A metaphor is a figure of speech that makes a comparison between two unlike things. One other rule about metaphors is that they cannot use the words "like" or "as" to make the comparison. If either of those words is used, then the comparison is a simile.
Page 73 might be a little bit different depending on what text edition you have. In my version, page seventy three is the very last page of chapter twenty. Maniac Magee is just about finished untying Cobble's Knot. The very last paragraph of the chapter has a metaphor that describes what the crowd's cheer was like after Maniac finished untying the knot.
". . . a huge roar went up, a volcano of cheers."
The metaphor comparison being made on page 73 illustrates that the crowd's cheering roar was super powerful. How powerful? It was basically a volcano exploding.