ES088 The Earth s Crust: Rocks Earth system forensics might well start with a disciplined look at materials and how they are formed. When you are given a piece of rock, it is important to remember that it is made of minerals or other naturally occurring materials. These materials help indicate the environment or conditions in which the rock formed. When early geologists looked at rocks and processes, they determined that some things were formed as the result of the cooling of hot liquids and related processes. These rocks were called igneous literally of fire because the liquids were hot enough to ignite grass, shrubs and trees as they flowed down the sides of volcanoes. In a simple igneous system, one in which the crystals grow from a hot, molten material called magma, crystalline contents within a liquid are under uniform stress. Because stress is uniform, the crystals grow in random orientations (Figure 1). As more and more crystals form they tend to form an interlocking texture (Figure 1). When cooling is very slow, large crystal grains some as large as a metre or more - can develop. At the other extreme, when cooling is extremely fast, liquids can be quenched, ultimately forming a natural glass without any crystal structure. Igneous rocks are formed in a continuum between these two extreme ends. Intrusive (also called plutonic) igneous rocks are formed within the crust and have crystal grains that are visible to the naked eye. The resulting texture is called phaneritic texture. Extrusive (also called volcanic) rocks are formed at, or on the earth s surface. The smallest mineral grains in extrusive igneous rocks are too fine to see in detail, even though some may be present. The resulting texture is called aphanitic texture. When magma migrates upward, crystals formed in the initial stages of cooling may be carried in it. A change in cooling rate can lead to the formation of crystals from the remaining magma, resulting in a rock with crystals of two markedly different sizes (large crystals formed during slow early cooling and small crystals formed during fast late cooling). Porphyritic texture is the result of this process. Porphyries are formed in both volcanic and plutonic igneous environments. The size of the smaller grains indicates the environment in which crystallization was completed. Figure 1. Random orientation of crystal grains in an igneous rock, granite.
Sedimentary rocks are formed from sediment - any material deposited at or near Earth s surface from a fluid such as liquid water, air, or ice. Sediment includes deposits of mineral or rock grains derived from pre-existing rock that has been weathered and eroded. However, some forms of sediment can be formed by the precipitation of minerals from solution (e.g. crystallization of salt from seawater), or can be composed of fossilized skeletal remains of organisms. Traces of biological activity may also be apparent in tracks and trails formed within the sediment. Over time, as the sediment grains are pushed together, water is squeezed out of sediment pores, and dissolved minerals are deposited in the pores (effectively cementing the grains together), a sediment body becomes sedimentary rock. Sedimentary rocks composed of fragments derived from pre-existing rock are called clastic sedimentary rocks. Such rocks grouped into categories according to the dominant grain size of their constituent sedimentary particles shale (dominated by clay), siltstone (dominated by silt), sandstone (dominated by sand) and conglomerate (dominated by granules, pebbles, cobbles, or boulders). Sedimentary rocks composed of minerals that were precipitated from solution (usually seawater) are called chemical sedimentary rocks. Chemical sedimentary rocks include some forms of limestone (containing the mineral calcite), dolostone (containing the mineral dolomite), and rock salt (containing the mineral halite). Sedimentary rocks that are largely composed of the remains of once-living organisms are called biogenic sedimentary rocks, and include some forms of limestone (e.g. fossiliferous limestone), and coal (made of organic matter derived from land plants) Sedimentary rocks indicate something of the environment in which they were deposited and the transporting medium. For example, clastic sedimentary rocks containing many types of minerals or rock fragments indicate that the constituent sedimentary particles had not undergone much transportation or sorting prior to being deposited. On the other hand, clastic sedimentary rocks containing one type of mineral (often quartz) generally indicates long travel distances or lengthy periods of agitation. Structures found in clastic sedimentary rocks such as ripples or crossbedding (inclined laminations) indicate significant agitation by waves or currents, whereas rocks dominated by clay and preserving delicate fossils indicate very little water movement during deposition (e.g. Figure 2). Furthermore, the presence of chemical sedimentary rocks, such as rock salt or gypsum, can indicate a warm environment with high evaporation rates and quiet-water conditions. Figure 2. A fossil fern found in black shale.
Metamorphic rocks are formed from previously existing rocks that have undergone a change in form due to increased temperature, pressure, or both. Metamorphic minerals may form as a result of break down, or reaction of earlier minerals. Like igneous rocks, metamorphic rocks generally have a crystalline appearance. However, metamorphic rocks can contain minerals that do not tend to form in igneous environments (e.g. calcite in the rock marble, inherited from its parent rock limestone). More obvious are metamorphic rocks that show a preferred orientation of crystals, resulting from crystal growth under conditions of increased pressure and temperature. The preferred orientation of platy/elongate mineral grains and banded appearance of some metamorphic rocks is called foliation. As discussed below, there are different types of foliation. Figure 3. Gneissic banding. The foliation is indicated by the alignment of the black mineral grains in the dark bands,.
