We have no basis to think other kinds of rock are relevant to the ocean floor spreading theory. We know that basalt makes up most of the ocean floor, but we don’t know other kinds of rock found in the ocean floor and whether those other kinds of rock are among the ones known to distort compass readings.

This strengthens by providing independent evidence of the timeline of earth’s magnetic reversals. We know that scientists determined the ages of earth’s reversals by examining the magnetite in continental rocks. And we observed a correlation between these reversals and the magnetic striping pattern in the ocean floor. But the pattern in the ocean floor is also based on magnetite — we look at the polarity of magnetite in various strips of the floor. It’s possible that the magnetite we examined in the continental rock and ocean floor do not reliably indicate earth’s actual polarity at different times. By providing independent (non-magnetite) verification of the ages of magnetic reversals, we have more reason to believe in the strong correlation between earth’s magnetic reversals and the striping pattern.

We have no reason to think that the fact some basalt is found on land affects the ocean floor spreading theory or the evidence supporting the theory.

We have no reason to think variations in height could affect the ocean floor spreading theory or the evidence supporting the theory.

We already know basalt makes up “much of the ocean floor.” It’s not clear why learning that some parts of the ocean floor not near the mid-ocean ridge are made up only of basalt would change our opinion about ocean floor spreading or the evidence supporting it.