If the molten rock comes out under water then it will cool very quickly. This is happening along the middle of the Atlantic Ocean where lavas are erupting out along huge cracks that run from Iceland in the north down to the Falkland Islands in the south. This is the longest volcanic mountain chain on Earth but not a lot of people know it is there because it is hidden beneath the sea. There are also other planets in the solar system with volcanoes on them. Mars has one of the biggest volcanoes - a mountain called Olympus Mons.
Scientists believe that all the volcanoes on Mars are inactive now.
Venus is almost completely covered in volcanoes. But scientists are yet to find a smoking one. One theory suggests that Venus burps out its heat from the mantle every 500 million years in a catastrophic volcanic event which covers the whole planet. The rest of the time it stays dormant.