Joanna V. Morgan, Sean P. S. Gulick, Timothy Bralower, Elise Chenot, Gail Christeson, Philippe Claeys, Charles Cockell, Gareth S. Collins, Marco J. L. Coolen, Ludovic Ferrière, Catalina Gebhardt, Kazuhisa Goto, Heather Jones, David A. Kring, Erwan Le Ber, Johanna Lofi, Xiao Long, Christopher Lowery, Claire Mellett, Rubén Ocampo-Torres, Gordon R. Osinski,, Ligia Perez-Cruz, Annemarie Pickersgill, Michael Poelchau, Auriol Rae, Cornelia Rasmussen, Mario Rebolledo-Vieyra, Ulrich Riller, Honami Sato, Douglas R. Schmitt, Jan Smit, Sonia Tikoo, Naotaka Tomioka, Jaime Urrutia-Fucugauchi, Michael Whalen, Axel Wittmann, Kosei E. Yamaguchi & William Zylberman (2016)
The formation of peak rings in large impact craters.
Science 354(6314): 878-882
Drilling into Chicxulub's formation
The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces.
Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.
Penny Barton (2016)
Revealing the dynamics of a large impact.
Science 354(6314): 836-837
Steady as a rock. We all know what to expect of rock. Rocks deform infinitesimally slowly. Earth scientists get excited at the prospect of “rapid” plate movements that average the same speed at which our fingernails grow. Humans don't make much impact on rocks, except at the most puny of scales. Sometimes nature does experiments for us that we could never do for ourselves: When a large meteorite hits the planet, interactions occur that are far outside our normal experience. The outer surface is deformed with a force and strain rate that we cannot begin to reproduce; rocks flow like fluid, very fast and on a huge scale. On page 878 of this issue, Morgan et al. (1) present results from a drilling expedition into the Chicxulub crater that reveal how the formation of peak rings in large impact craters occurs. Numerical simulations of the impact model the time scale of events: a rim of mountains, higher than the Himalayas, adjacent to a void 25 km deep and about 70 km wide, forming and collapsing within the first three minutes; the central fluidized peak rising and collapsing in minutes 3 to 4; and a shakedown period in minutes 5 to 10, leaving a shallow crater at the surface, an intensely deformed impact zone extending through the thickness of the Earth's crust and beyond, and the world changed forever.