The heat of Magma, when moves inside the earth, passes near or over a rock, changes its composition and structure and a new rock is formed. This process is called Thermal metamorphism
Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change).[1] The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism.[2] Metamorphism typically occurs between diagenesis (max. 200°C), and melting (~850°C).[3]
Three types of metamorphism exist: contact, dynamic, and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
Solid state recrystallization and neocrystalization
Metamorphic rocks can change without melting. Heat causes atomic bonds to break, and the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures (neocrystallization), or enabling recrystallization.[3] When pressure is applied, somewhat flattened grains that orient in the same direction have a more stable configuration.
Limits
The temperature lower limit on what is considered to be a metamorphic process is generally considered to be 100 – 200 °C;[4] this excludes diagenetic changes due to compaction and the formation of sedimentary rocks.
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is typically 700 – 900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not fully segregated from the refractory residue. Since the 1980s it has been recognized that rocks are rarely dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900 – 1100 °C. The metamorphic process usually has to be over pressure of at least 100 mega pascals but below 300 mega pascals, the depth of 100 mega pascals varies depending on what type of rock is applying pressure
Types
Regional
Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterized by paired metamorphic belts. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic