The Genesis of Rocks

The life story of rocks is reflected in the minerals they contain and their structure. Lithospheric researchers with their small-scale rock analyses provide the basis for understanding large-scale geological processes on our planet as well as for identifying extraterrestrial materials in the Earth’s crust.

Feb. 1, 2016

In the far north-east of Russia, on the Siberian peninsula of Chukotka, lies the 18 km-diameter El´gygytgyn crater. The formation, which was created by a meteorite impact 3.6 million years ago, has long been the focus of scientific interest. El’gygytgyn is the only impact crater known on Earth to have formed in acid volcanic rocks. “This gives us the unique opportunity to investigate the shock effects of acid rocks by studying core samples,” says Christian Koeberl, impact researcher and geochemist, who also serves as the Director-General of the Natural History Museum in Vienna.

New analysis methods allow us to take the fingerprints of meteorites in impact rocks and draw conclusions about the nature of the impact bodies.”

Christian Koeberl, Professor of Impact Research and Planetary Geology

With his Research Group Impact Research & GeoCosmoChronology at the University of Vienna, he has been involved in the “International Continental Scientific Drilling Program” (ICDP) project at El’gygytgyn since the beginning of the project. In a follow-up project funded by the Austrian Science Fund (FWF), the researchers analysed core samples taken from the crater lake and tried to find ways of distinguishing the volcanic bedrock from rocks influenced by the meteorite impact.

In 2016, another large ICDP and IODP (International Ocean Discovery Program) project will take the Viennese scientists to Yucatán, Mexico, to the famous Chicxulub impact structure. The impact of a particularly large asteroid that occurred there is widely considered the cause of the mass extinction event 65 million years ago that also resulted in the extinction of dinosaurs. The crater has been preserved in good condition, making it an important natural laboratory for impact research. In the new drilling project, the research partners – Christian Koeberl is one of the six principal investigators of the ICDP project – want to study the peak ring form of the crater and the behaviour of the rocks with regard to the impact as well as investigate the environmental changes caused by the impact, which are supposed to have led to global mass extinction.

From impact to geochemistry and cosmochemistry

In addition to impact research, Koeberl’s Group also studies geochemistry and cosmochemistry: “We try to understand not only how impact craters are formed but also the physical, chemical and geological processes that are associated with the impact.” To this end, the researchers use isotope analyses as well as geochemical and geochronological methods in dedicated laboratories. Recently, the Department was able to install the equipment for a method that is available only in very few places worldwide: osmium isotope analysis. It can be used to detect traces of extraterrestrial materials in the rocks affected by the impact. “It essentially gives us a kind of fingerprint of the impacting body,” he explains.

Large meteorite impacts generally influence the material of the Earth’s crust. This creates so-called impact breccias (fragments of various rocks that have subsequently become solidified in one piece) or melt rocks. Isotope analyses allow the researchers to detect the extremely small traces of extraterrestrial material – the majority of the meteorite is vaporised on impact – and distinguish them from the large amount of terrestrial rock. The largest class of meteors – chondrites – contain a considerably higher proportion of siderophile (or “iron-loving”) elements than terrestrial rocks. This includes platinum metals such as iridium or osmium, which occur at a rate 50,000 times higher than in crustal rocks. By measuring the enrichment of the platinum metals and the ratio of elements to each other (e.g. the rhenium (Re) – osmium (Os) ratio and the ratio of the isotopes ¹⁸⁷Os and ¹⁸⁸Os to each other), researchers can show the presence and amount of extraterrestrial components in the impact rock. “This also allows us to draw conclusions about the type of meteorite we are dealing with,” says Koeberl.

Rock analysis in the lab

The theoretical and experimental analysis of rocks, their properties and their formation history are the focus of the Research Group of the petrologist Rainer Abart: “Whether in terrestrial or lunar rocks, in meteorites, in slags from industrial smelters or in ceramics production: We are interested in all processes of rock formation,” says the Head of the Research Group Petrology. Our focus, he says, is on the high temperature range, as found in the Earth’s crust and mantle and applied in industrial processes.

For predicting the dynamics of the deep Earth, we need to understand the material it is made of.”

Rainer Abart, Professor of Theoretical and Experimental Petrology

In some ways, the work of the petrologist is similar to that of medical diagnostics: “Instead of taking tissue samples for histological analyses, we take thin rock cuttings for petrographic analysis,” says Abart. Optical microscopes with a resolution of down to 1 micrometre “already give access to many useful diagnostic indicators on the formation history of the rock”, he says. However, many structures in rocks are considerably smaller. Scanning electron microscopy allows the petrologists to access structures in the  nanometre range. Electron microprobes and scanning electron microscopes also provide information on the chemical composition of the material as well as on crystal structures and orientations.

“Processes such as magmatic crystallisation, rock metamorphism or deformation happen at the atomic level but have far-reaching implications on all scales: Understanding the microscopic processes underlying rock formation allows us to predict the behaviour of the bulk material,” Abart explains. Knowing how different types of rock react to changes in temperature and pressure is a prerequisite for understanding, e.g. tectonic plate movements. 

Materials science for geomaterials

Abart’s team regard their work as materials science focused on geological materials. Transferring concepts from materials science to geological systems was also the objective of the project “Nanoscale Processes and Geomaterials Properties” (2008-2016), which has been funded by the German Research Foundation (DFG) and the FWF. An interdisciplinary team studied how and at what rates substances move through geological material and how information about the formation of rocks is stored in their building blocks.

“We want to understand reaction mechanisms in geomaterials, particularly in the solid state, calibrate them experimentally and ultimately transfer them to natural rocks,” says Abart, who directs the project together with Wilhelm Heinrich from the German Research Centre for Geosciences (GFZ) in Potsdam. In 2014, the researchers were able to create crystal orientation maps based on experiments on crystals under directional stress. They show by which mechanisms crystals grow at different pressures.

In cooperation projects with industry partners, Abart’s Group also deals with artificial rocks such as refractories or slags. The focus is often on optimising production processes, e.g. in order to improve the high-temperature properties of ceramics . The principle is the same: The formation processes determine the material properties.