3.1 Introduction to Rock Textures

Rocks are aggregations of minerals. They must be coherent, solid, and found in nature. Rocks differ from each other by which minerals make them up and in what relative proportions (their mineralogical composition) and how those minerals are arranged (their texture). Identification of rocks is important for many practical situations and is fundamental to the study of geology.

We have just covered basic mineral identification, so we will apply that knowledge to determining the mineralogical composition of rock.

In this week’s lab, we learn about rock textures. Next week, we will apply our knowledge of Labs 2 and 3 to identify the names of common rocks. Before we apply a particular name to a rock, let us first try to determine which class of rock – igneous, sedimentary or metamorphic – that rock belongs to.

3.2 Igneous Rock Textures

Igneous rocks crystallize from molten material (magma) as it cools, either within the Earth or erupted onto Earth’s surface as lava. The minerals in an igneous rock crystallize with an interlocking texture, i.e., a crystalline texture (Figure 3.1).

Intrusive igneous rocks form when magma within the Earth cools, insulated by thick rock layers. This cooling is slow, allowing mineral grains to grow large enough to be seen without a microscope, called phaneritic texture.

This texture is classified into:

• coarse grained: >5 mm
• medium grained: 1-5 mm
• fine grained: < 1 mm

Extrusive igneous rocks form when lava cools rapidly on the surface of the Earth, as it is not insulated by surrounding rocks and is exposed to colder water or air.  Due to the rapid cooling, mineral crystals cannot grow very large (you would need magnification to see most of the minerals), which creates an aphanitic rock texture, or minerals may not even grow at all, which creates a glassy rock texture.

An important textural feature is the mineral shape, which describes how “perfect” their crystal shape is, or whether they have grown in an aggregate of interlocking crystals. Mineral grains that are well formed, i.e. close to perfect, are called euhedral. Minerals that are not perfect, but their shape is somewhat recognizable, are subhedral. Mineral grains that show no recognizable shape are called anhedral. This information can tell us about the sequence in which minerals crystallize.

Another textural feature is the grain sizes of all mineral grains. If all grains are about the same size, we call it equigranular. If there are two distinct grain sizes, it is porphyritic (Figure 3.2). Equigranular rocks imply (mostly) continuous cooling. Porphyritic rocks imply the rock had two separate cooling stages; at first, it cooled large euhedral grains (phenocrysts), followed by quicker cooling of the remaining magma, which forms a finer-grained groundmass. Groundmass may be phaneritic or aphanitic, depending on whether the second stage of cooling was intrusive or extrusive.

Finally, a special texture found in some extrusive igneous rocks is vesicles, small circular or tubular holes which represent gases or liquids which were trapped in the magma during volcanism. This texture makes the rock resemble an Aero chocolate bar, which we call a vesicular texture. After the rock is cooled, minerals may precipitate and fill in the vesicles, making them amygdules, which creates an amygdaloidal rock texture.

3.3 Sedimentary Rock Textures

Weathering

Weathering occurs when a rock is exposed to the “weather”, or forces and conditions, at Earth’s surface. These forces include temperature, oxygen (and other gases), water, etc. Mechanical weathering, also known as physical weathering, is the breakdown of rocks into smaller grains by physical means, without changing their chemical composition. Chemical weathering does alter the chemistry of minerals, either by dissolving minerals or by replacing elements. Mechanical and chemical weathering go hand-in-hand because mechanical weathering produces fresh surfaces that can be affected by chemical processes, and chemical weathering weakens the mineral, making it more susceptible to physical weathering.

Sediment Erosion, Deposition, and Lithification

After mechanical weathering, particles are moved by water, wind, and ice in a process called erosion. This can transport sediment hundreds of kilometres away. Chemical weathering typically precipitates ions out of a solution, which forms solid particles to be deposited.

In all cases, sediments undergo lithification to go from loose grains to solid rock. This is done through compaction and cementation. When grains are deposited, they have empty spaces called pores (porosity). Compaction reduces this porosity by forcing the grains closer together. This is similar to a full garbage can; you would typically push it down to make more room on the top. Sediments work the same way, but instead of a hand doing the compacting, it is the accumulation of sediment on top of the previously deposited material, which adds pressure to the sediment below. 

Cementation happens at the surface or during compaction. As sediments interact with water, minerals precipitate inside the pore space, which “glues” the grains together. In compaction, water is squeezed out of the pore spaces, which enhances cementation. Typically, cement is made of calcite, quartz, and pyrite.

Clastic Sedimentary Rocks

Clastic sedimentary rocks can be described based on their maturity, determined by rounding (grain shape) and sorting. Grain shape may be described on a spectrum from rounded to subrounded to subangular to angular (Figure 2-4). Weathered grains closer to their source material are angular and become increasingly rounded with distance and duration of transport. Sorting is also an important characteristic. Clastic sedimentary rocks which contain grains that are all about the same size are called well-sorted rocks. Rocks which have a wide spectrum of grain sizes are called poorly sorted rocks. As the distance from the source material increases, the size and angularity decrease, and their sorting increases (Figure 3.3).

