Molten Rocks are falling objects that appear in Ocarina of Time. During volcanic eruptions on Death Mountain, the rocks tumble down from the sky, harming Link while he is trying to climb up the higher elevation of the Death Mountain Trail. This will only occur when Link is a child.
The rocks break apart once they hit the ground, but if they hit Link, they will deal damage. If Link has a Hylian Shield, he can defend with it, causing the Molten Rocks to bounce off, preventing him from taking damage.