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For Elementary School Teachers

Metamorphism is the alteration of a preexisting rock (the parent rock) due to heat and
pressure caused by burial in the earth. The parent rock must adapt to the new conditions and it
does so by changing mineral composition and texture. These rocks with new texture and
composition are metamorphic rocks. Texture and composition are each discussed next.

METAMORPHIC TEXTURES AND COMPOSITIONS

Metamorphic Rock Texture
The new metamorphic textures are of two types, granular (also non-foliated)
where the grains are equidimensional, and foliated where the minerals are
layered .

Granular rocks are usually uniform in composition; they are all one mineral (e.g. quartz =
Quartzite, calcite = Marble; exception = Hornfels) so the minerals do not segregate into layers -
like the picture to the right. (We must be careful not to confuse mineral layering with color
banding; in some rocks color banding looks superficially like mineral layering, but color
banding without mineral layering is not foliation; for example in this Serpentinite.)

Foliated textures result when the new metamorphic minerals (many of which are platy micas
such as Biotite and Muscovite) line up producing a distinct layering in the rock. The layering
produces three distinctly different looking rocks, those with slaty cleavage (e.g. Slate),
schistosity (e.g. Schist), and mineral banding (or Gneiss Texture). Metamorphic rocks are
identified on these textures (click image for large version, or go to Metamorphic Texture for more
details).

One source of confusion is that we have both texture names and rock names, and although
they often refer to the same thing, there are exceptions. The table below shows all the
relationships. Since texture names and rock names do not line up perfectly it is important to
distinguish them, but from this point on we will refer to everything by rock names.
Metamorphic Rock Composition
Metamorphism is the alteration of a parent rock into a metamorphic rock. There are as many
parent rocks as there are igneous, sedimentary, and metamorphic rocks, since any of these can be
metamorphosed.
But, the simplest way to examine compositional changes during metamorphism is to start with
the three end products of the Simple Model For Sedimentary Rocks, shown in the illustration
below (click picture for full model). These weathering products are also the most common
sedimentary rocks and form a very large percentage of the metamorphic rocks we see.

These three end products (Attractors) in the sedimentary model (Quartz Sandstone, Shale, and
Limestone) become the three parent rocks of metamorphism. All the metamorphic rocks
discussed here, and included in the identification key (see below) come from these three parents.

The Changes We See

Compositionally simple sedimentary rocks, such as quartz sandstone and limestone (calcite)
do not change composition with metamorphism - their chemistry is too simple. Quartz remains
quartz, and calcite remains calcite. The rocks do recrystallize, though, forming granular rocks
with fused grains; the rocks quartzite and marble. It does not matter by what process you
metamorphose quartz sandstone and limestone, their composition does not change, and they
produce the same kind of metamorphic rock.
Rocks with a more complex chemical composition, such as shale (the mineral clay), however,
undergo many compositional, and textural changes. Shale produces the greatest diversity of
metamorphic rocks, so many changes in fact that it is sometimes hard to believe they could all
come from the same parent. We can see this in the chart below where sedimentary shale turns
into Slate, then Phyllite, then Schist, then Gneiss. These rocks look so different that it can be hard
to imagine they are all derived from the same shale parent (and this does not even include the
Hornfels produced when shale is contact metamorphosed).

Note that we keep referring here to the mineral composition of the rocks, and will make some
use of mineral composition in the key to identifying the rocks. If you are working with
elementary school students it is easy to introduce them to the few minerals needed, but you can
also work with just the general appearances of the rocks.

IDENTIFICATION KEY TO COMMON METAMORPHIC ROCKS

Pdf Version - What Is Pdf?
 Rocks are identified by making a series of decisions about their properties, such as texture,
composition, hardness, etc. This requires the ability to observe and recognize these properties -
one of the skills of science that must be learned for each group of rocks.
The Key To Common Metamorphic Rocks allows someone to identify a rock based on its
properties. We are able to do this because the properties do not overlap completely. You can see
this on the key through the color coding for the properties. None of the colors overlap
completely. [Note this key does print legibly in black and white.]
Go to Simple Metamorphic Identifiction Keys And Charts for suggestions on using the key
with students.

