mountain growth and ocean spreading. What is the nature of these planetary gradients? How can we study them and how do they punctuate Earth dynamics? Each gust of wind is determined by atmospheric pressure gradient: areas where atmospheric pressure is greater push air toward areas of lower pressure. The greater the gradient, the faster the winds. Cyclones, tornadoes, rain and snow are controlled by those gradients. For analogous reasons, in the two hemispheres, the Coriolis force plays opposite and the linear velocity gradient induces clockwise or counter clockwise atmospheric circulation. Rivers flow downstream because of a topographic gradient and therefore a gravitational one: gravity never stops working. Atmospheric disturbances determine heavy rains, which in turn lower the friction in the soil and rocks, causing landslides: it is a succession of gradients that shape the Earth’s surface, with a tendency to decrease or eliminate morphological irregularities.
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Volcanoes erupt violently because inside the magma chambers, where the lava accumulates and melts the embedded rocks, pressure gradients are created that break through the overlying Earth’s crust. The density gradient that is generated for the thermal expansion of molten rocks contributes to the ascent of magma together with gas bubble coalescence that, as soon as reaches the surface, finds an opposite gradient producing lava flows on the sides of the volcanic apparatus, or disperses with pyroclastic clouds. The higher is the viscosity of magmas, the greater is the pressure gradient that manages to accumulate, and the viscosity generally tends to increase with the silica content of the magma. The higher the magma viscosity, the greater the explosiveness and the episodic nature of volcanism: this is an example of chemical gradient affecting geophysical dynamics.
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Gradients are determined by variations in pressure, temperature, chemical and mineralogical composition, friction, rigidity, viscosity, electromagnetic field, etc. The greatest the gradient, the largest the energy involved, yielding a wide range of manifestations at different spatial and temporal scales. The surface of the Earth is featured by complex geological patterns produced by both endogenous and exogenous events playing as a mirror of phenomena acting since the dawn of our planet. The lack of direct investigations still makes Earth interior poorly understood and also prevents complete clarification of peculiar mechanisms ruling geodynamics and related phenomena. For the same reason, the effective roles of the different forces contributing to plate tectonics remain elusive.
