Types of Rocks Study Guide (page 2)
The dynamic earth churns new matter up to its surface and returns other matter back down to the mantle. Through all this activity, including through the actions of water and wind as well as deposition of sediments to form new types of rock, continents are created and changed. Continents are, in essence, made of rock, spanning scale of sizes from dust particles to mountains and the underlying bedrock of the continental masses themselves. Here we into look the science of rock.
Elements, Minerals, and Rocks
Different elements exist because of the different kinds of atoms that can be made from the same basic three atomic building blocks. Atoms are the most finely divisible parts of matter that possess the characteristics of a particular element—elements such as copper, gold, carbon, or hydrogen. Atoms alone (not in molecules or ions) are electrically neutral and contain equal amounts of positive and negative electrical charges. The positive charge is concentrated in a tiny central massive region called the nucleus. The negative charge is in one or more tiny electrons (the first of the atomic building blocks). Electrons whirr around the nucleus, bound to it by electrical attraction.
The nucleus of atoms contains protons and neutrons, which are the other two atomic building blocks. Protons are the carriers of the positive charge. Neutrons, as their name suggests, are neutral. Now, all the atoms of a particular element have the same number of protons in their nuclei (which determine the charge of the nucleus, thus the number of electrons around the nucleus, and thus the chemistry of the element). But atoms of the same element can vary in the number of neutrons in their nuclei. These variants are called isotopes and will be discussed more in other lessons in this book. In summary, the atoms of elements are made from the atomic building blocks of protons, neutrons, and electrons.
Table 7.1 outlines the amounts of different kinds of elements in Earth's continental crust.
Note that nearly of Earth's rock is oxygen and silicon. Furthermore, the top eight elements (down to and including potassium) make up 98% of all rock. What is element number 3? Number 4? From the table, we know that rocks will be mostly oxygen and silicon, and we can surmise that differences between kinds of rock will be found in how the oxygen and silicon are arranged and what kinds of small amounts of other elements are in the rock. Is calcium and manganese there? Is iron and phosphorus there?
This discussion already assumes that rocks are made of several elements. Note that it is rare to find elements by themselves, such as a vein of pure gold. We don't dig up elements as pure veins of calcium or aluminum, for example. So to understand more about the structure of rocks, we have to review a bit of chemistry.
The science of chemistry studies the interactions of atoms, how they form molecules, and the interactions of those molecules. Molecules range from simple ions, such as sodium dissolved in seawater, to complex organic molecules of life, such as proteins and lipids. The key concept in how atoms form molecules is the atomic bond, that is, the connection that joins atoms into molecules.
The atomic bond occurs between electrons of atoms. For example, electrons can be shared in what is called a covalent bond. In another type of bond, called the ionic bond, electrons are taken from one atom and are joined to another. The bonds between metals, say in copper or gold, are unique and are named the metallic bond. Whatever it is that allows certain types of bonds to form between particular kinds of atoms has to do with the number of electrons the atoms of an element possess. And, as we have seen, the number of electrons depends on the number of protons in the nucleus of an atom of an element. So the bonds among atoms in molecules depend directly on the atomic structure of the atoms.
Now, even though we have been discussing the fact that rocks are made of elements, an intermediate level of organization is crucial to the study of rock. In that level, we find what are called minerals. Geology is mainly concerned with the broad type of molecular organization of minerals. Minerals are always larger than a single molecule. Indeed, crucial to the existence of minerals is that they have crystalline structure. Having a crystalline structure means that the atoms create a network of repeating units, a crystal.
Common table salt is an excellent example of a crystal. The salt crystal is a network (or lattice) of sodium and chlorine atoms. Many types of crystals exist, and the terminology and geometry can get quite complex. These topics won't concern us here. Just be aware that rocks are made of minerals, and that minerals are solids made of certain elements drawn from the list of elements in Earth's crust. Minerals have specific chemical compositions and have crystal structures.
Now let's return to the fact that the two most abundant elements are oxygen and silicon. Silicon and oxygen alone can form the mineral called quartz. The chemical formula of quartz is SiO2. It has one atom of silicon (Si) and two atoms of oxygen (O). Of course, this basic unit of SiO2 is linked with others of the same formula to create the gorgeous crystals of quartz we have all seen.
