Submitted by Vincent Vertolli, Assistant Curator Geology
The igneous, sedimentary and metamorphic rocks exposed at Earth’s surface provide the only permanent record of Earth’s history and evolution and analogous to human history the further back in time you go the less you have to work with. Very old rocks, those greater than 3,200 million years old occur on all the continents but in small blocks generally less than 500 km across and until recently the oldest rocks known were found at Isua in south-western Greenland. These sedimentary and volcanic rocks are known to have originated underwater between 3,700-3,800 million years ago and most investigators assumed this was about the time conditions had stabilized on Earth’s surface to maintain oceans and molten rock could harden into a crust. It was believed that prior to this time Earth’s surface was a hellish and fiery place with enormous amounts of energy being released from Earth’s interior and the surface was being bombarded by massive meteorite strikes. Most scientists assumed that this environment lasted for as long as 500 million years and aptly named it the Hadean (Greek for Hell) Era. Radiometric age-dating of the Acasta Gneiss, a rock found in Canada’s Northwest Territories and the Jack Hills Conglomerate from Western Australia has now changed our thinking about Earth’s beginnings.
The Acasta Gneiss, a rock that formed from magma that cooled and solidified deep within Earth’s crust is the oldest rock yet dated at just over 4,000 million years old. The Jack Hills Conglomerate contains the oldest mineral yet dated at almost 4,200 million years old. Although the Acasta Gneiss can’t tell us any information about conditions on Earth’s surface since it formed deep within the crust it does indicate the presence of a crust and the Jack Hills Conglomerate contains minerals that were eroded from a pre-existing crust or landmass. What this means is that our previous model of what early Earth was like may have to change: perhaps continents or small landmasses could have appeared 400 million years earlier than generally thought.