Igneous Rocks Lab
From GeoClasses
Contents |
Introduction
A rock is defined as an aggregate of minerals. This aggregate (a collection of units or parts, in this case crystals or grains, which form a single unit) can contain a single mineral type, or it can contain multiple minerals in various proportions. Based upon a rock’s origin, its classification will fall into three broad categories: igneous, sedimentary, or metamorphic. The Rock Cycle illustrates the origins of formation of these three rock categories and is provided in Figure 2-2.
An igneous rock is the result of the cooling and crystallizing of magma or lava. As molten rock materials (magma or lava) cool, the elements in its mixture crystallize to produce a mineral aggregate—characterized by interlocking grains.
If a rock is exposed to the environmental elements on the Earth’s surface, it is broken into discrete particles or dissolved by weathering processes. Subsequently, these particles or dissolved materials may be transported by wind, water, or ice, and eventually deposited as sediment. These sediments, sometimes mixing with plant or animal remains, may become compacted or cemented: thus producing a sedimentary rock.
If a rock (any kind of pre-existing rock) is buried and subjected to elevated temperature and/or pressure, the minerals and/or size of its crystals are changed. The result of such a transformation is called a metamorphic rock.
This lab focuses on the identification of igneous rocks.
Lab objective
The objective of this exercise is to learn how to identify and classify some common igneous rocks.
Getting started
Read the following definitions and explanations before attempting to identify the provided rock specimens. First, you will learn a little more about the occurrence of igneous rocks. Then, you will learn how to classify igneous rocks by texture and by mineral composition.
How Do Igneous Rocks Form and Where Do They Occur?
For igneous rocks, crystallization takes place as magma or lava cools. In theory, if magma is subjected to increased pressure (within the earth), the same results are produced. Igneous rocks are subdivided into two major categories that are dependent upon the place of magma’s cooling, which consequently influences the rate of cooling and thus the texture and mineral composition. Bowen’s Reaction Series, provided in Figure 2-1, illustrates the temperature sequence of mineral crystallization from a magma.
The two categories of igneous rocks are intrusive (plutonic) and extrusive (volcanic). Intrusive rocks are formed when magma cools and consolidates below the earth’s surface, whereas extrusive rocks are formed when magma cools on the surface of the earth.
Plutonic rocks are found exposed in the deeply eroded cores of mountain ranges and in the ancient rocks of the Canadian Shield. Presently, volcanic rocks are found in mountain ranges like the Cascades of Washington and Oregon, the Andes, and the Hawaiian Islands. Additionally, volcanic rocks are found covering vast areas such as the Columbia Plateau of Washington and the Deccan Plateau of India. Volcanic rock also forms the sea-floor that is exposed on the mid-ocean ridge.
Classification of Igneous Rocks
Igneous rocks are classified on the basis of their texture and mineral composition.
Figure 2. The rock cycle . Igneous rocks -//| |\\ // \\ Melting // Melting \\ Weathering and erosion // \\ // \\/ Weathering and erosion Metamorphic rocks <====================> Sedimentary rocks Metamorphism
Classification by texture
The texture of an igneous rock directly corresponds with its cooling history. Volcanic (extrusive) rocks are produced by lava flows and shallow intrusions; their fine-grained or glassy texture is a result of rapid cooling in a low-pressure environment. Conversely, intrusive rocks are produced deep within the earth under conditions of high pressure. This environment of slower cooling and high pressure produces plutonic (intrusive) rocks, coarse-grained rocks with larger mineral constituents. Sometimes rocks show a cooling history with a change in their rate of cooling. Because of this change in cooling rate, these rocks usually have at least two markedly different-size mineral grains. Table 2-1 provides you with a quick reference for basic classification of igneous rocks.
Classification by mineral composition
The mineral composition of most igneous rocks is relatively straightforward. The following nine minerals, or mineral families, in varying combinations and proportions, fall under the two general categories of felsic and mafic. How do you remember which mineral composition goes into the felsic of mafic category? Just remember that the words felsic and mafic are acronyms.
The word “felsic” is derived from a combination of the words feldspar and silica. The most abundant minerals in felsic igneous rocks are generally light in color because they contain much silicon and may include feldspar.
Likewise, the word “mafic” is derived from a combination of the words magnesium and ferric. The most abundant minerals in a mafic igneous rock are generally dark in color because they contain magnesium and iron.
Table 2-2 lists the minerals that commonly make up felsic and mafic rocks.
Now that you are familiar with the different textures and mineral compositions of igneous rocks, start working on Lab Exercise II.
Lab II—Exercise
Overview
This is a three-step exercise. In Step 1 you will examine each rock specimen, determine its texture, and identify each specimen by origin, general category, and name. If the specimen is of phaneritic texture, then you will need to proceed to Step 2 to determine the approximate mineral composition of each phaneritic rock and identify each specimen by origin, general category, and name. In Step 3, you will verify the identity of the specimen. A worksheet is provided at the end of this chapter.
- Glassy texture indicates a rock that is obsidian, scoria, or pumice.
- Aphanitic texture indicates that the grains are too fine to see with the naked eye. Use color to identify the rock. A relatively light colored aphanitic rock is called a felsite; whereas, a very dark colored and heavy aphanitic rock is called a basalt.
- Porphyritic texture indicates that the phenocrysts must be examined and identified. If the phenocrysts are quartz, the rock is a rhyolite porphyry. If the phenocrysts are amphibole, the rock is an andesite porphyry.
- Phaneritic texture indicates that further determination of its mineral composition is necessary. If you have a phaneritic specimen, go to Step 2 of this exercise.
