Post on 27-May-2020
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