Name: EPS 50 Lab 2: Igneous Rocks Grotzinger and Jordan, Chapter 4 Introduction In the previous lab, we learned about mineral characteristics, properties and identities as well as the three basic rock types (igneous, sedimentary and metamorphic). In this lab, we will explore the basic classification scheme for igneous rocks based on texture and silica content. We will also investigate the relationships between igneous rock types and tectonic settings. Throughout the lab, remember that in spite of our tables of characteristics and rock classifications the diversity of rock in nature is generally continuous. Objectives Observe various igneous rocks to determine their characteristic minerals and properties, and interpret their origins. By the end of the lab period you should be able to identify, group and classify the major types of igneous rocks, understand which geologic processes they imply and be familiar with some of the manifestations of these processes in California. Answers Please answer the numbered questions on the separate answer sheet handed out during lab. Only the answer sheet will be graded. Explanations should be concise a few sentences or fewer. All answers should be your own, but we encourage you to discuss and check your answers with a few other students as you work through the exercises. Identifying Igneous Rocks There are far more rock types in nature than listed in the simplified classification tables that are common to geology. It is easy to be fooled by a rock if you do not make careful observations. Initial observation of hardness, streak, cleavage planes, etc. of individual minerals in a rock provides a starting point to rock identification. In practice, these initial observations are later complemented by a detailed microscopic and/or chemical analysis of the rock to make an unequivocal identification. All igneous rocks are classified by their mineral composition and crystal size. While the minerals reflect the chemistry of the original magmas, the sizes and shapes of the crystals (texture), indicate cooling time of the magma.
Texture Texture is used to subdivide igneous rocks into two major groups: intrusive or plutonic rocks, such as granite, have crystals large enough to be seen with the naked eye, indicating a slowly cooling magma that solidified within the Earth s crust; extrusive or volcanic rocks usually have microscopic crystals, because the magma cooled quickly when exposed to air or water on the surface of the Earth. This is largely a genetic classification based on the depth of origin of the rock: plutonic at depth and volcanic at or near the surface. 1) Although plutonic (intrusive) rocks are formed deep inside the crust of the Earth, can we see them exposed on the surface of the Earth? Explain your answer. (2 pts) 2) Before beginning with rock identification, recall the chemical formulae and descriptive features (color, cleavage, hardness, etc.) which may help you recognize common mineral constituents of igneous rocks. (14 pts) Composition Some igneous rocks contain abundant iron- and magnesium-bearing minerals and less silica; these rocks tend to be darker colored. We call these rocks mafic, for magnesium (Mg) and iron (Fe). Rocks with very low silica content are called ultramafic. These include mantle rocks such as peridotite and its rare volcanic equivalent komatiite (not seen in this lab). Other igneous rocks contain abundant silica-bearing minerals and less iron and magnesium; these rocks tend to be lighter colored. We call these rocks felsic, for feldspar and silica (SiO 2 ). These are endmember cases, and rocks can have an intermediate composition as well. Because the mafic minerals are very dense, mafic rocks tend to be denser than felsic rocks. The basic igneous rock types are listed below. Basic Igneous Classification Felsic Intermediate Mafic Ultramafic Extrusive Rhyolite Andesite Basalt Komatiite Intrusive Granite Diorite Gabbro Peridotite 3) Arrange the nine igneous rock samples in order from most felsic to most mafic based on composition and classify them as intrusive or extrusive based on texture. (9 pts) Plutonic Rocks A plutonic rock may be classified based on the relative proportions of its constituent minerals. While such a classification is desirable for petrologists, the average Earth scientist relies on a
much simpler scheme based on the silicate minerals comprising the rock. This scheme requires knowing merely which mineral phases are present, but not their proportions. While not as accurate or precise as the petrological classification, this method is adequate for field/lab studies. You will find the mineral property of cleavage helpful when identifying rock hand samples. Use your hand lens to look closely at the entire specimen. Examine the crystals, noting mineral luster, color, cleavage and any other diagnostic properties you can identify. Note the overall look, the graininess, the overall color, and surface flashes of light that indicate a cleavage face. - Quartz has no cleavage and will not show the flashes when you rotate the rock. Quartz often looks greasy or waxy. Rarely will you see a complete crystal or even a shiny crystal face, but you may see the characteristic conchoidal fracture of quartz. - Feldspar has two directions of cleavage and you can easily see the flashes, but not on all crystals. Plagioclase feldspar may have very tiny stripes of thin lines. These may be visible only with a hand lens and at certain angles of reflected light. Pink grains are almost always potassium feldspar, but ascertain that you are not looking at rust stains around an iron-bearing mineral (dark in color) such as pyroxene, amphibole or olivine. This is especially important when examining coarse-grained rocks. Granite: Samples 183, 184, 216 Granite forms when a magma rich in silica cools slowly underground; the high silica content of granite generally makes the rock light in color. The granitic parent magma may be derived from melted crust. Granites can look very different, but are still called granites because they have common grain texture and composition. However, petrologists use more specific names for granitic rocks based on their chemistry. 4) Examine the suite of granites. Estimate the average percentages of felsic and mafic minerals in these specimens (refer to chart at end of handout). (2 pts) 5) What minerals can you identify in these granites? Give names and descriptions. (4 pts) Diorite: Samples 457, 507, 640 Like granite, diorite forms by slow cooling of magma within the Earth s crust. However, the dioritic parent magma is more mafic and may have formed by partial melting of mafic rock in a subduction zone. Diorite is commonly formed in volcanic arcs and at subduction zones along the edge of a continent (e.g. the Andes Mountains). Dioritic parent magma erupted on the Earth s surface is called andesite. Diorite and andesite have essentially the same composition but very different textures: diorite has larger crystals, while andesite has smaller crystals.
