6.1 Igneous Rocks and Earth’s Layers

The upper layers (mantle and crust) of the Earth are mainly composed of igneous rocks, although sedimentary rocks are the most abundant on the surface. You probably have learned that these layers formed early in its history through the process of chemical differentiation, when denser elements sank towards the center of the Earth and less dense elements floated toward the surface. This led to density differences between the layers as well as chemical differences.

Exercise 6.1 – Determining the Density of Earth’s Layers

Your instructor has presented you with three examples of Earth’s layers: oceanic crust, continental crust, and the upper mantle.

1. Describe the differences between the three rocks.
   

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2. Measure the density of each sample as you did at the end of Chapter 1. Weigh each sample and record the mass, then record how much water is displaced when you place it in a graduated cylinder. Fill out table 6.1.
   

   Table 6.1 – Density calculation

   Rock	Mass (g)	Volume (cm3)	Density (g/cm3)

3. How do your measured densities compare with your observations in a?
   

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4. What do you think causes the varying densities of these rocks?
   

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5. On Figure 6.1, show where these rocks could come from.

 

6.2 Classifying Igneous Rocks

The classification of igneous rocks is based on chemistry and texture, which means it’s based on the minerals present and how large the mineral grains are (Figures 6.2 to 6.5). The magma’s chemistry and temperature determine which minerals will form. There are eight common minerals in igneous rocks (Figure 6.2), which can be separated into four different igneous compositions: ultramafic, mafic, intermediate, and felsic.

The size of the mineral grains is chiefly determined by how quickly the magma cools. Slow cooling rates (thousands to half a million years) make larger mineral grains, creating intrusive (plutonic) igneous rocks. Fast cooling rates (instantaneous to hundreds of years) produce smaller grains, creating extrusive (volcanic) igneous rocks. The distinction between fine and coarse grains is whether you can identify the minerals without magnification. This is important because two different rocks can have the same composition, but different textures (small grains vs. large grains). For example, diorite (large grains, intrusive igneous rock) and andesite (small grains, extrusive igneous rock) have the same minerals but different textures.

The common textures of igneous rocks are:

• Coarse (phaneritic) – grains are large enough to identify without magnification, indicative of slow cooling; intrusive rocks
• Fine (aphanitic) – grains are too small to identify without magnification, indicative of fast cooling; extrusive rocks
• Porphyritic – grains are a mixture of coarse and fine grains, indicative of two cooling rates, slow at first (intrusive) and then fast (extrusive)
• Glassy – very smooth texture with no minerals, cooled so quickly that minerals did not have time to form, commonly has conchoidal fracture; extrusive rocks
• Vesicular – rough, porous texture, cooled quickly, vesicles (holes) created from abundant gas in the lava; extrusive rocks

How would you identify minerals in an extrusive igneous rock if the grains are too small to see? There are a couple of tricks that geologists use to identify which type of igneous rock they have. Do you remember how color was not a diagnostic property for minerals? Well, color in an igneous rock can be diagnostic! Iron and magnesium-bearing minerals are either black or green in color, like olivine and pyroxene, and they are the primary minerals that make up mafic and ultramafic igneous rocks. Potassium, aluminum, and silica-rich minerals, like quartz and potassium feldspar, are lighter in color and make up intermediate and felsic igneous rocks. Geologists can use the color of igneous rocks and the relative amount of light- vs. dark-colored minerals to help identify them (Figures 6.2 and 6.3).

Glassy and vesicular textures are a little different (Figure 6.4). These rocks form when lava cools off too quickly for minerals to form. The conchoidal fracture in obsidian tells us that it has a high silica content (felsic) despite its dark color. The dark color comes from the tiny amount of iron present.

Exercise 6.2 – Igneous Rock Identification

Using figures 6.2 – 6.5, identify the set of igneous rocks given to you by your instructor. Fill in the table in Figure 6.6.

 

6.3 Creating Magma

The temperatures needed to melt a rock to create magma are between 600 and 1,300 ^oC  (1,100 and 2,400 ^oF), depending on the rock’s composition and its depth (pressure). With normal temperature gradients in Earth, it is almost impossible to reach these temperatures. In your lecture class, you’ll learn about decompression melting and fluid-assisted melting. It is impossible to demonstrate these in a lab setting. We can, however, simulate one melting process which results in creation of magmas.

Partial melting is when some minerals in a rock melt before others do. Most rocks are made up of different minerals, and each mineral has a different melting temperature. In general, minerals with more silica will melt before those with less silica. During partial melting, minerals with lower melting points will melt first; while minerals with higher melting points remain solid. The melt (magma) composition is “enriched” with the minerals that have melted, and the remaining solid (rock) is “depleted” in the minerals that have melted. For example, if a mafic rock undergoes partial melting, the magma created is intermediate. The resulting magma has more (enriched in) silica, and the remaining rock has less (depleted in) silica.

