Igneous Rocks and the Rock Cycle

OBJECTIVES

• Distinguish between minerals and rocks
• Recognize the identifying features of each rock type
• Understand the two characteristics that define igneous rocks
• Distinguish the unique textures and formation of these textures in an igneous rock
• Identify and name the dominant minerals in the common igneous rocks
• Recognize and name some of the common igneous rocks

MATERIALS

• Ruler, glass plate, hand lens, sedimentary particle size chart, several minerals: Olivine, pyroxene, amphibole, biotite mica, muscovite mica, two feldspars, and quartz (8 from the mineral lab)
• Suite of Igneous (Granite, Diorite, Gabbro, Rhyolite (ashy tuff), Porphyritic Andesite, Basalt, Pumice, Scoria, Vesicular Basalt, Obsidian), Sedimentary (Sandstone, Fossil Limestone) and Metamorphic rocks (Schist, Gneiss) - provided by your instructor

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 eruption from an explosive volcano. Some of you may have been lucky enough to have seen rocks form at volcanoes in Hawai’i.  As a result of the unique geologic processes that create each of the three rock types you can use these processes to help construct the history of the sample.  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 can identify a specific type of rock, 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) (Figure 1). 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, Luster.  Examples of minerals are quartz, feldspar, mica, amphibole, pyroxene, olivine, calcite, and halite.  

ROCKS ARE AGGREGATES OF MINERALS.

Figure 1. Granite is an igneous rock typically composed of the minerals, Amphibole (Hornblende - H), Quartz (Q), Feldspar (F), and Mica. Image from opentextbc.ca/geology license: Creative Commons 4.0  S. Earle.

Let’s Practice Comparing Rocks and Minerals

Your instructor will provide you with a variety of minerals that you have seen in the past few labs combined with a variety of new Igneous rocks.  Practice looking at the distinction between rocks and minerals. Try to identify and name the minerals.  Can you see any minerals in the Igneous rocks?

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.

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.

IDENTIFYING ROCKS

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 rock may include only one type of mineral such as Coal).  

IGNEOUS ROCKS

Igneous rocks form from magma (molten rock) that has either cooled slowly underground (Granite) or cooled quickly at the surface after a volcanic eruption (Basalt) (figures 3, 4, 5).  As a result of a variety of cooling environments Igneous rocks have a variety of textures and grain sizes.  The distinguishing feature of most igneous rocks is the large (visible) interlocking/crystalline arrangement of their mineral grains OR the fine grained-ashy texture indicative of a volcanic eruption.  When molten rock, (magma) cools slowly deep inside the Earth the grains can grow large or visible to the unaided eye.  When molten rock, (lava) erupts on the surface it may produce a very fine-grained (not visible to the unaided eye) vesicular, ashy or glassy texture. Igneous rocks also have a variety of silicate minerals such as Quartz, Feldspars, Micas, Amphiboles and Pyroxenes.  The combination of different silicate minerals produces a variety of color levels seen in Igneous rocks.

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. Image CC BY NPFlynn.

Figure 4. Volcanic Basalt showing stretched vesicular texture formed from 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.

Figure 5. Diagram representing some igneous magma locations and shapes relative to the surface: a, b, c, and d represent magma deep within the Earth.  The letter e represents magma that has erupted on the surface to form a volcano.  Image: opentxtbc.org Creative Commons Attribution 4.0 International License, author S. Earle.

SEDIMENTARY ROCKS

Sedimentary rocks, such as Sandstone, form when the weathered products (silt/sand/pebbles) of other rocks accumulate at the surface and are then buried by other sediments. The sediment produced by surface weathering is transported by erosional agents such as wind, water or glaciers and deposited in new locations, such as lakes, beaches, and deltas (figure 6).  The transportation and deposition of the sediment can leave distinctive layers that can aid in the identification of sedimentary rocks.  Fossils and organic material (shells) can also accumulate in sedimentary rocks. If the rock looks like it is made up of bits of silt or sand or fossils or has depositional layers it may be a sedimentary rock (figures 7 & 8). Many sedimentary rocks form in marine environments such as lakes and the oceans.  Evaporation of the water can leave deposits of the formerly dissolved elements such as minerals like gypsum and halite (figure 9).

Figure 6. Common depositional environments where sediments accumulate.  The specific sedimentary environment influences the types of particles that will deposit.    Image CC BY S.A. 4.0 international license. Johnson, Affolter, Inkenbrandt, Mosher. opengeology.org/textbook. Modified by S. Earle opentextbc.ca.

Figure 7. Fossiliferous Limestone. A sedimentary rock comprised of brachiopod and bryozoan fossils from the Kope Formation of Ohio. These fossils formerly lived in a marine environment.  Image opengeology.org CC BY S.A. 4.0 International license: Johnson, Affolter, Inkenbrandt and Mosher.

Figure 8. Depositional layers seen in the sedimentary rocks of the Badlands National Park, SD. Notice the horizontal layers exposed by erosion. Image CC BY NPFlynn.

