Gradient

GRADIENT, in meteorology and geophysics, is the rate of change of a certain element (temperature, pressure, electric field) in a direction perpendicular to level surfaces (i.e., surfaces of equal temperature, equal pressure, etc.). The geothermal gradient is the magnitude of the increase in the Earth's temperature with depth per unit of depth measurement. The magnitude of the depth at which the temperature increases by one degree is called the geothermal step. A constant, more or less regular, increase in temperature with depth begins not from the Earth's surface itself, but only from the so-called belt of constant temperature, located at a depth to which the periodic annual temperature fluctuations observed on the Earth's surface do not penetrate. The temperature of this belt is equal to or slightly higher than the average annual temperature of a given location. The depth of the belt of constant temperature depends, firstly, on the magnitude of the amplitude of temperature fluctuations on the Earth's surface, and secondly, on the physical and chemical properties of the rocks and the conditions of their occurrence. In general, it fluctuates within fairly significant limits—from 1 to 30 meters and more. Below the belt of constant temperature, the temperature increases on average by one degree every 33 meters. However, this value varies in different places, and even for the same place, it sometimes changes at different depths. The magnitude of the geothermal step is influenced by a whole range of factors, among which should be noted: 1) the varying magnitude of the thermal conductivity of rocks, 2) chemical processes occurring in the Earth's crust, 3) the nature of the rock bedding, 4) the circulation of groundwater, 5) the nature of the relief, 6) the uneven cooling of the globe in different places. The production of more systematic and scientific observations of the geothermal gradient began only in the second half of the 19th century. The very fact of the increase in temperature with depth was noted long ago in connection with the development of mining, which required the excavation of underground workings in deep layers of the Earth. At the present time, deep boreholes, mines, and tunnels passing at great depth under mountain ranges are used for systematic observations. One of the deepest boreholes is a borehole drilled in Prussian Silesia, near the town of Czuchów, to a depth of 2,239.72 meters. Temperature measurement in this borehole was carried out to a depth of 2,220 meters, at which depth the temperature reached 83.4°. How the temperature increases below the depths accessible to us is unknown. However, hot springs and volcanic lavas emerging from the Earth in various places on its surface clearly prove the presence of high temperatures in the depths inaccessible to us. According to the most popular hypothesis at the present time, the high internal temperature of the Earth is a remnant of its initial heat. However, at present, other views are also encountered. Thus, for example, many scientists are inclined to link the thermal regime of the Earth with the processes of decay of radioactive substances contained in the Earth's crust.