Caldearas are more or less circular depressions surrounded by steep cliffs. Calderas form when a magma chamber spews out its magma, and the mountain above it collapses.
Shield volcanoes are born from multiple vents that emit flowing (not exploding) lava. Mauna Loa on Hawaii, standing 10 kilometers from ocean floor to summit, is a classic shield volcano. Shield volcanoes are rare because they form over an oceanic hot spot, not at a subduction zone.
Seamounts are underwater volcanoes that appear over a mid-ocean ridge or a hotspot. The Loihi seamount near Hawaii, the world’s most active volcano, is expected to reach the surface in about 50,000 years. Loihi is more evidence that the Hawaiian islands were formed by volcanoes. As the oceanic plate has drifted northwest for millions of years, volcanoes mark where its passage over a magma pipe. As the plate drifts, older volcanoes die in the northwest and newer ones (like Loihi) forms in the southeast. Eventually the northwestern islands erode back into the sea.
Ocean floors are formed at the mid-oceanic ridges, where massive flows of lava ooze out and spread. The ocean floors are, in effect, slow-motion conveyor belts, moving from the mid-ocean toward the continents, where they dive underground and disappear (often feeding volcanoes like Mount St. Helens in the process).
Pillow lava forms during a slow flow of lava into water. As the outside of the flow cools, it hardens, and the structure inflates due to the entry of hotter, more fluid lava. Pillow lava is a sure sign that an area was once under water.
Hot springs and geysers often get their energy from magma. Old Faithful geyser, at Yellowstone National Park, erupts about every 65 minutes, sending water up to 60 meters into the air with a mighty roar. Yellowstone spewed 240 cubic miles of rock 600,000 years ago, forming a huge caldera.