ESS 210 Lab 5: Igneous Processes

Name: ______

Lab 5: Igneous Processes

Geologic material is constantly being recycled into different compositions and textures through a set of processes known as the rock cycle, illustrated below. The three major types of rock are igneous, sedimentary, and metamorphic. These rock types differ in both (1) source material and (2) the processes leading to their formation, both of which are constrained by the paths shown on the cycle. Therefore, identifying a rock’s type will enable us to infer the general history of a specimen. Note that rocks do not necessarily follow the path around the outside of the circle; they can follow any of the short circuits from one stage to another. As will be shown in the next several labs, a more precise knowledge of a rock’s history may be obtained from a more detailed classification scheme.

Igneous rocks are those that form directly from the cooling and solidification of molten rock, or magma. The composition and location of this magma at the time of solidification will determine the features of the resulting igneous rock. Therefore, description of the texture and composition of an igneous rock provides information about the processes that formed that rock. This lab introduces the common textures and compositions of igneous rocks and their relationship to igneous processes.

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Igneous Environments One comprehensive model for the occurrence of the various types of igneous rocks is based upon plate tectonics (A detailed account is included in your textbook.). The convection of the mantle and the corresponding movement of lithospheric plates produce upwelling of molten material at areas such as mid-ocean ridges, and result in burial of material at subduction zones to depths where it undergoes partial melting. Specific tectonic processes are confined to certain portions of the convection cycle, so the characteristic igneous rocks associated with each process are typically found in similar environments wherever they occur.

In particular, the composition of igneous rocks is sensitive to the environment in which they form. The primary constituent of most magmas is silica (SiO2), and the precise amount of silica in a magma exerts so much control on its behavior that the classification of magma into its common types is based on this criterion. Mafic magmas contain about 50% silica, tend to flow easily, and have high density. Mafic magmas are most often associated with mantle sources, such as mid-ocean ridges or oceanic hot spots. An example of a mafic rock is basalt. Felsic magmas contain about 70% silica, flow very poorly, and have low densities. These felsic compositions are associated with continental crustal sources, such as melted crustal rocks above continental hot spots or in subduction zones. An example of a felsic rock is rhyolite. Intermediate magmas contain about 60% silica and have intermediate densities and viscosities. These compositions are often associated with subduction zones where oceanic crust is partially melted and assimilates continental crustal material as it rises to the surface. The classic example of an intermediate rock is andesite.

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Bowen’s Reaction Series Common igneous minerals can be arranged according to their order of crystallization, as summarized in Bowen’s reaction series. Minerals with high crystallization/melting temperatures (around 1200°C) such as olivine and Ca-rich plagioclase, are at the top. These are the mafic minerals, rich in iron, magnesium, and calcium, and tend to be dark (green to black). Minerals with low crystallization/melting temperatures (around 600°C) such as quartz are at the bottom. These are felsic minerals, rich in potassium, aluminum, and silica, and tend to be lighter in color (pink to white to colorless). Igneous rocks are classified as mafic, intermediate, or felsic based on the abundances of these different types of minerals.

600° C

Visual guide to estimating percentages of mafic minerals.

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Igneous Structures The texture of an igneous rock – size and shape of mineral grains in the rock – is a result of how and where it crystallizes. Magmas that solidify underground are called intrusive and produce plutonic rocks. Magmas that reach the surface before solidifying (lavas) are called extrusive and form volcanic rocks.

Intrusive Rock Structures and Textures Intrusive magmas cool slowly, insulated by the surrounding rock, which provides time for their minerals to grow large enough for individual crystals to be seen with the unaided eye. This coarse-grained igneous rock texture is known as phaneritic. When and intrusive magma completely solidifies into a large body of plutonic rock, the rock structure is known as a pluton. Plutons are classified by their geometry, orientation, and size. A planar body that cuts through the layers of surrounding rock (country rock) is called a dike. Dikes usually mark the channels along which magma flowed upward. Cylindrical channels that feed volcanic vents are termed volcanic pipes or necks. Planar bodies that have intruded parallel to the surrounding rock layers are sills. If a sill flexes the overlying material upward to form a dome, then it becomes a laccolith. Very large plutons, which often serve as source reservoirs for all of the above features, are called batholiths. F

