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Metamorphic rocks are formed when pre-existing rocks are altered by heat, pressure, and/or chemically active fluids, resulting in changes to their mineralogy, chemistry, and texture. This process, called metamorphism, can occur deep within the Earth's crust, often under the influence of tectonic forces or magma intrusions. 

Here's a more detailed breakdown:

• Formation:

  Metamorphic rocks start as another type of rock (igneous, sedimentary, or even previously metamorphic) and are transformed by geological processes. 

  
  

• Causes:

  • Heat: High temperatures can cause minerals to recrystallize or form new minerals. 

  • Pressure: Pressure can cause minerals to realign, creating layering or foliation. 

  • Chemically active fluids: Hot, mineral-rich fluids can also alter the composition of rocks. 

• Examples:

  • Foliated rocks: These rocks exhibit a layered or banded appearance due to mineral alignment, such as schist and gneiss. 

    
    

  • Non-foliated rocks: These rocks do not have a distinct layering, such as marble and quartzite. 

    
    

• Types of Metamorphism:

  • Regional metamorphism: Occurs over large areas due to tectonic forces and deep burial. 

  • Contact metamorphism: Occurs when rocks are heated by magma intrusions. 

    
    
    
    

In essence, metamorphic rocks are the result of a transformation, a change in form, where existing rocks are modified into something new due to the influence of heat, pressure, and fluids within the Earth's crust.