Statement:
Onua absolutely can use a dirt ramp to lift Pohatu.
Proof:
First off, Pohatu is not running downward. He is running forward. the ramp doesn’t have to resist the force of his run, it just has to hold him up and keep him moving forward. His speed does not matter as much, and in fact would help keep him from sinking in to the dirt – each time he starts to sink, he moves before his feet sink very far.
Second, can he stop? The mask of speed probably allows him to slow to a stop, but not instantaneously stop. If you’ve ever tried to stop while running very fast, it isn’t easy to do. Stopping instantly, at the speed Pohatu is running, would probably damage him a lot.
So the dirt simply needs to act like the dirt Pohatu was running on before he hits the ramp: hold him up, and keep him moving forward. Can a dirt hill do that?
Probably not. In fact, it would likely fall apart even without Pohatu running on it. But this is not a normal dirt hill. It’s one being controlled by Onua.
Pohatu is going to sink into the dirt, sure, but at the speed he’s going, that isn’t an issue. The issue is that his feet will push the dirt backwards as he runs, effectively digging through the dirt. However, all Onua needs to do is keep pulling more in to replace it. Ironically, digging through the soft dirt with each step will also slow Pohatu down, but not enough to stop him: again he’s going too fast.
With each step, Pohatu will dig through the dirt, until his foot encounters enough resistance to propel him forward. Maybe not at full speed, but he’s running really fast already. If this were normal dirt, he’d eventually dig through it, but for all the dirt he displaces, Onua pulls in more to replace it. Pohatu does still dig a trench through it, and it can only go up so high before structural integrity makes the top unstable, but it still succeeds in its purpose: moving Pohatu’s trajectory upward, which causes him to go flying.
Also, and this is something none of you have taken in to account: there are tree roots entwined in this dirt. This will help the structural integrity even more, even if Pohatu’s feet never hit them, by providing more resistance.
And finally: even if an earth Toa can solidify their element (which is not necessary for this earth ramp trick) it still doesn’t defeat the purpose of separate elements. Onua and Pohatu can both create a wall of their element, but when they stop focusing on it, Onua’s wall crumbles while Pohatu’s wall stays up.
Now, with all that said, should the earth ramp have worked on the speeding boat in Voyage of Fear? No. Just no.
However, this time there was a Toa of stone present, who helped make the ramp. With the strength of rock mixed in, this ramp…
Still shouldn’t have made the boat fly over the whirlpool. Even if it lifted it up, which is still questionable. Boats don’t do that.