Magma 50%, because eventually the melted rock mass becomes a crystal-and-melt mush that can then ascend "en masse" as a diapir, which may then cause further decompression melting. The degree of partial melting is critical for determining what type of magma is produced. The degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements. Incompatible elements commonly include potassium, barium, caesium, and rubidium. Rock types produced by small degrees of partial melting in the Earth's mantle are typically alkaline (Ca, Na), potassic (K) and/or peralkaline (high aluminium to
Pitchstone that is very resistant to erosion. The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley. Pitchstone from the Isle of Arran was used as the raw material for making various items from the Mesolithic through the Neolithic to the Early Bronze Age. Mesolithic use appears to have been limited to the Isle of Arran itself, while in later periods the material or items made from it were transported around Britain. Pitchstone Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly.