Magma Mimicry – The ability to become magma.
Magmaniac (Image) is a supervillain composed of magma (The Official Handbook of the Invincible Universe #2).
The Molten Man (Marvel) is composed of magma (Amazing Spider-Man V1 #28).
Lava Lords – Savage Dragon
Magma Beast – D&D 4th Edition – Monster Manual 1
Molten Man-Thing (Marvel) – Tales of Suspense V1 #7
Volcano Man (DC) – Challengers of the Unknown V1 #27