Geology 101 Name Reading Guide for Chapters 1, 4, and 5 The purpose of the Reading Guides is to help you sort out the most important ideas in the text. I recommend answering the questions as you read the book. Chapter 1 (p.3): Most of the first chapter will be covered in other parts of the quarter, but I recommend that you review the contents of this chapter as an introduction to the topic. The questions below are of special importance: The Nature of Scientific Inquiry: As a science, geology involves the formation of hypotheses and theories. In our common, everyday language we use the words hypothesis and theory. From pages 9-11, describe what is meant by a hypothesis and a theory in science. This quarter we will study many processes that shape the Earth and influence our safety, but we will also take some time to examine what rocks and minerals tell us about the history of our planet. Read the short section on pages 21-23 that describes minerals and the three major types of rocks. What is a mineral? How are igneous rocks formed? How are sedimentary rocks formed? How are metamorphic rocks formed? The remainder of pages in this first chapter will not be on the first exam, but I recommend that you read this part because it will give you an overview of the field of geology, the scientific theories on the formation of Earth and the solar system, and the concept of geological time. If you have time, I recommend that you read a short section of Chapter 2 (pages 34-41) which will give you a brief introduction to plate tectonics. At this point we will use plate tectonics to explain features we see around the world, but we will wait until near the end of the quarter to understand the development of the idea and the evidence we have that plate tectonics exists. (You may wonder, why aren t we reading chapter 3? It is on minerals, the building blocks of rocks. If this course was designed for geology majors, we would read chapter 3, but since this a course for non-science majors, there is more detail in Chapter 3 than you will need in this course. Of course, if you are curious about minerals and their structures, please read chapter 3, but you will not be tested over chapter 3 directly).
Chapter 4: Please turn to page 96 and read the introduction to chapter 4. Magma: Parent Material of Igneous Rocks (p. 96) What does the term igneous mean? What is the difference between the terms magma and lava? Magmas consist of three components:, and. Crystallization is the opposite of. In crystallization, are established between atoms. When magma crystallizes at depth, it forms, also known as... Igneous rocks that form when molten rock solidifies at the surface are classified as. They are also called. Igneous Compositions (p.98): The most common two elements in the outer portion of the Earth are silicon and oxygen. Silica is the name we give this combination of silicon and oxygen. Instead of referring to rocks by silica percentage, we call them a name to convey the level of silica in the rock. Basalt is considered a silica- rock (mafic), while rhyolite (felsic) is a silica - rock. Rocks that form from magmas with a composition between mafic and felsic have an composition. Igneous Textures: What Can They Tell Us? (p. 100): The term texture refers to the: Not how it. Slow cooling (deep underground) promotes the formation of. So a rock of intergrown crystals (with no fine-grained part) is said to have a - texture. Whereas, cooling at or near the Earth s surface creates a -grained texture of tiny intergrown crystals or volcanic glass. In some igneous rocks there are well-developed crystals surrounded by a background of very fined-grained rock. This type of texture is known as a texture. This texture is formed by: 2
Imagine that you found a rock that looked like the rock illustrated to the right: Would this texture be aphanitic, phaneritic, or porphyritic? Explain your logic. The Swiss cheese-like texture of some volcanic rocks is called a texture. This texture develops in the upper part of the lava flow from expanding. Some rocks form from particles of volcanic fragments. Sometimes this type of rock forms inside the volcano as magma rips fragments of the existing rock and incorporates them into the magma. Other volcanic rocks with fragments can be from accumulations of fragmental debris on the slopes of the volcano. Welded tuffs are a type of pyroclastic volcanic rock that forms from hot ash particles. Naming Igneous Rocks (p. 103) Igneous rocks are classified based on their and. The rocks that are light in color and have the highest silica content are called (mafic/felsic)? If the magma cools slowly underground and forms all crystals we call this. If this same type of magma cools so fast that there is as least some finegrained part, we call this. This section uses many mineral names such as quartz, feldspar, amphibole, etc. We will spend time identifying these minerals in one of our labs later this quarter. At this point, spend more time on the relationships between the felsic rocks (light-colored) and the mafic rocks (dark-colored) as well as the the effect of cooling on the texture of the rock. Again, I will not expect you to know the mineral names on the first exam. Most of the time when