Identification of igneous rocks Chart 1 on the previous page provides a basic overview for identifying igneous rocks. The top of the page represents the highest temperatures at which minerals crystallize from magma. It is generally true that high temperature rocks generally contain lots of iron and magnesium and tend to be dark in colour. The latter is not always dependable though. Rely on your ability to identify minerals where possible. Notice that there are mineral names associated with different temperatures. Turn the page sideways and notice also that the terms intrusive and phaneritic are aligned with four rock names. Similarly, the terms extrusive and aphanitic are aligned with three other rock names. Phaneritic rocks have all grains within the visible size range (e.g., Figure 1). In aphanitic rocks the smallest grains are too small to see with out aid. Identification of sedimentary rocks Sedimentary rocks are sub-divided into clastic, chemical and biogenic. Clastic rocks, as described above, are composed of mineral or rock particles derived from pre-existing rocks and are classified on the basis of their dominant sediment grain size. The basic categories are shale (rock equivalent to clay), siltstone (rock equivalent to silt), sandstone (rock equivalent to sand) and conglomerate (rock equivalent to gravel). On a finer level, the mineral composition and grain shape of clastic rock components also factor into the name of the rock (e.g. jasper pebble conglomerate or quartz sandstone). Chart 3 below summarizes the characteristics of clastic sedimentary rock. Use this as a basic guide to identify clastic sedimentary rocks; you can estimate the size of the grains, based on the overall appearance of the rock (does it look like it is made of clay, silt, sand or gravel?). Chemical sedimentary rocks are formed by changes in the environment due to changes in the characteristics of the liquid from which they precipitate. Such changes usually occur due to increases in the concentration of certain components in water (for example, increased salt content due to evaporation). Chemical rocks are characterised by interlocking crystal grains that might be confused with an igneous texture. The mineralogy however is different. Chemical rocks include rock salt (composed of halite, alabaster/rock gypsum (composed of gypsum), some limestone (composed of calcite), (composed of dolomite) and chert (composed of very fine grained quartz). Biogenic rocks are formed by biological processes that primarily involve the organisms secreting shell material (calcitic remains such as shells then forming fossiliferous limestone), or in the case of land plants, leaving all of their tissues behind and producing coal. As with limestone, chert can be deposited as either a chemical sediment or biogenic sediment. In its biogenic form, its is largely composed of the silicabearing shells of micro-organisms such as diatoms and radiolarians, as well as some sponges. It is important to note that chert can look like limestone or shale, but unlike the latter rock types, will scratch glass and a streak plate; this is because of its quartz content. Chert also has a conchoidal fracture, just like its constituent mineral quartz. It should also be pointed out that with few exceptions, fossils are restricted to sedimentary rocks (sediments can only accumulate at or near Earth s surface). So if you see the remains of organisms in a rock, you can be more or less certain that the rock is sedimentary. Also, sedimentary rocks commonly show some layering (as one would expect for layering of sediment), and occasionally preserve tracks and trails of organisms, wave ripples, and other surface features.
Identification of metamorphic rocks Metamorphic rocks may be divided roughly into those with and those without foliation. Remember that foliation is the term used to describe the alignment of platy and/or elongate mineral grains, and in the case of the rock gneiss, the presence of alternating light- and dark-coloured bands. In metamorphic rocks devoid of platy/elongate minerals (i.e., are, instead composed of blocky, equidimensional mineral grains) foliation cannot develop and are referred to as being granoblastic. Distinguishing such rocks from rocks of other classes can be tricky, but keep in mind the more crystalline and denser appearance of metamorphic rocks relative to sedimentary rock and distinctive mineral content (e.g. calcite in marble a rock produced by metamorphism of limestone, and sugary-looking quartz crystals in quartzite a rock produced by metamorphism of quartz sandstone). Use chart 4 to identify metamorphic rocks that display a distinct foliation. This chart shows what happens when shale (a sedimentary rock) undergoes increasing temperatures and pressures. The earthy appearance turns into something dull (as seen in slate) and then it begins to exhibit a shinier lustre (as seen in phyllite) as mineral grains become larger and much more aligned. At medium grade, a very obvious sparkle appears because mineral grains are large enough to discern with the naked eye (producing a glittery-looking rock called schist) At high grade, not only do the mineral grains sparkle, but distinct banding is seen (as seen in rock type gneiss; e.g. Figure 3).
ES088 Rock EXERCISES Name Student # Part I: Use map 1 in order to locate the rocks to be classified in this part. You will find rock names associated with these specimens. Your job is to place each into its proper class. Map 1. Specimen Boulder collection St. Marys Cement Rock Garden. 1) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 2) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 3) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 4) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 5) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 6) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 7) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 8) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1)
9) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 10) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 11) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 12) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 13) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) 14) This rock is igneous/ metamorphic/ sedimentary (circle one class) (1) Part II Use map #2 to locate the necessary rocks for questions 15 through 22, which deal with rock names.. 15) This rock is dominated by the mineral calcite. What is the proper name for it? 15b) What evidence supports your choice of rock class? it? 16) This rock is dominated by the mineral dolomite. What is the proper name for
16b) What evidence supports your choice of rock class? 17) This rock is dominated by the mineral calcite. What is the proper name for it? 17b) What evidence supports your choice of rock class? 18) This rock is dominated by the mineral calcite. What is the proper name for it? 18b) What evidence supports your choice of rock class? 19) This rock is dominated by the mineral calcite. What is the proper name for it? 19b) What evidence supports your choice of rock class? 20) This rock is dominated by the mineral quartz. What is the proper name for it? 20b) What evidence supports your choice of rock class? 21) This rock is a mixture of quartz, biotite, plagioclase and orthoclase. What is the proper name for it? 21b) What evidence supports your choice of rock class? 22) This rock is a mixture of quartz, biotite, plagioclase and orthoclase What is the proper name for it? 22b) What evidence supports your choice of rock class? for it? 23) This rock is a mixture of plagioclase and amphibole. What is the proper name 23b) What evidence supports your choice of rock class?
Part III Use map #3 to locate the rocks 27 and 28. Map #3. Art Installation between North Campus Building and Staging building. 27) This rock is a mixture of quartz, biotite, plagioclase and orthoclase. What is the proper name for it? 27b) What evidence supports your choice of rock class? 28) This rock is dominated by the mineral quartz. What is the proper name for it? 28b) What evidence supports your choice of rock class?