Volcanos and Igneous Rocks in the National Parks

OBJECTIVES

• Connect rocks and their process of formation to the Rock Cycle
• Understand the two main characteristics than define igneous rocks
• Distinguish the unique textures and formation of these textures in an igneous rock
• Recognize and name the common igneous rocks
• Identify the various Volcano types and their potential dangers
• Connect specific National Park features to their Igneous rock locations

MATERIALS

• Ruler, glass plate, hand lens, protractor, several minerals: olivine, pyroxene, amphibole, biotite mica, muscovite mica, two feldspars, and quartz (8 from the mineral lab). 4 viscous fluids.
• Suite of Igneous (Granite, Diorite, Gabbro, Rhyolite (ashy tuff), Porphyritic Andesite, Basalt, Pumice, Scoria, Vesicular Basalt, Obsidian), various volcanic samples provided by your instructor
• PARKS FEATURED: Hawai’i Volcano NP, Craters of the Moon NM&P, Idaho, Mount St. Helens NVM, Oregon, Alaskan volcanoes NP’s and Mt. Rushmore, NM South Dakota

INTRODUCTION: Every Rock Tells a Story

Identifying rocks is an acquired skill. To obtain these skills you must make careful observations and describe the properties that define the rock, such as composition, minerals, size and shape of the grains and overall texture. Based on those properties, interpretations are made about how the rock formed. In most cases, with some exceptions, we rarely witness the actual formation of rocks. One such exception is the formation of volcanic rock from the crystallization of lava or an eruption from a volcano. Some of you may have been lucky enough to have seen rocks form at volcanos in Hawai’i. Hawai’i Volcanoes National Park, on the big island is a currently active shield volcano that can be safely viewed by visitors. As a result of the unique geologic processes that create each of the three rock types (Igneous, Sedimentary, Metamorphic) you can use these processes to help construct the history of the rock. For example, most Sedimentary rocks are formed from sediment. This sediment is the weathered product of other rocks. The type of sediment, such as clay or pebbles can tell you something about the rock’s geologic history, such as how it may have been deposited by a glacier or a mighty river or the wind. Metamorphic rocks form from other rocks through conditions of high heat and pressure. With some practice and your new observational skills, you too can tell the rocks story.

Before we identify a specific rock type, we must first know what a rock is.

CLASS QUESTION:

WHAT IS A ROCK? Hint as opposed to a mineral.

Write description here:

Rock vs. Mineral

Rocks are aggregates of one or more minerals or biochemical components (such as plants or fossils). In geology, a mineral has a very specific definition – recall from the previous lab on Minerals. A mineral is a naturally occurring, inorganic crystalline solid with a fixed chemical formula. Minerals have a set of diagnostic physical properties that can be used to identify the minerals, for example: Hardness, Cleavage, Fracture, Magnetism, Color, and Luster. Examples of minerals are quartz, feldspar, mica, amphibole, pyroxene, olivine, calcite, and halite.

ROCKS ARE AGGREGATES OF MINERALS.

CIRCLE SEVERAL DIFFERENT MINERALS ON THE IMAGE BELOW. NAME THEM.

Figure 1. Granite from Temple University. Notice the different colored minerals. This granite sample contains pale colored feldspars, grey vitreous glassy quartz, and black amphiboles. Image author.

The Rock Cycle

The rock cycle (Figure 2) is a summary of rock forming processes that occur within the Earth and on the Earth’s surface. The three main rock types illustrated in the rock cycle (igneous, sedimentary, and metamorphic) form through very specific geologic conditions. Furthermore, through specific processes such as weathering, lithification, metamorphism, melting, or crystallization, one type of rock can alter into another. In other words, rocks are recycled and formed into other rocks. Note the many arrows and directions of the processes. Geologists use the clues created by the formational processes to help

identify the three unique rock types. During this lab we will practice developing the skill to identify the features that distinguish the three rock types. A key point to remember is the difference between a mineral and a rock. A mineral is a pure substance with a specific composition and structure, while a rock is typically a mixture of several different minerals (although a few types of rocks may include only one type of mineral such as Coal and Limestone).

