Warmth of the Earth. Probable sources internal warmth

Geothermy- a science that studies the Earth's thermal field. The average temperature of the Earth's surface has a general tendency to decrease. Three billion years ago, the average temperature on the Earth's surface was 71 o, now it is 17 o. Sources of heat (thermal ) The Earth's fields are internal and external processes. The Earth's heat is caused by solar radiation and originates in the bowels of the planet. The magnitudes of heat influx from both sources are quantitatively extremely unequal and their roles in the life of the planet are different. Solar heating of the Earth accounts for 99.5% of the total amount of heat received by its surface, and internal heating accounts for 0.5%. In addition, the influx of internal heat is very unevenly distributed on Earth and is concentrated mainly in places where volcanism occurs.

External source is solar radiation . Half solar energy absorbed by the surface, vegetation and subsurface layer of the earth's crust. The other half is reflected into the world space. Solar radiation maintains the temperature of the Earth's surface on average about 0 0 C. The sun warms up the near-surface layer of the Earth to a depth of an average of 8 - 30 m, with an average depth of 25 m, the influence of solar heat ceases and the temperature becomes constant (neutral layer). This depth is minimal in areas with a marine climate and maximum in the Subpolar region. Below this boundary there is a zone of constant temperature corresponding to the average annual temperature of the area. For example, in Moscow, on agricultural territory. Academy named after Timiryazev, at a depth of 20 m, the temperature since 1882 has invariably remained equal to 4.2 o C. In Paris, at a depth of 28 m, the thermometer has consistently shown 11.83 o C for more than 100 years. The layer with a constant temperature is the deepest where perennial ( permafrost. Below the zone of constant temperature is the geothermal zone, which is characterized by heat generated by the Earth itself.

Internal sources are the bowels of the Earth. The Earth radiates more heat into space than it receives from the Sun. Internal sources include residual heat from the time when the planet was molten, the heat of thermonuclear reactions occurring in the bowels of the Earth, the heat of the gravitational compression of the Earth under the influence of gravity, the heat of chemical reactions and crystallization processes, etc. (for example, tidal friction). Heat from the interior comes mainly from moving zones. The increase in temperature with depth is associated with the existence internal sources heat - decay of radioactive isotopes - U, Th, K, gravitational differentiation of matter, tidal friction, exothermic redox chemical reactions, metamorphism and phase transitions. The rate of temperature increase with depth is determined by a number of factors - thermal conductivity, permeability rocks, proximity to volcanic centers, etc.

Below the belt constant temperatures There is an increase in temperature, on average 1 o per 33 m ( geothermal stage) or 3 o every 100 m ( geothermal gradient). These values ​​are indicators of the Earth's thermal field. It is clear that these values ​​are average and vary in magnitude in various areas or zones of the Earth. The geothermal stage is different at different points on the Earth. For example, in Moscow - 38.4 m, in Leningrad 19.6, in Arkhangelsk - 10. So, when drilling deep well on the Kola Peninsula at a depth of 12 km, the temperature was assumed to be 150 degrees, in reality it turned out to be about 220 degrees. When drilling wells in the northern Caspian region at a depth of 3000 m, the temperature was assumed to be 150 o degrees, but it turned out to be 108 o.

It should be noted that the climatic features of the area and the average annual temperature do not affect the change in the value of the geothermal stage; the reasons lie in the following:

1) in different thermal conductivities rocks that make up a particular region. The measure of thermal conductivity is the amount of heat in calories transferred in 1 second. Through a cross section of 1 cm 2 with a temperature gradient of 1 o C;

2) in the radioactivity of rocks than greater thermal conductivity and radioactivity, the lower the geothermal stage;

3) in different conditions occurrence of rocks and age of disturbance of their occurrence; observations have shown that the temperature increases faster in layers collected in folds, they are more likely to have irregularities (cracks), through which the access of heat from the depths is facilitated;

4) character groundwater: flows of hot underground waters warm up rocks, cold ones cool them;

5) distance from the ocean: near the ocean due to the cooling of rocks by the mass of water, the geothermal step is greater, and at the contact it is less.

