This is a composite Indus sign. A mountain range is combined with the heat of volcanoes. If we scale down both the size of the mountains and the intensity of heat, we get a special type of terrain, that of mud volcanoes.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1820 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 56: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Mud Volcanoes: Lynn Fawcett, 2020.