Figure 2. The Rock Cycle. There are three basic rock types: Igneous, Sedimentary and Metamorphic. They are connected by the many Geologic processes such as weathering, burial and melting. The rock cycle does not proceed in one direction. Each rock type is created and recycled by the various processes (see arrows) that connect them. Any rock has been and can be another rock over the 4.6 billion-year time history of the Earth. Notice the conditions that create sediment occur on the Earth’s surface while the conditions necessary to create both Igneous and Metamorphic rocks occur within the earth. Image from opentextbc.ca/geology license: Steve Earle Creative Commons 4.0. Image: opentextbc.ca/geology license: Steve Earle Creative Commons 4.0.

IGNEOUS ROCKS: Overview

Igneous rocks form from magma (molten rock) that has either cooled slowly underground (Granite, Gabbro) or cooled quickly on the surface from a volcanic eruption (Rhyolite, Basalt) (figures 3 & 4). When molten rock, (magma) cools slowly deep inside the Earth the grains can grow large or visible to the unaided eye. This slow cooling produces a distinguishing feature of large (visible) interlocking/crystalline arrangement of their mineral grains. When molten rock, (lava) erupts on the surface, via a volcano it may produce a very fine-grained (not visible to the unaided eye) vesicular (filled with holes), ashy, flowing structures or glassy texture. This distinguishing feature is the fine grained-ashy texture indicative of a volcanic eruption. These features are called TEXTURE.

Figure 3. Slowly cooled large crystalline grains of Granite. Sample shows dark colored Amphiboles, pink/orange Potassium feldspar, clear/grey glassy Quartz, and white Plagioclase feldspar from Colorado Springs, Colorado. Image CC BY NPFlynn.

Figure 4. Volcanic Basalt showing stretched vesicular texture formed from a volcanic eruption and the release of gases as the lava cools on the surface very quickly. Sample is from Idaho outside of Craters of the Moon NM&P. Image CC BY NPFlynn.

Molten rock underground is called magma, and molten rock that erupts at the surface is called lava (figures 5 & 6). Lava and pyroclastic ash are associated with surface volcanoes. Hawai’i Volcanoes National Park on the Big Island of Hawai’i is currently an active volcano producing igneous Basaltic lava and an enormous shield volcano. Mount St. Helens National Volcanic Monument in Washington erupted violently in May of 1980, producing copious volumes of pyroclastic Rhyolitic ash and Pumice from the explosive Stratovolcano (figure 7). Cooling of magma and lava produces igneous rocks. The variety of cooling environments such as fast, on the surface or slowly inside the Earth, results in the variety of textures and grain sizes seen in igneous rocks.

Figure 5. Lava flow in Hawaii. The black rock is called Basalt and is abundant in the Hawai’i Volcano National Park. The red/orange flow is liquid lava, that will cool to form a fine-grained basalt. Image: https://upload.wikimedia.org/wikipedia/commons/8/82/Pahoehoe_toe.jpg Hawaii Volcano Observatory (DAS), Public domain, via Wikimedia Commons. http://hvo.wr.usgs.gov/kilauea/update/archive/2003/May/main.html

Figure 6. Basaltic lava from Hawai’i Volcanoes NP the big island Hawai’i. Image: CC ITWFlynn

Figure 7. Mount St. Helens National Volcanic Monument, Washington is a classic explosive stratovolcano producing large volumes of pyroclastic ash and pumice. Image: author

MINERAL COMPOSITION: Igneous rocks are primarily formed from a variety of silicate minerals such as Quartz, Feldspars, Micas, Amphiboles and Pyroxenes. The combination of silicate minerals produces a variety of color levels seen in Igneous rocks. Some are very dark colored (mafic) due to an abundance of pyroxene and amphibole minerals that contain significant amounts of the elements Iron and Magnesium and very little Silica such as Basalt (Figures 4, 6 & 8). Others are pale or light colored (felsic) due to an abundance of Silica, Sodium and Potassium such as Granite (figure 3) and Pumice (figure 8).

Figure 8. Left Ropey Basalt from Craters of the Moon National Monument and Park, Idaho. Right Pumice from the Hoodoos in Yellowstone National Park near Minerva terrace, Wyoming. Images: author

Igneous Rock Classification

Igneous rocks are typically classified by TWO features: their texture (crystal size and arrangement) and chemical composition/color level (minerals present). The texture of an igneous rock reflects its cooling history. The composition of igneous rock, to a large degree, reflects its plate tectonic setting during formation as seen in the distinct minerals grouped together. The composition of an igneous rock can in many cases be inferred from its overall color level. The color levels are felsic (light colored), intermediate (medium), mafic (dark) and ultra-mafic (a rare black-greenish color).

