The first step in learning to read rocks is to understand their basic types and how they form. You already know the basics of the rock cycle (see Section 4.2.2), which describes how rocks can be changed between the three major types: igneous, sedimentary, and metamorphic. The diagram of the rock cycle from Chapter 4 is repeated here.

We will use the rock cycle to consider rock formation in more depth, but first we need to talk about a little more of the terminology of rocks. Let’s start with the terms “rock” and “mineral.” These terms are often used interchangeably, but in science, minerals are the building blocks of rocks. More specifically:

• Each different type of mineral has a distinct chemical composition, with its atoms arranged in an orderly way to give the mineral what we call its crystal structure.
• A rock is made from minerals. Some rocks contain only a single mineral type, but most rocks are a mix of two or more different minerals that have become stuck together.

Slide show 5.1.1–2 illustrates the definitions of and differences between rocks and minerals.

Slide Show 5.1.1–2. Study these slides to be sure you understand what we mean by rocks and minerals in Earth science.

Our second piece of terminology concerns how we describe the story that geologists can read from rocks. The general term for anything that preserves evidence of a past event is the word record (as a noun), which comes from a Latin term meaning “remember.” For example, your school records show your past performance in school, your medical records describe your past medical visits and treatments, and a music record (or recording) provides a way to hear a piece of music that someone performed in the past. With this in mind, geologists refer to the story that they can read from rocks and fossils as either the rock record, the fossil record, or the geological record.

• The rock record refers to the history of our planet that can be read from individual rocks or from layers of rocks. For example, we say that the walls of the Grand Canyon contain a rock record that stretches through hundreds of millions of years of time.
• The fossil record is very similar, but refers to the information that can be read from fossils of organisms that died in the distant past.
• The geological record encompasses both the rock record and fossil record.

Note that these three terms are often used in interchangeably, which makes sense because fossils are found within rocks, and both rocks and fossils are studied in geology.

Finally, a less crucial but sometimes surprising bit of terminology concerns whether we should talk about singular “rock” or plural “rocks.” The answer is that we tend to use both forms interchangeably, but there’s a subtle distinction in how we generally use the terms:

• When we talk about singular rock, we are generally referring to rock as a type of material that is solid and made from minerals.
• When we talk about plural rocks, we are referring to sets of objects that are made of rock.

As a simple example, when we refer to the “rock layers” of the Grand Canyon, we are talking about the fact that the layers are made of the material that we call rock. When we talk about the “rocks of the Grand Canyon,” we are talking about all the individual rocks that we could find there. Note, however, that because individual rocks could in principle break off from the walls at any time, it doesn’t really matter whether we refer to the “rock” or “rocks” that make up the Grand Canyon walls. That is why the singular and plural forms are often used interchangeably.

With the above terminology in hand, you are ready to investigate the three basic rock types and their formation in a little more detail.

Igneous Rocks

An igneous rock is one that formed when molten rock cooled and solidified. This tells us that the process of making an igneous rock requires only two basic steps:

• The first step is melting: Pre-existing rock within the Earth must melt to become molten rock.
• The second step is crystallization , in which solid mineral crystals form and grow as the molten rock cools and solidifies.

You are probably familiar with the basic idea of crystallization, because it occurs whenever water freezes into ice (Figure 5.1.1–3). As you know, ice formation begins when liquid water cools to a temperature at or below the freezing point (0°C), which means the crystallization of water into ice is just like the crystallization of igneous rock, except that it occurs at much lower temperatures.

Figure 5.1.1–3 – When we cool water enough, it crystallizes to make ice. In other words, what we usually call “freezing” is actually the process of crystallization. Credit: Yinan Chen, Dan Bruell.

Now, see if you can apply what you’ve learned from the examples of ice crystallization and the crystal growth activity to a few questions about the formation of igneous rock.

Discuss the following questions with a classmate. Then click to open the answers to see if they agree with what you came up with.

1. Where does rock melting occur?
   
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   Melting must occur underground, because there is not enough heat at Earth’s surface to melt rock. When rock melts, it generally forms a subsurface chamber of molten rock, like that you saw previously in Figure 4.18.
   

