13 - Birth of the solid earth

Summary

Astronomy tells us that the solar system is slightly more than 4.5 billion years old, the age of most meteorites. On earth, grains of the mineral zircon have been found that, although they are imbedded in much younger rocks, have an age of 4.3 by. The oldest rocks we know occur in the Slave Lake region of Canada and came into existence 3.96 by ago. Others only slightly younger are at Isua in Greenland, on the Limpopo River in South Africa, and in northeastern India. Staggering numbers that mean little to our own sense of time.

Distorted as they are by time and history, even the oldest rocks confirm the presence of an earth crust, of a sea where sediments were laid down, and of weathering and erosion on the land from which those sediments came. As our knowledge grows, it has become evident that the earth evolved in as little as 500 my from a ball of cosmic dust to a planet with many of its present properties, albeit in different configurations.

Those first 500 million pages are blank, but even the rest is difficult to decipher until we get close to the Phanerozoic. The reasons for this are common to all earth history, but greatly magnified in the Precambrian: our inability to tell time with precision, our frequent failure to read a record of which so much has been lost by erosion, burial, and a metamorphism often so intense that we cannot remove its overprint.