GEOL104: Exploring the Planets LAB 5: PLANETARY TECTONICS OBJECTIVES: I. Understand the three basic types of tectonic interactions that can occur II. Identify tectonic interactions on other planets MATERIALS: Two wooden blocks with attached wire mesh. Play Dough or clay Circular water bottle cap Ruler Rolling pin SAFETY WARNING: To prevent INJURY, please be aware of the potential for SHARP edges on the metal gratings screwed into the wooden blocks. INTRODUCTION: Tectonics is the study of large-scale movements in a planet. These movements have many products: Mountain belts such as the Appalachians, Rift valleys such as the Rio Grande, Great lateral-slip faults such as the San Andreas, The tectonic plates of the Earth, or Convection inside planets that allows planets to cool and that drive surface tectonic processes and products. An understanding of tectonics for the Earth matters because of a need to understand Earthquake risks as all major natural earthquakes occur on faults, The arrangement of faults and folds because they can control the location of natural resources such as oil, Changes in global climate due to the formation of large mountain belts, and For the purposes of this lab, how the Earth works as a physical engine, which we see as the movement of plates with the great trenches, the sea-floor spreading mountain ranges and the major, earthquake-prone lateral-slip faults An understanding of tectonics on other objects in the Solar System is important in order to gain a better understanding of: The geologic history of individual bodies in the Solar System The forces involved in the observed tectonic structures (internal forces versus external forces). The characteristics of subsurface materials. This may indicate the presence or absence of liquid H 2 O, which is necessary for life as we know it. In the context of moons and other planets in the Solar System, a key question is Are they like the Earth? We can ask that question about tectonics. We do not need a particular 1
answer, but rather just the answer. If they are like the Earth, then the similarity will be great. If they are different, then we can learn more about the way the planets and moons evolve as a function of size and also, position in the solar system. That has implications for the hunt for life, space exploration, and even such questions as Do these bodies have resources that the human race will need in the future? THINKING FROM A TECTONIC PERSPECTIVE The products of tectonics are produced by deformation, which can occur in one of four ways, or in a combination of the following ways: Translation: Change in position Rotation: Change in orientation Distortion: Change in shape Dilation: Change in volume In the figure to the left, the pink square is termed a strain marker (or deformation marker). Common small scale strain markers used in studying tectonic movements on Earth include mineral grains, fossils, sedimentary features, or any other feature that has a known initial shape. On other solar system bodies, only large scale strain markers are currently available to us. Large scale strain markers include rivers, craters, fractures, and ejecta blankets. In most cases, only the final deformation (Time 2 in the figure above) is available for observation. Using these deformed strain markers, it is our job to determine how these markers appeared previously (Time 1 in the figure above). If we are able to do this, then we have determined the materials tectonic history. The deformation on strain markers occurs due to the formation and evolution of certain geologic structures. Common geologic structures include (shown in the figure below): Fractures rocks fracture when their strength is exceeded, but without the material becoming displaced. Faults material that has been displaced. There are three main types of faults. These include: o Normal Faults: Commonly results from extension, resulting in thinning of the material and an increase in the ground surface area. o Reverse Faults (also called thrust faults if the fault angle is less than ~30.): Commonly results from contraction, resulting in thickening of the material and a decrease in the ground surface area. 2
o Strike-slip faults (also called transform faults if they define a tectonic plate boundary): Occurs due to lateral motion of two blocks of material. Lateral motion does not result in a change in ground surface area. Folds - rocks change shape and/or size through solid-state flow. Commonly occurs in regions of contraction. Folds may or may not result in a decrease in ground surface area. CRUSTAL TECTONIC MOVEMENTS: We can classify crustal tectonic movements according to the overall effects of these structures: Extension The surface area of the crust has increased, resulting in crustal thinning, commonly in the form of fractures and normal faults. Contraction (also called shortening) The surface area of a section of crust has decreased, resulting in folding or reverse faulting. Sometimes fractures may develop on the crests of folds. Lateral Motion Occurs when two blocks of crust move past each other horizontally, with no change in crustal surface area. This results in the formation of transform and/or strike-slip faults. The Earth contains all three types of tectonic motions along its tectonic plate boundaries as well as within each plate. 3
