Types of Rocks Study Guide (page 2)

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Updated on Sep 25, 2011

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.

Table 7.2 Particle Size

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!

Table 7.3 Metamorphic Rocks


Practice problems of this concept can be found at: Types of Rocks Practice Questions

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