6.4 Volcanoes

Volcanic eruptions are one of Earth’s most dramatic and destructive agents of change. Powerful explosive eruptions drastically alter land and water for tens of kilometers around a volcano. Also, droplets of sulfuric acid erupting into the stratosphere can temporarily change Earth’s climate. Volcanic eruptions often force people living near volcanoes to abandon their land and homes. Those living farther away are likely to avoid complete destruction, but these cities and towns, crops, industrial plants, transportation systems, and electrical grids can still be damaged by tephra, ash, lahars, and flooding.

Fortunately, volcanoes often exhibit unrest that allows eruptions to be anticipated and warnings sent to at-risk communities. The warning time preceding volcanic events typically allows sufficient time for nearby communities to implement response plans. Another way to predict future eruptions is to investigate the past eruptive history of a volcano.

Exercise 6.4 – Volcano Eruption Frequency

Some people have suggested that the number of volcanic eruptions is increasing and contributing to climate change. Let’s look at some data. Table 6.2 shows the number of volcanic eruptions worldwide from 1992 to 2008.

Table 6.2 – Yearly volcanic eruptions from 1992-2008

Year	Number of Eruptions
1992	27
1993	27
1994	32
1995	33
1996	35
1997	26
1998	32
1999	39
2000	39
2001	33
2002	41
2003	29
2004	49
2005	46
2006	44
2007	36
2008	45

1. Plot the volcanic eruption data from Table 6.2 on the blank graph in Figure 6.7.
2. Do you think there is a trend in this data? Have the number of volcanic eruptions per year been increasing? Explain.
   

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Sometimes people interpret data without looking at the big picture, either accidentally or by cherry-picking data points that support their point. You were only given a subset of the volcanic eruption data in Table 6.2.

3. Table 6.3 contains the remaining data from the years before and after the data you plotted. Plot this new data on the same graph in Figure 6.7. You can use a different color if you’d like.
4. Do you think there is a trend now that you have the complete data set? Have the number of volcanic eruptions been increasing, decreasing, or about the same? Explain.
   

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Table 6.3 – Yearly volcanic eruptions from 1960-1991, 2009-2022

Year	Number of Eruptions	Year	Number of Eruptions
1960	35	1984	33
1961	32	1985	27
1962	29	1986	41
1963	40	1987	32
1964	29	1988	27
1965	34	1989	25
1966	35	1990	30
1967	43	1991	37
1968	32
1969	32
1970	34	2009	31
1971	32	2010	33
1972	30	2011	36
1973	41	2012	39
1974	27	2013	38
1975	23	2014	42
1976	29	2015	41
1977	39	2016	34
1978	25	2017	32
1979	32	2018	37
1980	41	2019	27
1981	29	2020	27
1982	32	2021	33
1983	32	2022	31

The volcanic eruption data is from the Smithsonian Institution Global Volcanism Program.

Geologists monitor most of the world’s volcanoes for eruptive activity, but how can past eruptions be determined, especially for events before human memories? The simplest way is to look at a detailed map around a volcano. With each eruption comes a new layer of lava that turns to igneous rock. By mapping out the igneous rocks, you can determine the volcano’s eruptive history. One of the most visited volcanoes is the Big Island of Hawaii, which is actually made up of several volcanoes, with Kīlauea being the most active today. The oldest eruption on the island started about 1 million years ago and it may be actively erupting as you read this (Figure 6.8).

If you want to live near a volcano, knowing the types of hazards is critical to evaluate the risks you’ll be taking. Some volcanoes are dormant or extinct and no longer hazardous. Most countries with active volcanoes monitor their potential hazards. Active volcanoes are monitored for seismic activity (earthquakes), gases, deformation of the ground, gravity, magnetics, and thermal imaging. You can go to the Global Volcano Network to assess recent activity around the world.

Exercise 6.6 – Reading a Volcanic Hazard Map

Figure 6.9 contains a colored digital elevation model and a hazard map of the Mt. Rainier region in Washington State. For reference, Seattle is located near Puget Sound just north of this map.

1. Do lava flows and pyroclastic flows pose a significant risk to the surrounding population? Explain
   

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2. What do you notice about the path lahars tend to follow in this region?
   

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3. How far can these lahars travel? Measure on the map.
   

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4. What could cause the failure of the Alder Dam and subsequent flooding (located on the western side of the map near the town of Elbe)?
   

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5. Critical Thinking: What do you think is the source of water for the lahars?
   

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Additional Information

Exercise Contributions

Daniel Hauptvogel, Virginia Sisson, Kaitlin Thomas