Metamorphic Rocks
Metamorphic rocks form when a pre-existing rock, called the parent rock or protolith, is
subjected to some combination of three new “agents” of metamorphism:
1) Elevated Temperature – Different minerals are stable at different temperatures, as we see
from Bowen’s Reaction Series in igneous rocks. The same idea applies in metamorphic
rocks though the minerals that form are much more diverse and complex. The complexity
results because heat is not the only agent involved, usually.
2) High pressure, either confining (equally high in all directions) or directed (compressed from
two opposite directions). Pressure also controls which minerals are stable. You can get
carbon as hot as you want and it won’t turn into diamond. You can squeeze it as hard as
you want and it won’t turn to diamond. Both things must happen of there’s no diamond.
Pressure also aligns flat or elongate minerals in a metamorphic rock, giving it a texture
called foliation.
3) Water or “fluids”. Water’s primary role in the process is to move ions around. If there’s
plenty of water this is easy, if the rocks are very dry it is not. Ion mobility makes it easier to
change one mineral into another, particularly if it involves a chemical change. Water also
introduces ions from other rocks making minerals possible that weren’t possible with the
elements at hand. Finally, water can move ions out of a metamorphic region where they
can concentrate those elements. Much of the world’s store of metals is derived from such
“hydrothermal deposits”. They are therefore important, but we will not consider them any
further.
TEMPERATURE
Rocks that result from alterations
caused by elevated temperature alone
are called contact metamorphic rocks
because the heat source for the
change is usually a nearby magma,
with which the protolith was in
contact.
Such rocks will have minerals stable at
various temperatures but will not be
foliated, even if minerals are present
that could foliate. Mica, for example,
is very common. In a foliated rock all
the flat mica “books” are aligned
parallel to each other, giving the rock
an obvious layered appearance. Micabearing rocks resulting from contact
metamorphism do not have the
alignment, nor the layering.
The heart source is normally
surrounded by concentric zones or
aureoles of different temperature
minerals, as the diagram shows.
*Unmetamorphosed protolith
*Low temp. minerals
*Medium temp. minerals
*Highest temp. minerals
Igneous Rock
(hot magma at the time
of metamorphism)
PRESSURE
A directed compressional
pressure on the protolith
will cause platy or elongate
minerals to align
themselves perpendicular
to the direction of the
stress. This can happen in
three ways.
This rock is slate. Its protolith
was shale, and this is obvious
because it is made of clay
minerals, just like shale.
Remember that clays are tiny
sheet silicates like mica.
The original layering in the shale
is evident crossing the rock, as
indicated by the dashed red line.
This bedding probably resulted
from slightly different grain sizes
in the original mud. Otherwise it
would not have been preserved.
Instead it would have been
destroyed when all the clays
were realigned by the directed
pressure that created the
foliation obvious on the side of
the rock and indicated by the
yellow dotted line. This type of
foliation as called slaty cleavage
after the rock or simply rock
cleavage.
From which directions was the
compressional stress that
foliated this rock operating?
Way #1
Perpendicular to the foliation, of course.
Way #2
This is a rock called schist. You should be able
to recognize the muscovite that makes up the
bulk of the rock because lots of cleavage faces
– virtually all the crystals – are reflecting light
toward your face. This can only happen if the
crystals are all oriented the same way
Viewed from the side
the layering that results
from the alignment of all
the micas is obvious.
This type of foliation is
called schistosity after
the rock.
How was the stress oriented that foliated this rock?
Perpendicular to the foliation,
of course.
Way #3
This is a rock called gneiss and
the type of foliation is called
gneissic foliation.
In this case there are feldspars
(white – probably plagioclase)
and ferromagnesian minerals
(black – probably amphibole)
and they are fairly well
separated into different layers –
not perfectly, but almost.
How was the stress oriented that foliated this rock?
Perpendicular to the foliation,
of course.
The pressures along faults are directed, but oriented differently than in the previous slides.
The laterally directed stress pulverizes and grinds the rock along the fault zone, grinding it to a
powder called “mylonite”. Because the grains are so small they cannot be foliated, though a different
type of vague layering is sometimes apparent in them.
Mylonite is a metamorphic rock that results from this sort of pressure. We call this type of
metamorphism dynamic metamorphism.
BOTH TEMPERATURE & PRESSURE
The foliated rocks that we saw while talking about foliation do not result from pressure
alone. Our evidence for this is twofold. First, increasing pressure always increases
temperature as well. In the second case, except for slate (whose minerals are clay) none
of the minerals in foliated rocks can form without elevated temperature as well. Even the
clay in slate is often partly recrystallized to higher temperature minerals.
