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Igneous intrusions heat surrounding wall rock, creating contact metamorphic aureoles with temperature decreasing away from the contact. Temperature distribution depends on intrusion size, wall-rock thermal properties, and fluid flow. Mineral changes record maximum temperature rather than pressure, distinguishing contact from regional metamorphism.
Model heat diffusion from an intrusion using geothermal equations. Map mineral isograds in contact aureoles.
When a body of magma intrudes into cooler surrounding rock, it acts like a hot iron pressed against fabric — heat flows outward from the contact, transforming the wall rock in a zone called a contact metamorphic aureole. You already know from your study of metamorphic rocks that heat and pressure drive mineral transformations. In contact metamorphism, heat is the dominant agent, while pressure plays a secondary role. This is what distinguishes it from regional metamorphism, where both temperature and pressure increase together over vast areas during mountain-building events.
The aureole is not uniform. Closest to the intrusion, temperatures may reach 700°C or higher, producing high-grade minerals like garnet, pyroxene, or even partial melting. Moving outward, temperature drops and the metamorphic grade decreases in concentric shells. These shells are mapped using mineral isograds — boundaries where a particular index mineral first appears. For example, you might find a sillimanite zone nearest the contact, then an andalusite zone, then a biotite zone, and finally unaltered country rock. The pattern is like ripples spreading from a stone dropped in water, except here it is heat spreading through solid rock.
The width of the aureole depends on several factors you can reason about from your understanding of magma and melting. A large pluton stores far more thermal energy than a thin dike, so it heats a wider zone. The thermal conductivity of the wall rock matters too — rocks that conduct heat efficiently spread the thermal pulse farther but at lower peak temperatures, while poor conductors concentrate heat near the contact. Hydrothermal fluids released from the cooling magma can dramatically extend the aureole because convecting fluids carry heat much faster than conduction through solid rock alone. Where fluids are active, you may also see chemical changes — new minerals introduced by the fluid, a process called metasomatism — superimposed on the purely thermal effects.
One key principle to remember is that contact metamorphic minerals record the maximum temperature reached at each point, not the pressure. Because intrusions are typically emplaced at relatively shallow crustal depths, contact metamorphism occurs at low to moderate pressures. This is why the diagnostic aluminum silicate in contact aureoles is usually andalusite (the low-pressure polymorph) rather than kyanite (high-pressure). By mapping which minerals formed and at what distances from the contact, geologists can reconstruct the thermal history of the intrusion — essentially reading the temperature fingerprint that the magma left behind in the surrounding rock.