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
Hogg Rock Jefferson to Santiam Pass includes shield volcanoes, lava domes, and cinder cones. The high elevation of Matuyama-aged rocks east of Santiam Pass, coupled with exposure of Brunhes-aged rocks to the west, imply the presence of a northward-trending normal fault. Part of a mostly mafic chain of volcanoes between the Three Sisters and Mount Jefferson, Hogg Rock has magma with an intermediate composition. A tuya, Hogg Rock is also considered a small lava dome with the flat top typically observed for tuya volcanoes. Unlike most lava domes in the Cascades, which are made of dacite or rhyodacite, Hogg Rock is comprised