Igneous Textures

The texture of an igneous rock reflects how and where the magma cooled and crystallized to form minerals. The size of the crystals depends on the cooling rate. When magma cools slowly over tens of thousands to millions of years very large grains grow. The individual mineral grains can be several centimeters in size. When large plutons of magma cool deep inside the earth, they can latter get pushed to the surface by tectonics processes and exposed by erosion. Such rocks form some of the most spectacular national park mountains such as the Black Hills granites of Mt. Rushmore NM in South Dakota and the Sierra Nevada Granites in Yosemite NP, California.

Coarse-grained textures (Phaneritic) are produced by slow cooling of the magma (tens of thousands to millions of years). Phaneritic texture is defined by interlocking crystals (typically 1-10 mm) that can be seen with the naked eye (without a microscope). Cooling deep (many kilometers) underground insulates the magma, allowing it to cool slowly and form large crystals (Figure 9 - 11). Examples of common Phaneritic – course-grained igneous rocks include Granite, Diorite and Gabbro.

Figure 9. The rocks that form the famous Mt. Rushmore NM in South Dakota are a coarse-grained Phaneritic texture known as Granite. Notice the crystalline interlocking grains of the many individual minerals, the right sample shows the large grains of the Mt Rushmore granite. Images: Granite Mt Rushmore NM, SD. CC BY NP Flynn

Figure 10. Granodiorite Yosemite NP, CA. Part of the Sierra Nevada mountains. Image: Author CC

Figure 11. A Gabbro intrusive exposed in Acadia NP Maine near Mount Desert Island north of Bar Harbor. Right Gabbro from Troodos Cyprus. Image: CC BY NC SA the Vertebrate Paleontology Lab vby Alton Dooley https://vmnhpaleontology.wordpress.com/2009/05/12/acadia-national-park-paleozoic-rocks-2/. Right https://www.sandatlas.org/gabbro/

Fine-grained (Aphanitic), pyroclastic (Ashy), Vesicular and glassy (Quenched) textures indicate fast cooling (days to hours or minutes) on the Earth’s surface. Most fine- grained igneous rocks cooled at the surface during a volcanic eruption which can produce a variety of textures such as a pyroclastic/ashy from explosive strato volcanoes or loose and wavy from effusive (non-explosive) volcanic eruptions. Sometime lava contains gases which escape as the liquid is rapidly cooling which leave holes or vesicles in the cooling lava. If the lava cools extremely rapidly (in seconds) because it flows into water, few individual crystals can form, and a volcanic glass will form producing Obsidian with a quenched texture. It looks and acts like glass. A variety of volcanoes produce the different aphanitic rock types.

Fine-grained (Aphanitic) textures are produced by small interlocking crystals (typically <1 mm), which are too small to see with the naked eye (a microscope is necessary). This fine-grained texture can cause the rock to look very uniform in color. Fine-grained textures can also result from shallow intrusions, or if magma is injected into fractures in cooler rock. These injections are called dikes or sills. Examples of fined-grained aphanitic igneous rocks include Basalt, Rhyolite, and Andesite (figures 12 & 13).

Figure 12. Fine-grained Aphanitic texture seen in Basalt (left) from Acadia NP, Maine near Cadillac Mountain. Notice the pink phaneritic granite to the lower right of the image. Image left. Basalt CC BY NC SA Vertebrate Paleontology Lab by Alton Dooley. https://vmnhpaleontology.files.wordpress.com/2011/07/2009-05-12b.jpg

Figure 13. Large deposits of Rhyolite produce the unique yellowish aphanitic volcanic rock that gave Yellowstone NP, Wyoming its name. The right image represents a fine-grained Rhyolite. The rock can appear grey, beige, yellow to pinkish in color. Image left, NPFlynn author, right image Rhyolite https://upload.wikimedia.org/wikipedia/commons/7/7e/RhyoliteUSGOV.jpg

Pyroclastic (Ashy) texture is a subset of Aphanitic textures that are specific to volcanic eruptions. Some volcanic eruptions are very violent such as the Mt. St. Helens May 1980 eruption (figure 14) and the 1912 eruption of Novarupta volcano in Katmai National Park and Preserve, Alaska (figure 15). The sudden and violent volcanic eruption produced copious volumes of ashy material that settled and deposited around the mountain. Flow structures as the ash moves down the side of the mountain can produce a layered look and a rock that contains broken bits of rock from the explosive eruption (figure 16). This type of rock may feel light weight and ashy to the touch.

