UDC 551.21

GEOPHYSICS

G. N. KOVALEV

THERMAL POWER OF HYDROTHERMAL SYSTEMS AND ACTIVE VOLCANOES

(Presented by Academician V. S. Sobolev, 16 IX 1968)

The thermal effect of active volcanism and hydrothermal systems existing in regions of present-day intensive volcanic activity is realized mainly through convective heat transfer. Molecular thermal conductivity is of strictly subordinate importance. The motion of a deep-seated heat carrier is determined by the structure of the enclosing medium. It is advisable to consider the heat flow of a hydrothermal system together with elements of the geological structure that are in paragenetic connection with the source of deep heat.

The development of volcano-tectonic depressions, and of ring-shaped caldera subsidence structures in plan, is the result of processes apparently initiated by large stocks of upper-mantle material and magmatic chambers intruded into the Earth’s crust.

Table 1

As a rough approximation, we shall assume that the area of the ring structure (caldera) corresponds to the area of the generation center of deep heat; that is, we shall assume a vertical arrangement of weakened zones of tectonic faults associated with the formation of the given structure. Then the total natural thermal power of a number of local thermal manifestations located within the ring structure, referred to its area, may be regarded as an energy characteristic of the generation center. It is assumed here that the hydrothermal system is in a regime of quasi-stationary heat flow.

Table 1 compares the specific heat flows of several hydrothermal systems; by the area of a thermal anomaly is meant the area distinguished as a structural geological unit.

The data in the table show that for young thermal anomalies, whose lifetime amounts to tens and the first hundreds of thousands of years, the power of heat supply is practically the same and on average equals 800 kcal/km²·sec. As the activity of the heat-generation center dies down, the specific thermal power of the thermal anomaly correspondingly decreases.

Thus, for example, for the Nalachevo thermal anomaly (Kamchatka), which has existed for about \(10^7\) years, the thermal power is about \(10\ \text{kcal}/\text{km}^2\cdot\text{s}\), i.e., it is of the same order of magnitude as the regional heat flow.

Let us now compare the specific power of hydrothermal systems with the average power of active volcanic edifices, making use of the fact that the main share of the energy of volcanic eruptions (80–95%) is accounted for by the heat carried by eruption products \((^{11})\).

The age of the cone of the active Karymsky volcano (eastern Kamchatka), determined by the radiocarbon method, is about 7000 years (communication from B. V. Ivanov). The volume of the cone is \(3.35\ \text{km}^3\). Let us take the average density of the material composing the cone as \(2.4\ \text{g}/\text{cm}^3\), and the heat content of volcanic products as \(360\ \text{cal}/\text{g}\). The cone is situated in a caldera whose area is \(20\ \text{km}^2\). The explosivity coefficient for the volcanoes of Kamchatka is 50–60% \((^3)\); however, a considerable part of the pyroclastic material falls on the cone. It may be assumed that during formation of the cone, from 20 to 40% of the solid material was ejected beyond the caldera as a result of explosive eruptions. After calculation we obtain that the average thermal power of Karymsky volcano, referred to the area of its caldera, is \(750\text{–}1000\ \text{kcal}/\text{km}^2\cdot\text{s}\).

The specific thermal power for Krenitsyn Peak, located in the Tao-Rusyr caldera, Onekotan Island, ranges from \(520\) to \(700\ \text{kcal}/\text{km}^2\cdot\text{s}\). (The area of the caldera is \(42\ \text{km}^2\), the volume of the cone \(4.4\ \text{km}^3\), and its age 7000 years \((^2)\).)

The area of the volcano-tectonic depression of the Khukhaitup—Ostraya volcanoes (Sredinny Range, Kamchatka) is \(1500\ \text{km}^2\), and the volume of basalts erupted over the last \(1.5\cdot10^4\) years is \(600\ \text{km}^3\). For the Sedankinsky volcanic region of the Sredinny Range, the area of the depression is \(1000\ \text{km}^2\), the volume of basalts \(250\ \text{km}^3\), and the time of formation of the volcanic edifices \(10^4\) years \((^1)\).

Taking the average density of the erupted rocks as \(2.5\ \text{g}/\text{cm}^3\), and their heat content as \(400\ \text{cal}/\text{g}\), we obtain an average thermal power, referred to the areas of the Khukhaitup—Ostraya and Sedankinsky depressions, of \(850\) and \(800\ \text{kcal}/\text{km}^2\cdot\text{s}\), respectively.

