I would like to disagree with the post from earlier and shed some light on why Rock Head is largely regarded as the better option. I can't deny that Lightning Rod has a lot of applicable defensive utility for Alolan Marowak teams, as they tend to be rather weak to Rotom-H, but Rock Head is definitely still preferable for various reasons.So what ability have people been leaning towards for Marowak? Rock Head letting you spam Flare Blitz is powerful, but the utility you get from the electric immunity Lightning Rod brings could easily be better. I'm not sure since both have real advantages and figured I should ask.
The biggest reason to still use Rock Head is because you have to choose between taking a LOT of recoil damage from Flare Blitz or simply being too weak to effectively break down a bunch of teams with Fire Punch if you're running Lightning Rod. The former is something you really want to avoid because Alolan Marowak is already worn down really quickly between taking Stealth Rock damage and checking Pokemon like Magearna. On the flipside, running Fire Punch is simply undesirable because of its mediocre damage output, especially against Mandibuzz, which only needs to run very little investment in Defense to avoid being 2HKOed by an unboosted Fire Punch.