6) Examine the suite of diorites. Estimate the average percentages of felsic and mafic minerals in these specimens (refer to chart at end of handout). Describe two differences between the diorites and the granites that you have examined. (4 pts) Granodiorite: Samples 30, 61, 291 Granodiorite is an igneous rock intermediate between granite and diorite in composition and can form directly by partial melting or by mixing of different magmas. Granodiorite makes up the bulk of the Sierra Nevada batholith, and is therefore very important to California geology! 7) Examine the suite of granodiorites. Estimate the average percentages of felsic and mafic minerals in these specimens (refer to chart at end of handout). Given that granodiorite can closely resemble some types of granites or diorites, how would you confirm the classification of granodiorite? (4 pts) Gabbro: Samples 26, 413, 454 Gabbro is another phaneritic igneous rock, meaning that its constituent crystals are readily visible with the naked eye. It is mafic: its dark color indicates that it contains an abundance of magnesium (Mg) and iron (Fe). Gabbro forms from the same type of magma that creates basalt and scoria (more on those later). However, gabbro forms from slowlycooling magma inside the Earth. 8) Examine the suite of gabbros. Estimate the average percentages of felsic and mafic minerals in these specimens (refer to chart at end of handout). Which minerals dominate gabbro? What inferences can you make about the elemental composition of gabbro? (6 pts) Volcanic Rocks Volcanic rocks present a challenge. Since many of the mineral grains are not visible, using a mineralogical classification can be problematic. Ideally we would like to use chemical analyses. However, access to analytical facilities requires time and money, and a classification based on chemistry, although desirable, is often rather impractical. Most field classifications of volcanic rocks rely on the few macroscopic crystals we can see and the color of the rock. Slow cooling of magmas allows enough time for certain crystals to grow large within the melt. If the magma reaches the surface and erupts while it contains both melt and solid crystals, the molten portion then cools rapidly, forming a matrix (background) of tiny crystals that surround the larger, pre-formed crystals. The resulting porphyritic rock consists of a finer-grained groundmass that surrounds larger phenocrysts. You can often identify a fine-grained, porphyritic rock by identifying its phenocrysts through the rough correlations in the following table:
Phenocryst mineral Rock type Quartz Rhyolite Amphibole Andesite Olivine Basalt Potassium feldspar Rhyolite Plagioclase feldspar Andesite or basalt Note that both mafic basalt and felsic rhyolite may contain types of feldspar! Basalt: Samples 33, 263, 620 Basalt is the most abundant igneous rock in the Earth s crust. Most of the ocean floor is composed of basalt. It is a volcanic igneous rock, typically dark in color, because it contains large amounts of iron (Fe) and magnesium (Mg). Intra-plate volcanism (e.g. the Hawaiian Islands) commonly produces basalt. 9) Examine the suite of basalts. List some distinctive features of these basalts. Why do these hand samples look different from one another? (4 pts) 10) List and describe three differences between the basalts and the granites that you have examined. (6 pts) Andesite: Samples 440, 469, 494 Andesite is a volcanic igneous rock named after the Andes Mountains of South America, which contain many active volcanoes. Most continental volcanoes over subduction zones are composed of andesite in the form of either lava or pyroclastic material. 11) Examine the suite of andesites. Describe any visible minerals and try to identify these minerals. (4 pts) 12) Notice that andesite is not as dark as basalt. What does this indicate? (2 pts) Rhyolite: Samples 16, 17, 88 Rhyolites are extrusive igneous rocks that are extremely enriched in silica (>70%). As a magma, rhyolites are highly viscous due to polymerization of silica. They may be ejected violently from volcanoes as tephra (ash) or may form flows.
