<div class="quote"><i>
I can't look into it much right now, but I think igneous rock is actually pretty weak. At the very least, I know that pumice & obsidian are. </i></div>
<p>Yeah that's what came in my mind as well, but what made me rethink it was that, as Team Avatar fled from the northern air temple through this tunnel, Bolin lavabent to stop the lava because normal earthbending didn't stop the lava. Can you imagine what I mean? The flowing lava couldn't destroy the wall of cooled down lava, but it did destroy the barriers of normal earth
</p>