Seafloor spreading occurs due to addition of crustal materials underneath plates. Volcanoes brings up lava from below and create new seafloor.
Step-by-step explanation:
When axial erupted, the boiling hot water is moved out of the volcano followed by great amount of hot lava fills the gap between pacific plate and the jaun de fuca plate creating a new seafloor
The magnetic products that are brought up under the plates form new seafloor along the jaun de fuca ridge.
The shallow rift valley is associated with each jaun de fuca segment is 50 - 200m deep and 1-8km wide.