However, the cause of this mass extinction was keenly sought but not known until Alvarez et al suggested that it was caused by the impact of a huge asteroid (4) (5) .
Subsequent work has identified the Chicxulub crater as being the site of the bolide event which most closely corresponds in time to the K-T boundary (6) .
Zircon is an extremely hard and stable crystal, which incorporates uranium on crystallisation but little lead. Over time, radiogenic lead accumulates in the crystal providing a measure of age.
The age for an undisturbed crystal, using the U/Pb ratio from two uranium-to-lead decay schemes, plots on what is known as the concordia curve (2).
Furthermore these signs correlate with the degree of resetting of the U-Pb clock (8) .