Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Interpretation of Topographic and Geologic Maps
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Interpretation of Topographic and Geologic Maps
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Subject: Mining
Class: Senior Secondary 3
Term: 3rd Term
Week: 1
Theme: Basic Geology
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This topic introduces students to the fundamental principles and practical applications of topographic and geologic map interpretation, essential skills for prospective mining professionals. Understanding these maps is crucial for identifying potential mineral deposits, planning mining operations, assessing environmental impacts, and ensuring safety in diverse Nigerian geological settings. The ability to interpret these maps allows for informed decision-making in mineral exploration, exploitation, and land-use planning across various sectors in Nigeria, including agriculture, civil engineering, and urban development.
Basic Geology each marked contour point from the paper strip down to its corresponding elevation on the graph paper. Connect these points with a smooth curve to create the topographic profile.
2. Transfer Geological Contacts and Structures: Return the paper strip to the map, aligning it with the section line. Mark every point where a geological contact, fault, or fold axis intersects the section line on the paper strip. Label the rock unit symbols on the strip. Project these points vertically down onto the topographic profile you just drew.
3. Plot Strike and Dip (if applicable): For dipping beds, use the dip angle and direction. If the section line is perpendicular to strike, the dip angle can be directly drawn from the contact lines on the surface profile. If not, apparent dip must be considered (though simplified for SS3).
A common simplified approach for SS3: Assume the beds are dipping uniformly. Draw lines from the projected contact points on the topographic profile, extending into the subsurface, respecting the dip direction and angle.
4. Complete the Section: Draw the rock unit boundaries in the subsurface, extending them based on the interpreted dip and strike. Shade or colour each rock unit according to the legend on the geological map. Label the rock units and geological structures (faults, folds) clearly. Add a title, horizontal scale, vertical scale, and legend to the completed cross-section.
Worked Example 2: Interpreting a Simple Geologic Map for Mining Prospects A geologic map of an area in Edo State shows a sequence of sedimentary rocks: a sandstone unit (Qts) overlying a shale unit (Ksh), which in turn overlies a limestone unit (Jls). There is a fault (F1) cutting through all units, striking NW-SE. The limestone unit (Jls) is known to host significant reserves of cement-grade material, while the shale unit (Ksh) may contain coal seams.
Interpretation: The youngest rock is sandstone (Qts), the oldest is limestone (Jls). This indicates that limestone is likely found at depth. The fault (F1) could have displaced the rock units, meaning the limestone may be offset. This displacement needs to be considered when planning exploration drilling or quarrying, as it might create challenges or even serve as a structural trap for mineralisation. The presence of shale (Ksh) suggests potential for coal deposits. Exploration would focus on areas where the shale is at suitable depths and thickness. Limestone (Jls) being present indicates potential for cement production, a key industry in Nigeria. Detailed mapping would be required to delineate the economic thickness and quality of the limestone, particularly in relation to the fault.
3. Teaching and Learning Activities 3.
1. Teacher Activities Introduction (10 mins): Begin by displaying various maps (road map, Nigerian political map, topographic map, simple geologic map if available). Ask students to identify what each map shows. Introduce the concept of specialized maps for mining.
Explanation of Topographic Maps (20 mins): Present a large sample topographic map (e.g., a relevant section of a Nigerian topographic sheet). Explain contour lines, contour interval, spot heights, and how they represent relief. Use visual aids (e.g., a simple 3D model of a hill to demonstrate contouring). Demonstrate how to identify hills, valleys, ridges, steep and gentle slopes on the map. Walk through Worked Example 1 (Gradient Calculation) step-by-step, emphasizing the use of scale.
