In movies, being burnt by lava was a slow and agonizing process where the person exposed to the lava was slowly absorbed into it as if he were being pulled into quicksand. The truth of it was quite different.
Lava was thick and heavy, and being pressed against literally thousands of tons of it felt just about as solid as being pressed against a wall. Of course, this wall was on fire, and the air was on fire and the ground was on fire and basically everything other than Lex himself was on fire.
His protective talisman had turned on, and sandwiched between the lava and debris, Lex remained unhurt, even if only for now. Acting on instinct alone, Lex quickly pulled up his backpack and quickly opened it to find the acorns given to him by Karom.