INSIDE GeoLaB - The Trail of the Stones

Photo of Tromm granite. Typical apperance of granite from the Tromm region.

Typical apperance of granite from the Tromm region. © D. Scheuvens

Associate Professor Dr. Jens Carsten Grimmer is a geologist at the Institute of Applied Geosciences at the Karlsruhe Institute of Technology. His field of expertise is the geology of the Upper Rhine Graben and its bordering mountain ranges. Currently, he and his colleagues in the GeoLaB project are analyzing drill cores obtained from two exploratory boreholes at the Tromm in the Odenwald. Particularly important are fractures and cracks in the rock. And there has even been an interesting discovery!

Twelve tonnes of drill cores with a total length of 500 meters — why do you go to such lengths to study them?

We want to understand what the subsurface looks like. In order for us geologists to form an accurate picture, we have to drill into the ground and bring the rock to the surface. Only then can we examine it in the laboratory and, in parallel, develop computer simulations.

What kind of rock do the drill cores consist of?

In the upper 400 meters, we mainly drilled through granite. Granite is magma that cooled and crystallized deep underground — in other words, rock that was once molten. It is also the rock that lies at the surface here in the Odenwald on the Tromm. Granite occurs on all continents of the world in the Earth’s crust within the upper fifteen kilometers. But we also found something else: at a depth of about 410 meters, the research boreholes show a transition from granite to gneisses and other rocks. These different rock types can be used for the planned experiments in the rock laboratory, which expands the scope of our research.

Let’s take a journey back in geological time — when did this rock form?

The Tromm granite is 339 million years old and formed at a time when two continents collided. During this collision process, granitic melt (magma) rose from greater depths. At a depth of around 10 to 15 kilometers, the melt, as we geologists say, “took its place,” cooled, and crystallized. In the process, the typical, beautiful minerals with their shiny crystal surfaces formed — the minerals that make up granite.

What is the significance of the fractures in the Tromm granite?

At first glance, we can see one or more fractures in almost every drill core recovered. Detailed investigations with specialized probes have shown that there are well over 1,000 cracks and fractures around the boreholes. Some of them are capable of transporting water. These fractures are extremely important for us because we need them for our planned geothermal research. If geothermal energy is to be extracted from granite deep underground, thermal water must flow through cracks and fractures in the rock to absorb heat. We want to understand how water flows through fractures and what changes occur in the process. In GeoLaB, the processes that take place deep below the Earth’s surface in geothermal projects will be recreated and investigated in the rock laboratory.

From a drilling site on the Tromm, two exploratory boreholes were drilled. In one of them, intact rock was extracted along the entire length of more than 500 meters and brought to the surface as drill cores. These allow scientists to gain direct insights into the rock.
A drill bit used in the exploratory boreholes in crystalline rock. Various drill bits were used for the two boreholes, reaching depths of more than 500 meters.
Drilling operations: A steel pipe containing a drill core is being pulled out of the borehole.
A drill core is being pulled out of the steel pipe.
A drill core showing the typical Tromm granite.
A section of a drill core is labeled and placed in a special storage box.
Drill cores from different depths. They show the transition from magmatic granite to metamorphic gneiss.
Paths of the two exploratory boreholes. The drill cores and geophysical measurements revealed a transition from granite to gneiss at a depth of around 410 meters — an exciting discovery. The curved lines bear witness to the enormous tectonic forces that acted on the rocks during their formation and later transformation.