Many other minerals are combinations of silicon and oxygen along with other elements. In general, these minerals are silicon oxides, also known as silicates. Other elements join in to create different kinds of silicates, such as magnesium–iron silicates, magnesium–aluminum silicates, and so forth. Geologists have given the most common minerals names. Perhaps you have heard, for instance, of feldspar. The chemical formula for the abundant mineral called feldspar is KAlSi3O8. It has one atom of potassium (K), one atom of aluminum (Al), three atoms of silicon (Si), and eight atoms of oxygen (O). If you are shown the chemical formula for a mineral and know the symbols for the different elements in the formula, you should be able to say how many atoms of each element are in the basic molecular unit of the mineral. (As an example of a symbol, the chemical symbol for magnesium is Mg.) We just went through an example for feldspar.
Geologists have developed ways to tell minerals apart by classifying them according to a number of basic properties. Here is the classic list of the basic properties of minerals used in geology.
Properties of Minerals
- Cleavage. In what preferred direction does the mineral break?
- Luster. Is the surface of the mineral polished, glassy, or oily?
- Color and streak. When the mineral is rubbed on an unglazed porcelain plate, what is its color? (This can be quite different from the color of the mineral itself.)
- Density and specific gravity. How heavy is the mineral, compared to water of the same volume?
- Hardness. What is the mineral's number on the Mohs scale of hardness? Diamond, of course, is the most hard, with number 10. Talc is the softest, with number 1 on the scale. Here are other examples: Calcite has hardness 3, and quartz has hardness 7. Hardness is found by finding out what the mineral can and cannot scratch. Can it scratch glass (between 5 and 6)? Can it scratch a copper penny (between 3 and 4)? A fingernail (between 2 and 3)?
Mineralogists know of about 3,600 different minerals. Are minerals the same as rocks? We are almost at the end of our story that has gone from atomic building blocks, to atoms of elements, to bonds between atoms, to molecules, and to minerals. Yes, rocks can be pure minerals, but that is rare. It is always a treat for geologists in the field to find a rock that is a pure mineral, say a large chunk of quartz. But more often, rocks are made of many minerals. A rock will generally be a mixture of small particles of different minerals (which can be seen by the eye or better with a magnifying glass). Thus, a rock can contain quartz as one of its minerals. Rocks are assemblages of minerals. After the practice questions, we will take up the question of different types of rocks.
Igneous, Sedimentary, and Metamorphic Rocks
Three main types of rocks are recognized by geologists: igneous, sedimentary, and metamorphic. We will begin with igneous, because all rock is born, so to speak, as igneous rock.
Igneous rock is rock that was once very hot and molten (think "ignition"). Molten magma from under the earth's surface, when it cools and solidifies, becomes igneous rock (intrusive igneous rock). So does lava that flows or erupts from volcanoes (extrusive igneous rock). As we have seen, the ocean's crust emerges from volcanism at the mid-ocean ridges, so the ocean's floor is mostly igneous rock (the exception is the sediments that fall on to the ocean's floor). Importantly, most (95%) of Earth's continental crust is igneous rock although very little of it is exposed at the surface.
Types of igneous rock include granite, rhyolite, gabbro, and basalt. These and others are all different kinds of silicates and can be classified into types by how much silica are in them (as well as other properties). The discussion of these types and how they are formed can get complex (and are still debated by geologists!), so we will pass that issue by. But you will recall from the previous lesson that the less silica a magma or lava has in it, the less viscous the molten rock is.
Like all rocks, igneous rocks have crystals of minerals. However, in the case of igneous rocks, the crystals form when the magma cools to become rock. One important concept regarding crystal sizes in the igneous rocks should be noted: The slower the cooling, the larger the crystals. Therefore, crystals are larger in intrusive igneous rocks.
Logically, the next type of rock to discuss is sedimentary rock, because sedimentary rocks form when other rocks have been physically or chemically broken down and deposited as sediments. The sediments, usually in the shallow waters of continental shelves, get piled up more and more over time. The sediment load can get heavy, even pushing the edge of the continent crust downward, allowing the buildup of more sediments on top. Eventually, the pressure can get so great that the sediments are fused into solid rock. Sedimentary rock is basically recycled rock, and is formed at temperatures and pressures near those at the surface of the earth.
