To transform an igneous rock into a metamorphic rock, several conditions and processes must be involved:
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Heat: Increased temperatures are needed to facilitate the metamorphism process. This heat typically comes from the geothermal gradient (the increase in temperature with depth in the Earth) or from nearby molten magma.
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Pressure: Elevated pressure is also essential. This pressure can result from tectonic forces, such as those found at convergent plate boundaries where tectonic plates collide.
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Time: The process of metamorphism can take thousands to millions of years. The minerals within the igneous rock will gradually alter and realign under heat and pressure, leading to the formation of new metamorphic minerals.
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Chemical Activity: The presence of fluids, particularly water and other volatiles, can facilitate the metamorphic process by enabling the movement of ions and aiding in the recrystallization of minerals.
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Parent Rock Composition: The original composition of the igneous rock will determine the types of minerals that can form during metamorphism. Different protoliths (parent rocks) will yield different metamorphic rocks under similar conditions.
When these factors come together, the original igneous rock undergoes physical and chemical changes, ultimately resulting in the formation of a metamorphic rock. Examples of such transformations include granite becoming gneiss or basalt transforming into schist or amphibolite.