The "memory" of the stones on the trail

Smallest variations in the Earth's gravity field and Earth's magnetic field allow conclusions about the structure and development of the Earth

UNESCO World Natural Heritage Mine Messel © Franz-Jürgen Harms, Senckenberg
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The gravity and the magnetic field of the earth influence our everyday life - be it by falling objects or by hikes with the compass. But for GGA Institute staff, in-depth knowledge of the Earth's gravity field and Earth's magnetic field also provides basic insights into the structure and evolution of our planet. Gravimetry and magnetics are the methods by which geophysicists can look into the interior of our earth.

"For some years, for example, we have been using both methods to study specific volcanic structures, called maars, " explains Gerald Gabriel of the Institute for Geoscientific Community Tasks (GGA Institute) in Hanover. "Thus, the analysis of the gravimetric and magnetic anomalies of the UNESCO world natural heritage Grube Messel near Darmstadt showed that this structure is probably a maar". This was subsequently confirmed in cooperation with the Senckenberg Institute through a 433 meter deep research well.

Conclusion on rock properties

Gravimetry - the determination of the gravitational field of the earth - could be used to estimate the thickness and distribution of sediments, which were deposited near the surface in the former Maarsee. Because these are characterized by a very low density compared to the much older rocks of the environment. "Such variations in density cause even the smallest differences in gravimetric measurements and allow appropriate conclusions on the rock properties, " said Gabriel.

LaCoste & Romberg Gravimeter G-1086 © GGA Institute

Highly sensitive measuring devices, so-called gravimeters, record the relative gravity changes up to the eighth decimal place. This value is not only determined by the geological structure, but it is also time and place-dependent: the geographical position, the height, topography or the tides play a role. But if the readings are corrected accordingly, they describe anomalies that are due solely to the inhomogeneous density distribution within the Earth's crust.

"For example, the lake sediments in the Messel pit cause a negative anomaly, " says Gabriel. This means that the position of the absolute minimum of gravity also marks the position of its maximum power. The interpretation of the anomalies is then generally done by model calculations. The required density values ​​of the different geological units are derived from borehole geophysical measurements and the weight of the cores. display

Change in the geomagnetic field

But in addition to the gravitational field, the Earth's magnetic field also provides valuable information about the Earth's interior. The Messel Pit used magnetics to capture the volcanic rocks at deeper depths. If certain rocks, such as the drilled lapillites, contain magnetic minerals, build up their own magnetic field as soon as they are brought into an external magnetic field: they receive an induced magnetization, which depends on a certain magnetic property, the so-called susceptibility t, Gabriel describes the investigation method. This magnetization strengthens the Earth's magnetic field at the location and generates positive anomalies. These in turn provide information about what it looks like inside the earth

Messel pit: gravimetric and magnetic anomalies (data: University of Mainz, GGA Institute) GGA Institute

Paradoxically, the corresponding investigations in Messel have revealed a negative magnetic anomaly. For the geophysicists, this was an indication that the Maerosparking must have taken place at a time when the prevailing magnetic field was opposite to today. This constant change in the geomagnetic field also makes magnetics exciting as a geophysical process. Because under certain conditions, rocks have a gift and store the magnetic field of the past.

They then acquire a so-called remnant magnetization which, depending on the polarity of the magnetic field at the time of rock formation, counteracts or amplifies the magnetic field of today, Gabriel explains the basic principle. Laboratory measurements have finally confirmed that the Messeler lapillites have a remanent magnetization. Their age of almost 48 million years falls into a phase of inverse polarity of the Earth's magnetic field. This explains the observed negative anomaly.

Fields of application Volcanism and plate tectonics

Of course, gravimetric and magnetic data are also of great interest in the exploration of many other geological structures, such as large mountain ranges or sedimentary basins. But also in terms of geodynamic processes such as mass movements inside a volcano or plate tectonics, they can provide important information, "Gabriel describes the fields of application. Therefore, work is currently being carried out at the GGA Institute on compiling Germany-wide gravimetric and magnetic anomaly data with the aim of making this information available to the geosciences in Germany for scientific work. An updated map of magnetic anomalies will be completed this year.

(Gerald Gabriel, Institute for Geoscientific Community Tasks (GGA Institute), 01.06.2007 - AHE)