Metamorphic grades. Diagram illustrating the relative changes in heat and pressure that different types of metamorphic rock undergo during their formation. The horizontal arrow indicates increasing temperature from left to right. The vertical arrow indicates increasing pressure from top to bottom. The rock types are positioned in the diagram according to the conditions in which they form. At top right is Hornfels. The rocks types across the centre are (from left to right): shale, slate, schist, gneiss and migmatite. The rock type at bottom left is blueschist.