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El’gygytgyn: a very special meteorite impact crater
Website of the FWF project: P21821-N19 "Studies of the El'gygytgyn Impact Crater"

Christian Koeberl
(University of Vienna, Austria; christian.koeberl@univie.ac.at)

Summary

The El’gygytgyn crater formed 3.6 million years ago by the impact of a large meteorite or asteroid, about 1 km in diameter. The crater and the lake that fills most of it are of scientific interest for two main reasons. First, this is the only known meteorite impact crater that formed in acid volcanic rocks and thus it offers the unique possibility to study the impact and shock effects on such rocks, which has implications for comparative planetology. Second, the about 300 meter thick deposit of lake sediments that were laid down on top of the impactites constitutes a unique climate archive of the largely unknown Arctic climate history. Investigating the lake sediment drill cores will provide important constraints on cause and effect of climate changes in the polar regions. Such work has great implications for the understanding of future climate change.


Introduction

Impacts of asteroids and comets represent the most energetic and spectacular geologic process currently known. The study of impact craters on Earth and on the Moon, as well as astronomical investigations of the orbits of asteroids and other solar system bodies, have allows the determination of the cratering rate for Earth (how many craters of which size form). Impact craters are actually the dominant land form on all bodies of the solar system that have a solid surface. Even on bodies that have an atmosphere, impact cratering is an important process. In contrast, such craters are not necessarily an obvious land form on the Earth. The reason is that on Earth active geological processes (ranging from plate tectonics to volcanism, erosion, and interaction with the hydro- and atmosphere) lead a rapid obliteration of any craters that form on the surface of our planet. Nevertheless so far about 175 impact craters have been recognized on Earth (Fig. 1).

Fig. 1: Examples of simple and complex impact craters on Earth. From left to right; top row: (A) Twaing, South Africa (1.2 km diameter, age 250,000 years); (B) Wolfe Creek, Australia (1 km diameter, age 1 million years); (C) Meteor Crater, Arizona, USA (1.2 km diameter, age 50,000 years); middle row: (D) Lonar, India (1.8 km, age ca. 50,000 years); (E) Mistastin, Canada (28 km diameter, age ca. 38 million years); (F) Roter Kamm, Namibia (2.5 km, age 3.7 million years); bottom row: (G) Clearwater double crater, Canada (24+32 km diameter, age ca. 250 million years); (H) Gosses Bluff, Australia (24 km diameter, age 143 million years); and (I) Aorounga, Chad (18 km diameter, age <300 million years).

The nature of the impacting bodies has been resolved – they are mainly asteroids and a few comets. The formation of a typical impact crater is an extremely fast process (Fig. 2).

Fig. 2: Schematic diagram of the formation of an impact crater, from top left to bottom right. An extraterrestrial body hits the surface of the Earth with cosmic velocity (about 10-70 km/s), resulting in a shock wave that hemispherically propagates into the ground, while the impactor is destroyed by melting and vaporization and some of the near-surface material is ejected at very high speeds. Then a release wave travels back towards the surface, which initiates the actual crater formation by mass flow and ejection of material above the crater to great altitudes. A high-energy explosion is a good analogy to this process. At the end, during the crater modification stage, the crater rim collapses inwards and fallback material fill part of the so-called transient cavity, resulting in a crater fill of brecciated material and melt rock. The crater floor below this breccia lens is intensely fractured. The main phase of crater formation lasts just a few minutes.

Fig. 3: Correlation between the frequency of impact crater formation (expressed in years between events, as well as the annual cumulative impact probably) and the diameter of the impactor. The probabilities for the formation frequency of craters similar in diameter to a variety of well known impact craters, including El’gygytgyn, is shown; also shown are the minimum estimates for a hypothetical Permian-Triassic impact event, even though no indications of an impact event at that time are known.

First, an asteroid hits the surface of the Earth (or any other planetary body) with very high velocity (about 10-70 km/s) and creates a shock wave that penetrates into the ground, where it leads to irreversible changes in the rocks and minerals of the target, and the impacting body is vaporized. Then the release wave runs back to the surface, and causes the ejection of large amounts of material out of the crater. In large craters, a central uplift forms by rebound. The whole process takes only a few minutes even in large craters. Basically a very small body hits the surface with high velocity and causes the formation of a crater that is about 20 times the diameter of the impactor. The relation between impact frequency and size is straightforward – smaller impacts happen more often than large ones (Fig. 3).

The physical effects of large-scale impact events are severe, ranging from burning due to the expanding fireball, seismic effects, possible tsunamis, and ejection and deposition of large amounts of rock and dust from the impact site. In very large events, these effects are global. An example of an impact event that had global implication is the formation of the about 200-km-diameter Chicxulub impact structure in Mexico, 65 million years ago, at the end of the Cretaceous. This event led to a severe mass extinction, in which more than half of all of the then living species (fauna and flora) became extinct. Despite the well-documented link between the impact and the mass extinction in this case, there is – so far – no clear link between other mass extinctions (e.g., the end-Permian or end-Triassic extinctions) and impact events. Nevertheless, the importance of impact events of the geological and biological evolution of the Earth is undeniable.