Figure 14. Mt St. Helens National Volcanic Monument, Washington showing the explosive violent blast that reduced the size of the mountain considerably in May of 1980. Image NPFlynn author.

Figure 15. Katmai National Park eruption of Novarupta. It is frequently compared to Mt. St. Helens eruption. Image USGS Photo https://www.nps.gov/katm/blogs/volcanic-leftovers-at-brooks-camp.htm

Figure 16. Ashy Pyroclastic texture of two Rhyolites. Welded fragments and flow banding are seen in the image to the left from Inyo National Forest, Hot Creek CA, near Mammoth lakes, while flow banding is seen in the right image. Left image: CC BY NPFlynn. Right image skywalker.cochise.edu CC BY SA R. Weller/Cochise College.

Vesicular: Vesicles are holes in volcanic rock that form as lava solidifies around gas bubbles. These are good indicators of volcanic rocks (phaneritic rocks don’t have vesicles). The size of the bubbles is often dictated by the viscosity (thickness) of the magma, with bigger bubbles forming in less viscous magma because they can coalesce more easily. Examples of this texture are seen in Pumice and Scoria. Pumice has a lot of little bubbles, almost foamy and very light weight - pumiceous texture. Pumice often has enough bubble space that it will float (figure 17 - left). Scoria has larger, less abundant bubbles in a denser reddish-black rock - scoriacous texture (17 - middle). Basalt also forms with a vesicular texture and is given the name Vesicular Basalt (17 right & 18).

Figure 17. Pumice (left), Scoria (middle), Vesicular Basalt (right). All three show visible holes called vesicles that are produced from escaping gas as the lava cools rapidly. The Scoria and Vesicular Basalt samples are from Craters of the Moon National Monument in Idaho. Images: Pumice CC BY SA 4.0 International JukoFF https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Images Scoria and Vesicular Basalt Craters of the Moon NM Idaho CC BY NPFlynn.

Figure 18. Craters of the Moon National Park and Preserve, Idaho shows large scale Scoria and Vesicular Basalt flows. Image: NPFlynn author.

Glassy (Quenched): Volcanic glass is called Obsidian and lacks visible crystals. Rocks that are very glassy show a characteristic conchoidal (curved) fracture pattern (Figure 19). Glassy texture forms when lava cools so quickly or is so viscous that ions cannot migrate through the melt and become arranged in an ordered pattern to form crystals.

Figure 19. Obsidian showing glassy texture and flow banding from Inyo National Forest, California Mono-Inyo craters and right conchoidal fracture. Image: left Inyo National Forest, Obsidian Dome CA CC BY NPFlynn, Right oer.galileo.usg.edu Karen Tefend CC BY-SA 3.0 oer.galileo.usg.edu/geo-textbooks/1/ CCBYSA 4.0 international

Porphyritic, (a two-stage cooling process), indicates slow initial cooling followed by rapid final cooling. Specifically, crystals >2mm in size are called phenocrysts, and they are embedded in a groundmass made of fine-grained crystals (often called a matrix). Porphyritic rocks are interpreted to have undergone two stages of cooling: the phenocrysts would form while the magma is slowly cooling deep underground, and, when the magma erupts, the groundmass cools quickly when exposed at the surface. Examples include Porphyritic Rhyolite and Porphyritic Andesite. Devils Tower National Monument in Wyoming is a famous example of a porphyritic andesite igneous rock (figure 20 & 21).

Figure 20. Devils Tower National Monument, Wyoming is an example of a remnant magma chamber that partially cooled slowly under the ground then stalled inside the volcanoes internal system. This results in a two-stage igneous rock texture called porphyritic. Image NPFlynn author.

Figure 21. Close up of the porphyritic andesite from Devils Tower NM, Wyoming. Notice the large whitish feldspars and the fine-grained matrix surrounding them. Image NPFlynn author.

Let’s practice connecting location to igneous textures

Texture is used to indicate whether the magma cooled at the surface (volcanic/extrusive) or deep underground (plutonic/intrusive) (figure 22). Use the Block diagram below to connect Magma/Lavas locations to possible textures produced. LOCATIONS = TEXTURES:

• A________
• B,D,E ________
• C________
• F________

How would C and A differ in texture?