KINDS OF METAMORPHISM
Although there are basically Five Kinds Of Metamorphism, most introductions to
metamorphism focus on just two: Barrovian (" regional" ) and Contact. We examine each of
these below.

Barrovian Metamorphism
Barrovian metamorphism takes place during mountain building
events when very large areas of sedimentary rocks are buried,
squeezed, and heated. Sometimes the metamorphism occurs just
because the burial is deep enough, but often it is also associated with
major igneous intrusions that supply most of the heat. For example,
the small sketch to the right shows a collision between two continents Clicking on it will take
you to a larger drawing of a continent-continent collision showing regions of Barrovian
metamorphism.
Barrovian metamorphism produces both textural changes and compositional changes in the
rock. The compositional changes take place because sedimentary minerals are stable only at the
earth's surface. As they become buried the sedimentary minerals become unstable and transform
into new metamorphic minerals. For example, clay goes to Chlorite, and chlorite eventually goes
to Quartz, Feldspar, and Mica, as well as other minerals. At the upper end of metamorphism the
rock melts and becomes igneous.
But the texture of the rock also changes during metamorphism. The rocks become foliated,
that is the texture sequence of slaty cleavage to schsitosity, to banded, or the rock sequence slate
to phyllite to schist to gneiss.
Thus, metamorphic rocks not only tell us the kind of metamorphism, they are also a measure
of the intensity of metamorphism. The closer we get to the source of heat and pressure the more
altered the rock becomes.

Contact Metamorphism (click image for enlargement)

Contact metamorphism occurs in the " country rock" (the rock
intruded by and surrounding an igneous intrusion). Rocks are "
baked" into a ceramic from heat escaping from intrusives, often
enhanced by hot fluids. However, unlike Barrovian
metamorphism, pressure is not a significant factor in the contact
process, especially directed pressure. Without directed pressure
foliation does not develop. So, the clay (shale) which in
Barrovian metamorphism develops a strong foliation (slate,
phyllite, schist, gneiss) in contact metamorphism develops a
granular texture (the rock hornfels). The analogy is putting a clay
pot in a kiln; the clay is simply baked.
Contact metamorphism can occur next to any igneous intrusion, although it is most easily
seen next to smaller intrusions. The intensity of metamorphism decreases with distance from the
intrusion, until at some distance away the rock is unaltered country rock. However, large
intrusions such as batholiths usually alter the country rock so much that other styles of
metamorphism override the contact metamorphism.
 The Relationships Between Rock and Process
One final observation. It would be nice if there
was a simple relationship between the processes of
metamorphism and the rocks produced. For example,
if all schist rocks were produced by the same kind of
metamorphism. Or, if a particular kind of
metamorphism produced rocks not found in any other
kind of metamorphism. Unfortunately, such simple
relationships do not exist. True, some rocks are
confined to one kind of metamorphic process, but
other rocks result from more than one kind of
metamorphism. Eventually these relationships have
to be learned, but they can be easily summarized in
the table to the right for this simple classification.
Observe that hornfels, quartzite, and marble are contact metamorphic rocks, but that hornfels
is the only strictly contact metamorphic rock. And, note that quartzite and marble can be formed
by both contact and Barrovian metamorphism.
One lesson might be that simple parent rocks undergo simple metamorphism, regardless of the
kind or intensity of metamorphism. Another lesson is, quartzite and marble cannot tell you the
kind of metamorphism, although hornfels, slate, phyllite, schist, and gneiss can.

Associated with this page:

Key To Common Metamorphic Rocks
Simple Metamorphic Identification Keys And Charts
Hardness And Acid Reaction Tests For Minerals And Rocks

Go on to more detailed pages

An Introduction To Metamorphism
The Mechanisms Of Metamorphism
Kinds Of Metamorphism
Metamorphic Rock Classification

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Last Update: 10/25/00 e-mail: (Fichtels@Jmu.Edu)