The oldest materials on the planet are tiny zircon grains that
were washed into conglomerate in Western Australia about 2650 to
3050 Ma ago. It wasn’t the fact that the grains are zircons,
which are among the most durable materials around, but the range
of ages that they revealed when routinely analysed. U-Pb dating
of detrital zircons is a well tested means of finding the
provenance of sedimentary materials as an indicator of orogenic
and igneous events that formed the crust from which they were
eroded. In the original study of the Jack Hills zircons some
showed ages that might reasonably have been expected from late
sediments in an Archaean craton: around 3.5 billion years is
about the maximum age for orogenic events there. What astonished
all geoscientists was that a proportion of the grains gave ages
of more than 4 billion years, some as old as 4.4 Ga: here was a
window on the missing first half billion years of Earth history,
the Hadean. Subsequent work on yet more zircons confirmed the
original age span