Figure 22. Block diagram showing magma cooling locations for practice. Image CC BY S.A. 4.0 International License Opengeology.org/textbook – public domain.

LET’S PRACTICE IDENTIFYING TEXTURE IN IGNEOUS ROCKS

Look at the Igneous rocks and divide them into the 5 different textural groups: Phaneritic/coarse grained, Aphanitic/fine grained/Ashy, Quenched/Glassy, Vesicular, Porphyritic. Your TA will guide you in this exploration and check that you have correctly identified the various textures. Write a few descriptive words to help you remember each texture.

Did you notice that the rocks in each texture group all have different color levels?

ONCE WE HAVE ADDED THE SECOND CLASSIFYING FEATURE (COLOR LEVEL/COMPOSAITION) WE CAN NAME THE ROCK.

Igneous Compositions

COLOR is a quick way to estimate the chemical composition of most (but not all) igneous rocks. The color of most igneous rocks is controlled by the minerals present. To generalize, a darker color (mafic) commonly indicates that the rock is composed of minerals with a higher iron/magnesium content and lower silica content which are dark in color such as Basalt. A lighter/paler color (felsic) indicates the rock contains minerals with high silica content and low iron/magnesium content resulting in minerals with a light color such as Granite. There are some exceptions such as the dark color seen in black obsidian that indicate the lack of crystals. In these rocks, the black color is a result of the light being absorbed by glass rather than being reflected back to our eyes from mineral grains.

Most magmas can be grouped into four broad categories based on their chemical composition and resulting minerals. These categories are ultra-mafic (rare mantle rocks), mafic (less silica, more Mg and Fe), intermediate, and felsic (more silica, less Mg and Fe) (Figure 23). The average mineral compositions of felsic, intermediate, mafic and ultra-mafic igneous rocks are shown in the crystallization sequence developed by Norman L. Bowen in the 1920’s (Figure 24). This graph reveals that most igneous rocks are composed of similar minerals but in different amounts. Note the minerals that comprise igneous rocks are mostly Silicates and that the elements Silicon and Oxygen are the two most abundant elements in the Earth’s crust. The overall silica content (not the mineral quartz) of igneous magmas is very important because it influences the viscosity of the magma, which determines the fluidity of the magma/lava. There is a fourth relatively rare ultra-mafic category. Contrary to the name these rocks are not ultra-dark. They often appear green or lighter in color than the mafic group. The rocks in this category represent rare (to the earth’s surface) mantle rocks and are often very dense. Your instructor will show you an example of this rare type of Igneous rock.

Figure 23. Color and compositional variation within the three Igneous rock groups. Basalt and Gabbro are examples of Mafic, Andesite and Diorite are examples of Intermediate, and Rhyolite and Granite are examples of Felsic Igneous rocks. Image Steven Earle license CC by SA 3.0 opentextbc.ca

Table 1. The minerals found in each of the four Igneous rock classifications.

Figure 24. Bowens reactionary Series showing the four compositional Igneous rock groups and their corresponding mineral groups. Image Steven Earle CC BY SA 3.0 opentextbc.ca/

Using the Igneous rocks, separate them into the three-color chemical composition groups. Ultra-mafic rocks are rare, and your instructor will show you one. Once your instructor has checked your groupings use the graph below to add names to the rocks, by connecting color level and texture. PLACE THEM IN A GRID STRUCTURE.

LET’S PRACTICE IDENTIFYING COLOR AND CHEMICAL COMPOSITION CATEGORIES OF IGNEOUS ROCKS – Then we can name them.

HOW DID YOU DO?? Try to answer the following questions.

QUESTION: Which rock is both Mafic and Phaneritic (Course-grained)?

QUESTION: Felsic Igneous rocks always contain this mineral?

QUESTION: In a Granite – the pink mineral is likely to be this mineral.

QUESTION: Vesicles are created from this?

QUESTION: Which rock is both Felsic and Phaneritic (Course-grained)?

QUESTION: Compare and Contrast Granite and Rhyolite. How are they the same/different.

QUESTION: How is Obsidian formed?

QUESTION: Explain how a Porphyritic texture forms.

QUESTION: How would you distinguish a Basalt from a Gabbro?

QUESTION: What texture is represented in this image? Figure 25. opentextbc.ca license: Creative Commons 4.0.

