The student should add the label "heat and pressure" to the arrow from the picture of obsidian to the picture of slate.
Obsidian is an igneous rock that forms from the cooling of lava, and slate is a metamorphic rock that forms from the alteration of shale (a sedimentary rock) through heat and pressure. Thus, the transformation from obsidian to slate would involve processes related to heat and pressure in the context of rock formation.