A calculation of the total area occupied by 28 active volcanoes of Kamchatka gives \(4\cdot10^3\ \text{km}^2\). Taking the specific power of active volcanism to be \(800\ \text{kcal}/\text{km}^2\cdot\text{s}\), we obtain the average intensity of losses of deep heat for the entire zone of active volcanism of Kamchatka through volcanic edifices as \(3.2\cdot10^6\ \text{kcal}/\text{s}\). This value, in order of magnitude, agrees with the result obtained in \((^5)\), \(5.4\cdot10^6\ \text{kcal}/\text{s}\).

A comparison of the data presented makes it possible to conclude that, in an energetic sense, modern hydrothermal systems in regions of active volcanism and the active volcanic edifices of central volcanoes are equivalent, despite the extraordinary diversity of forms in which volcanic activity is manifested.

The close values of specific thermal power for modern hydrothermal systems and volcanic edifices indicate the commonality of the processes underlying volcanic activity.

The established magnitude of the specific power of active volcanism makes it possible to make quantitative estimates of certain volcanic phenomena.

Paroxysmal eruptions of Shiveluch volcano (Kamchatka) occurred in the Holocene, on average, once every 150 years, and during each eruption \(10^9\text{–}10^{10}\) tons of material were ejected \((^4)\). Taking a heat-flow value of \(800\ \text{kcal}/\text{km}^2\cdot\text{s}\), we obtain an area of the heat-generation focus of \(400\ \text{km}^2\), which corresponds to a magmatic body \(22\ \text{km}\) in diameter. The validity of this estimate will be shown by geophysical measurements, if such are carried out in the future.

Avachinsky volcano: the diameter of the magma chamber, determined by geophysical methods, is 6 km; the area of the horizontal projection of the chamber is 30 km². With a specific power of 800 kcal/km²·sec, we obtain a heat flux supplying the volcano of 24,000 kcal/sec. Through the fumaroles of the crater, 18,000 kcal/sec is discharged ⁵. Consequently, about 6000 kcal/sec accumulates in the volcano. The last eruption of the volcano occurred in 1945. It may be expected that the thermal effect of a future eruption, if it occurs, for example, in 1970, will amount to \(5 \cdot 10^{12}\) kcal (\(2 \cdot 10^{23}\) ergs). Such a thermal effect corresponds to the effusion of a lava flow 3 km long, 150 m wide, and 10 m thick.

Proceeding from an average specific thermal power of 800 kcal/km²·sec, it is also possible to predict in advance the magnitude of the natural thermal discharge of some hydrothermal systems. For example, for the Golovnin caldera on Kunashir Island, calculations give a heat flux on the order of \(10\,000 \pm 2000\) kcal/sec (caldera area 12 km²). The results of actual measurements in the Golovnin caldera, which will be carried out in the near future, will be the subject of a subsequent communication.

CITED LITERATURE

¹ V. N. Vinogradov, N. V. Ogorodov, Problems of the Geography of Kamchatka, No. 4 (1966).
² T. S. Gorshkov, Volcanism of the Kuril Island Arc, “Nauka,” 1967.
³ I. I. Gushchenko, in the collection Contemporary Volcanism of Northeastern Siberia, “Nauka,” 1964.
⁴ I. I. Gushchenko, Ashes of Northern Kamchatka and the Conditions of Their Formation, “Nauka,” 1965.
⁵ B. G. Polyak, Geothermal Features of the Region of Contemporary Volcanism, “Nauka,” 1966.
⁶ T. E. W. Barth, Volcanic Geology, Hot Springs and Geysers of Iceland. Corneg. Inst., 1950.
⁷ F. Durr, UN Nations Conf. on New Sources of Energy, Roma, 1964.
⁸ J. Healy, N. Z. Geol. Geophys., 7, No. 1 (1964).
⁹ R. A. Sonder, Studien uber heiss Quellen und Tektonik in Island. Zürich, 1941.
¹⁰ D. E. White, Geol. Survey Circular, 1965, p. 519.
¹¹ I. Yokoyama, Bull. Earthquake Res. Inst. Tokyo, 35, Part 1 (1957).
¹² K. Yuhara, A. Tomosada, S. C. Geophys. Inst. Kyoto Univ., No. 5, 33 (1965).