Knowledge of the specific value of the geothermal step is of great practical importance.

1. This is important when designing mines. In some cases, it will be necessary to take measures to artificially lower the temperature in deep workings (temperature - 50 o C is the limit for humans in dry air and 40 o C in humid air); in others, it will be possible to carry out work at great depths.

2. Great value has an assessment of temperature conditions during tunneling in mountainous areas.

3. The study of the geothermal conditions of the Earth's interior makes it possible to use steam and hot springs emerging on the Earth's surface. Underground heat is used, for example, in Italy, Iceland; In Russia, an experimental industrial power plant was built using natural heat in Kamchatka.

Using data on the magnitude of the geothermal step, we can make some assumptions about temperature conditions deep zones of the Earth. If we accept average value geothermal stage for 33 m and assume that the temperature increases uniformly with depth, then at a depth of 100 km there will be a temperature of 3000 o C. This temperature exceeds the melting points of all substances known on Earth, therefore at this depth there must be molten masses. But due to the enormous pressure of 31,000 atm. Superheated masses do not have the characteristics of liquids, but are endowed with the characteristics of a solid.

With depth, the geothermal stage apparently should increase significantly. If we assume that the level does not change with depth, then the temperature in the center of the Earth should be about 200,000 o degrees, and according to calculations it cannot exceed 5,000 - 10,000 o.

2. Thermal regime of the Earth

The Earth is a cold cosmic body. The surface temperature depends mainly on the heat coming from outside. 95% of the heat of the Earth's upper layer is external (solar) heat, and only 5% is heat internal , which comes from the bowels of the Earth and includes several energy sources. In the interior of the Earth, the temperature increases with depth from 1300 o C (in the upper mantle) to 3700 o C (in the center of the core).

External heat. Heat comes to the Earth's surface mainly from the Sun. Each square centimeter of surface receives about 2 calories of heat within one minute. This quantity is called solar constant and defines total quantity heat coming to Earth from the Sun. For a year it amounts to 2.26·10 21 calories. The depth of penetration of solar heat into the bowels of the Earth depends mainly on the amount of heat that falls per unit surface area and on the thermal conductivity of rocks. The maximum depth to which external heat penetrates is 200 m in the oceans, and about 40 m on land.

Inner warmth. With depth, an increase in temperature is observed, which occurs very unevenly in different areas. The temperature increase follows an adiabatic law and depends on the compression of the substance under pressure with the impossibility of heat exchange with the environment.

The main sources of heat inside the Earth:

Heat released during the radioactive decay of elements.

Residual heat retained since the formation of the Earth.

Gravitational heat released during the compression of the Earth and the distribution of matter by density.

Heat generated due to chemical reactions occurring in the depths of the earth's crust.

Heat released by tidal friction of the Earth.

There are 3 temperature zones:

I – variable temperature zone . Temperature changes are determined by the climate of the area. Daily fluctuations practically die out at a depth of about 1.5 m, and annual fluctuations at depths of 20...30 m. Ia - freezing zone.

II – constant temperature zone , located at depths of 15...40 m depending on the region.

III – temperature rise zone .

The temperature regime of rocks in the depths of the earth's crust is usually expressed as a geothermal gradient and a geothermal step.

The amount of temperature increase for every 100 m depth is called geothermal gradient. In Africa at the Witwatersrand field it is 1.5 °C, in Japan (Echigo) - 2.9 °C, in South Australia - 10.9 °C, in Kazakhstan (Samarinda) - 6.3 °C, on the Kola Peninsula – 0.65 °C.

Rice. 3. Temperature zones in earth's crust: I – variable temperature zone, Ia – freezing zone; II – zone of constant temperatures; III – zone of temperature increase.

