GRE reading problem?

Question

Now astronomer James Head's higher resolution models demonstrate that collisions making craters only three kilometers across can jettison 10 million fragments, each about 10 entimeters across, into space, a distribution sufficient to cause some of them to be found on Earith. Sections of the planet covered by debris(thius likely to be made up of older terrain) would require larger and hence rarer impacts, and thus meteorites which reach Earth are predictably biased toward younger ages.
well, can any one tell me what does that old terrans and debris talking about, I need some highly illustrated description to have a solid understanding, thanks. pal

Answer 1

Hi. Older terrain gets swallowed up in subduction zones and eroded by weather. The ice sheets in Antarctica are the best source for meteorites of fairly recent vintage.