Using the crust’s electrical conductivity and density to study the Earth’s innards

The lower crust and topmost mantle are coloured by density anomalies at depths between 28 and 39 kilometres. The colour red represents a liquid conducting phase with a density below average, whereas the colour blue suggests a neutral to slightly over normal density with a dry melt or solid conductive phase. Yellow stars indicate the location of the Yellowstone Hotspot. Credit: Geophysical Research Letters (2022). DOI: 10.1029/2021GL096336

Understanding the structure and composition of the Earth’s crust is essential for comprehending the dynamics of the planet and its environment. For example, in plate tectonic processes, the presence or absence of melt or fluids has a significant impact in the process. Geophysical surveys have provided us with the majority of our information in this domain. The link between quantifiable geophysical characteristics and the real conditions in the Earth’s interior, on the other hand, is frequently vague and difficult to establish. This state of affairs has been addressed by Max Moorkamp, a geophysicist at LMU who has devised a novel way in which data on the distribution of electrical conductivity and density in the Earth’s crust is merged and processed using a method adapted from medical imaging. “The advantage of this approach is that the connections between the two factors are included in the study,” explains Moorkamp. “This is absolutely novel in terms of geophysical applications.”

Moorkamp was able to demonstrate, using the new technique, that earlier assumptions regarding the geographical distribution of magma and fluids in the western United States may have been oversimplified in their assumptions. Researchers had previously concluded that molten rock (magma) and fluids were prevalent in geologically young and dynamic places based on electrical conductivity studies, but older and more stable locations were assumed to be completely devoid of molten rock and fluids. “However, the new findings paint a more nuanced picture,” Moorkamp explains. While the electrical conductivity of molten rock and fluids is remarkably similar to that of solid graphite and sulphides, melts and fluids, in contrast to solid graphite and sulphides, indicate that geologic activity had occurred recently.

The method used by Moorkamp allowed him to distinguish between these two types of structures for the first time. This allowed him to demonstrate that, even in the highly active region around Yellowstone, there are fluid-dominated structures that are directly adjacent to fluid-free areas that contain graphite and sulphides. This leads the geophysicist to the conclusion that, when contrasted to present geologic activity, geologic history—specifically older plate tectonic processes—has a far bigger effect on the placement of fluids than was previously thought possible. The results of previous studies, not only in the United States but throughout the world, may need to be revised as a result. Furthermore, the approach may prove to be quite beneficial in the quest for geothermal energy or mineral reserves.

In Geophysical Research Letters, the findings of the investigation are reported.

Further information: Max Moorkamp, Deciphering the State of the Lower Crust and Upper Mantle With Multi‐Physics Inversion, Geophysical Research Letters (2022). DOI: 10.1029/2021GL096336

Journal information: Geophysical Research Letters

Source: Ludwig Maximilian University of Munich