It depends on the temperature of the lava and the material that's melting. For example, geologists can collect lava samples by tossing a hammer head on a chain into the lava, pulling it out, and throwing it in a bucket full of water. But if they just threw the hammer in there, yes it would melt eventually.
Sure; Melts away ground in dirt.... Some Wal*Mart stores still carry it. They keep it in the Automotive section next to the hand cleaners. I'm not sure if it's still a Proctor and Gamble product though.