Simplify the following text for a Grade 10 native English-speaking person.
Original: Hotspots are thought to be caused by a narrow stream of hot mantle convecting up from the Earth 's core -- mantle boundary called a mantle plume , although some geologists prefer upper-mantle convection as a cause .
Simplified: It was later postulated that `` hotspots '' are fed by narrow streams of hot mantle rising from the Earth 's core-mantle boundary in a structure called a mantle plume . Whether or not such mantle plumes exist is currently the subject of a major controversy in Earth science .