QUESTION: Describe how this texture forms? Figure 26. Image NPFlynn author

QUESTION: What is this samples color level? Can you assume any minerals based on the color level? Figure 27. Image NPFlynn author

PART II. IGNEOUS ROCKS AND VOLCANOES IN THE NATIONAL PARKS LAB

TASK: Take out the many Igneous rocks provided by your instructor.

Separate them into two groups: Course-grained (Phaneritic) and Fine-grained (Volcanic).

Discuss the differences and what geologic condition leads to the difference.

Mix the samples again and separate them into the other distinguishing feature based on mineral and chemical composition (three color levels).

What minerals are typically found in each of the three compositional color groups.

TRY TO NAME THEM.

NOW: Lets focus on the Volcanic rocks. Put the Phaneritic igneous rocks to the side.

Using just the Igneous-Volcanic you can see that they are different in their mineral and chemical composition. This means that they formed from different types of Volcanic events, producing different volcanic landforms.

VOLCANOES ARE FORMED FROM IGNEOUS ROCKS.

ARE ALL VOLCANOES THE SAME?? Explore the two images below. How are they different?

Figure 28. Top Mount St. Helens Volcanic NM, Washington, Bottom Kilauea volcano the big island Hawai’i. Image top Author, bottom ITWFlynn both CC.

MAGMA VARIATIONS:

THE CONNECTION TO VOLCANOES, THEIR LANDFORMS AND EXPLOSIVITY

The types of volcanic eruptions and landforms produced depends on several factors relating to the magma: Viscosity, Silica content, Gas, and Temperature. The most important is the amount of silica content in the magma which is directly related to the plate tectonic setting that creates the magma. This portion of the lab will explore plate tectonic setting and the different magma types produced. Then we will connect the magma type to the explosivity level of the volcano and the resulting landform that is created.

VISCOSITY: Since magma is literally liquid rock it can flow easily or sluggishly, and this characteristic is described as viscosity (the resistance to flow) (figure 29). High viscosity (viscous) magmas resist flow and move slowly, the more silica the magma contains the greater the viscosity of the magma (thicker). The higher the silica content the more interconnected (polymerized) the minerals are resulting in greater viscosity, producing a thick sticky magma. Thick like cold peanut butter. Low viscosity magmas have low resistance to flow and as such move/flow easily. The less silica the more easily the magma flows. Therefore, viscosity is controlled by the magma’s chemical composition (silica), fluid vs. mineral content, and temperature. For example, magma viscosity increases as it cools (the magma thickens). Think of the viscosity of a magma or lava as its thickness. The viscosity of a magma strongly influences the type of volcano produced and the explosivity of the volcano.

Figure 29. A representation of different viscosities or thickness of fluids. The yellow liquid on the left is less viscous (thinner) and the clear liquid on the right. Image: https://wiki.anton-paar.com/en/basic-of-viscometry/

SILICA content and GAS CONTENT: The variation in silica content also influences whether the volcanic eruption will be quiet or violently explosive. More viscous magmas contain more complex bonds and a higher percentage of silica overall. The increased silica present in a magma result in more bonding and a thicker more viscous magma. The connection to explosivity and higher silica content begins with the fact that all magmas contain dissolved gasses, usually between 1 and 6 % water vapor (H₂O) and some carbon dioxide (CO₂). As a magma rises toward lower pressures near the surface, the gases separate (exsolve) from the magma. In the case of the low silica (mafic Basalts) melts, the gases can escape easily and bubble out of the molten material, due to their lower viscosity (flowing easily). An analogy is the ease with which bubbles can be blown in a glass of milk or thin fluid. In the case of higher silica magmas (felsic Rhyolites), gases continue to accumulate in the melt, building up pressure until it reaches a critical level, at which point it is released in a violent explosive eruption. The thicker magma prevents the easy escape of the dissolved gasses (think about a carbonated beverage erupting). An analogy is the difficulty with which bubbles cannot be blown in a thick milkshake. Therefore, thicker more viscous magmas tend to be able to hold onto and store more dissolved gas, resulting in an explosive eruption. Thicker more viscous magmas also contain greater quantities of silica bonded minerals.

EXPLORE THE VISCOUS FLUIDS PROIVIDED BY YOUR TEACHER.

TRY TO MAKE BUBBLES IN THEM BY TURNING THE BOTTLES OVER.