A. Pluton B. Laccolith C. Sill D. Dike E. Volcanic neck or feeder F. Lava flows or pyroclastic deposits

Extrusive Rock Structures and Textures Magmas cool quickly upon exposure to air or water (most present-day volcanic activity is submarine) so that only small crystals form, leading to a fine-grained, aphanitic texture. If the magma cools so rapidly that no crystals form, then the texture is said to be glassy. If the magma produces gas bubbles that fail to escape during cooling, then the resulting frothy texture is called vesicular and the bubbles are known as vesicles. Basaltic lavas have low viscosity, require only low pressures to erupt, and tend to flow gently from their vents and spread widely into low, broad structures known as shield volcanoes, such as Kilauea in Hawaii. Andesitic and rhyolitic lavas, with their higher viscosity, require higher pressures to force an eruption. This usually culminates in a sudden catastrophic explosion, often ejecting material into the atmosphere where it cools and descends as pyroclastic debris. This material forms a steep pyroclastic cone around the vent. If eruptions continue to occur from the same vent, repeated layers of flows and pyroclastic debris will accumulate into a stratovolcano such as Mount Rainier.

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Complex Cooling Histories and Texture Magmas can have complex cooling histories, during which different minerals will nucleate (begin to form) and grow at different pressures and temperatures. This can lead to rocks with crystals of more than one size. This multi-modal texture is porphyritic. For example, a basaltic magma may remain underground cooling slowly, forming large olivine crystals (olivine is stable at high temperatures so it crystallizes early in the cooling process). If the magma, with its large olivine crystals, then erupts so that the remaining melt cools rapidly, then the other minerals will be small. The large crystals are called phenocrysts and the smaller grains constitute the groundmass. The order and rates of crystallization for the minerals in an igneous rock can often be determined by textural relationships such as this.

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Part I. Igneous environments and compositions

1. List the minerals you recognize in each of the following specimens. Determine the percentage of mafic minerals. Your eyes will tend to emphasize the dark-colored minerals, leading you to overestimate their abundance. Use the abundance diagram to help compensate for this. Identify each specimen as mafic, intermediate, or felsic (composition).

Specimen Minerals Present Percent Mafic Composition Minerals

1

2

6

2. Mt. St. Helens is now spewing ash with an intermediate composition and has in the past produced lavas ranging from rhyolitic to basaltic in composition. What does this indicate about the regional tectonic environment of the Cascades and why?

3. What type of lava is most common globally: basaltic, andesitic, or rhyolitic? What are two distinct reasons for this? (Hint: One is related to tectonic environment and source, the other to a physical property of the magma.)

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The Palisades Sill, located in New Jersey, is a classic natural example of fractional crystallization.

4. Use Bowen’s reaction series and the figure above for the following questions:

A. What is the overall composition of this sill (mafic, intermediate, or felsic)?

B. Using the letters on the figure, fill in the order of crystallization of the different units:

Crystallized first ______Crystallized last

C. After the olivine layer crystallized, did the remaining melt have a similar, more mafic, or more felsic composition than the original melt? Briefly explain.

D. After the basalt layers crystallized, did the remaining melt have a similar, more mafic, or more felsic composition than the original melt? Briefly explain.

E. At approximately what temperature did the sill finish crystallizing?

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Part II. Igneous structures and textures

5. Identify textures (aphanitic, phaneritic, porphyritic, vesicular, glassy), cooling rates (slow, moderate, fast, two-stage), and origin (intrusive, extrusive, intrusive-extrusive) for the following samples.

Specimen Texture Cooling Rate Origin

1

2

3

6

7

10

11

6. Sample 10 has a felsic composition. Why is it so dark?

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7. Use the following figure to answer the questions: F

a. Which unit will have the largest crystals? Why?

b. Which unit will have the smallest crystals? Why?

c. Superficially, a sill (C on the figure) exposed on the surface and a lava flow (F on the figure) can look very similar: a horizontal or nearly horizontal “sheet” of igneous rock. Describe two ways to tell the difference between them

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Part III. Synthesis

8. Based on the compositions from Question 1 and the textures from Question 5, choose a likely setting for each sample from the diagram below (pick a letter).

Specimen Setting

1

2

6

8

10

10