The age of the earth is constrained to 4.55 b.y. based on many age determinations of meteorites.  Prior to a decade ago, the oldest rocks you could hold in your hand were 3.8 b.y. old, significantly younger than the age of the earth.  Remnants of old rocks in Canada, the Acasta Gneiss, were subsequently found to be 3.96 b.y. old.  In the last year, however, based on work by Prof. John Valley in this department, old mineral grains (the mineral zircon) that have ages as old at 4.4 b.y. old have been found in Australia.  Moreover, the composition of the zircon grains suggests that liquid water was present on the surface of the Earth at this time, a remarkable conclusion considering the melting of the surface of the earth that must have occurred only a few 10's of millions of years before due to formation of the earth's core and the moon.

Click on the thumbnails below for larger image.

The photo above is a microscope image of the oldest mineral grain in the world, with the ages (in m.y.) of the spots that were analyzed by Prof. Valley's group.

This is the locality in Western Australia where the very old mineral grains were found - not too impressive!

Photo of the group involved in this research; Prof. Valley is on the left.

The implications for this discovery are profound, and greatly compresses the time line for the first billion years of earth history.

So, we are left with a model where the early earth went from a completely melted outer surface to having an ocean in only 10's of millions of years.  The illustration above depicts such an earth, where the moon may have still had a magma ocean on it when the earth itself was cool enough for a liquid water ocean.

The figure above shows how this revised history of the early earth fits into the better known history of the earth in its last 3 b.y.

Spring 2003 Semester