If sedimentary rocks form primarily underwater, why is it that substantial portions of the land area of the United States are made of sedimentary rock? Also, consider the fact that sedimentary rock is found 5 miles high in the Himalayan mountains. Indeed, if we consider the surface of the earth, it turns out that 75% of all rock at the surface is sedimentary and only 25% is igneous. In these numbers, geologists include the third type of rock, metamorphic rock, depending on whether the metamorphic rock was derived from sedimentary or igneous rock. (There will be more about that in a couple paragraphs.) Compare these numbers to the percentages for all rock in the crust: 95% igneous and 5% sedimentary. At the surface, the numbers are turned around. Thus, more of the rock exposed at the surface is sedimentary, even though most of crustal rock is igneous.
The secret to the abundance of sedimentary rock at the surface is, once again, plate tectonics. Tens of millions of years ago, plate tectonics smashed the once separate continent of India into what is now Tibet and lifted the Himalayas. Plate tectonics brings parts of continents that were formed underwater (sedimentary rock, in other words) to the surface. Finding a piece of shale (a common type of sedimentary rock) when you are hiking inland in the hills shows you that vast changes have taken place over hundred of millions of years.
Let us return to the part of the story in which pieces of sediment are moved from the continents to the oceans, to be deposited and eventually become sedimentary rock. Elements in these pieces are shifted from rock to the ocean by two different processes: physical weathering and chemical weathering. In physical weathering, particles of rock are sloughed off and transported by rivers to the ocean. Deposited in the ocean and eventually cemented together into rock, the particles create different kinds of rock, depending on the size of the particles. You can see this in Table 7.2.
The second way that matter is transported from continent to ocean is by chemical weathering. In chemical weathering, minerals are actually dissolved in water and transported as ions that are invisible to our eyes. Dissolved salt, for example, cannot be seen, even though sodium and chloride ions are dissolved in the water.
Very often, the dissolved ions are precipitated out from the water, creating deposits in the sediments on the ocean floor. Salt, for example, can come out from the seawater when the water in a lagoon evaporates away and leaves the salt. If this happens time after time, sedimentary rocks of the types called halite (salt) and gypsum (calcium sulfate) are formed.
Life is a powerful force in precipitating dissolved elements from water and creating deposits of sediments. The most important of these sediments come from shells of creatures such as coral and certain single-cell organisms. Two types of sedimentary rock are made primarily from this biological precipitation: limestone (from the mineral calcite) and dolostone (from the mineral dolomite). Calcite and dolomite are calcium carbonate and calcium-magnesium carbonate, respectively. Thus, the shells were fused into rock. Examples of limestone are the white cliffs of Dover in England and much of Indiana, Illinois, and Florida.
Sedimentary rock is the kind of rock in which we find fossils, from ancient cells to dinosaurs. Sedimentary rock that contains fossils is called fossilferous. Our best current evidence for the origin of life comes from sedimentary rocks, formed 3.5 to 3.9 billion years ago. It must be so, because life began in the water; life needs water.
The third main type of rock is called metamorphic. Metamorphic rock, in a sense, is a recycled rock, too, but not in the direct way that sedimentary rock is. Metamorphic rock is created when either igneous, sedimentary, or other metamorphic rock is subjected to great heat and pressure and transformed (metamorphosed). How does this happen?
The simplest way is called contact metamorphism. Contact metamorphism occurs when magma intrudes into another rock, say a sedimentary rock. The places where the sedimentary rock touches the magma, which is itself now cooling into igneous rock, become so hot that they turn into metamorphic rock.
Another way to create metamorphic rock is by deep burial. Older rock can be covered by new rock and get deeper and deeper. The heat and pressure turns the deepest rock into metamorphic rock. One final way to create metamorphic rock takes place in mountain building during continental plate collisions. Sedimentary rock, for example, can be squeezed, twisted, and folded during the collision. So much heat and pressure occurs that the rock can be turned into metamorphic rock.
Table 7.3 offers a few examples of some of the most well-known kinds of metamorphic rock and the kind of rock from which the metamorphic rock was derived. Note that some kinds of metamorphic rocks are made from other metamorphic rocks!
Practice problems of this concept can be found at: Types of Rocks Practice Questions
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