1. Intrusive Igneous Activity:
* Plutons: Large, deep-seated bodies of magma (granite, diorite, etc.) intrude into existing rocks. The heat from the magma cooks the surrounding rocks, transforming them into metamorphic rocks. This is the most common setting for thermal metamorphism.
* Dikes and Sills: Smaller intrusions of magma that cut through pre-existing rock layers. The heat from these intrusions also causes metamorphism in the adjacent rocks.
2. Regional Metamorphism:
* While regional metamorphism is primarily driven by pressure and heat, the heat component can be significant. In these settings, rocks are buried deep within the Earth's crust, and the geothermal gradient (increasing temperature with depth) causes the rocks to undergo thermal metamorphism alongside the pressure-driven changes.
3. Contact Metamorphism:
* A specific type of thermal metamorphism where the heat source is directly adjacent to the rock being metamorphosed. This can happen at the contact between:
* Magma and country rock: A narrow zone of metamorphism around an igneous intrusion.
* Lava flows and sedimentary rocks: The hot lava can transform the rocks below it.
4. Hydrothermal Metamorphism:
* This is a type of metamorphism driven by both heat and chemically active fluids. While the fluids are the primary cause of change, the heat from these fluids can contribute significantly to the transformation process. Hydrothermal fluids are often associated with:
* Volcanic activity: Water heated by magma reacts with existing rocks.
* Fault zones: Hot water circulates along fractures in the Earth's crust.
5. Subduction Zones:
* Deep in the Earth's crust, along subduction zones, the heat from the descending plate contributes to thermal metamorphism of rocks on both the descending and overriding plates. This can occur along with the pressure-driven changes of regional metamorphism.
Key Point:
In all these settings, the heat is the main factor driving the transformation of the minerals and texture within the original rock, leading to the formation of metamorphic rocks.
