Deep heat: Earth’s crust beneath Germany is unexpectedly warm, new mapping reveals. According to this, the heat flow underground is on average 78 milliwatts per square meter – and thus around 20 percent higher than previously assumed. A particularly large amount of heat from the earth’s interior penetrates near the surface in the Ore Mountains, Upper Rhine Graben and Black Forest.

On the other hand, the subsoil is rather cool in the Bavarian and Palatinate Forest. This reveals the basics about the geology, but also the potential for geothermal energy.

Subsurface heat flow is a crucial feature of our and other terrestrial planets. Because it reveals how much heat is generated inside the earth and what the layers are like through which the heat reaches the surface.

Measuring the heat flow in the earth’s crust also allows conclusions to be drawn about geological and tectonic processes and structures, such as weak zones, massive mountain roots or volcanic-hydrothermal zones.

It is all the more important to know the heat flow underground as precisely as possible. Therefore, the International Heat Flow Commission (IHFC) is currently working to review and standardize the data on heat flow in different regions collected in a global database.

The problem, however, is that in Germany, many of the temperature measurements carried out since the 1960s in deep drillings, well drillings, mines or lake sediments have not used standardized or undocumented methods.

“This causes significant quality differences in the heat flow data, which lead to misinterpretations and distortions when calculating the depth temperatures,” explain Sven Fuchs from the German Research Center for Geosciences Potsdam (GFZ) and his colleagues.

In order to be able to adequately determine the heat flow, you need data on the depth-dependent temperature gradient and on the thermal conductivity of the rock at the sample location. The latter depends on the type of rock, but also on its water content. But in the past, thermal conductivity was often determined on dry laboratory samples.

“This leads to an underestimation of thermal conductivity, especially in porous sedimentary rock,” says the team. The measurement of the depth-specific temperature to determine the gradient was also handled differently.

This is where Fuchs and his team come into play. They re-analyzed the existing heat flow data and assessed its quality. This included both the systematic review of all available measurements since the 1950s and new measurement data not yet contained in the databases.

“The new database is a significant improvement for exploration of the geothermal field in Germany and a strong contribution to the revision of the European heat flow data as part of the ongoing Global Heat Flow Data Assessment project,” says Fuchs.

The result of the new analysis is a new heat flow map for Germany. It shows that the average heat flow in Germany varies from around 66 milliwatts per square meter to 83 milliwatts per square meter, depending on the geological region. The researchers determined 78 milliwatts per square meter as the area-weighted average heat flow density in Germany. “The terrestrial heat is 20 percent higher than previous estimates,” report Fuchs and his colleagues.

How warm it is underground depends primarily on the tectonic-geological conditions. The heat flow is particularly high in regions with granite rocks, for example. These include the Ore Mountains, the Fichtel Mountains, the Franconian Basin and parts of the Swabian Jura and the Black Forest. Tectonic weak zones such as the Upper Rhine Graben or former volcanic areas such as the Vogelsberg are also warmer than average.

In contrast, the heat flow is significantly lower in the Saale/Nahe area, in parts of the Upper Palatinate Forest and the Bavarian Forest. The geologists partly attribute the cooler subsoil in the two south-eastern German regions to the fact that the boundary between the cooler, firm lithosphere and the underlying soft, warm part of the mantle lies at a depth of around 115 kilometers. This transition is therefore significantly deeper than, for example, in the Rhine Graben with around 80 kilometers or parts of northern Germany with 90 to 100 kilometers.

For many regions in Germany, however, there is no reliable heat flow data at all, for example in north-western Germany, northern Saxony, southern Brandenburg and Thuringia and in parts of Bavaria. For some of these regions, the previous data did not pass the quality assessment, in many other cases no heat flow measurements have been carried out at all. This means that an important parameter for understanding the thermal field of the subsoil is missing.

“In view of the drastically increased demand to use the subsoil for various geoenergy applications, we have to close these large data gaps in Germany as quickly as possible,” emphasizes Fuchs. “These efforts will help to complete our picture of Germany’s geothermal resources.” The GFZ team is already preparing a new heat flow measurement campaign for the whole of Germany. This will allow the map of the underground temperature and heat flow field to be further populated. (Earth Science Reviews, 2022; doi:10.1016/j.earscirev.2022.104231)

Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences

This article was written by Nadja Podbregar

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The original for this article “Germany’s underground is warmer than expected” comes from scinexx.