From Lexicanum

Magma Vents are Tyranid plants that burrow deep into the planetary crust down to just shy of the the mantle. These plants then vitrify to form hollow tubes. These tubes release the gas trapped in this planetary layer. The gases further the consumption of the planet. The relationship between Magma Vents and Magma Corers is implied, but not substantiated.

Sources

• ¹White Dwarf 254(us) "Hostile Terrain" by Pete Haines
• ²White Dwarf 254(us) "Tyranid Planetary Assimilation Analysis" by Sherman Bishop

See also

• Species of Tyranid