Frequency and Recognition of Impact Craters

Fig. 4: Microphoto of a shocked quartz grain from a polymict impact breccia found within the El’gygytgyn crater. The multiple sets of straight, parallel lines are actually planes that penetrate the crystals and are called planar deformation features and are characteristic for an origin by shock. The presence of such shocked minerals is unambiguous evidence for the impact origin of a geological structure. (Width of image 1 mm, plane-polarized light.)

Even though on Earth impact craters (and other shallow land forms) are rapidly destroyed by erosion and other geological processes, it has been possible to identify about 175 such meteorite craters so far. So the question is, how does one recognize impact craters? How can they be distinguished from other (circular) geological features? On the Moon and other atmosphere-less bodies simple morphological criteria can be used. On Earth, however, there are numerous geological processes that can result in circular features. There are only two generally accepted criteria for the distinction of meteorite impact craters from other features on Earth: a) the presence of shock metamorphic effects in rocks and minerals (which is a consequence of the interaction of these rocks with the impact-induced shock wave), e.g., in the form of shocked minerals (Fig. 4), and/or b) the identification of traces of meteoritic material in breccias or melt rocks within or around the crater.

The frequency of impacts depends on the size of the impacting bodies. Small bodies hit the Earth relatively often, whereas large collisions are rare. For example, a crater the size of “Meteor Crater” in Arizona with a diameter of about 1 km (formed by the impact of an about 40 meter diameter iron meteorite) occurs about once every few thousand years. A crater the size of El’gygytgyn (18 km diameter, about a kilometer-sized asteroid) happens about once every million years, and really large events, such as that at the end of the Cretaceous, 65 million years ago (which led to the formation of the 200-km-diameter Chicxulub crater and the extinction of the dinosaurs and about half of all living species on Earth a that time), only occur about once per hundred million years (Fig. 3).

The Importance of the El’gygytgyn Impact Crater

The 18-km-diameter El’gygytgyn crater is located on the Chukotka peninsula, northeastern Russia (Figs. 5 and 6. It formed about 3.5 million years ago by the impact of an about kilometer-sized asteroid. This crater is of great interest for both, impact cratering studies and paleoclimate investigations, because it is located in an area of the high Arctic for which few paleoclimate data exist (Fig. 7). The deep basin that forms during a meteorite impact is an ideal location for the accumulation of lake sediments that carry the climate information. A major aspect of its importance is that it represents the only currently known impact structure formed in siliceous volcanic rocks, including tuffs. The impact melt rocks and target rocks provide an excellent opportunity to study shock metamorphism of volcanic rocks. The shock-induced changes observed in porphyritic volcanic rocks from El’gygytgyn can be applied to a general classification of shock metamorphism of siliceous volcanic rocks. That El’gygytgyn is an impact crater was already confirmed in the late 1970s by studies of the geologist Evgeniy Gurov from Kiev and his team, who found shocked minerals (Fig. 4) and impact glasses (Fig. 8) at the crater. However, the impact rocks on the surface are almost totally removed by erosion, and so the deep drilling project provides the unique opportunity to study the crater-fill impactites in situ and determine their relations and succession. The investigations are expected to provide information on the shock behavior of the volcanic target rocks, the nature and composition of the asteroid that formed the crater, and the amount of energy that was involved in the impact event. This will also allow constraining the effects this impact event had on the environment.

Fig. 5: Satellite image of the El’gygytgyn impact crater, Arctic Russia (NASA Aster image). The images shows the 12-km-diameter Lake El’gygytgyn, which is asymmetrically located with the 18-km-diameter impact crater (the crater rim is indicated on the radar image – inset on upper left).

Fig. 6: Landsat satellite image of the El’gygytgyn impact structure (NASA).

Fig. 7: Location of the El’gygytgyn structure in the northeastern corner of Siberia, at the Chukotka Peninsula, with respect to the Arctic (extent of sea ice in summer of 2008; NASA image). The study of the lake sediments will provide valuable information on the development of the climate in the area during the past 3.5 million years.

Fig. 8: Microphoto of an impact glass from the El’gygytgyn crater, which clearly shows the deformation and twisted layering that resulted from high-temperature melting at temperatures of around 2000°C. (Width of image 1 mm, plane-polarized light.).


The ICDP drilling project at the El’gygytgyn impact crater


During the last years, El’gygytgyn became the target of a large international drilling project at that is coordinated by the International Continental Scientific Drilling Program (ICDP) and that was recently successfully concluded. The main goals of the project are to obtain, from analyses of the drill cores, new information the formation of the impact crater, as well as to derive a climate history of the Arctic. The principal investigators of this project were Christian Koeberl (University of Vienna) together with Prof. Julie Brigham-Grette (University of Massachusetts-Amherst, USA), Prof. Martin Melles (Univ. Cologne, Germany), and Dr. Pavel Minyuk (Russian Academy of Sciences, Magadan, Russian Federation). The investigation of the impact breccia drill cores will be coordinated by Christian Koeberl, and studies in Vienna are funded by the Austrian Science Foundation FWF through grant P21821-N19.

