Module 6

Metamorphism is the process by which pre-existing rocks (igneous, sedimentary, or older metamorphic rocks) are altered in texture, mineralogy, and sometimes chemical composition due to exposure to: High temperature – enough to change minerals but not melt the rock. High pressure – from burial deep in the crust or tectonic forces. Chemically active fluids – such as water with dissolved ions, which can accelerate mineral changes Key features: Foliation, shiny

• Recrystallization Recrystallization occurs when existing minerals change size or shape without changing their chemical composition. • Phase Change A phase change happens when minerals transform into new minerals with the same chemical composition but a different crystal structure due to increased pressure or temperature. Example: Carbon can exist as graphite or diamond, depending on pressure conditions. • Neocrystallization • Pressure Solution Minerals dissolve where pressure is highest (such as where grains touch) and re-precipitate in lower-pressure areas. This can change mineral shapes and help produce layered textures. • Plastic Deformation Under strong pressure, minerals can bend, stretch, or flatten without breaking. • Development of Foliation Directed pressure causes minerals (especially platy minerals like mica) to align perpendicular to the greatest stress.

Geologists classify metamorphic rocks mainly based on two characteristics: Texture – the size, shape, and arrangement of mineral grains Mineral composition – the types of minerals present in the rock The most important textural distinction is whether the rock is foliated or non-foliated.

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