Chemical Sedimentary Rocks

Chemical sedimentary rocks precipitate from saturated solutions (under the right conditions). Chemical sedimentary rocks have a crystalline rock texture, not a clastic texture. It is comparable to some igneous rock textures, as both precipitate from solutions (whether the solution is magma or a saline lake). For these rocks, we can say the grain size ranges from granular (similar to the “phaneritic equigranular” texture in igneous rocks) to fine-grained (similar to “aphanitic” texture in igneous rocks).

Other Sedimentary Rocks

Biochemical sedimentary rocks consist of minerals that were formed by the actions of once-living organisms. The rocks may consist of aggregations of shells or skeletons of various creatures, including plankton. These body parts are often made up of calcite or quartz.

Organic sedimentary rocks, which include such rocks as coal and oil shale, are made primarily of former organic material that has been transformed by heat and pressure during burial. Labs for this course will only include clastic, chemical and biochemical sedimentary rocks.

3.4 Metamorphic Rock Textures

Metamorphic rocks form when pre-existing rocks transform under heat and pressure. During the process of metamorphism, igneous, sedimentary, and even other metamorphic rocks get buried deep in the Earth’s crust, where temperature and pressure are significantly higher. Heat and pressure can cause several changes in rocks, such as recrystallizing minerals, creating new minerals, and orienting minerals in a direction perpendicular to differential pressure. The original rock, or the protolith, recrystallizes into new, larger grains and forms a crystalline texture.

Under directed pressure, minerals may align themselves and create a layered appearance, also known as foliated texture. This foliation is commonly observed in aggregates of micaceous minerals such as biotite, muscovite, or talc. Several types of foliation are commonly seen in metamorphic rocks: gneissic banding, slaty or rock cleavage, and schistosity (which will be revisited next lab). As pressure increases, so does the degree of foliation. However, some minerals, such as quartz, do not change much during metamorphism. Therefore, when only heat causes metamorphism, there is no preferred orientation of minerals; it has a nonfoliated equigranular crystalline texture (Figure 3.4).

Some new minerals that crystallize during metamorphism may precipitate as well-formed crystals set in a finer-grained groundmass of softer minerals. The texture may resemble a porphyritic igneous rock, but it does not imply stages of magma cooling. In metamorphic rocks, this texture is called porphyroblastic texture. The larger euhedral crystals are called porphyroblasts, which are set in a finer-grained (possibly foliated) groundmass of other minerals.

Since metamorphic rocks are crystalline, we can use the same grain size classification we use for igneous rocks. However, grain size for metamorphic rocks is only descriptive; it does not have the same meaning that it would have for igneous rocks, i.e. it does not imply crystallization rate.

3.5 The Rock Cycle

The rock cycle is the traditional way geologists demonstrate the relationship between the three rock types and the processes that tie them together (Figure 3.5). Igneous, sedimentary, and metamorphic rocks can transform from one type to another through several processes, including weathering, erosion, deposition, volcanism, intrusions, uplift, burial, metamorphism, and melting, all of which occur simultaneously. These processes influence the textures of each rock type.

3.6 Process of Rock Identification

Use this chart to make a tentative determination of which class of rock a sample belongs: igneous, sedimentary or metamorphic. Your instructor has given you a tray with several samples. Using the information above, complete Table 3.1.

Table 3.1 – Rock Identification Chart, Ferreira and Young (2018)

3.7 Why Study Rocks?

Why should you study different types of rocks? Well, they all contain clues about what the Earth was like in the past. Perhaps one area may have sedimentary rocks which experienced an environmental change from a desert to a swamp to a coral reef under the sea. Different rocks form under only certain conditions, and even the dullest gray rock can tell us something important about the past. Some types of things that rocks can tell us about our planet, as well as other planets, are:

• Was there a lake or a volcano present where the rock was found?
• Was there a mountain range or a sea?
• Was it hot or cold?
• Was the atmosphere thick or thin?

For example, by understanding where volcanoes have occurred in the past, we have a much better idea of where they are likely to occur in the future and can be prepared for them. Second, by gaining an understanding of how planets work, we can better predict how the Earth will react to changes. If we understand how the Earth and its life responded to temperature changes in the past, we might better understand the effects of the global warming that is happening today.

So the basic point is to better understand our world. This helps us to better coexist with nature and reap the benefits that it has to offer.

Exercise 3.5 – Rock Types in North America

Look at the distribution of rock types at the surface in North America (Figure 3.6) and answer the following questions. Remember that these maps show you the rocks at the surface of the Earth.

1. What is the most abundant rock type in North America? ___________________
2. Describe the distribution of each rock type. For example, do you see any patterns, or are those rocks only located in certain areas?
   

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3. Critical Thinking: These maps only show what is exposed at the surface. Why does one rock type cover most of North America?
   

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4. Critical Thinking: Cratons are the nuclei of continental plates composed of intrusive igneous and metamorphic rocks. Where do you think the North American craton is located? What is your reasoning?
   

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Additional Information

Exercise Contributions

Lab 3: How Do Rocks Form? was modified by Jessica Kristof and Ricardo L. Silva from the original Chapter 5: The Rock Cycle by Daniel Hauptvogel and Virginia Sisson, 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 in Hauptvogel et al. (2024) and the original Lab 3: Introduction to Rocks 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