Drilling Project Picture Gallery

The logistically very challenging drilling project was successfully concluded during the first half of May 2009. Just the planning of this project, from the scientific concept to the logistical planning, application for funding, and obtaining all the necessary permits, took over 8 years. Several hundred tons of equipment had to be transported to the very remote drilling location. Drilling was done from the top of the frozen lake, where it turned out, for example, that the actual ice sheet had to be strengthened by pumping more water to the surface where it froze to increase the ice thickness, so that the about 75-ton drilling platform and all the supporting vehicles were safe above the 170-meter-deep lake. The closes town is Pevek at the Arctic Ocean, at a distance of about 350 km from the drilling location. In Pevek is a port (ice-free only for a few months in the summer) and an airport, which is connected to Moscow by only one flight every two weeks. The complete drilling equipment was sent by ship to Pevek during the summer of 2008 and then transported over land, on a specially constructed snow road, to El’gygytgyn. Personnel and scientists, as well as sensitive equipment, were transported to the lake by cargo helicopter.

At the end of 2008, a shallow hole was drilled near the shore of Lake El’gygytgyn to study the permafrost characteristics. In early 2009, the first scientists and technicians arrived at the lake together with the equipment for the deep drilling on the frozen lake. The drilling equipment was assembled and ready in late February 2009. Temperatures around -30°C and snow storms of up to 100 km/h, resulting in wind-chill factors of -50°C, made the working conditions difficult. Drilling started on March 18, 2009 with the retrieval of the first sediment cores. Technical problems caused some delays due to the drill string getting stuck and then sheared off, at depth of 117 and 143 m below the lake floor. After spare parts were flown in, drilling continued in April 2009.

On April 14, 2009, at a depth of about 312 meters below the lake floor (total depth 482 meters) the drilling reached the transition zones between the post-impact lake sediments and the impact breccia deposits. This also represented the time marker of 3.6 million years. This important moment was the conclusion of a long and difficult planning and drilling process. In total the drilling reached a depth of 517.3 meters below the lake floor, or a total depth, from the lake surface, of 687.3 meters. In the fall of 2009, the cores were transported from Pevek via St. Petersburg to Europe, where they are divided into sedimentary cores for paleoclimate studies (mainly to be done by the colleagues in Germany and the USA), and the impactite cores.

Initial plans called for three drill cores to be retrieved (Fig. 9), but due to technical and logistical limitations, only two – the permafrost core on the lake shore, and the deep core at location D1 in the center of the frozen lake, were drilled. During the drilling project it was indeed possible to retrieve impact breccia samples below the post-impact lake sediments. Immediately underneath the lake sediments there is a unit of so-called suevitic breccia, which has a thickness of at least 50 meters. A suevite is a glass-bearing polymict impact breccia, which contains fragments of a variety of rocks that represent different layers in the target that are cemented in a fine-grained matrix (Fig. 10). The glasses form by melting of the target rocks at very high temperatures. Such breccias are uniquely characteristic of impact craters on Earth, and not found in any other geological location. Underneath the suevites is a unit of shocked and locally brecciated volcanic target rocks, which was uplifted during the impact event. During the formation of such central peaks, which are typical for impact craters of this size (which are also called “complex craters”), deeper layers of target rocks rebound towards the surface and then solidify – thus a mountain of several kilometers in diameter is uplifted within less than a minute by over a kilometer; truly a spectacular geological process. The over 200 meters of impactite drill core will allow the detailed study of all these processes that occurred during the formation of the El’gygytgyn impact crater. Besides a detailed study of the shock behavior of the volcanic basement rocks, our group in Vienna will attempt to determine the composition and nature of the meteorite that formed the crater. In addition, it is hoped that the studies will allow constraining the energy that was released during the impact, which in turn has implications for the environmental effects of the impact event.

Fig. 9: Schematic cross section of the El’gygytgyn crater ans lake (image: Univ. Cologne), showing the locations of the three planned drill cores. Due to technical and logistical limitations, only the D3 (permafrost; end of 2008) and D1 (Spring 2009) were drilled.

Fig. 10: A core segment from the current drilling project at the El’gygytgyn impact crater, showing a suevitic breccia – which is a melt-bearing polymict impact breccia (i.e., a rock that consists of broken pieces of many different target rocks, with a little impact melt rock mixed in), from about 316 m below the lake floor, just below the transition from the post-impact lake sediments. The glassy melt rock, which forms during the impact when some of the rock is heated to over 2000°C, is the dark gray frothy inclusion in the center of the core segment.
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