7.2: Rock Types of the Cascades

Rocks Classified by Composition

In section 2.4: Igneous Rocks, you learned that igneous rocks are usually classified mainly by two attributes: first by texture, whether they are intrusive (plutonic) or extrusive (volcanic), then by composition. This chapter focuses primarily on extrusive or volcanic rocks. For more discussion of intrusive igneous rocks, see 9.2: Basement Geology of the Sierra Nevada: The Core of an Ancient Volcanic Arc. The major types of volcanic rocks, therefore, are distinguished and identified from one another based on composition. In truth, volcanic rock composition varies continuously along a spectrum, from mafic to felsic, yet to classify these rocks and give them names, we must separate them into distinct categories. Typically, an introductory geology student learns to divide volcanic rocks into three such categories: basalt (mafic), andesite (intermediate), and rhyolite (felsic). However, if you travel in the Cascades and read interpretive signs and visit visitor centers, you will likely encounter a fourth volcanic rock named ‘dacite’. Dacite describes a volcanic rock with a composition that falls between intermediate and felsic.

The overall composition of an igneous rock can be determined by identifying the minerals that make up the rock (see 2.4: Igneous Rocks). However, in volcanic rock, the texture is usually aphanitic, meaning that many of the mineral crystals are too small to be seen (and definitely too small to be identified) with the naked eye. Geologists who study volcanic rocks often use microscopes to look at thin sections of rocks. Thin sections are slabs of rock ground thin enough for light to pass through them. This makes identifying the minerals using a microscope easier. Unfortunately, it is oftentimes difficult to have a microscope in the field, which makes classifying rocks by “hand sample” useful. When information about the mineral composition is not directly available, the overall color or tone of the rock can provide useful information. Felsic minerals tend to be lighter in color and the common felsic mineral, potassium feldspar often appears pink. Therefore, the overall tone of rhyolite, a felsic rock, is usually light and it can have a pinkish tint. By contrast, unweathered basalt (mafic) is black or very dark gray. Dacite and andesite fall somewhere in between (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Color Comparison of Rhyolite Andesite and Basalt. This figure by Emily Wright is licensed under CC BY-NC 4.0. Original photos: "Rhyolite (Tertiary; Castle Rock, Colorado, USA) 2", "Andesite (Tertiary; Yellowstone, Wyoming, USA) 1", and "Olivine basalt (Cedar Canyon, Iron County, Utah, USA) 13" by James St. John are licensed under CC BY 2.0.

When using color to identify the composition of a rock, it is important to keep two things in mind. First, color is most useful as an identifying characteristic on unweathered surfaces. Freshly broken surfaces are least likely to be weathered, and this is one of the reasons that geologists carry rock hammers. The second thing to keep in mind is that composition is not defined by color; rather, color is merely a useful tool for approximating mineral composition when minerals can’t be seen. Nature does not always follow perfect rules and sometimes the color of rocks can be misleading.

In some cases, minerals can be identified in volcanic rocks. Although the texture of volcanic rocks is usually aphanitic, sometimes it is also porphyritic, meaning that there are larger phenocrysts, surrounded by an aphanitic groundmass. Phenocrysts can often be large enough to be identified in hand samples by the naked eye. Magmas which are closer to the felsic end of the composition spectrum have had more time to evolve. Therefore, they have also had more opportunity for growth of phenocrysts. Basalt generally has few phenocrysts, and the most common is olivine, the first mineral to cool according to Bowen’s Reaction Series (see 2.4: Igneous Rocks). Andesite has more phenocrysts, usually composed of plagioclase feldspar or hornblende, two common intermediate minerals. Rhyolite often has many phenocrysts, composed of a variety of minerals. In particular, quartz phenocrysts can be useful for identifying rhyolite, as quartz is one of the last minerals to cool. Practice identifying a volcanic rock using both color and identification of phenocrysts in the activity, “Rock Identification: Chaos Jumbles”.

Query \(\PageIndex{1}\)

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Rocks Classified by a Special Volcanic Texture

There are a few notable volcanic rocks that are not named using the standard igneous rock naming scheme, but instead are named for their special volcanic textures. Aphanitic texture forms when rocks cool quickly at the surface of the earth and the resulting mineral crystals are too small to be seen. In some cases, lava can cool in a way that no crystals form at all. The resulting material is volcanic glass. Obsidian is a rock consisting of volcanic glass. Obsidian has excellent conchoidal fracture, similar to the mineral quartz (see 2.2: Mineral Properties). Obsidian is, however, a rock with a color that can be misleading. Obsidian is usually felsic in composition but almost always appears dark in color (Figure \(\PageIndex{2}\)). However, a thin shard of obsidian can often appear translucent. Like manufactured glass, obsidian is mostly clear, but obsidian often has enough dark impurities to make the translucent material appear dark.

Figure \(\PageIndex{2}\): Obsidian from Glass Mountain, part of the Medicine Lake Volcano in the California Cascades. The white to gray pumice in the background is similar in composition, but very different in color. "Glass Mountain in northern California in summer 2012 (15)" by Chmee2 is licensed under CC BY-SA 3.0.

