v20n3 From the Editors
By Janne Blichert-Toft, Sumit Chakraborty, Tom Sisson, and Esther Posner | June, 2024
This “Cratons to Continents” issue of Elements provides insights into the multiple ways geoscientists explore the 45% of Earth history encompassed by the Hadean and Archean eons. The contributions provide examples of geochemical, petrologic, and geophysical approaches to unraveling the origin and evolution of continents. Our authors explore the craton to continent journey from the darkly shrouded mysteries of the Hadean from 4.6 to 4.0 billion years ago through the merely old Archean eon, 4.0 to 2.5 billion years ago. Over these roughly two billion years, the Earth accreted from planetesimals, and developed its core, mantle, oceans, and earliest crust. The nature of the Hadean crust can be explored only indirectly as none has survived. Archean cratons preserve a legacy of crust with deep mantle lithosphere keels composed of greenstones and high grade felsic gneiss. Toward the end of the Archean, these rock suites gave way to continental crust we’d recognize today. Archean cratons are the oldest pieces of continental crust on the planet and form the nuclei of many continents, as shown in the image depicting the cratonic nuclei of Africa (bright green), which stand out against the background of younger crust. To quote the Beatles, “it has been a long and winding road!”
v20n2 From the Editors
By Janne Blichert-Toft, Sumit Chakraborty, Tom Sisson, and Esther Posner | April, 2024
Subduction, where one plate dives beneath another, controls long-term whole-Earth cycling of rocks, fluids, and energy. Plates subduct faster than they heat up, making them the coldest parts of the Earth’s interior. Fluids released from these cold plates rise into hotter overlying rocks, forming magma that feeds surface volcanism. Cold deep conditions associated with subduction complemented by hot shallow conditions under volcanic arcs are reflected in the presence of pairs of metamorphic belts, representing sites of ancient subduction. This issue of Elements guides readers through a premier example of paired metamorphism: the Cretaceous SanbagawaRyoke metamorphic pair of Japan. Estimates of pressure, temperature, the age and duration of metamorphism, and the tectonic framework in which metamorphism took place help us to develop quantitative models—both for the evolution of SW Japan and subduction systems in general.
v20n1 From the Editors
By Becky Lange, Janne Blichert-Toft, Sumit Chakraborty, Tom Sisson, and Esther Posner | February, 2024
Extraterrestrial organic matter—organic molecules beyond our planet—sparks curiosity and fuels the search for their nature, origin, and distribution due to their importance for cosmochemistry and for life. From the dusty surfaces of asteroids and the icy realms of comets, to planetary systems being born, galaxies far away, and atmospheres of exoplanets, scientists have uncovered a cosmic cocktail of carbonbased compounds. These extraterrestrial molecules, akin to the very essence of life on Earth, serve as enchanting hints that the building blocks of life may not be exclusive to our planet. Meteorites, cosmic dust, and other planetary samples carry the fingerprints of “out of this world” organic chemistry, which are, in a sense, cosmic secrets encoded in carbon compounds. As telescopes peer into distant locations and space rovers explore different landscapes, the quest for the origin of organic matter, building blocks of life, intensifies. Analytical developments and access to some of the most pristine organic-rich planetary samples are driving scientists into new frontiers. Thus, more groundbreaking revelations are inevitable.