Can you make the same amount and size bubbles in each?

Which do you think will make a more explosive eruption? Explain.

Order the fluids from least to most viscous 1-4. Draw in the bubble size.

TEMPERATURE also plays a role in the viscosity of a magma. Mafic/Basaltic magmas tend to form at very high temperatures: 1400-1600 ⁰C. While Felsic/Rhyolitic magmas are produced at a much lower temperature 700-800 ⁰C.

HOW DO YOU THINK TEMPERATURE INFLUENCES VISCOSITY??

HOTTER MAGMAS ARE ________________________

MAFIC/BASALTIC MAGMAS ARE _____________________THAN FELSIC/RHYOLITIC MAGMAS

BECAUSE_____________________________ AND _______________________

Figure 30. An animation showing two liquids with different viscosities. Image: By Synapticrelay - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50627718

PLATE TECTONIC SETTING: Volcanoes are closely connected to the subduction of thin oceanic crust at convergent plate boundaries – such as Mount St. Helens and Mt. Rainier (Washington). Volcanoes are also formed at non plate boundaries known at Hot spots – such as Hawai’i. Very different magma types are produced at these two conditions resulting in two of the main volcano types: Stratovolcanoes and Shield volcanoes.

Figure 31. A generalized image of plate tectonic boundaries. “The Ring of Fire” results in explosive volcanic activity as well as many strong earthquakes. Image: Provenance
https://www.nps.gov/subjects/geology/plate-tectonics.htm

Why does the Oceanic crust subduct at convergent boundaries?

Figure 32. The oceanic crust is basaltic in composition which is denser than the continental crust. Image:

https://www.nps.gov/subjects/geology/plate-tectonics-subduction-zones.htm

Volcano Types - landforms

Stratovolcanoes (composite cone)

The majority of the world’s land-based volcanoes are Stratovolcanoes also called composite cone volcanoes. Their name comes from the interlayering of beds of lava with beds of cinders, ash, volcanic breccia and mudflows (lahars). The Cascade mountains contains several National Volcanic parks: From the southernmost Lassen Peak in Lassen Volcanic National Park, California, Crater Lake National Park Oregon, Mount St. Helens National Volcanic Monument, Washington, Mount Rainier National Park, Washington to Mount Baker near North Cascades National Park (figure 33).

Figure 33. The Cascade Range containing several national parks and active stratovolcanoes – Mt Rainier, Mount St. Helens and Crater Lake. Image: https://www.oregonencyclopedia.org/articles/cascade_mountain_range/

The subduction zone of the Pacific Ocean floor underneath Washington and Oregon drags water into the mantle and causing melts that are viscous and very cold (figure 34).

Figure 34. The subduction of the thin oceanic crust melts producing a viscous cold fluid rich magma. This magma creates explosive stratovolcanoes of the Cascade Range. Image:

https://www.nps.gov/para/learn/nature/magma-melts-and-eruption-types.htm

PBS. NPS.gov public domain.

Figure 35. Mount St. Helens Volcano erupting July 22, 1980. The eruption sent pumice and ash 6-11 miles into the air. Image: https://volcanoes.usgs.gov/vsc/glossary/explosive_eruption.html

Figure 36. Ash from Mt St. Helens eruption on May 18, 1980, collected 24 miles downwind. Image: https://volcanoes.usgs.gov/vsc/glossary/ash_volcanic.html

USING THE VOLCANIC ROCKS IDENTIFY THE FELSIC ASHY LOOKING ROCKS.

Carefully look for evidence of an explosive eruption: ash, small glassy charred bits, vesicles from escaping gases or flow structures from a pyroclastic flow.

Use a protractor and measure the angle of the steepness of the volcano.

Connect the Rhyolite, Pumice, Obsidian to the samples.

Now let’s look at the mafic volcanic rocks.

NON-EXPLOSIVE VOLCANIC ERUPTIONS – Hotspots - Shield cones

Not all volcanoes or magma are produced at subduction zones, though most are. Some volcanoes called hot spot volcanoes are produced by very deep in the mantle, hot, dry and silica poor magmas. As a result, they are dark in color (mafic). The deep hot spot source produces a hot, low viscosity (thin and runny), Silica poor and dry magma. This magma type is non-explosive since it is so thin and runny it cannot store any dissolved gases, so flows as large volumes of lava with out exploding. Hawai’i Volcanoes National Park on the Big Island of Hawai’i is an example of a shield volcano that is non-explosive (figures 37 & 38).

