They discovered the microtektite layer above the sediment layer that recorded the reversal and estimated the impact occurred 30,000 years after the beginning of the reversal.
Scientists proposed this theory 20 years ago when they noted that a turnover in the geomagnetic field 730,000 years ago coincided in timing with an impact, marked in the geologic record by tiny, glassy grains called microtektites.
Natural microtektites are small glass droplets derived from melting and quenching of terrestrial rocks during hypervelocity meteorite impacts on the Earth (see  and refs.
However, a variety of industrial processes also may produce broadly similar glassy microspheres, and there still are risks in mistaking man-made products for natural microtektites.
Analysis of Probable K-T Boundary Microtektites, Mississippi Embayment, Southeastern Missouri.
Microtektites were found at a "kitty litter" strip-mine in the K-T boundary deposits of the Mississippi Embayment in Stoddard County, Missouri.
This equation is consistent with the higher specific gravities of microtektites and it may show the role that gas bubbles play in determining specific gravity.
Glass (13) reports specific gravities for microtektites with diameters of 125-550 micrometers; two of the microtektites from that study (samples 95-1 and 95-2) have [SiO.
These also appear to be microtektites
, they report.
Smit proposes that the spherules are microtektites, little balls formed when a meteorite or comet strikes the planet and sends a shower of molten rock specks into the atmosphere, where they cool quickly and fall back to Earth.
Because of their composition, the purported microtektites suggest the extraterrestrial body crashed into a basaltic rock, probably on the ocean floor.
These include a cluster of craters formed at about the same time in the Eocene and the finding in Eocene sediments of at least two globally distributed horizons of microtektites
, small glassy spheroids created during an impact.