What does this line mean?

[belly_ttt]

During the first days of October when the eruption peaked, billions of gallons of meltwater filled the lake to bursting, prompting officals(1)to warn that an explosive glacial flood was imminent.

1) why do we use gerund here at (1)?
2) And what does it mean by the words in bold?

 

[shalstudent]

1) In the first sentence, when we say bursting, it means "filled to the top" or "brimming up to the top."

So we can say, "During the first days of October when the eruption peaked, billions of gallons of meltwater filled the lake to the top
OR
billions of gallons of meltwater brimmed up to the top of the lake.

2) "Prompting" in this sentence means "bidding" or " directing".
"Officials" would mean "the Government" or the governing body or "ministry."

Thus the entire sentence can be put forth as:

During the first days of October when the eruption peaked, billions of gallons of meltwater filled the lake to the top, directing the Government to warn (the people or population) that an explosive glacial flood was imminent.