Metamorphic Rock
Metamorphism refers to changes in mineral composition or rock geometry that occurs
in SOLID rocks with increasing temperature and pressure
It is important to remember that changes producing metamorphic rocks take place
while the rock is solid
1) ANY part of the rock that is melted will eventually cool and form an
igneous rock.
2) Sedimentary rocks and igneous rocks can be metamorphosed if they are
subjected to high temperatures and/or pressures.
When rock has changed so much that it is not easily recognized as sedimentary,
geologists talk about metamorphism.
A) HIGH temperatures and/or pressures are responsible for changing one
rock to another
B) Metamorphism occurs below the Earth’s surface.
The basic idea behind metamorphism is that a crystal of a mineral can grow only
in a certain range of temperature and pressure. This is why the minerals in a
metamorphic rock are usually very different from the minerals in the original rock
Everywhere on Earth, temperature increases with depth. There are two reasons for this: 1)
the deep interior still has heat left over from the origin of the Earth, 2) and the radioactive
decay of radioactive materials in deep rocks continues to produce heat.
The weight of overlying rocks causes pressure to increase with depth in the Earth.
The ORIGINAL rock from which a metamorphic rock is formed is called a
PROTOLITH of the metamorphic rock.
Interpreting the protolith of a metamorphic rock is difficult and uncertain.
a) Marble is almost certainly started out as a limestone
b) Quartzite almost certainly started out as either quartz-rich sandstone or
chert.
New minerals grow at the expense of the original minerals
Metamorphic Rock are described by Geologists as: Low Grade, Intermediate Grade, or
High Grade.
This is characterized by the nature of the mineral assemblage.
The two scales for rock geometry are TEXTURE and STRUCTURE
Texture is used for rock geometry on the scale of grains; size of grains, mutual
geometrical arrangement of grains are aspects of texture.
Structure is used for rock geometry on a scale much larger than grains; layering
is an example of rock structure.
Metamorphism alters both texture and structure of rocks
Foliation: the tendency for a metamorphic rock to split along parallel planes (looks like
layers)
1) One specific and very important textural change involves the growth of new
sheet silicate minerals in a metamorphic rock.
2) Most kinds of metamorphic rocks, change in mineral composition is
accompanied by strong shearing deformation.
Deformation can change the shape of grains in a rock.
3) Strong shearing tends to obscure features such a stratification in a sedimentary
or volcanic rock.
Regional Metamorphism and Contact Metamorphism
Metamorphism that affects enormous (large amounts) volumes of rock in large regions of
the Earth’s crust by an increase in temperature and pressure during deep burial is called
Regional Metamorphism
1) Subduction zones are the most important setting for regional metamorphism.
2) Extreme deformation (change in the shape of a material) is common during
regional metamorphism
3) In many metamorphic rocks, all kinds of features and structures are “smeared
out” by transposition to become nearly parallel planes. Forces within the
Earth can also stretch and compress the rock.
Rocks in the immediate vicinity of a newly emplaced intrusive body of magma are heated
as the magma loses heat to its surroundings. If the temperature becomes sufficiently high,
new minerals that are stable at high temperatures can grow at the expense of the
preexisting minerals. This is called Contact Metamorphism
Contact metamorphism is most common when the intruding magma contains
abundant water, which tends to be carried out into the surrounding rock and
promotes changes in mineral composition.
Goals:
1. Identify and classify several metamorphic rocks using a rock chart
2. Describe two agents of metamorphism
3. Use a geologic map to search for evidence of past metamorphism in a
community
4. Understand that properties of materials can change over time.