Final answer:
The agents that cause metamorphism include heat, pressure, and chemically reactive fluids, but not biological activity. Metamorphism occurs under subsolidus conditions, resulting in the transformation of rocks without melting them. The process is identified by changes in mineral content and textures, such as foliation.
Step-by-step explanation:
The agents that cause metamorphism include all except biological activity. Metamorphism is the process whereby the minerals and textures of a rock are transformed through heat, pressure, or the action of chemically reactive fluids. These changes take place in a solid state, under subsolidus conditions, where the temperatures and pressures are high but not enough to cause the rock to melt. The most significant agents of metamorphism are heat from magma chambers or the geothermal gradient, and pressure from the overlying rock or tectonic forces, particularly during plate collisions.
Metamorphic rocks can either show foliation, where minerals are aligned perpendicular to pressure, due to the re-orientation under pressure, or they can be non-foliated when minerals change very little under metamorphic conditions.
Non-foliated metamorphic rocks, like quartz, can be difficult to distinguish from igneous rocks. The texture, which refers to the orientation of minerals, and the mineral content, altered by the metamorphic process, are the primary ways to identify these rocks.