   View Figure 4.18

2. You know that molten rock can erupt as lava from a volcano. Why does this molten rock turn into igneous rock?
   
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   Once the lava reaches the surface, it is exposed to the cool temperatures of the surrounding air or water (if it is in a lake or ocean). This causes the lava to cool and solidify fairly quickly , making igneous rock.

3. Not all molten rock erupts out of volcanoes. Sometimes, it cools and solidifies to make igneous rock while still underground. How would you expect the time it takes molten rock to cool and solidify underground to compare to the time it takes to cool after eruption on the surface?
   
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   Cooling and solidifying underground takes much longer than at the surface. At the surface, cooling is very rapid because of the cool air or water temperatures around the molten rock. Underground, cooling and solidifying often takes thousands or even millions of years as the molten rock slowly rises upward to regions of lower temperature.

4. Suppose you find an igneous rock in which you can clearly see large mineral crystals (like those in the granite from slide show 5.1.1–2). Do you think this rock cooled and solidified underground or after eruption from a volcano? Explain how you know.
   

   View Granite Slide

   
   
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   The answer is underground. Crystal growth takes time (as you will have seen if you did the crystal growth activity above), and volcanic rock cools too quickly to allow large crystals to form. Therefore, rocks that have visible crystals (like granite) must have cooled and solidified underground, so that there was enough time for their mineral crystals to grow to sizes large enough to see by eye.

Figure 5.1.1–4 summarizes the basic processes through which igneous rocks form .

All igneous rocks share the same basic formation process of melting and crystallization, but they can still differ greatly from one another. As you’ve already seen, the cooling rate — which depends on whether the rock solidifies underground or from a lava flow —will affect how crystallization occurs within a particular rock. Other factors that will affect the characteristics of an igneous rock include the chemical composition of the rock that melted to make it (because different chemicals can combine to make different minerals) and the temperatures at which it melted and solidified (because different minerals melt and crystallize at different temperatures).

Slide Show 5.1.1–5 shows several famous igneous rock formations. Note that while it’s fairly obvious how igneous rock from volcanic eruptions ended up on Earth’s surface, you might wonder how igneous rock that cooled and solidified underground (such as granite) got to places where we can see it. The answer is what geologists call uplift , in which rocks are pushed upward from where they originally formed. We’ll discuss processes that cause uplift later in the chapter, but you may already recognize that it is very common. For example, uplift also explains how layers of the Grand Canyon that formed beneath the sea ended up at their current high elevations.

Slide show 5.1.1–5 – Take a quick tour of some of the world’s most famous igneous rock formations.

Sedimentary Rocks

If you pour sand into a jar of water, the sand will soon settle to the bottom of the jar. This turns the sand into an example of what we call a sediment , which means a material that has settled out in a liquid such as water. In fact, the word sediment comes from a Latin word meaning “settling” or “sinking down.”

• The first step in forming sedimentary rock requires erosion , in which existing rocks are worn down and their material is then carried away, usually by wind, water, or ice.
  Note: A closely related term that you should know is weathering , which refers to the wearing down of rocks or other materials. The subtle difference between weathering and erosion is that weathering is only the wearing down, while erosion includes the transport to a new location. For example, you might hear someone say that they need to repaint their house because the old paint has been broken down by weathering.
• In order to become part of a sedimentary rock, the eroded material must settle down somewhere as sediment, so that more sediment can pile on top of it. This settling and piling usually occurs in water such as in a stream, lake, or sea, but it can also be driven by wind (such as in sand dunes) or the gradual movement of glaciers . The processes by which sediments settle down can be called either deposition (because the sediments are deposited in their new location) or sedimentation (from the fact that the word sediment literally means material that has settled).
• The final step is to turn the pile of sediments into solid rock. This requires compression of the sediments by the weight of other sediments or rock above them , so that they become pushed close enough together to make them stick to each other (since otherwise they’d still be separate sediments, like loose sand, instead of a solid rock). This “sticking together” part of the process is often called cementation , because it is similar to the process of making cement. Just as when a construction worker pours cement for a floor or sidewalk, cementation typically happens as water helps to crystallize minerals that can hold together adjoining bits of sediment.

Figure 5.1.1–6 summarizes the basic processes that form sedimentary rock.

