4.1 Classifying Igneous Rocks

Igneous rocks are classified based on the proportion of their minerals and their texture. The proportion of minerals reflects the rock composition, which can suggest where the magma was sourced from. The texture reveals information about a magma’s cooling history. Magma within the Earth cools slowly; therefore, crystals may grow to a mostly uniform size of more than a centimetre in diameter. In contrast, lava extruded from the Earth cools quickly, so grains have a short time to grow and are typically too small to be seen with the naked eye. The texture of most igneous rocks is crystalline, identified as a network of interlocking crystals.

Texture of Igneous Rocks

The general physical appearance or character of an igneous rock can be subdivided into three parts:

1. Crystallinity: the proportion of the rock that is crystalline as opposed to glassy
2. Granularity or grain size: size of mineral grains or crystals:
   1. aphanitic
   2. phaneritic: coarse-grained, medium-grained, fine-grained
3. Fabric: the relationship between crystallinity and granularity in a rock
   1. shape of mineral grains: euhedral, subhedral, and anhedral
   2. distribution of mineral grains: equigranular or porphyritic

Igneous rocks may have special textures as well. For example, rocks can have vesicles, making them vesicular. Another example is the rock pumice, which has a pumiceous texture. Although there are many more, you will only need to know vesicular and pumiceous textures.

Composition of Igneous Rocks

Igneous rocks mainly consist of a few minerals, including quartz, feldspar minerals (plagioclase and K-feldspar), amphibole, pyroxene, olivine, and mica minerals (biotite and muscovite). These minerals make up over 95% of all igneous rocks. The order of crystallization allows minerals with the highest melting point to crystallize first and develop euhedral faces. Minerals that crystallize at lower temperatures are forced to grow in the space between the earlier formed crystals and are commonly anhedral in shape. Texture helps identify a general order of crystallization (Figure 4.1). This order is fundamental to the understanding of igneous rocks.

Bowen’s reaction series, in Figure 1, shows that igneous minerals are not always compatible. For example, the mineral olivine would not occur with muscovite.

• Felsic igneous rocks are rich in light-coloured silicate minerals such as feldspar and quartz (silica). The name “felsic” is derived from feldspar + silica.
• Intermediate igneous rocks have nearly equal amounts of felsic and mafic minerals. They have moderate amounts of silica.
• Mafic igneous rocks are rich in magnesium and iron. The name “mafic” is derived from magnesium + ferric iron. They are low in silica and usually contain olivine, pyroxene, amphibole, and biotite.
• Ultramafic igneous rocks are usually composed of very high amounts of mafic minerals and are very rich in magnesium and iron. They have very low amounts of silica.

Classification of Igneous Rocks

How would you identify minerals in an extrusive igneous rock if the grains are too small to see? There are a couple of tricks that geologists use to identify which type of igneous rock they have. Do you remember how colour was not a diagnostic property for minerals? Well, colour in an igneous rock can be diagnostic! Iron and magnesium-bearing minerals are either black or green in colour, like olivine and pyroxene, and they are the primary minerals that make up mafic and ultramafic igneous rocks. Potassium, aluminum, and silica-rich minerals, like quartz and potassium feldspar, are lighter in colour and make up intermediate and felsic igneous rocks. Geologists can use the colour of igneous rocks and the relative amount of light- vs. dark-colored minerals to help identify them (Figures 4.2 and 4.3).

Glassy and vesicular textures are a little different (Figure 4.4). These rocks form when lava cools off too quickly for minerals to form. The conchoidal fracture in obsidian tells us that it has a high silica content (felsic) despite its dark colour. The dark colour comes from the tiny amount of iron present.

4.2 Identification of Sedimentary Rocks

Sedimentary rocks form at Earth’s surface from the physical and chemical breakdown of older rocks and the recombination of these products of weathering into new rocks. They are the most common rocks on Earth’s surface, often covering igneous and metamorphic rocks that they come from. Sedimentary rocks tell us about the environment in which they formed on the surface of the Earth, even millions or billions of years ago. The components and their relationships may provide important clues about the past:

1. Source of the sediment
2. Type and amount of weathering
3. How the sediment was transported and the duration of transport
4. Physical, chemical and biological environment of sediment deposition
5. Changes that may have occurred after deposition

In Lab 2, you have already learned that sedimentary rocks can be subdivided based on how they formed: Clastic sedimentary rocks are composed of fragments of pre-existing material, typically the result of physical weathering. Chemical sedimentary rocks are formed by chemical and organic reprecipitation of the dissolved products of chemical weathering. Biochemical sedimentary rocks are a type of chemical sedimentary rock that has a biological component to their origin. Organic sedimentary rocks contain a large amount of organic carbon material. Labs for this course will only include clastic, chemical and biochemical sedimentary rocks.

