Rock microstructure intrusions are a result of uninterrupted slow cooling. When a rock cools too quickly the liquid freezes into a solid glass, or crystalline groundmass. Often vapor loss from a magma chamber will cause a porphyritic texture. "Embayments" or 'corroded' margins to phenocrysts infer that they were being resorbed by the magma and may imply addition of fresh, hotter magma. Ostwald ripening is also used to explain some porphyritic igneous textures, especially orthoclase megacrystic granites. A crystal growing in a magma adopts a habit (see crystallography) which best reflects its environment and cooling rate. The usual phenocryst habit is the ones
Pressure ridge (lava) Pressure ridge (lava) A pressure ridge or a tumulus (plural: tumuli), and rarely referred to as a schollendome, is sometimes created in an active lava flow. Formation occurs when the outer edges and surfaces of the lava flow begin to harden. If the advancing lava underneath becomes restricted it may push up on the hardened crust, tilting it outward. Inflation also takes place and is a process where the plastic layer of lava underneath expands as it cools and small crystals form. The end result is a raised mound of hardened lava rock, usually a relatively narrow but long ridge.