The depth at which the temperature rises by 1 degree is called geothermal stage. The numerical values ​​of the geothermal stage are not constant not only at different latitudes, but also at different depths of the same point in the region. The size of the geothermal step varies from 1.5 to 250 m. In Arkhangelsk it is 10 m, in Moscow - 38.4 m, and in Pyatigorsk - 1.5 m. Theoretically, the average value of this step is 33 m.

In a well drilled in Moscow to a depth of 1630 m, the temperature at the bottom was 41 °C, and in a mine drilled in the Donbass to a depth of 1545 m, the temperature was 56.3 °C. The highest temperature recorded in the USA was in a well 7136 m deep, where it was 224 °C. The increase in temperature with depth should be taken into account when designing deep structures. According to calculations, at a depth of 400 km the temperature should reach 1400...1700 °C. The highest temperatures (about 5000 °C) were obtained for the Earth's core.

People have long known about the spontaneous manifestations of gigantic energy hidden in the depths of globe. The memory of mankind preserves legends about catastrophic volcanic eruptions that killed millions human lives, which have changed the appearance of many places on Earth beyond recognition. The power of the eruption of even a relatively small volcano is colossal; it is many times greater than the power of the largest power plants created by human hands. True, there is no need to talk about the direct use of the energy of volcanic eruptions: people do not yet have the ability to curb this unruly element, and, fortunately, these eruptions are quite rare events. But these are manifestations of energy hidden in the bowels of the earth, when only a tiny fraction of this inexhaustible energy finds release through the fire-breathing vents of volcanoes.

The small European country of Iceland (“the land of ice” in literal translation) is completely self-sufficient in tomatoes, apples and even bananas! Numerous Icelandic greenhouses receive energy from earth's heat, there are practically no other local energy sources in Iceland. But this country is very rich hot springs and famous geysers - fountains hot water, with the precision of a chronometer bursting out of the ground. And although Icelanders do not have priority in using the heat of underground sources (even the ancient Romans brought water from underground to the famous baths - the Baths of Caracalla), the inhabitants of this small northern country the underground boiler room is operated very intensively. The capital city of Reykjavik, where half the country's population lives, is heated only by underground sources. Reykjavik is the ideal starting point for exploring Iceland: from here you can go on the most interesting and varied excursions to any corner of this unique country: geysers, volcanoes, waterfalls, rhyolite mountains, fjords... Everywhere in Reykjavik you will feel PURE ENERGY - the thermal energy of geysers gushing from underground, the energy of purity and space of a perfectly green city, the energy of cheerful and incendiary nightlife Reykjavik all year round.

But people draw energy from the depths of the earth not only for heating. Power plants using hot underground springs have been operating for a long time. The first such power plant, still very low-power, was built in 1904 in the small Italian town of Larderello, named after the French engineer Larderelli, who back in 1827 drew up a project for using the numerous hot springs in the area. Gradually, the power of the power plant grew, more and more new units were put into operation, new sources of hot water were used, and today the power of the station has already reached an impressive value - 360 thousand kilowatts. In New Zealand, there is such a power plant in the Wairakei area, its capacity is 160 thousand kilowatts. 120 km from San Francisco in the USA, a geothermal station with a capacity of 500 thousand kilowatts produces electricity.

Geothermal energy

People have long known about the spontaneous manifestations of gigantic energy hidden in the depths of the globe. The memory of mankind contains legends about catastrophic volcanic eruptions that claimed millions of human lives and changed the appearance of many places on Earth beyond recognition. The power of the eruption of even a relatively small volcano is colossal; it is many times greater than the power of the largest power plants created by human hands. True, there is no need to talk about the direct use of the energy of volcanic eruptions - people do not yet have the ability to curb this rebellious element, and, fortunately, these eruptions are quite rare events. But these are manifestations of energy hidden in the bowels of the earth, when only a tiny fraction of this inexhaustible energy finds release through the fire-breathing vents of volcanoes.