people have heard of volcanic glass before, they have heard of obsidian, the black or reddish glass used by Native Americans to make arrowheads and other hunting implements. Your text states that obsidian comes from lavas that cool instantaneously. However, it is apparent when you look at obsidian lava flows, such as some of the flows near Bend, Oregon, that they did not cool faster than other types of lava flows. The shiny, glassy texture of obsidian is primarily a function of the high silica content of the magma. The authors of your text mention that another type of frothy, glassy texture exists in the form of pumice, a rock so porous that it will commonly float in water. There are more gas bubble holes in pumice than there is volcanic material. (The gas bubble holes are called vesicles. If a rock is highly vesicular and light in color we refer to it as pumice. If some vesicular material is very dark in color (basaltic in composition) it is called scoria. I will not expect you to know the difference between pumice and scoria on a test!) 3
From the information up to this point, complete the following chart: silica content color volcanic rock type plutonic rock type felsic highest granite relative viscosity of magma intermediate andesite mafic dark ultramafic you do not need to know this one peridotite you do not need to know this one An aside: (There are six basic rock types noted in the text: granite, diorite, gabbro (intrusive rocks) and rhyolite, andesite, and basalt (extrusive rocks). In lecture, I will add two more to this list: granodiorite and dacite. Granodiorite and dacite are from the same composition of magma, but the granodiorite magma did not reach the Earth's surface and the magma cooled slowly to produce crystals. Dacite, the volcanic rock, was the type of magma erupted by Mt. St. Helens in 1980. Granodiorite (plutonic) and dacite (volcanic) are the rocks types that originate from magmas that have a composition between granite and diorite (or rhyolite and andesite)). Origin of Magma (page 107): We will defer this section until near the end of the quarter when we study plate tectonics. You are welcome to read it now, but it will not be covered on this first exam. How Magmas Evolve (page 109): The main concept in this section is that as minerals (the basic building blocks of the rock) crystallize at different temperatures as the magma cools, the composition of the magma can increase in silica percentage. There are many mineral names in this section, but I will not ask you to know these names. Use this reading guide and the lecture information to get the level of information you need from this section. Do not worry about the differences in the continuous and the discontinuous series in the Bowen s Reaction Series. The basic principle in this section is that as a magma cools, crystals begin to form and those minerals with the highest melting temperatures (same as crystallization temperature) will form first from the magma. The minerals that form first have the lowest silica content, so as the magma cools the remaining liquid will have a higher percentage of silica. In general, rocks that are mafic (dark colored) are composed of minerals that have (lower/higher) crystallization temperatures (same as the melting temperatures), while rocks that are more felsic (light colored) are composed of minerals that have melting temperatures. When a magma body has become more silica-rich as crystals have been removed, we term that process fractional crystallization. Another process that may affect the composition of magma is when blocks of the existing rock (termed the country rock ) are assimilated by the magma. Normally the continental crust is higher in silica than the 4
magmas which originate in the upper mantle, so the blocks of country rock tend to increase the silica content of the magma in a manner similar to crystal fractionation. One of the important ideas from Ch. 4 is that intrusive rocks (plutonic rocks) do not erupt! Igneous is not the same as volcanic magmas that are trapped underground, cool slowly and are found only at the surface when erosion has stripped away the overlying rock layers. So, don t fall into the trap of thinking igneous and volcanic mean the same! Partial Melting and Magma Composition (page 112): Again, we will defer this section to the end of the quarter. Feel free to read it now if you are curious. Intrusive Igneous Activity (page 113): An intrusion that is parallel to the preexisting rock is a (sill/dike) An intrusion that cuts across the existing layers is a (sill/dike) This is not covered in the text and will not be on the quiz over this chapter: How could one determine whether an igneous rock (such as a basalt layer) is either a sill or a lava flow? True or False: Dikes and sills may have columnar jointing and fine-grained textures. Explain your answer. A mass of intrusive igneous rock greater than 100 square kilometers (about 6 miles by 6 miles) is termed a. Normally, these intrusions are a result of several intrusions, not just one event. An intrusion smaller than 100 square Km is termed a. In what way is a laccolith different than a sill? The information in section 4.9 to the end of the quarter will not be covered in this unit, however it is interesting to learn more about deposits of valuable minerals in the earth. 5