Foliated rocks occur over large refions of the Earth. The Appalachian region – both the
Piedmont and Blue Ridge – are regionally metamorphosed rocks, and the Piedmont is
partially buried beneath young sediments of the Coastal Plain. The range extends from
Alabama to Canada and has metamorphic rocks throughout.
The foliation in the Appalachian rocks, as is true in all metamorphic belts in all mountain
chains, suggest that thee directed pressure that made the foliation was directed across
the long axis of the mountains – roughly SE/NW. Other indicators of stress direction
indicate the same thing. In the next few slides we’ll see one way to refine our
interpretation and decide on only one direction of stress.
So what is big enough to squeeze the entire side of North America hard enough to
metamorphose the rocks all along the eastern ¾ (or more) of the mountains?
Metamorphic Grade
We looked have at three foliated metamorphic rocks
and I introduced them in the order I did for a specific
reason. The most common protolith for all three is
shale (or mudstone), so why do you get different rocks
by metamorphosing the same thing?
Slate is formed at T&P that can foliate the rock, but not change
the minerals (much). Schist requires higher T&P in order for the
clay to be altered to mica. As you’ll recall from Bowen’s Reaction
Series amphibole and plagioclase are even higher temperature
minerals than muscovite, so the T&P must have been even higher.
Metamorphosing a shale to higher and higher T&P would change
it into each of these rocks in series, starting with slate and ending
with gneiss. Even higher T would melt the rock! (Unless the P
increased even faster and kept it from melting.)
We have created the idea of metamorphic grade to make it easier to suggest that
both T and P are changing, without having to say, “both T & P” every time.
Low grade rocks form at low T & P, mid-grade rocks at moderate T & P, and high
grade rocks at the highest T & P.
Like a lot of things in the natural world the rocks we know don’t divide the possible
range of T&P conditions into equal parts. It would be nice if they did, but they don’t.
There is only a small range of T&P’s that can form both slate and gneiss, and a big fat
middle range that can form schist.
So we have discovered a series of minerals that are formed within the schist range,
but at increasing grade within that range. These minerals are called metamorphic
index minerals.
There are seven of these index minerals shown here
arranged by grade from the bottom left.
There is considerable overlap in the stability fields of
these minerals so one rock might have more than one of
them in it. In that case it is the highest
grade mineral that tells the grade
of the rock.
Sillimanite is often
white and fibrous,
like asbestos.
Kyanite is usually
a very pretty blue
Staurolite has a distinctive
shape in cross-section
Garnet occurs as roundish,
soccer-ball shaped crystals
Biotite you already know
Muscovite you already know
Chlorite – a Green Mica
Staurolite also sometimes
has two different crystals
intergrown to form a “twin
crystal” known colloquially
as a “fairy cross”
You should see two different minerals in this rock. Both are index minerals. Before
you go on, see if you can identify them from the previous picture and indicate which is
the higher grade mineral.
Garnet is the higher grade mineral of the two, therefore this rock reached a T&P adequate to form garnet.
A geologic map of an area in Scotland by George Barrow in 1893. This was the first time that anyone had recognized that various
minerals in metamorphic zones Were not distributed randomly. The “isograds” were picked at the boundary between rocks that had
and rocks that did not have a particular mineral in them. For example, the rocks north of the staurolite isograd have staurolite, those
to the south do not. (“Isograd” just means “same grade”). This version of the map was modified slightly for the textbook: Long, L.,
1974, Geology, McGraw-Hill, NY, 526 p.
Rocks 1,2,3, and 4 are younger than the metamorphic rocks we are interested in because they disrupt the continuity of those older
rocks and faults. In other words, the faults that broke the older rocks did not break the rocks labeled 1-4. Notice that all of Barrow’s
isograds are between two faults, which I’ve emphasized in red. The foliation in the rocks would be roughly parallel to the isogads,
indicating that the rocks were squeezed by a compression coming from the NNW and SSE as the arrows indicate. (The rocks were
later deformed in a different direction, bending them). Which way do you think the main push was actually coming from? Why?
This way. The highest grade rocks, indicating, if you will, the worst T&P
regime for preserving the original rocks unaffected is on this side of the belt.
(Compare the mineral photos with the index minerals a few slides back.)
High Grade
Low Grade
We see something similar in the mountains of north Georgia.
Which way to the zone of maximum T & P?
Blairsville
Blue Ridge
Ellijay
Staurolite Isograd
Only kyanite
Dahlonega
Kyanite + staurolite
And in the Piedmont near Carrollton, GA.
Which way to the zone of maximum T & P?
Garnet isograd
Kyanite isograd
Increasing grade indicates this way to the
zone of highest T&P.
Remember the earlier question: What
can do this kind of damage to the edge
of a continent?
Any ideas?