A geyser is a hot spring that spews its water at a regular or irregular height, like a fountain. The name comes from the Icelandic word for “to pour.” The appearance of geysers requires a certain favorable environment, which is created only in a few places on earth, which makes them quite rare. Almost 50% of geysers are located in national park Yellowstone (USA). The activity of a geyser may cease due to changes in the subsoil, earthquakes and other factors. The action of a geyser is caused by the contact of water with magma, after which the water quickly heats up and, under the influence of geothermal energy, is thrown upward with force. After the eruption, the water in the geyser gradually cools, seeps back into the magma, and gushes out again. The frequency of eruptions of different geysers varies from several minutes to several hours. The need for high energy for the geyser to operate – main reason their rarity. Volcanic areas may have hot springs, mud volcanoes, fumaroles, but there are very few places where geysers are found. The fact is that even if a geyser was formed in a place of volcanic activity, subsequent eruptions will destroy the surface of the earth and change its condition, which will lead to the disappearance of the geyser.

Earth energy (geothermal energy) is based on the use natural warmth Earth. The bowels of the Earth contain a colossal, almost inexhaustible source of energy. The annual radiation of internal heat on our planet is 2.8 * 1014 billion kW * hour. It is constantly compensated by the radioactive decay of certain isotopes in the earth's crust.

Geothermal energy sources can be of two types. The first type is underground pools of natural coolants - hot water (hydrothermal springs), or steam (steam thermal springs), or a steam-water mixture. Essentially, these are ready-to-use "underground boilers" from which water or steam can be extracted using conventional boreholes. The second type is the heat of hot rocks. By pumping water into such horizons, it is also possible to obtain steam or superheated water for further use for energy purposes.

But in both use cases main drawback lies, perhaps, in a very weak concentration of geothermal energy. However, in places where peculiar geothermal anomalies form, where hot springs or rocks come relatively close to the surface and where, when immersed deeper for every 100 m, the temperature increases by 30-40 ° C, concentrations of geothermal energy can create conditions for its economic use. Depending on the temperature of water, steam or steam-water mixture geothermal springs are divided into low and medium temperature (with temperatures up to 130 – 150° C) and high temperature (over 150°). The nature of their use largely depends on the temperature.

It can be argued that geothermal energy has four beneficial distinguishing features.

Firstly, its reserves are practically inexhaustible. According to estimates from the late 70s, to a depth of 10 km they amount to a value that is 3.5 thousand times higher than reserves traditional types mineral fuel.

Secondly, geothermal energy is quite widespread. Its concentration is mainly associated with belts of active seismic and volcanic activity, which occupy 1/10 of the Earth's area. Within these belts, we can identify some of the most promising “geothermal areas”, examples of which are California in the USA, New Zealand, Japan, Iceland, Kamchatka, North Caucasus in Russia. Only in former USSR By the beginning of the 90s, about 50 underground hot water and steam pools had been opened.

Thirdly, the use of geothermal energy does not require large costs, because in this case we're talking about about “ready-to-use” energy sources created by nature itself.

Finally, fourthly, geothermal energy is completely harmless from an environmental point of view and does not pollute the environment.

Man has long been using the energy of the internal heat of the Earth (remember, for example, the famous Roman baths), but its commercial use began only in the 20s of our century with the construction of the first geoelectric power plants in Italy, and then in other countries. By the beginning of the 80s, there were about 20 such stations in the world with a total capacity of 1.5 million kW. The largest of them is the Geysers station in the USA (500 thousand kW).

Geothermal energy is used to generate electricity, heat homes, greenhouses, etc. Dry steam, superheated water or any coolant with a low boiling point (ammonia, freon, etc.) are used as a coolant.

The main sources of thermal energy of the Earth are [, ]:

• heat of gravitational differentiation;
• radiogenic heat;
• tidal friction heat;
• accretion heat;
• frictional heat released due to the differential rotation of the inner core relative to the outer core, the outer core relative to the mantle and individual layers within the outer core.

To date, only the first four sources have been quantified. In our country, the main credit for this goes to O.G. Sorokhtin And S.A. Ushakov. The data below is mainly based on the calculations of these scientists.

