What is the meaning of "N'vuv Luchos"?
Rashi, Rashbam, R. Bachye, Seforno, Targum Onkelos and Targum Yonasan: It means that the Mizbe'ach was made of planks of acacia-wood (and was overlaid with copper ? Tanchuma) and was hollow 1 in the middle. 2
Tanchuma: With reference to answer #1 ? It was hollow but it had a cover of acacia-wood and copper on which they burned the Korbanos.
R. Bachye: From the Lashon "ve'Ish N'vuv Luchos" (Iyov 11:12).
If, as the commentaries explain, the Mizbe'ach had no cover, how did the sacrifice on it?
Rashi, Rashbam and Targum Yonasan: Whenever they encamped, they filled it with earth. 1
If the Mizbe'ach had a cover of wood and copper, why did the fire not burn the wood?
R. Bachye (citing the Tanchuma): Moshe posed this question to Hakadosh-Baruch-Hu, who answered that 'The G-d who commands fire to burn would command the fire on the Mizbe'ach not to burn through the copper' ? despite the fact that it burned constantly and despite the thinness of the copper. 1
R. Bachye, ibid.: Just as Moshe had passed fiery angels on Har Sinai and even into the voicinity of Hashem Himself ? who is described in Devarim 4:24 as 'a Consuming fire' and came out unscathed.