Chapter 5: Volcanoes and Volcanic Hazards (page 126) Please read section 5.1 on Mt. St. Helens and Kilauea. We will cover most of this information in the lectures. If you have questions, please let me know. An aside: On page 128 the authors state: Mount St Helens erupted with tremendous force. The blast blew out the entire north flank of the volcano We will discuss this statement in class. The missing part of the mountain is primarily due to the collapse of the bulge area caused by a moderate earthquake. Almost two months of continuous bulging weakened that area so when an earthquake happened on May 18, the north side of the mountain collapsed. The Nature of Volcanic Eruptions (page 129): is the resistance of a liquid to flow. If magma has a high viscosity it moves (slow/fast), while magma with a low viscosity moves. Something like honey is (higher/lower) in viscosity compared to water. In what way does the silica content of the magma influence the viscosity of the magma (lava)? The higher the temperature of magma, the (higher/lower) the viscosity of the magma (lava). The lower the temperature of magma, the (higher/lower) the viscosity of the magma (lava). The primary reason that magma sprays into the air in lava fountains is due to the presence of. If a magma comes to the surface and there is very little gas in the magma, the eruption is normally very (explosive, quiet). If magma has a high viscosity, how does this affect the ability of the magma to release gases? How does this affect the explosiveness of the magma? So, (low/high) viscosity magmas are usually more explosive and viscosity magmas are usually less explosive. Explain why some eruptions are quiescent while others are very explosive. 6
Applying the ideas to this point: We could summarize that there are three primary variables that influence the character of a volcanic eruption: (all three are in this section) 1. 2. 3. Materials Extruded During an Eruption (page 131): What is the term for very blocky angular lava flows (pahoehoe/aa)? Which of these two types of lava flows has the lowest viscosity?? Lava tubes tend to occur when the sides and the top of the lava flow solidify before the. Distinctive rounded masses of lava known as pillow structures are formed when the lava is erupted into. Summary: Normally, andesitic lava flows flow (slower/faster) than basalt. This is because andesite lava usually has a viscosity. Rhyolitic magmas tend to crystallize forming granite, so eruptions of rhyolite are rare. The viscosity of most rhyolitic magmas is very. When rhyolite does erupt, it usually erupts very, producing large amounts of ash. Anatomy of a Volcano (page 135) In the introduction to this section read about the common features associated with a volcanic cone. The authors correctly note that a crater is a feature associated with the material ejected from the volcano. Craters are small and are commonly found with cinder cones and composite volcanoes. However, if I was an author of this book I would have included the term caldera at this point in the chapter. Calderas are formed by the collapse of the upper part of the volcano and are much larger. Calderas are discussed on page 145 in this chapter. Calderas may be a type of volcano by themselves, but most commonly they are associated with other types such as a shield or composite cone. I encourage you to jump to page 145 and read about calderas at this point. Is Crater Lake in Oregon a crater or a caldera? How did it form? Please read about shield volcanoes, composite cones, and cinder cones in this section. Then jump to page 147 and read about Fissure Eruptions and Volcanic Domes. To me all of these types of volcanoes or volcanic features should be in the same section. 7
Explain the difference between a composite cone and a cinder cone. If you observed a cone that you believed to be a cinder cone, how would it differ in appearance from a composite cone? Once you have read about shield volcanoes, cinder cones, composite cones, fissure eruptions, and volcanic domes match the statements with the proper terms to the right: 1. A large depression created by the collapse of a volcano A. crater 2. The principle rock type erupted from fissures B. composite cone 3. When mixes with basaltic magmas, the C. caldera eruption may be very violent 4. Volcano formed primarily of basaltic lava flows. Has D. cinder cone relatively flat slopes. 5. This type normally erupts dacite or rhyolite. Lava flows not E. basalt common. 6. A depression created by the explosion of a volcano. F. volcanic dome 7. A small, steep-sided volcano built mostly of tephra- usually G. sea water basaltic in composition 8. Mt. Rainier is an example of this type of volcano H. shield volcano We will examine these various types of volcanoes in one of our first labs. Go back to page 141 (Volcanic Hazards- section 5.8): I. rhyolite J. fissure eruption What is a pyroclastic flow? What types of volcanoes are most likely to have a pyroclastic flow? In what ways is a lahar different than a pyroclastic flow? Plate Tectonics and Volcanic Activity (page 150): this section will not be covered on the first exam. 8