Physical Geology 101
15. Metamorphic Rocks I
(p. 223-233)
New Rocks from Old
What is the Greek origin of the word metamorphism?
“meta” means _________________; “morphe” means __________________.
Definition: metamorphism refers to all the changes that occur in the minerals, chemistry, and
structure of solid rocks as a result of the physical and chemical conditions deep in the crust.
So metamorphism involves changing existing rocks to make new ones.
This is different to igneous and sedimentary rock processes:

Igneous rocks: made from molten magma or lava.

Sedimentary rocks: diagenesis is the lithification process that binds the particles of sediment
together to form rock.
When rocks undergo metamorphism, we say that they have been ______________________.
Does metamorphism involve any rock melting? YES or NO ?
Changes that occur during metamorphism:

_______________: existing minerals convert into new minerals

_______________: minerals change their appearance because of the forces that are involved in
metamorphism.
Depth of Metamorphism
Metamorphic rocks form in the Earth's crust underneath where sedimentary rocks form near the surface
(i.e. where the process called ________________ occurs), but above the depths where magma begins to
form through the process of __________________ (where present).
The depth range of metamorphism will thus depend on the local ____________________________.
What is the lower temperature limit of metamorphism? ______________
What is the deepest depth (approximately) that diagenesis occurs? ______________
Pressures at this depth are more than ____________ times greater than atmospheric.
We have to have high enough pressures AND temperatures for metamorphism to occur, by
recrystallization (the growth of new minerals).
What is the upper temperature limit of metamorphism? ___________________
What mineral starts to melt at this temperature, if present in the rock? _____________________
1
Physical Geology 101
The three grades of metamorphism are:
___________________ (only occurs at shallower depths than ___________)
___________________
___________________
Do low-grade metamorphic rocks resemble the original rock? YES or NO
Do high-grade metamorphic rocks resemble the original rock? YES or NO
Controlling Factors
The type of metamorphic rock that forms is very dependent on what the original rock is, but the exact
changes that occur to the original rock will vary depending on what the metamorphic conditions are. The
controlling factors are:
1. ____________________
2. ____________________
3. ____________________
4. ____________________
Pressure
Pressure refers to the effect of the increasing weight of the overlying rocks as you go deeper and deeper
into the crust. This pressure is referred to as the:
____________________________
This pressure is the same from all directions, just like water pressure deep in the ocean.
If pressure is the same from all directions:
-
do objects decrease in volume because of the pressure?
YES or NO
-
do objects change their shape because of the pressure?
YES or NO
Under lithostatic pressure, rocks decrease in volume- the mineral grains get more and more tightly
packed and they start to ________________, forming a metamorphic rock.
If forces are stronger in one direction than other directions (unlike pressure), we use the term
_____________ to talk about the forces acting on the rock. This may happen because of the forces of the
tectonic plates pushing against each other.
In particular, if there is a greater push from one direction compared to another direction results in a type of
stress called __________________________.
Because of these unequal stresses, rocks get squished in one direction more than others. This causes
platy minerals (e.g. ___________), which grow in sheets, to align with each other perpendicular to the
direction of the largest stress.
This alignment of minerals in metamorphic rocks is called ________________.
2
Physical Geology 101
Because it is a physical characteristic of the rock, we refer to this foliation as a type of metamorphic
____________________.
In low-grade metamorphic rocks, the grain sizes are usually very small and the foliation may be difficult
to see. A tell-tale clue is that the rock breaks apart easily along the foliation, just like with cleavage in
minerals, forming flat, plate-like fragments.
This characteristic is called _________________________.
In intermediate-grade metamorphic rocks, recrystallized mica and chlorite grain sizes are bigger and the
foliation is easier to see and is often wavy in appearance.
This type of foliation is called ________________________.
In high-grade metamorphic rocks, recrystallized minerals segregate into light and dark bands, forming a
__________________________.
This type of foliation is also called ________________________.
Temperature
Heat is a crucial part of metamorphism because chemical reactions occur faster as temperatures:
INCREASE
or DECREASE ?
What are the two sources of heat for metamorphism?
1) ____________________________
or
2) ______________________________
Eventually the temperatures will get high enough that a certain rock type can no longer remain solid at the
existing pressures and partial melting begins. If partial melting doesn't melt all of the rock, we can end up
with rocks that are part metamorphic and part igneous (usually granite). These type of rocks are called
____________________.
Fluids
What are 2 examples of holes or openings in rock that can contain fluids or volatiles?
1. ___________________ (gaps between grains in sedimentary rocks)
2. ___________________
What types of minerals contain water in their chemical makeup? __________________ (e.g., clay)
What types of minerals contain OH in their chemical makeup? __________________________
(e.g., mica and amphibole- can be used to make H2O in chemical reactions)
What are 2 examples of fluids or volatiles that are used in metamorphic reactions?
____________________
and
_____________________.
3
Physical Geology 101
Fluids are important for metamorphic reactions because they allow existing minerals to dissolve, or they
allow new minerals to crystallize from the chemical ingredients present in the fluid.
Do fluids speed up or slow down chemical reactions during metamorphism?
SPEED UP
or
SLOW DOWN
Time
How long does it usually take for one silicate mineral to fully convert into a different silicate mineral (even
when the temperatures and pressures are very high)?
____________________________
Types of Metamorphic Rocks
We can broadly divide metamorphic rocks up into two categories. Those that have a foliation and those
that don't.
In other words: __________________ and ______________________
Foliated
In order from the lowest to the highest metamorphic grade:

__________________: forms when shale undergoes low-grade metamorphism. Develops a foliation
called a _____________________.
What products are made from slate? _________________________________

__________________: slightly higher grade metamorphism of shale and forms an obvious foliation.

__________________: intermediate-grade metamorphism of shale, with big mica crystals that form a
pronounced foliation called a ___________________.

__________________: form from high-grade metamorphism of clay-rich sedimentary rocks and
granite. The metamorphic minerals segregate into light and dark bands producing a
___________________texture.

__________________: forms when ______________ undergoes low-grade metamorphism. The
resultant rock is similar to a schist that develops from a shale, but in the case of basalt, contains
mostly green chlorite minerals.

__________________: intermediate-grade metamorphism of basalt. Contains amphibole.

__________________: high-grade metamorphism of basalt. Contains pyroxene.
Non-Foliated

__________________: metamorphic rocks with large recrystallized grains that show no foliation.
4
Physical Geology 101

__________________: forms when _____________ gets metamorphosed. The original quartz grains
recrystallize into tightly packed, reorganized quartz crystals.

__________________: forms from _______________. It is entirely made up of the mineral
_______________.
All the original mineral grains recrystallize, obliterating all sedimentary features, and forming an often
pure white rock with a sugary look to it. The purity of the rock is why it is used for gravestones and for
sculptures (calcite is soft and easy to sculpt).
FINAL QUESTION:
What famous and intricate Michelangelo sculpture was made out of marble?
____________________
5