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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