ELI5: How do volcanoes form?

A volcano forms when melted, bubbling rock deep inside the Earth forces its way up to the surface and piles up into a mountain.

Imagine the Earth is like a giant, hard-boiled egg. The hard eggshell is the cold ground we walk on, called the crust. Deep beneath this shell, it is so blazing hot that rocks melt into a thick, glowing, gooey liquid called magma, just like warm melted chocolate or boiling soup.

Here is how that gooey liquid turns into a volcano, just like things you see at home:

• The Cracked Shell: The Earth's surface is not one whole piece; it is cracked like a puzzle, with giant slabs called tectonic plates. Imagine leaving a cracked egg on the counter—sometimes, cracks pull apart or bump into each other, leaving weak spots.
• The Shaken Soda Can: Inside the hot Earth, bubbling gas gets trapped in the melted rock. Think of a bottle of soda that you shook really hard. The bubbles want to escape, creating huge amounts of pressure.
• The Pop and Squeeze: When the pressure gets too big, the bubbly magma rushes up through the weak cracks in the ground, just like soda spraying everywhere when you finally twist the cap off. When it shoots into the open air, we call it lava.
• Building the Mountain: Think about playing with hot wax from a birthday candle. When the hot wax drips, it cools down and turns hard. In the exact same way, every time a volcano squirts out hot lava, the lava cools, hardens into rock, and stacks up.