I think that geologists think that the Earth has been here more than 6, 000 years because the land has scars that have been made and it cant be made in 6000 years. The forms of the land of the rocks it cant be made in a short period of time it takes time for them to form in there shape.They also might of looked at the worlds population. The remains of things that used to be, along time ago that have been found could also be involved because when they found bones of dinosaurs or other they could of found that it is much older than 6000 years.