Explanation of Geologic Maps (25 mins): Present a large sample simple geologic map (can be a simplified diagram if actual maps are scarce). Explain rock unit symbols, geological contacts, faults, and folds. Introduce strike and dip symbols and their significance in determining subsurface orientation. Explain the relevance of these features to mineral exploration (e.g., faults as mineralizing pathways, specific rock types hosting deposits). Refer to Nigerian examples like coal in Enugu (shale/sandstone sequences) or limestone in Ewekoro (sedimentary sequences). Discuss Worked Example 2 (Interpreting a Simple Geologic Map). Demonstration of Geological Section Drawing (20 mins): Using a large projected geologic map and a chosen section line, physically demonstrate the step-by-step process of drawing a topographic profile first. * Then, is on a 200m contour line, and Point B is on a 300m contour line. The measured distance between A and B on the map is 4 cm.
1. Determine Vertical Interval (VI): VI = Elevation of B - Elevation of A = 300m - 200m = 100m.
2. Calculate Horizontal Equivalent (HE): Map scale = 1:25,000, meaning 1 cm on map = 25,000 cm on ground. 25,000 cm = 250 m. So, 1 cm on map = 250 m on ground. Measured distance on map = 4 cm. HE = 4 cm 250 m/cm = 1000 m.
3. Calculate Gradient: Gradient = VI / HE = 100m / 1000m = 1/
1
0. This can also be expressed as 1:10 or 10% (100m/1000m 100%).
Interpretation: For every 10 meters travelled horizontally, the elevation changes by 1 meter. This indicates a relatively gentle slope, potentially suitable for establishing haul roads in a mining area in say, Kogi State where limestone quarries operate. 2.
3. Geologic Maps Definition: Maps that show the distribution of different rock types and geological structures at or near the Earth's surface.
Key Features and Interpretation: Rock Units (Formations): Represented by different colours or patterns, often with letter symbols (e.g., 'K' for Cretaceous, 'ls' for limestone). A legend provides detailed descriptions.
Geological Contacts: Lines separating different rock units.
Conformable Contacts: Represent continuous deposition.
Unconformable Contacts: Represent a break in the geological record (erosion or non-deposition).
Faults: Fractures in the Earth's crust along which there has been significant movement. Represented by solid or dashed lines, sometimes with symbols indicating type (normal, reverse, strike-slip). Faults are important as they can act as conduits for mineralizing fluids, common in areas like the Nigerian Younger Granites where tin and tantalite are found.
Folds: Bends in rock layers due to compressional forces. Represented by lines showing the fold axis (e.g., anticline, syncline) with arrows indicating plunge direction. Folds can create traps for petroleum or concentrate economic minerals.
Strike and Dip: Fundamental measurements of the orientation of rock layers.
Strike: The compass direction of a horizontal line on an inclined rock layer. It is perpendicular to the direction of dip.
Dip: The angle of inclination of the rock layer from the horizontal plane, measured perpendicular to strike. It includes both the angle and the direction (e.g., 30° SE).
Symbol: A 'T' or 'long T' symbol on the map. The long bar indicates the strike direction, and the short bar (perpendicular to the long bar) points in the direction of dip, with the dip angle written beside it.
Mineral Deposits: Often indicated by specific symbols (e.g., Au for gold, Sn for tin, Fe for iron ore) or associated with specific rock units or structures.
Cross-sections: A vertical slice through the Earth's crust, showing the subsurface distribution of rock units and structures along a specific line on the map. 2.
4. Drawing Geological Sections Purpose: To visualize the subsurface geometry of rock layers and structures, which is critical for understanding mineral deposit orientation and depth.
Step-by-Step Procedure:
1. Draw a Topographic Profile: Place a strip of paper along the chosen section line (e.g., A-A') on the topographic map. Mark the ends of the section line (A and A') and every point where a contour line intersects the paper strip. Label the elevation of each marked contour. Draw a horizontal base line on graph paper, representing the horizontal length of the section. Establish a vertical scale on the graph paper (e.g., 1 cm = 100m). Project each marked contour point from the paper strip down to its corresponding elevation on the graph paper. Connect these points with a smooth curve to create the topographic profile.
