Earth is the only planet know to havecontinents , although precisely how these distinct landmasses come to exist remains something of a enigma . Among the most exciting hypothesis is the idea that a barrage of giant impacts during Earth ’s babyhood led stable continental portions calledcratonsto form , and investigator have now cater the strongest grounds yet for this hypothesis .
“ By try out tiny crystals of the mineral zircon in rocks from the Pilbara Craton in Western Australia , which represents Earth ’s best - preserve remnant of ancient encrustation , we find evidence of these giant meteorite impacts , ” explicate study writer Dr Tim Johnson in astatement .
“ Studying the musical composition of atomic number 8 isotopes in these zircon crystals revealed a ‘ top - down ’ cognitive process start with the melt of John Rock near the surface and progressing deeper , coherent with the geologic effect of gargantuan meteorite impacts . ”
antecedently , researcher had tentatively mooted the notion that Earth ’s cratons may have been mold during theLate Heavy Bombardment , when the internal solar system was purportedly battered by a disproportionately large number of asteroid . establish on the eld and distribution of craters on the moonshine , scientists consider that the impingement pace declined importantly between 3.9 and 3.5 billion years ago .
“ That the age of the oldest continental crust in most cratons also cross the clip menstruum 3.9–3.5 [ billion year ago ] beg the question of whether this is concurrence or if there is a causal relationship , ” drop a line the study generator in the journalNature .
To enquire , the researchers looked for change in the denseness of a fussy oxygen isotope called oxygen-18 within thePilbara Craton . Previous studies on large impact craters have revealed that such events cause meaning melting of the shallow mantle , resulting in a diminution in oxygen-18 relative to lighter isotopes .
Their analysis revealed that the Pilbara Craton formed in three stages , the first of which occur between 3.6 and 3.4 billion years ago . Zircon vitreous silica find out within this layer were isotopically lightheaded , indicating that they may have “ crystallize following an initial giant impact that finally led to formation of the Pilbara Craton . ”
agree to the generator , the hit that set off this effect would have regard a monumental space rock measuring “ several tens to hundreds of kilometres in diameter . ” Significantly , the researchers note that a similar isotope pattern can be learn in the zircon record of some of Earth ’s other craton , including the Yilgarn Craton of Western Australia .
“ Our enquiry provides the first whole evidence that the processes that ultimately take form the continents begin with gargantuan meteorite impacts , similar to those responsible for the defunctness of the dinosaurs , but which occurred 1000000000 of years originally , ” says Johnson .
“ Data related to other areas of ancient continental crust on Earth appear to show patterns similar to those recognised in Western Australia . We would like to examine our finding on these ancient rock to see if , as we distrust , our mannequin is more wide applicable . ”