Figure 5.1.1–6 – This figure summarizes how sedimentary rock forms in water, but these layers can also be laid down by wind.Credit: Zofostro Science.

The fact that sediments pile up over time explains why they are typically found in layers like those in the walls of the Grand Canyon. These layers are often called strata , which is just a fancy but common synonym for “layers.” So if you hear teachers or scientists talk about the “rock strata” of the Grand Canyon, you’ll know they just mean the rock layers that we see there .

Slide show 5.1.1–7 – Take a quick tour of some of the world’s most famous sedimentary rock formations.

Metamorphic Rocks

You can begin to make sense of the third major category of rock, metamorphic, by noticing that its name is very similar to the word metamorphosis, which is used to describe a caterpillar changing into a butterfly (Figure 5.1.1–8). Both words include two roots that go back to Latin and Greek: meta, which means “changed,” and morph, which means “form” or “shape.” In other words, when we say that a caterpillar undergoes metamorphosis to become a butterfly, we are simply saying that it becomes changed in its form.

In a similar way, a metamorphic rock is a rock that has undergone a change in form , but not of composition. The change in form must be very significant to count as metamorphic, but not so significant as to completely melt or destroy the original rock. Metamorphic rocks are therefore sometimes said to have undergone deformation , because they still have the same composition but now look deformed compared to their original shapes or mineral structures.

It’s not easy to deform a rock. In general, deformation requires some combination of high heat and high pressure, which means it almost always occurs deep underground. This heat and pressure must be applied for a very long time— typically for millions of years — to alter the rock. This tells us the basic process by which most metamorphic rocks are formed:

• First, an existing rock (of any type) becomes buried underground. This can happen, for example, if a mountain range builds up over the place where the original rock was located, or because the rock is carried downward as one tectonic plate slides under another.
• Then, over millions of years, heat and pressure transforms the old rock (without melting it) so much that its mineral arrangement is significantly changed (but its composition remains the same), making it a new rock.

Figure 5.1.1–9 summarizes the basic process by which metamorphic rocks are made, and Slide show 5.1.1–10 shows examples of metamorphic rock formations.

Slide show 5.1.1–10 – Take a quick tour of some of the world’s most striking metamorphic rock formations.

The Rock Cycle Revisited

Let’s look once more at our figure of the rock cycle, now that you have learned a little more about the three major rock types. The arrows indicating the different ways in which rocks can change between these types should now make more sense.

Try the following quiz to make sure that you understand the major ideas of the rock cycle. After you complete the quiz, do the group discussion and the activities that follow to help further strengthen your understanding of the rock cycle and its importance to our planet.

Reading the History of a Rock

Identifying a rock as igneous, sedimentary, or metamorphic gives us a basic picture of how a rock formed. The details of any rock can tell us far more. We’ve already seen a few examples of these details. For example, the sizes of mineral grains in igneous rocks tell us whether they crystallized underground or after a volcanic eruption at the surface, and sedimentary rocks can tell us that they formed at a time when their location was underwater.

Geologists have learned to read much more through detailed examination of a rock’s structure and content. For example, the precise mineral composition and arrangement in a rock can tell us the temperature and pressure that the rock was exposed to as it formed. In this way, geologists can learn how far underground a rock once resided. To understand this idea, consider two well-known minerals that are both made of nearly pure carbon: graphite and diamond (Figure 5.1.1–11). The crystal structure of graphite can form fairly easily, which is why graphite is common. Note that its structure also makes graphite very soft for a mineral, which is why it used in pencils. In contrast, the very hard crystal structure of diamond can form only under great pressure, telling scientists that diamonds must form at least 150 kilometers underground. This means that diamonds are made in the mantle, well below Earth’s crust. This, in turn, explains why diamonds are rare and valuable: Their formation at great depth means they are rarely uplifted all the way to the surface.

Figure 5.1.1–11 – Graphite and diamond are both made of nearly pure carbon, but their crystal structures have different arrangements of the carbon atoms. Diagram credit: Steve Earle, from Physical Geology – Second Edition (Victoria, B.C.: BCcampus, /opentextbc.ca/physicalgeology2ed/).