By Robert Hazen, Ph.D., George Mason University
Igneous, sedimentary, and metamorphic rocks form the three great classes of rocks that make up the rock cycle. They are transformed by temperature and pressure. This means that one can start with an igneous rock and make a metamorphic rock, or start with a sedimentary rock and make a metamorphic rock, or indeed one can metamorphose metamorphic rocks themselves.
The Rock Cycle
Each of the three rock types can be transformed into the other two, through the processes of the rock cycle. We know that matter moves from one reservoir to another and that these motions are driven by energy. It is interesting to see how the Earth’s atoms are continuously cycled from one reservoir to another.
The rock cycle is also interconnected with other cycles, such as the water cycle and the atmospheric cycle, as well as human activities.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Geology
The Earth is a physical system; it has a long, complex, and largely unknowable history. That history conformed to all the physical and the chemical laws, but it was influenced by countless random and unpredictable events that we cannot know in retrospect. The best opportunity to know the Earth lies in studies of the rocks, a field of study which is referred to as geology.
When studying geology, we’re studying this very rock cycle. It includes the formation, the alteration, and the destruction of the solid portions of the Earth’s crust. It’s epic both in terms of time and in terms of scale.
Rocks include almost all the materials that make up what we call the solid Earth. If we look at most rocks carefully, we’ll see that they’re composed of interlocking grains of several different minerals, of different chemical compounds of specific composition and structure. Those are the mineral grains that make up these rocks.
Learn more about the doctrine of uniformitarianism.
Common Rock Forming Minerals
If we pick up a typical rock, or perhaps a handful of sand, we’re going to see light grains, dark-colored grains, and sometimes even colored grains. Each grain is a mineral, and the uniform compound, with well-defined composition and well-defined structure, makes up the catalogue of some 3,000 different known minerals. Yet, in all the rocks one is likely to pick up, there are only about 50 common rock-forming minerals.
If we look at a typical rock specimen, we will see some of these grains; we will see reddish feldspars, colorless quartz, and maybe some dark-colored minerals like hornblende or pyroxene.
Quartz and Feldspar
Quartz is the common mineral that makes up beach sand; it also makes up power crystals. It’s composed of silicon dioxide, which is the principal chemical element in most glass. If we look at Pikes Peak granite, for example, we will see the colorless grains of Pikes Peak quartz.
Nonetheless, the commonest crustal mineral, by far, is feldspar. It is an oxide of silicon, but it also has aluminum, sodium, and calcium. A geology professor once said that if ever quizzed on what minerals might occur in a particular hand specimen, one should automatically say feldspar; most of the time they would be right.
Pikes Peak Granite
In Pikes Peak granite, feldspar makes up the reddish minerals that occupy most of the granite. There are also invariably a few dark grains which typically are iron-bearing denser minerals that are often black in color.
In Pikes Peak granite, there are a few black grains of a mineral called amphibole. These individual grains form an interlocking mass of crystals to make the solid rock. This interlocking mass is so strong that when one breaks most rocks, one ends up breaking right across the grains themselves, rather than separating the grain boundaries.
Learn more about geochemical cycles.
Transforming Rocks
Over geological time, through deep burial, through baking, through squeezing, in the pressure cooker of the Earth, these minerals, these very rocks are transformed in dramatic ways. They are destined to erode away, to gradually return their atoms back to the Earth, where they will cycle again and again and again, for billions of years.
Thus, every rock, every specimen, every outcrop has a rich and ancient history; every rock has an amazing story to tell and it is the science of geology which allows us to read that story.
Common Questions about Rocks: Composition and the Rock Cycle
Rocks are transformed by temperature and pressure. This is known as the rock cycle. This means that one can start with an igneous rock and make a metamorphic rock, or start with a sedimentary rock and make a metamorphic rock.
Quartz is the common mineral that makes up beach sand; it also makes up power crystals. It’s composed of silicon dioxide, which is the principal chemical element in most glass.
In Pikes Peak granite, feldspar makes up the reddish minerals that occupy most of the granite. The few black grains in the granite are of a mineral called amphibole.
