Metamorphic rocks Lab
From GeoClasses
Contents |
Introduction
Metamorphic rocks, the third category of rocks, form when preexisting rocks are transformed beneath the earth’s surface. This process of transformation, called metamorphism, changes the mineralogy (or mineral composition) and texture of rocks and forms new rocks as a result. The term metamorphism comes from the Greek terms meta, meaning change, and morpho, meaning shape or form. Any kind of rock, including a metamorphic rock, has the potential to undergo change and become a new metamorphic rock.
Metamorphism occurs deep within the earth under conditions of high temperatures and pressure; the presence of fluid within this same environment accelerates the metamorphic process. A rock’s metamorphism, however, occurs while it is in a solid form—not a liquid form. The type of metamorphic rock that will form depends on three factors: the parent (source) rock, metamorphic agents, and the amount of time that the parent rock is subjected to these agents.
Lab objective
Students will learn to identify and classify some common metamorphic rocks.
Getting started
Read the following text before beginning your identification of the provided rock specimens. First, you will learn more about how metamorphic rocks form and where they occur. Then, you will learn how to classify metamorphic rocks primarily by texture.
How Do Metamorphic Rocks Form and Where Do They Occur?
There are three types of metamorphism: contact (thermal), dynamic, and regional. Because the first two types affect relatively small volumes of crustal rock, they are not pertinent to the identification process in this lab exercise. Regional metamorphism—resulting from deep burial, high heat, and pressure—produces profound changes in texture and mineral composition. This lab focuses on rocks that are transformed by regional metamorphism.
Metamorphic rocks form in regions of lithospheric plate convergence (where the earth’s plates move towards each other), lithospheric plate divergence (where the earth’s plates move away from each other), and near large plutons (massive, intrusive, igneous formations within the earth). They are also prominent in the Shield (the nucleus around which a continent grows, which is also a massive area of exposed ancient rock) areas of the world where rocks are more than 600 million years old. In short, metamorphic rocks are rocks that form by mineral and textural transformations in a solid state; they are produced in the orogenic regions of the world where mountain building processes such as folding and thrust faulting occur.
Classification of Metamorphic Rocks
For this lab exercise, the classification of metamorphic rocks relies primarily upon texture, which is defined by the size of a rock’s grains and its fabric—the arrangement of a rock’s grains. Because definitive classification of metamorphic rocks by mineral composition generally requires the use of polarizing microscopes, mineral composition serves simply as a default classification for this lab. For this particular lab exercise, if a rock cannot be classified by texture, field identification of the predominant mineral will suffice.
Metamorphic Rock Textures
Foliation, which produces a banded appearance in rocks, is a primary indicator of a metamorphic rock, although metamorphic rocks occur in either foliated or non-foliated forms. Parallel arrangement of mineral grains, which are usually tabular or elongate in their shape, or parallel segregation of mineral grains, caused by pressure, produces foliation in most metamorphic rocks.
There are three main types of foliation in metamorphic rocks: schistose banding, slaty cleavage, and gneissic banding. Schistose banding (also known as schistosity) is a type of foliation that is due to mineral orientation within a rock; minerals (which can usually be seen with the naked eye) are arranged in a parallel orientation in the rock. If minerals are too fine to be seen with the naked eye, the type of foliation is called slaty cleavage. Rocks with a slaty cleavage can be easily split into parallel planes. Slate is a desirable building rock for roofs, walls, and paving, because of its slaty cleavage. Gneissic banding (also known as gneissosity) is characterized by minerals of different colors segregated into alternating parallel bands within a rock.
If a rock does not show any foliation, then it is a non-foliated rock. Non-foliated metamorphic rocks have minerals that are equidimensional in their shape and, as a result, do not align, orient, or segregate into bands.
Lab IV—Exercise
Overview
This is a short, two-part exercise. First you will examine each metamorphic rock specimen and determine if the specimen exhibits a foliated texture and, if it does, also determine the type of foliation. If the specimen does not exhibit a foliated texture, you will determine its predominant mineral composition. Then you will refer to Table 4-1 and use the information that you have to identify each specimen.
- If the rock is foliated, use the descriptions given above and the assistance of your lab instructor to determine which of the following types of foliation it exhibits:
- Slaty cleavage
- Schistose banding (schistosity)
- Gneissic banding (gneissosity)
- If the rock is non-foliated, determine its mineral content using the field methods you have learned in the previous exercises. Determine if the specimen is predominantly quartz or calcite
