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Metamorphic rock

Metamorphic Rock of Stone

Metamorphic rock is one of the three main groups in the rock cycle and includes popular building and flooring stones such as marble and Slate.

Metamorphic rock is formed by the changing of a pre-existing rock, known as a protolith, into a new rock. The protolith, meaning 'first stone' in greek, can be igneous, sedimentary or another metamorphic rock. These changes happen without melting which is a characteristic of igneous rock formation. Its name derives from the greek 'metamorphosis' meaning 'change in form'.

There are three main processes that can create metamorphic rock:

  • High heat and pressure due to the rock being deep below the Earth's surface
  • Friction and distortion created by tectonic plate movements
  • Intrusion of magma which subjects the rock to high heat

Particle sizes in the rock change size during the metamorphism in a process called recrystallisation. High temperatures cause atoms in the crystals to migrate meaning they can re-organise and create new minerals. Differing crystal growths and the type of pressure which is applied will create two possible visual textures, foliated and non-foliated:

Foliated: Certain mineral crystals have a tendency to grow perpendicular to the level of stress or pressure applied to them. In this case the rock will become foliated if a strong one directional force is present during recrystallisation.

Non-Foliated: When the pressure applied to a recrystallising rock is uniform or if the mineral crystals do not have distinctive growth patterns, the texture of the metamorphic rock will be non-foliated. Slate, probably the most well known foliated rock, originates from the sedimentary rock shale. The growth of calcite crystals under high heat and pressure causes Limestone to become the non-foliated rock, Marble Quartz crystals grow during the metamorphism of Sandstone resulting in the formation of the non-foliated rock, Quartzite.

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