LAB R EARTH MATERIALS

Transcription

1 Introduction LAB R EARTH MATERIALS Rocks are the most common of all earth materials. You see rocks everywhere, from the mountains that provide a good hiking area or a scenic vista to the rocks used for landscaping your yard. Rocks such as granite, limestone, and slate are used as building materials and artisans use fine marble for sculpting. To a geographer, earth materials (rocks) are important because rocks form or are eroded to form landforms that make up the land that we see around us today. The objective of this lab is to help familiarize you with the origins, characteristics, and appearances of different rock types. You will use the criteria of igneous, sedimentary, or metamorphic origin to classify a rock. Materials needed: pencil, rock samples (provided) Part I. Three rock classes: Igneous, Sedimentary and Metamorphic Igneous: Igneous rocks form from magma (molten material) that has solidified. Molten material found beneath the surface of the Earth cools slowly and the resulting rock is called intrusive. This slow rate of cooling permits the mineral crystals to grow large, making them visible to the naked eye. Intrusive rocks are said to have a coarse-grained texture (e.g., granite). If the magma comes out onto the surface of the Earth, it is then called lava. The rock that forms from the rapid cooling of lava is called extrusive, and mineral crystals do not have time to develop. This rock has a fine-grained texture (e.g., rhyolite). These are two different types of rock textures, resulting from differences in the cooling history of the molten material. Other igneous textures exhibited by extrusive rocks include vesicular texture (lots of holes) as in pumice, or a glassy texture as exemplified by obsidian (volcanic glass). Chemical composition of the rock is also important and can be determined to some extent by looking at the rock s color. The minerals found in igneous rocks are most commonly composed of a light colored material called silica (Si), as well as dark colored iron (Fe) and dark colored magnesium (Mg). Therefore, rocks that are high in silica and low in iron and magnesium tend to have an overall light color (e.g., granite and rhyolite), whereas dark colored igneous rocks are high in iron and magnesium (e.g., basalt and gabbro). Sedimentary: These rocks form from the accumulation of sediment. Sediment is produced through the process of weathering, which is the break down of any type of rocks. Additionally, organic materials such as parts of plants or shells of marine organisms can be classified as sediment as well. These materials can then be transported by the various erosive agents such as running water, waves, wind, gravity, or ice and eventually deposited in horizontal layers. The process of lithification (compaction and cementation) transforms the sediment into hard rock (clastic sedimentary rocks such as sandstone or shale). Sedimentary rocks can also form from materials that were dissolved in liquid (water) and then are precipitated out of the solution (chemical sedimentary R-1

2 rocks like limestone). Organic sedimentary rocks are made up of plant or animal remains. Examples of this type of sedimentary rock include peat and coal. Metamorphic: This group of rocks have been changed below the Earth s surface where they have been exposed to very high temperatures, pressures, or both. The new metamorphic rock will have a different texture and structure than the original (parent or precursor) rock. Metamorphic rocks were previously igneous, sedimentary, or even metamorphic rocks that have gone through this modification process. They are generally denser and less porous than the parent rock. The rock s appearance (texture) changes as the mineral crystals tend to become elongated and align themselves perpendicular to the compressional forces. This causes the rock to appear layered or banded a texture called foliation. Examples of foliated metamorphic rocks include: gneiss, schist, phyllite, and slate. Nonfoliated metamorphic rocks are identified by their mineral composition. They lack a layered or banded appearance. Examples include marble and quartzsite. A. IGNEOUS ROCK IDENTIFICATION Igneous rocks are classified according to composition, texture, and color. For each rock describe the sample in terms of these characteristics. 1. grains/crystals are visible? (Y/N) 2. color (lt, med, dk) 3. origin (intrusive or extrusive) 1. grains/crystals are visible? (Y/N) 2. color (lt, med, dk) 3. origin (intrusive or extrusive) 1. color (lt, med, dk) 2. texture (glassy, vesicular, ashy) 3. weight (light, heavy) Group A Group B Group C Most Si Intermediate Si Least Si Least Fe and Mg Intermediate Fe and Mg Most Fe and Mg Granite Diorite Gabbro Rhyolite Andesite Basalt Group D Other igneous extrusive rocks Obsidian Pumice Ignimbrite R-2

3 B. SEDIMENTARY ROCK IDENTIFICATION Sedimentary rocks are classified according to grain size and shape, texture, and origin of the materials. For each rock describe the sample in terms of these characteristics. 1. rock shape (rounded or angular) 1. grain size (visible like sand; not visible like clay) 2. roughness (rough, smooth) Group A Clastic sedimentary rocks Conglomerate Shale Breccia Sandstone 1. roughness (rough, smooth) 2. color Group B Chemical sedimentary rocks Chemical Limestone Chert 1. origin (plant, animal) Group C Organic sedimentary rocks Skeletal Limestone Peat Coal R-3

4 C. METAMORPHIC ROCK IDENTIFICATION Metamorphic rocks are classified according to parent (precursor) rock material, banding, foliation or the lack of foliation. For each rock describe the sample in terms of these characteristics. 1. size of bandings (large, small, none) 2. precursor rock 1. precursor rock Group A Foliated metamorphic rocks Gneiss Schist Slate Group B Nonfoliated metamorphic rocks Marble Quartzite Part II. Unlabeled Rock Identification Based on what you have observed and learned in the previous section, you will now examine several rock samples. Answer the following questions based on these samples and see if you can identify these mystery rocks. Sample A 2. How would you describe the texture of this rock? 3. Did it cool slowly or quickly? 4. Is this rock higher in silica or in iron and magnesium? 5. What is the rock type? Sample B 2. Is it organic or inorganic? 3. Based on the rocks you see, were the larger rock pieces shaped by fluvial or colluvial processes? 4. Name this rock. Sample C 2. What is your evidence for this conclusion? 3. Name the rock type. R-4

5 Sample D 2. What is the evidence for your conclusion? 3. How would you describe the texture of this rock? 4. Did it cool slowly or quickly? 5. Is this rock higher in silica or in iron and magnesium? 6. What is the rock type? Sample E 2. Is it intrusive or extrusive? 3. How can you tell? 4. Is it higher in silica or in iron and magnesium? 5. What is the rock type? Sample F 1. Are the materials in this sample organic or inorganic? 2. Based on what you see, what kind of environment were these sediments probably deposited in? (ocean bottom, riverbed, nearshore / shallow sea, marsh, desert) 3. Name this sample. Sample G 2. What type of texture does this rock have? 3. Based on what you see, what conditions were necessary to form this rock? 4. Where might you find these conditions? 5. Name this rock? R-5