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Metamorphic Rocks
Metamorphic rocks started out as some other type of rock, but have been substantially
changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic
rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or,
more commonly, some combination of these factors. Conditions like these are found deep
within the Earth or where tectonic plates meet. In metamorphic rocks some or all of the
minerals in the original rock are replaced, atom by atom, to form new minerals.

Metamorphic rocks are often squished, flattened out, and folded. Despite these
uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would
become igneous rocks!

Foliated metamorphic rock (Foliated means having thin leaf-like layers)
Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they
become aligned. These rocks develop a platy or sheet-like structure that reflects the direction
that pressure was applied in. Slate, schist, and gneiss (pronounced 'niece') are all foliated
metamorphic rocks.

Non-foliated metamorphic rock
Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several
ways that non-foliated rocks can be produced. Some rocks, such as limestone are made of
minerals that are not flat or elongated. No matter how much pressure you apply, the grains
will not align! Another type of metamorphism, contact metamorphism, occurs when hot
igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked
by the heat, changing the mineral structure of the rock without addition of pressure.

Here are some things to look for in metamorphic rocks.

Texture is the main thing to look for. Some of the forms you find may be;

Slatey;  Fine crystalline rock in which minerals, such as mica, are aligned parallel to one
another, which means that the rock splints easily between the layers.
Slate is another example of this.
Schistose;  Minerals such as mica, chlorite and hornblende are aligned in easily visible
parallel bands and, because of their platy alignment, the rock splits easily. These are called
Gneissose;  These are characterized by a coarse foliation with individual bands several
centimeters across. The foliation may wrap around larger crystals, as in Augen gneiss, and
all the minerals are very coarse and grainy and easily identifiable. These are called Gneiss
Granoblastic;  Mainly large mineral grains that have crystallized at the same time, and
therefore, penetrate each other with the grains remaining large enough to be identified easily.
Hornfels;   Compact, finely grained rock that shatters into sharp fragments. These are called
Banded;  Components occur in well-defined bands.

Here are some photographs of some common metamorphic rocks. Look for these same
forms, patterns and structure in the rocks you find during your rock hunting trips. See…you
have made another step on the road to  becoming a genuine rockhound. When you find your
rocks, you can number them as we do and make a list to remember their names.

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Common Metamorphic Rocks