> He starts off with saying that "there is a line of magma that is 15km long and 9 miles in length." The real explanation is rather prosaic: bad transcription.

> He starts off with saying that "there is a line of magma that is 15km long and 9 miles in length." The real explanation is rather prosaic: bad transcription. He doesn't use the word "and." An accurate transcription would be: > there is a line of magma that is 15 km long—about 9 miles in length. He's just (parenthetically) converting the units.