13) List and describe three differences you expect to observe between basaltic and rhyolitic lava flows in the field (hint: what are the consequences of differing viscosities)? (6 pts) 14) What characteristics distinguish rhyolitic lava from a rhyolitic tephra deposit? (2 pts) Obsidian: Samples 22, 535, 650 Obsidians are quenched magmas that cool too quickly for minerals to form and are therefore glassy (amorphous) in texture, although microscopic minerals called microlites may form during the rapid cooling. In general, one cannot distinguish mafic from felsic volcanic glass by sight. However, volcanic glass is generally dominated by SiO 2 and therefore is commonly classified as rhyolite. Be careful: the obsidian specimens might be sharp. 15) What type of fracture does obsidian have? (2 pts) 16) Most obsidian is black (despite being a felsic rock), but obsidian can also appear red, brown or grey. Speculate about what properties lend obsidian its observed color. (2 pts) Pyroclastic Rocks During many volcanic eruptions, explosions blast and convection columns of hot gases carry fragments of lava or rock into the air. These fragments fall back to earth on and near their source vent, forming a pyroclastic fall or tephra deposit. Explosive eruptions that produce voluminous tephra deposits often also produce pyroclastic flows. Pumice and scoria: Samples 142, 262 Both pumice and scoria are hot, welded, medium-sized pyroclastic ejecta. As magma approaches the earth's surface, volatiles (gases, including H 2 O) that were dissolved in the melt at higher pressures begin to exsolve, forming bubbles, and the viscosity of the magma increases. Silica- rich magmas are more viscous than basaltic magmas and tend to be more explosive upon gas exsolution. Pumice (sample 142) is the term generally used for the felsic version. Scoria (sample 262) looks similar to pumice and forms in a similar fashion but contains more iron (Fe) and magnesium (Mg) and less silica. It is the explosive equivalent of basalt. Both pumice and scoria may be glassy and may also commonly contain crystals, glassy shards and crystal fragments of minerals that were formed prior to eruption. 17) Examine the pumice sample with your hand lens and sketch what you see. Contrast this with the scoria sample. Include labels in your sketches. (6 pts) 18) Explain the differences in vesicle size as a function of magma viscosity. (3 pts)
Texture Terminology The texture of an igneous rock does not refer to the roughness or smoothness of the surface, but rather to the rock s crystalline properties, including grain size. Grain size is controlled largely by how fast or slow the melt cools when it solidifies. Finegrained rocks are aphanitic, whereas coarse-grained rocks are phaneritic in texture. A variety of textures to become familiar with are listed below. Pegmatitic: composed of very large crystals (larger than 2-3 cm). Phaneritic: entirely composed of crystals that are large enough to see but smaller than pegmatitic texture. Porphyritic: composed of crystals of two different sizes: large crystals called phenocrysts are visible, while smaller crystals that make up the groundmass are not. Aphanitic: a fine-grained texture with crystals too small to see without a microscope. Glassy: composed entirely of glass; few if any crystals are visible. Vesicular: formed when lava solidifies before gas bubbles escape; the result is a "bubbly" appearance. Pyroclastic: composed of volcanic fragments; these fragments or clasts can be very fine ash, coarse lapilli or very coarse bombs and blocks. 19) Examine the diverse textures of the seven samples. Ascribe a texture to each sample and note whether the texture of each sample is indicative of an intrusive or extrusive origin. (10 pts) Bowen's Reaction Series Much of our basic understanding of igneous rocks is based on the original work of a scientist named Norman L. Bowen who experimentally determined the melting behavior of silicate minerals at near-surface pressures. For now you only need to consider his 'first order' result: when you slowly cool a silicate melt similar in composition to common igneous rocks, the common rock-forming minerals appear at progressively lower temperatures in the following order:
One can use this relationship to place sets of related igneous rocks into a differentiation series of rocks that formed by progressive crystallization upon cooling of a high-temperature 'parent' silicate melt. Become familiar with Bowen's reaction series. Minerals common in large amounts that crystallize early in the Bowen sequence are most likely to be found in all igneous rocks and provide immediate knowledge about the relative timing and rate of magma crystallization. 20) In #3 you arranged nine samples from most felsic to most mafic. If you did this correctly, they are also ordered from the lowest-temperature to the highest-temperature in terms of Bowen's reaction series. Given the qualitative nature of your observations, consider the major-element composition of the common minerals and describe how the chemistry of these samples changes as a consequence of progressive crystallization. In other words, what happens to the major-element chemistry? (6 pts) 21) Based on Bowen's reaction series and your knowledge of basalt mineralogy, explain why we do not expect to find quartz in basalt. (2 pts)