2. Transfer Geological Contacts and Structures: Return the paper strip to the map, aligning it with the section line. Mark every point where a geological contact, fault, or fold axis intersects the section line on the paper strip. Label the rock unit symbols on the strip. * Project these points vertically down onto the topographic profile you just drew. Interpretation of Topographic and Geologic Maps Term: 3rd Term Week: 11 ---
1. Overview and Learning Objectives This topic introduces students to the fundamental principles and practical applications of topographic and geologic map interpretation, essential skills for prospective mining professionals. Understanding these maps is crucial for identifying potential mineral deposits, planning mining operations, assessing environmental impacts, and ensuring safety in diverse Nigerian geological settings. The ability to interpret these maps allows for informed decision-making in mineral exploration, exploitation, and land-use planning across various sectors in Nigeria, including agriculture, civil engineering, and urban development. Upon completion of this lesson, students will be able to: Accurately read and understand the various features and symbols on topographic maps, such as elevation, slope, and landforms. Effectively interpret geological maps to identify rock types, geological structures (e.g., faults, folds), and their distribution, which is vital for locating mineral resources in Nigeria. Construct simple geological cross-sections from a given geological map, a practical skill for visualizing subsurface geology and planning drilling programs in mining exploration. Apply map interpretation skills to real-world scenarios in Nigeria, such as identifying suitable sites for quarrying, understanding regional geology for mineral prospecting (e.g., identifying schist belts for gold in southwestern Nigeria), and planning infrastructure development in challenging terrains.
2. Key Concepts and Explanations 2.
1. Introduction to Maps in Mining Maps: A two-dimensional representation of a three-dimensional portion of the Earth's surface, showing features like topography, geology, infrastructure, and boundaries.
Importance in Mining: Maps are indispensable tools for: Exploration: Identifying potential mineralized zones.
Planning: Designing mine layouts, access roads, waste disposal sites.
Operation: Guiding drilling, excavation, and production activities.
Safety: Identifying geological hazards (faults, unstable slopes).
Environmental Management: Assessing and monitoring environmental impacts. 2.
2. Topographic Maps Definition: Maps that show the shape and elevation of the land surface, often using contour lines.
Key Features and Interpretation: Contour Lines: Lines connecting points of equal elevation above a datum (usually sea level).
Closely spaced contours: Indicate steep slopes.
Widely spaced contours: Indicate gentle slopes.
Concentric closed contours: Represent hills (higher elevations towards the center) or depressions (lower elevations towards the center, often marked with hachures).
V-shaped contours pointing upstream: Indicate valleys (the V points towards higher elevation).
U-shaped contours pointing downhill: Indicate ridges (the U points towards lower elevation).
Contour Interval: The vertical distance (difference in elevation) between adjacent contour lines. This value is usually constant for a given map and stated in the legend (e.g., 20m, 50ft).
Relief: The difference between the highest and lowest elevations on a map. High relief implies rugged terrain; low relief implies flat terrain.
Spot Heights: Specific points on the map with their exact elevation marked (e.g., •150m).
Drainage Patterns: Rivers, streams, and lakes indicate surface water flow and can reveal underlying geological structures.
Scale: The ratio of a distance on the map to the corresponding distance on the ground.
Representative Fraction (RF): e.g., 1:50,000 (1 unit on the map equals 50,000 units on the ground).
Verbal Scale: e.g., "1 cm represents 500 meters." Graphic Scale (Bar Scale): A line marked with distances, useful for direct measurement.
Gradient Calculation: Formula: Gradient = Vertical Interval (VI) / Horizontal Equivalent (HE)
Vertical Interval (VI): Difference in elevation between two points.
Horizontal Equivalent (HE): Actual horizontal distance between the two points on the ground, calculated using the map scale.
Worked Example 1: Calculating Gradient A topographic map with a scale of 1:25,000 shows two points, A and B. Point A is on a 200m contour line, and Point B is on a 300m contour line. The measured distance between A and B on the map is 4 cm.
1. Determine Vertical Interval (VI): VI = Elevation of B - Elevation of A = 300m - 200m = 100m.
2. Calculate Horizontal Equivalent (HE): Map scale = 1:25,000, meaning 1 cm on map = 25,000 cm on ground. 25,000 cm = 250 m. So, 1 cm on map = 250 m on ground. Measured distance on map = 4 cm. HE = 4