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Name: ________________________________ Hour: ______ Sedimentary Rocks Lab Refer to Exercise #2 in you lab manual for reference. (pgs 19-30) Objectives: 1. Separate common sedimentary rocks into detrital or chemical based on their properties. 2. Use a classification key to identify sedimentary rocks. 3. Identify the environments that form sedimentary rocks. Introduction To an Earth scientist, rocks represent much more than usable substances. They are the material of the Earth; understanding heir origin and how they change allows us to begin to understand earth and its processes. “The history of the Earth is written in the rocks” (Tarbuck, Lutgens, Pinzke pg 19) You will investigate some of the common sedimentary rocks that are found on and near the earth’s surface by identifying them based on their properties. Sedimentary rocks form from the accumulation of weathered rock particles called sediments. These particles may be solid particles or material that was formerly dissolved and then precipitated by either inorganic or organic processes. The process of lithification transforms the sediment into hard rock. Since sedimentary rocks form at, or very near the Earth’s surface, they often contain organic matter, or fossils, or both. The layering (or bedding) that develops as sediment is sorted by, and settled out from a transporting material (usually water or air) helps make sedimentary rocks recognizable. Sedimentary Rocks Identification Refer to Figures 2.10-2.17 of your lab manual for pictures of common sedimentary rocks. Sedimentary rocks are made from either detrital or chemical materials, and sometimes both. Detrital material consists of mineral grains or rock fragments derived from the process of mechanical weathering that are transported and deposited as solid particles (sediments). Rocks formed in this manner are called detrital sedimentary rocks. The mineral pieces that make up a detrital sedimentary rock are called grains (or fragments). The identification of a detrital sedimentary rock is determined primarily by the size of the grains or fragments. Mineral composition of the rock is a secondary concern. Chemical material was previously dissolved in water and later precipitated by either inorganic or organic processes. Rocks formed in this manner are called chemical sedimentary rocks. If the material is the result of the life processes of water-dwelling organisms- like a shell- it is said to be of biochemical origin. Mineral composition is the primary consideration in the identification of chemical sedimentary rocks. In this investigation, sedimentary rocks are divided into two groups, detrital and chemical, based upon the type of material found in the rock. Part 1: Examining Sedimentary Rocks Examine the sedimentary rock specimens 1-4 in your tray. These rocks are made of pieces or fragments and are called detrital sedimentary rocks. They do not include any rocks that have abundant shells or shell fragments. Use these rocks to answer Q1 & 2. 1. Pick up each detrital sedimentary rock specimen and rub your finger over it to feel the size of the grains or fragments. How many of your detrital specimens feel rough like sand? How many feel smooth like mud or clay? ________ specimens feel rough ________ specimens feel smooth 2. Two minerals that often comprise the grains of detrital sedimentary rocks are quartz and clay. Quartz is a hard (scratches glass) mineral with a glassy, transparent to translucent luster. Clay is a soft (doesn’t scratch glass) mineral that contains tiny particles. How many of your detrital sedimentary rock specimens are made of quartz and how many appear to be made of clay? ________ have quartz ________ have clay The remaining sedimentary rocks (specimens 5-9) are chemical sedimentary rocks. They are composed of shells or shell fragments as well as crystals. Use these specimens to answer Q 3. 3. As a result of their method of formation, many chemical sedimentary rocks are fine to coarse crystalline, while others consist of shells or shell fragments. How many of your chemical sedimentary rocks are crystalline, and how many contain abundant shells or shell fragments? ________ specimens are crystalline ________ specimens contain shell or shell fragments Part 2: Sedimentary Rocks and Environments Sedimentary rocks are extremely important in the study of Earth’s history. Particle size and the materials from which they are made often suggest something about the place, or environment, in which the rock formed. The fossils that often are found in a sedimentary rock also provide information about the rock’s history. Think of sedimentary rocks as representing a “place” on Earth where sediment was deposited. Figure 2.30 shows a few generalized environments (places) where sediment accumulates. Often, an environment is characterized by the type of sediment and life forms associated with it. Use Figure 2.30 to answer Q 4 & 5. Use pg 30 from your labe manual for a colored image of Figure 2.30. 4. Imagine you are holding the sedimentary rock sandstone. Sandstones are formed from sand. Where on Earth do you find sand, the primary material of sandstone, being deposited today? ____________________________________________ 5. Name the environment(s) where, in the past, the sediment for the following sedimentary rocks may have been deposited. Part 3: Sedimentary Rock Identification Use the Sedimentary Rock Identification Key below to identify the sedimentary rocks in your tray. Notice that the primary subdivisions for detrital rocks are based upon grain size, whereas composition is used to subdivide the chemical rocks. Sedimentary Rock Identification Key Detrital Chemical Texture Sediment Name Rock Name Composition Texture Rock Name Coarse Gravel (rounded) Conglomerate Calcite, fizzes in HCl Fine Limestone Medium Sand Sandstone Calcite, fizzes in HCl Shell Fragment Coquina Medium Sand Banded Sandstone Quartz Fine, very light colored Chert Medium Sand (with feldspar) Arkose Sandstone Gypsum Fine light to pink Rock Gypsum Fine Mud Shale Plant Fragments Fine very light weight Bituminous Coal Using the Identification Key above, fill in the identification chart below. Rock # Detrital/Chemical 1 2 3 4 5 6 7 8 9 10 Texture (grain size) Sediment Name (Detrital) or Composition (Chemical) Environment formed? (use Fig 2.30) Rock Name