On the flanks of Mauna Loa, lava flows run downhill to the ocean, forming new land at the edge of the sea. Mauna Loa on Hawaii, standing 10 kilometers from ocean floor to summit, is the classic shield volcano. Seamounts are underwater volcanoes that appear over a mid-ocean ridge or a hot spot. Seriously, Loihi is more evidence that the Hawaiian islands were formed by volcanoes. The oceanic plate under Hawaii has been drifting toward the northwest for millions of years.
Volcanoes form where it passes over a hot spot carrying magma from deep below.
Eventually the islands in the northwest erode back into the sea. Pillow lava forms during a slow underwater lava flow. As the outside of the flow cools, it becomes elastic, and the structure inflates with hotter, more fluid lava from the inside. Pillow lava is a sure sign that an area was under water at some point in geologic history. Hot springs and geysers often get their energy from solid magma that's still carrying enough energy to produce an eruption.