Metamorphic Rocks What is a metamorphic rock? The word metamorphic can be broken down to help explain its

meaning. The prefix meta- means after, and -morph means shape or form, so in combination,

the word metamorphic means “after shape” which fits exactly what metamorphic rocks are.

When a rock is exposed to the extreme heat and/or pressures that exist beneath the surface of

the Earth, its physical properties change, and indeed, its shape may as well. Most of the time

the chemical makeup of the rock remains the same, and it resembles very closely its parent

rock, or the rock it was before metamorphism. Mineral colors are often a good indicator of

what the parent rock looked like before metamorphism.

So how does metamorphism happen? Metamorphism occurs in one of two ways: regionally,

or locally (contact). Regional metamorphism occurs over large areas as rocks are exposed to

extreme pressures and heat. Typically regional metamorphism is found along folded mountain

belts and during tectonic plate collisions where pressures can exceed millions of tons.

Local or contact metamorphism is much different and is also on a much smaller scale. If an

intrusion of magma makes its way through layers of rock, it will heat its surroundings. This

heat is sufficient enough to change the characteristics of the rock around the intrusion causing

the formation of metamorphic rocks. Local metamorphism is common in areas with volcanic

activity and along plate boundaries.

IN THIS ISSUE

Plate Tectonics P.1 Foliated Rocks P.1

Contact Metamorphism P.2 Non-foliated Rocks P.2

Foliated Rocks

When minerals align from directed

heat and pressure during metamor-

phism they fall into the classification

of foliated rocks. Foliation charac-

teristics range from flaky, crystals

composed of the mineral mica in the

metamorphic rock schist, to extreme

bands of alternating light and dark

minerals in the metamorphic rock

gneiss.

METAMORPHIC ROCKS THE EARTH SCIENCE JOURNAL

ISSUE 12 NOVEMBER 2013

Non-foliated Rocks

Non-foliated metamorphic rocks do

not have the mineral alignment that

foliated rocks do and will often have

a color and texture similar to that of

their parent rock. The sample below

is quartzite, which is a metamorphic

rock formed from a quartz sand-

stone. Quartzite is slightly more

dense and very resistant to weather-

ing, so it is a popular choice for use

in building materials.

Shale to Slate

Sandstone to Quartzite

The diagram above depicts an igneous intrusion pushing through horizontal rock strata

(layers). Along the intrusion are a series of hachure marks that indicate contact metamor-

phism of the surrounding sedimentary rocks. In order to identify what rocks the preexisting

sedimentary rocks change into, you must use both the Scheme for Sedimentary Rock Identifi-

cation and the Scheme for Metamorphic Rock Identification. First identify the sedimentary

rock that the intrusion goes through. Then look under the ‘Comments’ column on the Scheme

for Metamorphic Rock Identification chart and determine which metamorphic rock formed

from the previously identified sedimentary rock. A few examples are below.

Contact Metamorphism

THE EARTH SCIENCE JOURNAL

Created By: Dave Burgess

Rochester STEM High School

I do not own any rights to the images in this journal.

All images used for educational purposes only.

Limestone to Marble

Conglomerate to Metaconglomerate