Heat of Earth's gravitational differentiation

One of the most important patterns in the development of the Earth is differentiation its substance, which continues to this day. Due to this differentiation, the formation occurred core and crust, change in the composition of the primary mantle, while the division of an initially homogeneous substance into fractions of different densities is accompanied by the release thermal energy, and the maximum heat release occurs when the earth's matter is divided into dense and heavy core and residual lighter silicate shell - earth's mantle. Currently, the bulk of this heat is released at the boundary mantle - core.

Energy of gravitational differentiation of the Earth over the entire period of its existence, it stood out - 1.46*10 38 erg (1.46*10 31 J). This energy for the most part first goes into kinetic energy convective currents of mantle matter, and then in warm; the other part of it is spent on additional compression of the earth's interior, arising due to the concentration of dense phases in the central part of the Earth. From 1.46*10 38 erg the energy of the Earth's gravitational differentiation went into its additional compression 0.23*10 38 erg (0.23*10 31 J), and was released in the form of heat 1.23*10 38 erg (1.23*10 31 J). The magnitude of this thermal component significantly exceeds the total release of all other types of energy in the Earth. Time distribution total value and the rate of release of the thermal component of gravitational energy is shown in Fig. 3.6 .

Rice. 3.6.

The current level of heat generation during gravitational differentiation of the Earth is 3*10 20 erg/s (3*10 13 W), which is from the size of the modern heat flow, passing through the surface of the planet at ( 4.2-4.3)*10 20 erg/s ((4.2-4.3)*10 13 W), is ~ 70% .

Radiogenic heat

Caused by the radioactive decay of unstable isotopes. The most energy-intensive and long-lived ( with half-life, commensurate with the age of the Earth) are isotopes 238U, 235 U, 232 Th And 40 K. Their main volume is concentrated in continental crust . Current level of generation radiogenic heat:

• by American geophysicist V. Vaquier - 1.14*10 20 erg/s (1.14*10 13 W) ,
• by Russian geophysicists O.G. Sorokhtin And S.A. Ushakov - 1.26*10 20 erg/s(1.26*10 13 W) .

This is ~ 27-30% of the current heat flow.

From the total heat of radioactive decay in 1.26*10 20 erg/s (1.26*10 13 W) in the earth's crust stands out - 0.91*10 20 erg/s, and in the mantle - 0.35*10 20 erg/s. It follows that the share of mantle radiogenic heat does not exceed 10% of the total modern heat losses of the Earth, and it cannot be the main source of energy for active tectono-magmatic processes, the depth of their origin can reach 2900 km; and the radiogenic heat released in the crust is relatively quickly lost through the earth's surface and practically does not participate in heating the deep interior of the planet.

In past geological epochs, the amount of radiogenic heat released in the mantle must have been higher. Its estimates at the time of the formation of the Earth ( 4.6 billion years ago) give - 6.95*10 20 erg/s. Since this time, there has been a steady decrease in the rate of release of radiogenic energy (Fig. 3.7 ).

Over all the time in the Earth, it has been released ~4.27*10 37 erg(4.27*10 30 J) thermal energy of radioactive decay, which is almost three times lower than the total heat of gravitational differentiation.

Tidal Friction Heat

It stands out during the gravitational interaction of the Earth primarily with the Moon, as the nearest large cosmic body. Due to mutual gravitational attraction, tidal deformations arise in their bodies - swelling or humps. The tidal humps of the planets, with their additional attraction, influence their movement. Thus, the attraction of both tidal humps of the Earth creates a pair of forces acting both on the Earth itself and on the Moon. However, the influence of the near swelling, facing the Moon, is somewhat stronger than that of the distant one. Due to the fact that the angular speed of rotation modern Earth (7.27*10 -5 s -1) exceeds the orbital speed of the Moon ( 2.66*10 -6 s -1), and the substance of the planets is not ideally elastic, then the tidal humps of the Earth seem to be carried away by its forward rotation and noticeably advance the movement of the Moon. This leads to the fact that the maximum tides of the Earth always occur on its surface somewhat later than the moment climax Moon, and an additional moment of force acts on the Earth and Moon (Fig. 3.8 ) .

