Metamorphic Rocks

Metamorphic Rock Identification

Introduction

Rocks are made of minerals. Rocks can be a mixture of different kinds of minerals, a mixture of many grains of the same kind of mineral, or a mixture of different grains of rocks. When you split a rock into very small pieces, the pieces are different from each other. For example, when you break granite apart, you get small pieces of quartz (clear), feldspar (pink or white), and mica (black). When you split a mineral into pieces, you still have pieces of the same mineral. If you break a big chunk of quartz into smaller pieces, you still have pieces of quartz.

There are three basic rock types: Igneous, sedimentary, and metamorphic.

Metamorphic Rocks

Metamorphic rocks (changed rocks) are made when existing rocks are subjected to high temperatures and high pressures for long periods of time. Metamorphism (meta = change, morph = form) happens when molten rock intrudes other rocks and bakes the contact zone where the molten rock touches the preexisting rock. Metamorphism also happens when rocks are buried deeply during the process of mountain building. The kind of metamorphic rock made depends on the kind of original rock; for example, sandstone is turned to quartzite, shale is turned to slate, and limestone is turned to marble. Other kinds of metamorphic rock are named for the kinds of minerals present, the size of the grains and other textures. For example, mica schist has very thin layers of mica, and garnet gneiss (pronounced like nice) has garnet crystals in thick layers of quartz and feldspar. The amount of time, amount of pressure, and highness of temperature determine what types of metamorphic rocks are made.

How Metamorphic Rocks are Made

1. Pressure from the weight of overlying rocks or from stresses of mountain building rearranges the minerals in rocks into bands or rearranges the atoms of the minerals into new minerals.
2. Heat from the intrusion of a large igneous mass can metamorphose a large area.
3. Heat from the intrusion of a dike or sill or flow can bake the adjoining rocks in a contact metamorphic zone.

Composition

The mineral composition of the rock can be determined based on observations with a hand lens and if needed, physical or chemical tests. Start by identifying and list all visible minerals present in the rock.

Probable Parent Rocks

All metamorphic rocks are derived by the action of heat and/or pressure on pre-existing igneous, sedimentary, or metamorphic rocks. The pre-existing rock is called either the parent rock or the protolith. Your textbook incorrectly uses the term “source rocks” for the pre-existing rock. The term source rock used in sedimentary deposits to describe the rock from which petroleum is derived, or the rock that erodes to produce sediment and later sedimentary rock. Read the descriptions in your lab manual or textbook for the source rock of each of the metamorphic rocks that you identify. Indicate in the name(s) of the probable parent rock(s) in the last column of the metamorphic rock identification form.

Texture

The term texture refers to the size, shape, and boundary relationships of the minerals, particles, and other substances that make up a rock. There are two major textural groups in metamorphic rocks: Foliated and Non-Foliated.

Foliated (Banded) Metamorphic Rocks

In this texture, the mineral crystals in the rock are aligned with each other. This alignment may be displayed as parallel planes along which the rock splits, by overlapping sheets of platy minerals such as micas, by the parallel alignment of elongate minerals such as amphiboles, or by alternating layers of light and dark minerals. Foliated texture is further subdivided based on the presence or absence of pronounced color banding in the rock. Rocks without distinct alternating bands of light and dark minerals are described a nonlayered, whereas rocks with alternating bands of dark and light minerals are described as layered. Layered is also referred to as gneissic foliation. Foliated textures are further described on the basis of the grain (crystal) size in the rock. Examples of complete descriptions of foliated metamorphic rocks include: foliated, nonlayered, very fine grained for slate, foliated, layered, coarse grained for gneiss, and foliated, nonlayered, fine grained for phyllite. Foliated textures produced by shearing and breaking, such as in a fault zone or a meteor impact crater, are referred to as mylonitic.

Non-Foliated Metamorphic Rocks

In this texture the mineral crystals in the rock have grown in many directions and do not show alignment. As a result, nonfoliated rocks commonly appear massive and structureless, with only a few lines of impurities through the rock. These rocks may break across, rather than around, mineral grains to produce a scaly surface on the specimen. Nonfoliated textures are further described on the basis of the grain (crystal) size in the rock. Examples of complete descriptions of nonfoliated metamorphic rocks include nonfoliated, medium grained for quartzite, or nonfoliated, coarse grained for marble.