Fire Chief Wallace Drysdale said the victim was helping pave a parking lot in this suburb just north of Winnipeg.
"I believe (the truck) dumped off way too much asphalt unexpectedly," Drysdale said.
"It was extremely hot asphalt. Our crews, when we were digging (the boy) out, had to shuttle different members in and out in about four- or five-minute intervals because our feet were burning," the chief said.
Richard Hill, who lives nearby, said he heard the boy scream and ran over. Hill said he and another man started digging with shovels but the boy wasn't moving. "We pretty much knew there wouldn't be any hope because of the heat and the weight of that" asphalt, he said.
Royal Canadian Mounted Police said they were withholding the boy's identity at the request of his family.