What role did barrels of benzol play in the explosion of the Mont-Blanc?

On the morning of 6 December 1917, the French cargo ship SS Mont-Blanc collided with the Norwegian vessel SS Imo in the waters of Halifax, Nova Scotia, Canada. The Mont-Blanc, laden with high explosives, caught fire and exploded, devastating the Richmond district of Halifax. At least 1,782 people were killed, largely in Halifax and Dartmouth, by the blast, debris, fires, or collapsed buildings, and an estimated 9,000 others were injured. The blast was the largest human-made explosion at the time.[1] It released the equivalent energy of roughly 2.9 kilotons of TNT (12 TJ).[2]

Mont-Blanc was under orders from the French government to carry her cargo from New York City via Halifax to Bordeaux, France. At roughly 8:45 am, she collided at low speed, approximately one knot (1.2 mph or 1.9 km/h), with the unladen Imo, chartered by the Commission for Relief in Belgium to pick up a cargo of relief supplies in New York. On the Mont-Blanc, the impact damaged benzol barrels stored on deck, leaking vapours which were ignited by sparks from the collision, setting off a fire on board that quickly grew out of control. Approximately 20 minutes later at 9:04:35 am, the Mont-Blanc exploded.

The impact of the SS Mont-Blanc and the SS Imo damaged barrels of benzol which were stored on the deck of the Mont-Blanc.  The damaged barrels leaked vapours which were ignited by sparks which set off a fire.