Exodus 40:34-38 (KJV)
34 Then a cloud covered the tent of the congregation, and the glory of the LORD filled the tabernacle.
35 And Moses was not able to enter into the tent of the congregation, because the cloud abode thereon, and the glory of the LORD filled the tabernacle.
36 And when the cloud was taken up from over the tabernacle, the children of Israel went onward in all their journeys:
37 But if the cloud were not taken up, then they journeyed not till the day that it was taken up.
38 For the cloud of the LORD was upon the tabernacle by day, and fire was on it by night, in the sight of all the house of Israel, throughout all their journeys.
As you can see; they did not travel continuously, but only if the cloud was 'not taken up from the Tabernacle'.
That is even harder to explain via a volcano...or a comet.
One must always keep in mind that the Bible uses numbers for shorthand. Forty years or Forty days or Four Hundred xxx = 4*10*x.
10 signifies divine completeness. 4 signifies god's creative works. Thus, 40 signifies the creative completeness of god's works. 40 is also the product of 5 & 8 and signifies god's grace manifest, ie, the completeness of the probationary period. 5 = grace; 8 = renewal or revival.
See E.W. Bullinger, "Number in Scripture".