During his island upgrade Hephaestus was able to gain some inspiration on how to evolve his island priesthood. So he started cultivating just like one of his past reincarnation who manage to become an immortal. He stayed in a room that have a bed that was made from a plant that he found dyrung his exploration and some beast that he slayed.
During his cultivation he taped into his past reincarnation that major in fire cultivation. It gave him many inspiration. Espacially from vulcan who was from a world that produce fire that have wonderfull abilities similar to Hestia's fire of purification.
He also paid special attention to Vlarga who was one of his past reincarnation who was born as a fire spirit which is a race born from the fire element. Vlarga live in a plane made of fire which has giant volcanos, lava rivers, etc which strengthen his inspiration.
So he made a plan. He will create lava and volacanoes for the world.
(Note : turn of your IQ real quick)
He went to an island near him that was full of earth and fire. He manipulated the fire and earth there while using his priesthood to connect with the will of the world. He made the fire on the island to go deep within the earth. Then he communicated with the earth using his divine power to comress the fire. From it lava was born. It was also the first fusion of elements in the world. Hephaestus then made the newly created lave travel to the dept of the earth. And after countless years of helping the world produce lava and make it flow according to his plan. He made the first eruption in the world.
It was a horrifying but wonderfull day. The earth especially place that have a lot of fire element started to explode. It created a wonderfull eruption tahtd estoyed anything.
This act allowed Hephaestus to evolve his fire priesthood while also gaining some authorities of the earth and metal since during his lave manipulation many minerals was bron wihtin the earth.