rebecca traister, farida jalalzai, thank you both. >> thank you. >> woodruff: stay with us.ing up on the newshour: a closer look at the details of the republican plan to replace obamacare; and, a new exhibit explores america's enduring fascination with poet emily dickinson. but first, there's a giant crack in an ice shelf of the antarctic. that's been the source of much attention this winter, and is raising new concerns over the larger stability of the ice there. miles o'brien has the story, the focus of this week's "leading edge." >> reporter: if glaciology is all that it is cracked up to be, this long, fast growing rift in the antarctic ice is a huge iceberg in the making, one of the largest ever seen-- about the size of delaware. welcome to the larsen c ice shelf, teetering on the edge of breaking away from the glacier. so we've known about this for quite a while, right? >> we've certainly known about the bulk of this rift for a while, it's this accelerated propagation that's really new. >> reporter: it's moving fast. >> moving fast. >> reporter: kelly brunt is a glaciolo