The phrase "large plume" means a big and dense cloud of something, like smoke, steam, or water vapor, that rises into the air. It typically suggests that the plume is significant in size and highly visible. Full definition
Rather it shows where large plumes of smoke were lofted several kilometers up into the atmosphere. (earthobservatory.nasa.gov)
These could provide enough energy for punchier volcanism, perhaps producing larger plumes — and heat would of course be beneficial for any bacteria that may be lurking below the surface. (newscientist.com)
They noticed that the waves traveled faster in two vast arc - shaped regions under the margins of the Pacific Ocean (blue in the illustration), while they slowed down in two equally large plume - shaped regions below the central pacific and Africa (red). (scientificamerican.com)