As far as I can tell the verb is still "allows" so if you take out the "snow to accumulate and" you have "A surplus allows for the pressure of snow accumulated over the years to transform buried snow into glacial ice with a depth great enough for the ice to flow."
I am not sure why it would use just "allows" and then add a "for" instead of using "allows" or "allows for" for both and I agree that it was confusing - I myself had to take a moment to sort out what they were trying to say. At the same time, this could very easily be the wrong answer - it's just a poorly worded sentence.