What is metamorphism?
Metamorphic = (Greek) “to change form”
Metamorphism is the bulk change in chemical composition and/or
crystal structure of rocks and minerals due to exposure to high
temperatures, pressure, and/or external chemical agents.
Metamorphism has two primary forms:
1. Change in chemical composition
2. Change in crystal structure
Agents of Metamorphism:
Heat – The most important of the agents of metamorphism.
Heat supplies the energy required to drive the chemical
reactions and reorganization of the crystal structure of
minerals.
Ex. The recrystallization of clay materials will result in the
formation of feldspar under conditions of high temperature
Pressure – Forces the parent rock to become denser, or more
closely packed. The realignment and growth of mineral
crystals perpendicular to the direction of the applied pressure
results in the formation of foliation and banding.
Chemical Activity – Minerals often react with dissolved
minerals that are introduced by an external agent such as
water. Water is the most common external chemical agent.
The presence of water may also affect the crystallization of
minerals.
Types of Metamorphism:
Contact (Thermal) Metamorphism – Metamorphism that
takes place when native rock comes into contact with
magma. The boundary region is the most highly
metamorphosed with lower grades of metamorphism
occurring at progressively greater distances from the
boundary. The zone of metamorphosed rock that surrounds
the magmatic intrusion is known as an aureole or “halo”
Contact metamorphism is dominated by conditions of low to
moderate pressure and very high temperature.
Regional Metamorphism – Metamorphism that takes place
as large sections of the Earth’s crust collide and push against
one another. This process is dominated by conditions of
moderate to high pressure and moderate to low temperature.
This process takes place of a wide region and generally
occurs deep within the Earth’s crust.
Hydrothermal (Metasomatic) – Metamorphism that takes
place as a result of the introduction of new elements and
minerals by hot water under great pressure.
Solid State Metamorphism – A form of chemical
substitution in which elements migrate from one rock to
another as a result of thermal agitation.
Classification of Metamorphic Rocks:
Grades of Metamorphism – The higher the conditions of
temperature and pressure required to produce a particular
metamorphic change the higher the grade of the resulting
metamorphic rock.
Metamorphic Textures –
Foliated – Foliated metamorphic rocks have the
appearance of having layers. This layering effect is the
result of the recrystallization of the minerals of a parent
rock perpendicular to the applied pressure. This has the
affect of creating sheet-like mineral structures. Minerals
such as mica and feldspar are particularly susceptible to
this type of recrystallization.
Banded – Banded metamorphic rocks are formed in a
manner very similar to that of the foliated metamorphic
rocks. In the case of banding, minerals recrystallize into
long needle like crystals. The formation of needle like
crystals is the result of the recrystallization taking place
perpendicular to pressure applied from two directions
simultaneously. Banded rocks are always of high
metamorphic grade.
Non-Foliated – Non-foliated metamorphic rocks form
when the minerals undergoing recrystallization have no
preference for the direction in which they grow. The
individual crystals grow in random directions
interlocking with one another. Non-foliated rocks often
have a grainy, sugary texture that is created as a result
of the interlocking mineral grains.
Common Metamorphic Rocks:
Parent Rock
Low Grade
LowIntermediate
IntermediateHigh
High Grade
Shale
Slate
Phyllite
Schist
Gneiss
Limestone
Marble
Sandstone
Quartzite
Basalt
Amphibolite
Granite
Granite Schist
Granite Gneiss