A new study suggests that a rift in Kenya and Ethiopia has reached a critical stage in the split-up process, and that water may flood it in a few million years Kenya's Great Rift Valley Shankar S. from Dubai, United Arab Emirates, via Wikimedia Commons under CC BY 2.0 Around 250 million years ago, Earth’s modern-day landmasses were united in a supercontinent called  Pangea . But the planet’s shifting outer layer—the crust—eventually broke it apart, according to the well-established scientific theory of plate tectonics .   The theory proposes that huge rock slabs called tectonic plates float atop a molten mantle layer. These plates can move away from each other at locations called rifts, allowing magma to rise and become part of the crust and form new geologic features. Now , researchers have found that a rift in eastern Africa, which spans Kenya and Ethiopia, seems to be further along in the breakup process than previously thought—and that an ocean will eventually fill the gap.…