Metamorphic rocks can be difficult to identify due to the very nature of their origin. Metamorphosis of one rock into another rock is a unique event of temperature, pressure, percolating hot fluids, and time. However, enough basic similarities

exist to allow for the process of using a few simple characteristics to classify some of the more common metamorphic rocks.

To begin, click here.

FoliatedNon-foliated

sandstone

limestone

Is slate:

Slate

or

Think of the sedimentary rocks you know. Which one would most readily be metamorphosed into slate?

shale

or

or

Both are right?? Click here to check it out.

Which characteristic makes a thin, flat piece of slate a great roofing material?

Water absorption rate

Hardness

or

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What’s foliation? Click here About 25 cm (10 in)

Foliated is another way to say that a metamorphic rock splits into layers. Notice how the layering is revealed along the edges of the slate after it has weathered on a roof for a while. If you had a hammer and a chisel do you think you might split off a thin layer of slate from one of the roof tiles? If so, then it is a foliated metamorphic rock.

If the metamorphic rock could not be split into flat layers then it would be a non-foliated metamorphic rock.

Metamorphic foliation and sedimentary layering are not the same thing nor formed the same way. Sedimentary layering forms by the process of different events depositing sediment which is then compressed into rock by the weight of overlaying layers of newer and younger sediments.

Metamorphic foliation requires the higher temperatures and pressures that accompany tectonic activity. Because foliation often occurs 90 degrees to the compression direction, studying it can provides clues to the direction(s) from which the tectonic pressure came.

Metamorphic Foliation

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About 25 cm (10 in)

First. slate may be rather easily split into thin, regular sheets. In fact, producing slate roofing tiles can be done by hand with minimal machinery. Slate’s foliation contributes to its ability to be cleaved in thin plates.

Second, the rock can be cut into shape and retain the shape. This is due to the rocks inherent internal strength.

Third, it is heavy and will not blow away.

Fourth, it is fire proof.

Fifth, and maybe most importantly, slate’s fine-grained texture provides a smooth surface that is nearly impervious to water and sheds rain water very quickly.

Slate’s Advantage as Roofing Material

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Misconception: “Roof slate” is not found in West Virginia coal mines. In spite of its common usage, “roof slate” is actually shale.

About 25 cm (10 in)

Non-foliated FoliatedIs phyllite or

Fine GrainedCoarse GrainedIs phyllite or

Based on your answers to the previous question, would you expect to be able to see individual minerals in phyllite? Explain your answer.

One of the best ways to identify phyllite is to look at the rock’s luster. Which term best describes phyllite’s luster:

DullGlossy Silky

Take a guess – is phyllite a metamorphosed version of shale?

Phyllite

Two right answers?? Do you know what luster means?

Click here to find out.

Click here to see

Click here to confirm.

No, not really! Phyllite is a metamorphosed version of slate. But, if you said yes you were not that far off. Actually, if a shale were to be subjected to a metamorphic environment it would first become slate. If more pressure were added the slate would become phyllite.

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FoliatedNon-foliatedIs gneiss

or

This is a nice rock! Really, its name is GNEISS but it is

pronounced like nice.

What is gneiss’ most distinguishing visual characteristic?

Alternating coloration

BandingLayering

All metamorphic rocks come from some other preexisting rock. Looking at the picture, can you speculate which igneous rock may be changed into a gneiss? Hint, which common, course-grained, igneous rock contains different minerals that could be moved around to form the bands seen in gneiss?

Can you explain the origin and geologic significance of gneiss’ most distinguishing visual characteristic?

Click here to find out

Click here to find out

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FoliatedNon-foliated

Fine GrainedCoarse Grained

Yes, that is it’s name. Get over it!

orIs schist

What is its texture?

or

How would you describe the foliation in the schist?

or

Schist

Laminated WithMinerals

Distinct Bands

Click here for an explanation

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Don’t know what “texture” means? Click here.

Non-foliated FoliatedIs greenstone or

Look at the photograph. Trick question - What is a greenstone’s most distinguishing characteristic?

Greenstone

Click here for the answer

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Quartzite and Marble

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Look at the photograph to the right. One rock is quartzite and the

other is marble.

One is red and one is white. Sometimes color is helpful, but in

this case, not so much.

One seems to be fizzing and the other is not.

Both rocks have a sugary texture and both are non-foliated.

Which is which?

If you have studied mineral or sedimentary rocks you may know

how to identify them.

If not, the photograph provides a clue.

Click here for more help.

More clues.

• Quartzite is a metamorphosed version of sandstone.• Sandstone is a sedimentary rock that contains a lot

of the mineral quartz. • Quartz is very resistant to chemical weathering.• Strong acids barely affect quartz.

• Marble is a metamorphosed version of limestone. • Limestone is a sedimentary rock that contains a lot

of the mineral calcite. • Calcite is NOT very resistant to chemical weathering. • In fact, rain water, which is only slight acidic, will

slowly dissolve the mineral calcite, the sedimentary rock limestone, and the metamorphic rock marble.

Look close. What’s happening?

An Answer – The Fizz Test!

Did you notice the bubbles? The bubbles on the marble are the fizz test at work! One or two drops of a 5% concentration of dilute hydrochloric acid is dissolving the rock. The chemical reaction is producing carbon dioxide bubbles. It’s actually more impressive when you see and hear it first hand. Ask your teacher to demonstrate or to provide you with the material to try it yourself. Remember the goggles! The simple fizz test is an extremely valuable geologic tool. Bac

k

Non-foliated

FoliatedAnthracite is

or

Anthracite’s luster is

Non-metallic

Metallic

or

Anthracite

? - Click here

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?Look at the photograph again. If you pick up a piece of anthracite will your hands get dirty? Take a guess.

Yes No

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LAST PAGE…KNOWLEDGE APPLICATION

You task is to identify the three metamorphic rocks on this page. You’ll have to get someone to check your answers!

A. The texture of this rock is ____________. It may be marble but the only way to find out is to ___________________.

B. This is a foliated, fine-grained, metamorphic rock with a silky luster. Is it anthracite or phyllite?

C. Sometimes its nice just to look at a photograph of a ___________ metamorphic rock.

A B C

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