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A distinctive feature of contact metamorphism is the formation of metamorphic aureoles, which are zones surrounding an igneous intrusion where the country rock has been subjected to metamorphism. Imagine igneous rock as a hot iron, and the surrounding rocks as clothes that change texture when heat is applied.

Metamorphic aureoles are like concentric rings, with the highest grade (most altered) metamorphic rocks closest to the heat source, transitioning outward to lower grade rocks until the unaffected rock is reached. The thickness and extent of the aureole depend on factors such as the size and temperature of the intrusion, the original composition of the country rock, and the duration of heating.

In a metamorphic aureole, we can observe zonal banding that reflects varying degrees of metamorphism. These zones provide valuable information about the peak temperatures and pressures the rock experienced, which helps in understanding the geological history of the area.

Contact metamorphism brings about transformation through recrystallization of minerals, which means that existing minerals in the rock change their shape and size, and in the process may form new, stable mineral assemblages. This happens without the rock ever melting; the high temperatures simply cause the minerals to recrystallize.

During this process, new crystals can grow larger than the original grains, which is why rocks subjected to contact metamorphism often have a coarser texture compared to their precursors. The new crystal structures are typically more stable under the high-temperature conditions and can often be identified by their size, shape, and arrangement.

As the rocks undergo recrystallization during contact metamorphism, the textures of the rocks evolve too. The term 'texture' in geology refers to the relationship between the minerals within a rock, including the size, shape, and arrangement of the grains.

There are several typical textures associated with metamorphic rocks formed by contact metamorphism. For instance, a nonfoliated texture is common in rocks where the heat has been the primary change agent, without directed pressure, leading to an equigranular texture where mineral grains are roughly the same size. In contrast, if there is some directional pressure during contact metamorphism, a foliated texture like that found in schist or gneiss can develop, with minerals aligning perpendicular to the pressure. These textures are not just a physical attribute; they reveal information about the environmental conditions the rock has undergone.

An exciting outcome of contact metamorphism is the formation of new minerals that weren't present in the original rock. Under the influence of high temperature, new stable minerals form as a response to the thermal environment. These mineralogical changes are distinct indicators of the metamorphic conditions.

For example, the mineral andalusite can form in shaly rocks under certain temperature conditions. The presence of such index minerals can provide a geothermal indicator, helping geologists to understand the maximum temperatures reached during the metamorphic event. It's like a thermal footprint left within the rock that maps the intensity of the heat over time. By studying these new minerals and their growth patterns, scientists gain insights into the thermal history and the processes that formed the Earth's crust.