Figure 37. The main island of Hawai’i is the current volcanic location of the hotspot islands that created the islands in the Hawaiian ridge up to the Emperor Seamount chain island which are over 65 million years old. Image: https://www.nps.gov/havo/learn/nature/volcanoes.htm

Figure 38. A simplified cross-section of the main island of Hawaii hot spot producing a shield volcano. Image: https://www.nps.gov/havo/learn/nature/volcanoes.htm

FIND THE MAFIC VOLCANIC ROCKS: Basalt, Vesicular Basalt, Scoria.

Figure 39. Basaltic lava flows on the Big Island of Hawai’i. Lava flows spilled across the kolei pali escarpment near the chain of craters road. Image: Public Domain https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/thumbnails/image/Hawaii%20Volcanoes24.jpg

https://www.usgs.gov/media/images/hawaii-volcanoes-lava-flows-spilled-across-kolei-pali

Figure 40. Close-up of lava tongues forming pa’hoe’hoe flows on Kilauea is about 1600 C. Image: USGS 2002 public domain. https://www.usgs.gov/media/images/hawaii-volcanoes-a-tongue-lava

CINDER CONE VOLCANOES NATIONAL PARKS

Cinder cones are an intermediate type of volcano. They are neither explosive nor are they produced from hot spots. This intermediate type of volcano produces a very small cinder cone. It can produce lava flows, but it primarily produces loose cinder materials that are toward the basaltic type in composition. There are many National parks with cinder cone formations. You can safely hike them in an hour or so and descend into the central vent as they are generally extinct. Lassen Volcanic National Park in California, Capulin Volcano National Monument in New Mexico and Sunset Crater National Monument in Arizona are a few examples of Cinder cones in the United States (figures 41-43).

Figure 41. Lassen Volcanic National Park, California is a classic small cinder cone volcano. Image: https://www.nps.gov/articles/000/cinder-cones.htm

Figure 42. Capulin Volcano National Monument, NM is a cinder cone structure that is 100 to 150 meters built of cinders with a well-defined central vent. Loose cinders and scoria. Usually from a single or limited eruptive event. Image: https://www.nps.gov/articles/000/cinder-cones.htm

Figure 43. The slope of Sunset crater volcano national monument Arizona shows the loose cinder material. Image: John st James flickr public domain.

FIGURE 44 BELOW IS AN OVERVIEW OF THE CONNECTION BETWEEN VISCOSITY, EXPLOSIVITY AND VOLCANIC LANDFORM

Figure 44. Image: NPS Astrid Garcia https://www.nps.gov/flfo/learn/education/learning/make-your-own-lava-lamp.htm

TASK:

USING THE IMAGES AND ROCK SAMPLES SET UP AROUND THE ROOM TO IDENTIFY THE VOLCANIC LANDFORM: STRATOVOLCANO, SHIELD CONE OR CINDER CONE.

CONNECT THE ROCK TYPE TO THE SPECIFIC LANDFORM

IMAGE 1.

https://pubs.usgs.gov/of/2004/1007/volcanic.html

IMAGE 2.

https://volcanoes.usgs.gov/vsc/glossary/composite_volcano.html

USING YOUR KNOWLEDGE OF PLATE TECTONICS, WHAT TYPE OF BOUNDARY IS SHOWN HERE IN IMAGE 3 - BELOW AND WHAT TYPE OF VOLCANO WOULD BE PRODUCED? Image: https://www.nps.gov/articles/aps-18-1-7.htm

IMAGE 4. MEASURE THE ANGLE OF THE STEEPNESS OF THE VOLCANO WITH A PROTRACTOR. What type of Volcano has this steep sides? Image: KF Bull April 2009 courtesy of AVO/USGS image id 47211. https://www.nps.gov/articles/aps-18-1-7.htm

IMAGES 5 & 6. WHAT TYPE OF LAVA IS THIS? IDENTIFY THE TYPE OF VOLCANO THAT PRODUCES THIS TYPE OF LAVA. Image: Hawaii image ITWFlynn

https://www.usgs.gov/media/images/hawaii-volcanoes-a-view-mauna-loas-summit

Mauna Loas summit from the kau desert trail along highway 11. Public domain USGS classic shield cone shape.