Figure 9. Massive Halite (salt) deposits in Utah near the Great salt lake area of the Bonneville Salt flats.  This type of sedimentary rock material forms from the evaporation of formerly salted marine waters. Image CC BY NPFlynn.

METAMORPHIC

Metamorphic rocks form when either igneous, sedimentary, or metamorphic rocks are heated and squeezed to the point where some of their preexisting minerals become unstable and new minerals form to create a different type of rock. Forces such as heat and pressure often create a unique banding texture as the rock is being folded and reformed (Figure 10). This texture is called foliation and looks a lot like layers.  Also, certain unique minerals may form during the heating and pressure process such as the micas and possibly garnets. These new minerals are known as Index minerals and can indicate the range of temperature and pressure needed to form the new metamorphic rock. Plate tectonic forces are the primary mechanism leading to metamorphism (figure 10).  Metamorphic rocks, such as Schist and Gneiss show very distinct features and minerals (figures 11 & 12).

Figure 10. Regional metamorphism of the subduction of oceanic crust under continental crust by temperature and increased pressure (left). Metamorphism of the ocean crust by (hot water) hydrothermal processes (right). Image opentextbc.ca. Creative Commons BY 4.0 International license, S. Earle

Figure 11. Schist is a metamorphic rock typically composed of the minerals quartz, feldspar, mica and garnet.  Notice the foliated (looks like layers) texture due to the compressive forces of metamorphism and the shiny appearance. Image author.

Figure 12.  Notice the banded foliation of this metamorphic Gneiss from the Wissahickon Valley area, PA. This sample was squeezed and heated to the level that it began to partially melt and bend like taffy over long periods of geologic time. Image CC BY NPFlynn.

When trying to distinguish between the three rock types there are a few features to look for.  

1. Can you see the minerals/grains with your naked eye, or do you need a microscope? Are particles all the same size or a mixture of different sizes?
2. Arrangement of mineral grains is a property that you will eventually use to tell if the rock is igneous, sedimentary, or metamorphic in origin.
3. Are there organic materials such as fossils present?
4. Are individual particles cemented together?
5. Are there holes in the rock from the escape of gas bubbles? Does the rock have a squashed look?
6. Do the minerals in the rock appear to have a preferred alignment?

NOTE: In geology, the texture of a rock refers to the characteristics of the material that makes up the rock, as opposed to the feel of the outer surface. Rock texture is the size of the minerals (or fragments), their shape, and how they are stuck together (Figures 13-16). The texture of the rock is perhaps the most important tool used to determine whether the origin of a rock is igneous, sedimentary or metamorphic. The texture reflects the geologic processes involved in the formation of the rock.

Igneous, Sedimentary, and Metamorphic Textures

Careful examination of rock texture places most (not all) rocks into one of three categories that depend on how they formed

Interlocking crystals Common texture: crystalline

Rock crystallized from magma

IGNEOUS

Particles cemented together Common texture: clastic

Lithification of sediment produced rock - SEDIMENTARY

Banded minerals Common texture: foliated

Rock subjected to increased pressure & temperature causing parallel alignment of minerals METAMORPHIC

Figure 13. Clastic texture - Conglomerate.        Figure 14. Foliated banding - Gneiss.

Figure 15. Interlocking Crystalline texture - Granite. Figure 16. Porphyritic texture – Prophyritic Basalt. Images in Figures 13,14,16 from opentextbc.ca license S Earle: CC 4.0 Image 15 CC BY SA 3.0 Wikimedia Mikenorton, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Let’s Practice Distinguishing the Three Rock Types

Your instructor will provide you with several Igneous, Sedimentary and Metamorphic rock samples.  Practice using the rock cycle to separate them. Write a few comments about each of the three groups.  Can you distinguish the three groups?

Igneous rocks: Granite, Pumice, Obsidian, Porphyritic Andesite, Basalt

Sedimentary rocks: Sandstone, Fossil Limestone

Metamorphic rocks: Schist, Gneiss

IGNEOUS ROCKS

Igneous rocks originate from molten rock - magma. “Igneous” is derived from the Greek word for “fire” (think of ignite). Molten rock underground is called magma, and molten rock that erupts at the surface is called lava. Lava and pyroclastic ash are associated with surface volcanoes.  Cooling of magma and lava produces igneous rocks (Figure 17).

Magmas can flow easily or sluggishly, and this characteristic is described as their viscosity (resistance to flow). High viscosity (or viscous) magmas resist flow and move slowly. Low viscosity magmas have low resistance to flow and move easily. Viscosity is controlled by the magma’s chemical composition, 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.

Figure 17. Lava flow in Hawaii. The black rock is called Basalt.  The red/orange flow is liquid lava, that will cool to form a fine-grained basalt. Imagehttps://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

Igneous Rock Classification

Igneous rocks are typically classified by TWO features: their texture (crystal size and arrangement) and chemical composition (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.