Clastic Sedimentary Rocks

Most sedimentary rocks have a clastic texture, characterized by clasts, or grains, of rocks, minerals or fossils (Table 4.1). In clastic rocks, grains are not intergrown with each other but are generally cemented together by a chemical precipitate. The particles are commonly derived from outside the basin of deposition, but where the particles are fossils or reworked chemical sediments, they may have instead formed in the basin itself. If the clastic grains are mostly fossils or fossil fragments, the rock is called bioclastic. The clasts may be composed of individual minerals (quartz, for example, is a common constituent of sandstone) or may be small fragments of pre-existing rock.

Rocks with a clastic texture have the following components:

• Framework grains: particles that support one another at the points of contact and form a rigid arrangement of supporting open pore spaces
• Matrix: finer-grained material enclosing or filling the interstices between the framework grains
• Cement: mineral material crystallized between individual particles, binding them together
• Porosity: percentage of the bulk volume of the rock that remains occupied by open spaces

Crystalline Sedimentary Rocks

Chemical sedimentary rocks, as well as non-clastic biochemical sedimentary rocks, commonly exhibit recognizable crystalline textures (Table 4.2). A primary crystalline texture forms during or shortly after deposition. Secondary crystalline textures result from recrystallization or replacement of existing minerals by new ones after lithification. Some sedimentary rocks, such as chert, are so fine-grained that it is impossible to see much detail within them in hand samples. These rocks are typically microcrystalline: they are made of crystals so small that they can only be seen with a microscope.

Crystalline sedimentary rocks are named based on mineralogy and texture. Common minerals that make up crystalline sediments are calcite, gypsum, halite and quartz. For example, calcite forms the rock limestone. Calcite can precipitate under varied conditions, so textural information can be more informative than just “limestone”. Here are the types of crystalline sedimentary rocks that you will be expected to learn in the labs for this course:

4.3 Identification of Metamorphic Rocks

Metamorphic rocks form when pre-existing rocks transform under heat and pressure. This transformation occurs because of high temperature and/or pressure at depth or through tectonism. It involves the development of a new mineral assemblage, or composition of minerals present in the rock, which is different from the protolith. A metamorphic rock is classified by (1) the composition of the original rock, (2) the type of metamorphism involved, and (3) the intensity of metamorphism.

Classifying Metamorphic Rocks

Metamorphic rocks are classified based on two key features: I.texture and II. mineral composition (Table 4.3).

I. Texture

Foliated Textures

Foliated rocks show layering or alignment of minerals due to pressure.

• Slaty cleavage – Very fine-grained, flat layers; smooth, compact appearance. Lowest grade (e.g., slate).

• Phyllitic foliation – Silky sheen or slight wrinkling from fine-grained platy minerals. Low to intermediate grade (e.g., phyllite).

• Schistosity – Visible flakes or crystals (e.g., mica), aligned in layers. Intermediate to high grade (e.g., schist).

• Gneissosity – Coarse mineral banding; alternating light (quartz/feldspar) and dark (mica/amphibole) layers. High grade (e.g., gneiss).

Nonfoliated Textures

• Granoblastic – Equally sized, interlocking grains; typically fine- to medium-grained (e.g., quartzite, marble).

• Porphyroblastic – Large metamorphic crystals (porphyroblasts) in a fine-grained matrix.

II. Mineral Composition

Foliated Rocks

Named by texture plus key minerals:

• Garnet-muscovite schist – schist with garnet and muscovite.

• Quartz-hornblende gneiss – gneiss with quartz and hornblende.

• Mica schist – schist mainly made of biotite and muscovite.

Nonfoliated Rocks

Named only by mineral content:

• Marble – metamorphosed limestone (calcite).

• Quartzite – metamorphosed quartz sandstone (quartz).

4.4 Feel like a Geologist already?

Additional Information

Exercise Contributions

Lab 4: Why Do Geologists Like To Stare At Rocks? was modified by Jessica Kristof and Ricardo L. Silva from the original Chapter 6: Igneous Rocks by Daniel Hauptvogel, Virginia Sisson, Kaitlin Thomas, Chapter 7: Weathering and Sedimentary Rocks by Daniel Hauptvogel, Virginia Sisson, Michael Comas, and Chapter 8: Metamorphic Rocks by Virginia Sisson and Daniel Hauptvogel in Hauptvogel et al., (2024) and from the original Lab 4: Rock Identification in Ferreira and Young (2018). 

References

Ferreira, K. and Young, J. (2018). GEOL 1340 The Dynamic Earth Lab Manual. Winnipeg, MB: Department of Geological Sciences, University of Manitoba

Hauptvogel, D., Sisson, V., and Comas, M. (2024). Investigating the Earth: Exercises for Physical Geology. Houston, TX: UH Libraries