The absolute values ​​of the tidal interaction forces in the Earth-Moon system are now relatively small and the tidal deformations of the lithosphere caused by them can reach only a few tens of centimeters, but they lead to a gradual slowdown of the Earth’s rotation and, conversely, to an acceleration of the orbital movement of the Moon and to its distance from the Earth. The kinetic energy of the movement of the earth's tidal humps turns into thermal energy, due to internal friction of matter in tidal humps.

Currently, the rate of tidal energy release is G. Macdonald amounts to ~0.25*10 20 erg/s (0.25*10 13 W), while its main part (about 2/3) is presumably dissipates(dissipates) in the hydrosphere. Consequently, the fraction of tidal energy caused by the interaction of the Earth with the Moon and dissipated in the solid Earth (primarily in the asthenosphere) does not exceed 2 % total thermal energy generated in its depths; and the share of solar tides does not exceed 20 % from the effects of lunar tides. Therefore, solid tides now play virtually no role in feeding tectonic processes with energy, but in some cases they can act as “triggers”, for example earthquakes.

The amount of tidal energy is directly related to the distance between space objects. And if the distance between the Earth and the Sun does not assume any significant changes on a geological time scale, then in the Earth-Moon system this parameter is variable. Regardless of ideas about almost all researchers admit that early stages development of the Earth, the distance to the Moon was significantly less than modern, but in the process of planetary development, according to most scientists, it gradually increases, and according to Yu.N. Avsyuku this distance experiences long-term changes in the form of cycles "coming and going" of the Moon. It follows from this that in past geological epochs the role of tidal heat in the overall heat balance of the Earth was more significant. In general, over the entire period of the Earth’s development, it has evolved ~3.3*10 37 erg (3.3*10 30 J) tidal heat energy (this is subject to the gradual removal of the Moon from the Earth). The change in the rate of release of this heat over time is shown in Fig. 3.10 .

More than half of the total tidal energy was released in catarchaea (shit)) - 4.6-4.0 billion years ago, and at that time only due to this energy the Earth could additionally warm up by ~500 0 C. Starting from the late Archean, lunar tides made only a negligible impact on the development energy-intensive endogenous processes .

Accretion heat

This is the heat retained by the Earth since its formation. In progress accretion, which lasted for several tens of millions of years, thanks to the collision planetesimals The Earth experienced significant heating. However, there is no consensus on the magnitude of this heating. Currently, researchers are inclined to believe that during the process of accretion the Earth experienced, if not complete, then significant partial melting, which led to the initial differentiation of the Proto-Earth into a heavy iron core and a light silicate mantle, and to the formation "magma ocean" on its surface or at shallow depths. Although even before the 1990s, the model of a relatively cold primary Earth, which gradually warmed up due to the above processes, accompanied by the release of a significant amount of thermal energy, was considered almost universally accepted.

An accurate assessment of the primary accretion heat and its fraction preserved to the present day is associated with significant difficulties. By O.G. Sorokhtin And S.A. Ushakov, who are supporters of the relatively cold primary Earth, the amount of accretion energy converted into heat is - 20.13*10 38 erg (20.13*10 31 J). This energy, in the absence of heat loss, would be enough for complete evaporation earthly matter, because the temperature could rise to 30 000 0 С. But the accretion process was relatively long, and the energy of planetesimal impacts was released only in the near-surface layers of the growing Earth and was quickly lost with thermal radiation, so the initial heating of the planet was not great. The magnitude of this thermal radiation, going in parallel with the formation (accretion) of the Earth, these authors estimate at 19.4*10 38 erg (19.4*10 31 J) .

In the modern energy balance of the Earth, accretion heat most likely plays a minor role.