Pumice is another rock made of volcanic glass, but it is usually white, gray, or otherwise light colored. Pumice forms when escaping gas bubbles become trapped in the solidifying lava to create a vesicular texture, with holes called vesicles (Figure \(\PageIndex{3}\)) present throughout the rock. The abundance of vesicles in the translucent glass make it appear white, much like the white water in a raging river or broken ocean wave, or the white color of meringue made from whipped egg whites. Vesicular texture is also the defining feature of the rock scoria. Scoria is a mafic rock with more than 50% vesicle by volume. Rocks that are vesicular, but are more rock than vesicle are described by their standard igneous rock name, preceded by the term ‘vesicular’, such as ‘vesicular basalt’. Scoria and vesicular basalt are contrasted in figure \(\PageIndex{4}\).

Figure \(\PageIndex{3}\): Airfall pumice from the 1065 A.D. eruption of Medicine Lake Volcano's Glass Mountain eruptive center. "Airfall Pumice" by James St. John is licensed under CC BY 2.0.

Figure \(\PageIndex{4}\): Scoria and vesicular basalt. Scoria is a mafic rock containing vesicles, in which half or more of the volume is air (filling the vesicles). Vesicular basalt is similar to scoria, but there is more rock than vesicle."Scoria Macro" by Jonathan Zander (Digon3) is licensed under CC BY-SA 3.0. "Vesicular porphyritic olivine basalt" by James St. John is licensed under CC BY 2.0.

When volcanoes erupt explosively, vast amounts of lava, rock, ash, and gasses are thrown into the atmosphere. The solid parts, called tephra, settle back to earth and cool into rocks with pyroclastic textures. ‘Pyro’, meaning fire, refers to the igneous source of the tephra and clastic refers to the rock fragments. Tephra fragments are named based on size—ash (<2 mm), lapilli (2-64 mm), and bombs or blocks (>64 mm). Pyroclastic texture is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. Rock formed from large deposits of tephra fragments is called tuff (Figure \(\PageIndex{5}\)). If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded tuff.

Figure \(\PageIndex{5}\): A sample of volcanic tuff contains fragments of pumice, volcanic glass and other rocks. Red scale bar is 2.0 cm. "Welded tuff" from An Introduction to Geology is licensed under CC BY-NC-SA 4.0.

Box \(\PageIndex{1}\): Fantastic Lava Beds

Figure Box \(\PageIndex{1.1}\): Basalt from the Fantastic Lava Beds with quartz xenocrysts. "Quartz basalt" by James St. John is licensed under CC BY 2.0.

Any good student of Bowen’s Reaction Series (see 2.4: Igneous Rocks) knows that visible quartz crystals in a basalt should never exist. Quartz is one of the last minerals to cool; thus, when mafic lava cools, the whole rock will crystallize into mafic minerals before the lava cools to the crystallization temperature of quartz. Yet this very phenomenon can be found in the Fantastic Lava Beds, which erupted from Cinder Cone in Lassen National Park. Clear, white quartz crystals sparkle within a dark basalt rock (Figure \(\PageIndex{6}\)) . How can this be?

In fact, the quartz crystals in the Fantastic Lava Beds are not phenocrysts, which crystalize as the magma cools, but instead are xenocrysts, mineral crystals that were incorporated into the magma from the surrounding country rock and never melted in the first place. You may be familiar with the term xenolith; a xenolith is formed similarly to a xenocryst, but xenocrysts consist of individual mineral crystals, rather than large pieces of country rock.

Acknowledgments

Parts of the text on this page were taken with some editing from existing Open Educational Resources by Earl (2019) and Johnson et al. (2017). Links to the original text can be found in the reference section on this page.

References

1. Clynne, M. A. (2011). Stratigraphy and compositional evolution of Cinder Cone, a composite monogenetic cone in Lassen Volcanic National Park, California. 2011, V13C-2621.
2. Clynne, M., & Muffler, L. (2010). Geologic Map of Lassen Volcanic National Park and Vicinity, California: U.S. Geological Survey Scientific Investigations Map 2899 [Map]. https://pubs.usgs.gov/sim/2899/
3. Earle, S. (2019). Chapter 4 Volcanism. In Physical Geology (2nd ed.). BCcampus. https://opentextbc.ca/physicalgeology2ed/part/chapter-4-volcanism/
4. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2017). Chapter 4: Igneous Processes and Volcanoes. In An Introduction to Geology. https://slcc.pressbooks.pub/introgeology/chapter/4-igneous-processes-and-volcanoes/
5. Szymanski, M. E., & Teasdale, R. (2015). Groundmass Crystallinities of Proximal and Distal Lavas from Cinder Cone, Lassen Volcanic Field. AGU Fall Meeting Abstracts, 2015, V23B-3106. https://ui.adsabs.harvard.edu/abs/2015AGUFM.V23B3106S/abstract