You probably already know that igneous rocks form from magma or lava. Did you know that the term is derived from the Latin word “ignis” meaning fire. Before doing this lab exercise, you probably know about basalt and granite, but there is a wide variety of igneous rocks that have unique chemical, mineral, and textural properties, as you just learned in the rock cycle exercises. Many have been fascinated by igneous rocks as they make up mountains, particularly volcanos. These are eruptions can be devastating, such as the Vesuvius eruption in 79 A.D. that buried Pompeii or 1815 eruption of Tambora, which caused global climate change for several years. Geoscientists study them to  insights into the lower crust and upper mantle as well as tectonic processes.  Nowadays, they are investigated as sources of lithium for batteries and rare earth elements for magnets for computers and cell phones.

6.1 Igneous Rocks and Earth’s Layers

During the rock cycle exercises, you learned that sedimentary rocks are the most abundant rock type on Earth’s surface, but most of the crust and upper mantle are composed of igneous rocks. You might remember learning these layers formed early in Earth’s 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.

6.2 Igneous Rock Classification

The classification of igneous rocks is based on texture and composition of the rock. Texture refers to how large or small the mineral grains are, and composition refers to which minerals are present..

Texture

The size of mineral grains in igneous rocks is primarily determined by how quickly the magma cools. Slow cooling rates (thousands to half a million years) make larger mineral grains. Slow cooling rates are common in the crust and upper mantle, creating intrusive (plutonic) igneous rocks. Fast cooling rates (instantaneous to hundreds of years) occur when lava is at the surface and produces 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. 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 (Figure 6.2) 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
• Pyroclastic –  angular rock fragments with fine ash ejected during volcanic eruptions.

Composition

The composition of an igneous rock is determined by the chemistry and temperature of the magma it formed from. There are eight common minerals in igneous rocks, which can be separated into four different igneous compositions: ultramafic, mafic, intermediate, and felsic (Figure 6.3). Identifying minerals in rocks is not easy for a beginner geologist, and identifying minerals in fine-grained rocks is impossible without magnification, but there are a few tricks that geologists use to identify the composition. 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 or the relative amount of light- vs. dark-colored minerals (Figure 6.4) to help identify the rock’s composition.

It can be difficult to determine the composition of glassy and vesicular textures. (Figure 6.5). These textures 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. Scoria, a vesicular rock, is mafic and gets it’s dark coloring from the iron present, whereas pumice is typically felsic and lighter in color. Scoria is also denser than pumice, and some pumice can float in water.

One of the obstacles to discussing rocks are classification schemes that emphasize rock names. Historically, geoscientists who study rocks have used 100’s of names. But, don’t panic as we want you to learn how to observe their important features and not obsess over their names. Just like the minerals chapters, our approach is to emphasize observtions you need to make and won’t worry about whether you can identify a rock as an ankaramite (a type of basalt with clinopyroxene that one of the authors worked on in Venezuela) or another obscure rock name. For igneous rocks, we have whittled it down to only eleven common names and based mostly of composition and a few others that have unique textures (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 (Figure 6.8). 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 Tectonic Setting of Magmas

Magmas can be found in every modern plate tectonic setting as well as within plate interiors. Some magmas occur in every tectonic setting, such as mafic rocks (basalt and gabbro), which occur in all three tectonic settings as well as hotspots. The minerals or textures of igneous rocks cannot determine their plate tectonic setting. Instead, geoscientists use the chemistry of the magmas. In lecture, you may have discussed how the silica (quartz or SiO₂) content decreases from felsic to ultramafic rocks. There are also changes in mineral abundances. These differences are reflected as changes in the rock chemistry. Additionally, since each mineral may contain trace elements, different magmas exhibit variable trace element chemistries. Geoscientists have created a worldwide database of igneous rock chemistries, EarthChem. Using this data, they can relate the chemistry of the magma to various plate tectonic settings.

6.5 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 6.8 – 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 6.7 shows the number of volcanic eruptions worldwide from 1992 to 2008.

Table 6.7 – 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 6.7 on the blank graph in Figure 6.10.
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 6.7.

3. Table 6.8 contains the remaining data from the years before and after the data you plotted. Plot this new data on the same graph in Figure 6.10. 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 6.8 – 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 6.11).

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 6.10 – Reading a Volcanic Hazard Map

Mount Rainier (Figure 6.12) is considered a dangerous volcano due to the type of volcano it is, its elevation, year-round snow/ice at its peak, and its proximity to the heavily populated regions of Tacoma and Seattle Washington that have almost 5 million people. Figure 6.13 contains a colored digital elevation model and a hazard map of the Mt. Rainier region in Washington State; Seattle is located near Puget Sound just north of the 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

Reference

Shervais, J, 2022, The petrogenesis of modern and ophiolitic lavas reconsidered: Ti-V and Nb-Th, Geoscience Frontiers, 13, 101319, https://doi.org/10.1016/j.gsf.2021.101319