amphiesmal

Amphiesmal is a term used in geology to describe a texture found in igneous rocks. This texture is characterized by the presence of two distinct sizes of mineral grains within the same rock. Typically, it consists of larger crystals, often called phenocrysts, embedded in a finer-grained matrix. The larger crystals are usually formed first, during an earlier stage of cooling, while the finer grains crystallize later as the magma cools more rapidly. Amphiesmal texture can indicate a two-stage cooling history for the magma. This often occurs when magma rises from a deeper reservoir and undergoes initial slow cooling, forming larger crystals, before erupting to the surface or intruding into shallower crust where it cools more quickly, resulting in the fine-grained groundmass. The term is derived from the Greek words "amphi" meaning "around" or "both," and "esma" meaning "thread" or "web," possibly referring to the appearance of the finer matrix surrounding the larger crystals. It is a textural classification that helps geologists understand the cooling processes and emplacement history of igneous rocks.