movie - Why wasn't Superman naked when he was lifting Luthor's artificial continent?

In the end of Superman Returns, Superman entered the hot magma below the artificial continent to lift it up. Why wasn't Superman's costume burned up due to the tremendous heat of the magma? I think, Superman's powers aren't automatically inherited by his costume.

Answer

His suit is made from the blanket which he was wrapped in when he first landed on Earth. It's alien properties make it nigh indestructible.

Additionally, the faq on imdb for the movie states:

In the original Superman movies, based on the silver age comic book superhero, Superman's costume was woven from blankets that arrived with him in the rocket that brought him from Krypton.

Under the Earth's yellow sun, they became invulnerable, just as he does.

And from the wikipedia of Superman Returns:

The film serves as an alternate sequel to Superman (1978) and Superman II (1980).

So the explanation for the suits invulnerability was already given in the previous movies.