Seafloor Spreading: Harry Hess  

Geophysicist and geologist Tanya Atwater talks about seafloor spreading, an idea first articulated by Harry Hess that many consider to be the linchpin of plate tectonics.
Subject: lava
Transcript: below, as it rises it melts And new lava comes out and that fills a little crack. And that one cracks in half, new lava fills in, And that onecracks in