A fresh look at catastrophic impact-cratering: how do melt-bearing impact-deposits form?

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When asteroids collide with Earth they shock the crust causing the target area to vaporize, melt, and fragment in seconds. Vast quantities of searing-hot melted rock, rock-fragments and dust are ejected through the atmosphere, leaving craters <300 km across and draping the surrounding landscape with debris. This, and the attendant catastrophic air blasts, earthquakes and tsunami, devastate the environment, biota and ecosystems. Impacts change climate, cause global mass-extinctions, and arguably represent the greatest natural hazard to humanity. They have actually shaped Earth history and they produce diamonds and useful sources of precious metals. Its therefore surprising that fundamental aspects of the process are not understood: until we better understand what happens near the point of impact we won't properly understand the wider effects. Luckily there hasn't been a recent large impact, so our understanding has to be pieced together from forensic-style investigations of the products of old ones. Impact craters on Earth are really important as they provide hands-on access to internal structures, the ejecta deposits and the melt-fragments and minerals that yield vital clues to help reconstruct the extreme events. We will investigate how hot impact-melt is fragmented and transported rapidly across the landscape from the impact site. Deposits known as 'suevite' and 'impact-melt breccia' have proved immeasurably useful, telling us much of what we have learned about impacts, e.g the phenomenal high pressures involved. Yet surprisingly impact-melt fragmentation and transport remains one of the least-understood and controversial aspects of impact-cratering. Scientists who study volcanoes have long been tackling how hot melt breaks and how the resultant melt fragments are ejected across the landscape. They have developed increasingly sophisticated field and laboratory approaches to take-apart evidence preserved in deposits. Yet astonishingly such methods have not yet been adapted for the study of asteroid impacts. Impacts clearly differ from volcanoes (e.g. the initial temperatures and pressures) but melt fragmentation and transport must follow the same laws of physics. We propose to bring together, for the first time, world-leading physical volcanologists and impact-specialists, to integrate state-of-art approaches to improve understanding and to spearhead a new wave of research that will transform the field. Focusing on Earth's best-preserved large impact-crater, Ries (Germany) and selected other sites, we will reappraise impact-melt bearing rocks to determine, with fresh eyes, the processes they reveal. We will adapt 3 state-of-art methods from volcano-investigations: 1. the fine-art study of examining internal variations in subtle layering and grain sizes in granular rocks to reveal how the particles were transported and deposited from high-velocity ground-hugging density currents; 2. how the shapes and patterns of bubbles in glass shards produced by the fragmentation of visco-elastic melt - like rapidly-stretched custard - can be used to reveal the physical properties of the hot melt when it broke apart; and 3. how orientations (unpicked using rock-magnetism) preserved in deposits of catastrophic currents can be used to reveal hitherto-hidden information about how the impact-crater slopes morphed and shifted just after the initial impact through to the final stages of gravitational collapse and cooling. The work is very timely given the current state of flux in this field, and because it will draw on the very latest experimental data from volcanology, computer-simulations of impacts, a 2017 geophysical survey (Ries) and 2017 borehole (Chicxulub). From the results we will reconstruct how hot impact-melt behaves and moves as the developing impact-crater subsides. This will improve our understanding of how impact craters are created, and how hot material is transported across the planet, causing immense environmental damage.