A mountain that forms when melted rock (magma) from deep inside the Earth pushes up and sometimes flows out as lava or bursts with ash.

Why it matters: Many volcanoes are under the ocean, building new seafloor and even growing islands. Their ash and minerals can feed plankton, and their heat helps make special seafloor habitats.

Fun fact: The longest volcano chain on Earth is the mid-ocean ridge, a huge seam that wraps around the planet, hidden underwater.

Say it like this: vol-KAY-no

Activity twist: Island-Maker! Mound damp sand into a cone on a tray and pour water around it (your “ocean”). Spoon a little baking-soda paste into the cone, then add a splash of vinegar (adult help). Watch your bubbly “lava” overflow.