Igneous Textures

The texture of an igneous rock reflects how the magma cooled and crystallized to form minerals. The size of the crystals depends on the cooling rate:

        Coarse-grained textures (Phaneritic) indicate slow cooling (tens of thousands of         years) underground.

Fine-grained (Aphanitic), pyroclastic (Ashy), Vesicular and glassy (Quenched) textures indicate fast cooling (days to hours or minutes) on the Earth’s surface and produces a wide varieties of appearances.

Porphyritic, (a two-stage cooling process), indicates slow initial cooling underground, followed by rapid final cooling on the surface.

Texture is used to indicate whether the magma cooled at the surface (volcanic/extrusive) or deep underground (plutonic/intrusive) (figure 18).  

Figure 18.  Block diagram showing Intrusive magma cooling locations at 1, 2, 3, A – E which produce coarse-grained phaneritic igneous rocks, as they cool slowly over thousands of years.  Locations 4 and F produce fine grained aphanitic rocks as they cool rapidly on the Earth’s surface.  Image CC BY S.A. 4.0 International License Opengeology.org/textbook – public domain.

The following are common Igneous textures:

Coarse-grained (Phaneritic): Interlocking crystals (typically 1-10 mm) that can be seen with the naked eye. Great thickness of overlying rock insulated the magma, so that it cooled slowly to form large crystals. Igneous rocks exhibiting this texture cooled deep underground (Figures 3, 15 & 19). Examples of common Phaneritic – course-grained igneous rocks include: Granite, Diorite and Gabbro.

Figure 19. Coarse-grained Phaneritic texture seen in two samples of Granite.  Notice the crystalline interlocking grains of the many individual minerals. Image on the left: opentextbc.ca license: Creative Commons 4.0. Right Image: Granite Mt Rushmore NM, SD. CC BY NP Flynn

Fine-grained (Aphanitic): 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. Most fine-grained igneous rocks cooled at the Earth’s surface after being erupted from a volcano, producing ashy material. 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 (Figure 18). Examples of fined-grained aphanitic igneous rocks include: Rhyolite, Andesite and Basalt (Figure 20).

Figure 20. Fine-grained Aphanitic texture seen in Basalt left. The right image represents a fine-grained Rhyolite. Image left. Basalt James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons. Right image Rhyolite https://upload.wikimedia.org/wikipedia/commons/7/7e/RhyoliteUSGOV.jpg

Pyroclastic (Ashy): When molten material is erupted from a volcano it can solidify before it hits the ground, yet still be hot enough to weld to other erupted material (Figure 21). This forms a rock called tuff, which is composed of fragments (of other rocks) and crystals, all embedded in a matrix of ash. A light weight ashy/powdery rock can also form.

Figure 21. Ashy Pyroclastic texture of two Rhyolites. Welded fragments and flow banding are seen in the image to the left from 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 (plutonic rocks don’t have vesicles). This term can be used as an adjective (e.g. this is a vesicular basalt). The size of the bubbles is often dictated by the viscosity 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 22 - left).  Scoria has larger, less abundant bubbles in a denser reddish-black rock - scoriacous texture (figure 22 - middle). Basalt also forms with a vesicular texture and is given the name Vesicular Basalt (figure 22 - right).

Figure 22. 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. Images: Pumice and Vesicular Basalt CC BY SA 4.0 International JukoFF https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Image Scoria Craters of the Moon NM Idaho CC BY NPFlynn.

Glassy (Quenched): Volcanic glass is called Obsidian and lacks visible crystals. Rocks that are very glassy show a characteristic conchoidal (curved) fracture pattern (Figure 23). 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 23. Obsidian showing glassy texture and flow banding from Mono-Inyo craters CA and right conchoidal fracture. Image: left 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: An igneous rock texture that is composed of two different distinct crystal sizes (Figures 16 & 24). 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.

Figure 24 Porphyritic textures showing larger slowly cooled phenocrysts inside of a uniform fine grained rapidly cooled matrix of an Andesite. The large phenocrysts are the mineral hornblende in image A and plagioclase feldspar in image B.  Image: oer.galileo.usg.edu/geo-textbooks/1 CCBYSA 4.0 international Karen Tefend  

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?

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 types of minerals present. To generalize, a darker color 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 color indicates the rock contains minerals with high silica (SiO2, mineral name Quartz) 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, or nearly black andesite may indicate the lack of crystals (Figure 18). 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) (Figures 25 and 26). 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 27). This graph reveals that most igneous rocks are composed of similar minerals 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 25. Some minerals present in the four classifications of Igneous compositions.

Figure 26. Color and compositional variation within the three Igneous rock groups. Basalt and Gabbro are examples of Mafic Igneous rocks, Andesite and Diorite are examples of Intermediate Igneous rocks, and Rhyolite and Granite are examples of Felsic Igneous rocks. Image Steven Earle license CC by SA 3.0 opentextbc.ca

Figure 27. 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/

Let’s Practice Identifying Color and Chemical Composition Categories of Igneous Rocks–Then we can name them.

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.

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?