How old is the Earth?
Thanks to meteorites from space, rocks brought back by the Apollo astronauts from the Moon, and sundry other long-distance readings (mostly from satellites) taken of planetary bodies throughout the solar system, scientists have been able to calculate the age of the Earth. They believe the planets, including the Earth, formed between 4.54 to 4.58 billion years ago. In general, most scientists say that the Earth formed somewhere in between-about 4.55 to 4.56 billion years ago. (For more information about the Earth’s age, see “The Earth in Space.”)
The reason for the reliance on other space bodies to determine the Earth’s age is simple: The movement of the lithospheric plates around our planet has recycled and destroyed the Earth’s oldest rocks. If there are any primordial rocks left on Earth, they have yet to be discovered. Therefore, scientists must use other means to infer the age of our planet, including the absolute dating of planetary rocks that probably formed at the same time as the Earth.
What are some of the oldest rocks so far discovered on Earth?
Scientists have found rocks exceeding 3.5 billion years of age on all the Earth’s continents. But the oldest rocks uncovered so far are the Acasta Gneisses in north-western Canada near Creat Slave Lake, which has been dated at about 4.03 billion years old. Others that are not as old include the lsua Supracrustal rocks in West Greenland (3.7 to 3.8 billion years old), rocks from the Minnesota River Valley and northern Michigan (3.5 to 3.7 billion years old), rocks in Swaziland (3.4 to 3.5 billion years old), and rocks from western Australia (3.4 to 3.6 billion years old). These ancient rocks are mostly from lava flows and shallow water sedimentary processes. This seems to indicate that they were not from the original crust, but formed afterward.
The oldest materials found on Earth to date are tiny, single zircon crystals uncovered in younger sedimentary layers of rock. These crystals, found in western Australia, have been dated at 4.3 billion years old, but the source of the crystals has not yet been discovered.