GEOL 104: Exploring the Planets Image credit: Smithsonian Institution, This Dynamic Planet; used under fair use as defined by copyright law. http://mineralsciences.si.edu/tdpmap/ EXTENSION: Extensional tectonism is the process whereby two crustal blocks are pulled apart. On Earth, locations of spreading are called spreading centers or rifts. On other planets, extensional tectonic features are often referred to fissures. The most common place where extensional tectonism occurs on Earth is at spreading centers on the ocean floor. Rifts also can occur on land, such as the East African Rift. As crustal blocks spread apart, the surface between the blocks assumes a lower elevation. Thus, rifts may start on land and then, with increasing spreading, become flooded by the ocean. The Red Sea is an example of a flooded continental rift. Image credit: Smithsonian Institution, This Dynamic Planet; used under fair use as defined by copyright law. 4 http://mineralsciences.si.edu/tdpmap/
Extensional tectonism may also occur as a fault. The block diagram below shows a normal fault. The pull-apart motion of the two crustal blocks causes slippage along the normal fault between the two blocks. Note that the downthrown block creates a valley that is filled with sediment. Image Credit: Nicholas Costello and Micah Jessup, Earth and Planetary Sciences Department, University of Tennessee Knoxville Two normal faults that face each other comprise a graben. Graben, which are the downthrown crustal blocks, are commonly bounded by horsts, or the up-thrown blocks. The combination is a set of so-called horst and graben, as shown below in the image of the Ceraunius Fossae, Mars. Image Credit: Google Mars 5
SHORTENING: Shortening is the tectonic process whereby two crustal blocks come together. On Earth, shortening occurs in mountain building zones along thrust faults. Block diagram B shows a thrust fault. Image Credit: Nicholas Costello and Micah Jessup, Earth and Planetary Sciences Department, University of Tennessee Knoxville Folds may develop independent of faults, but for this lab, we particularly want to focus on folds that develop with thrust faults during crustal shortening. The image below from Solis Planum, Mars, shows a buried thrust fault, which is a reverse fault with a very shallow angle of dip, with a resultant fold that is created by the faulting. This faultrelated fold is called a wrinkle ridge. Sharp et al. (2006) Lunar Planet Sci. Conf. XXXVII, abstract 1966 6
LATERAL MOTION: Two crustal blocks can also slide past each other laterally. Block diagram C shows a lateral-slip fault, where sideways motion occurs. Note the offset river-lake system. Image Credit: Nicholas Costello and Micah Jessup, Earth and Planetary Sciences Department, University of Tennessee Knoxville 7
Part 1: EXPERIMENT: You will conduct three clay model experiments as instructed below. Observe the types of structures formed and the order of their formation for each type of motion. Part 1a: Extension 1. Using a rolling pin, smooth out the clay provided until the clay is about 0.5cm in thickness. The clay must have a consistent thickness. 2. Place two wooden blocks with the attached wire mesh together, with the handles on the outside and facing each other. 3. Place the clay across both boards and gently using the rolling pin to attach the clay to the wire mesh. 4. Gently use the bottle cap to create at least 6 circular impressions (very lightly) across the clay. Make sure that some of the impressions lie across the clay overlying where the two boards meet. The impressions should not cut through the clay. 5. Gently use the ruler to create at least 3 linear impressions in the clay and overlying both boards. 6. Measure the length, width and thickness of the block of clay. 7. On the answer sheet provided, draw the clay block with the impressions of circles and lines, and label your measurements in map view and cross section. 8. Make a hypothesis by drawing a diagram predicting the deformation sequence that will result as you pull the blocks away from each other. (How do you think the clay will deform?). 9. Slowly pull the blocks apart about 5 cm. 10. Re-measure the length, width, and thickness of the clay block, as well as the long and short axes of each bottle cap imprint. 11. Draw in both map-view and cross-sectional view what you observe, with the measurement labeled. 12. In a short paragraph, write a conclusion. What did changes did you observe? You may have to use the back of the paper for more writing space. 13. Also write how your conclusion differed from your original hypothesis. You may have to use the back of the paper for more writing space. Part 1b: Contraction 1. Using a rolling pin, smooth out the clay provided until the clay is about 0.5 cm in thickness. The clay must have a consistent thickness. 2. Gently use the bottle cap to create at least 6 circular impressions (very lightly) across the clay. The impressions should not cut through the clay. 3. Gently use the ruler to create at least 3 linear impressions in the clay and overlying both boards. 4. Place the two wooden blocks about 5 cm apart. 5. Place the clay across both boards and gently using the rolling pin to attach the clay to the wire mesh. 6. Measure the length, width and thickness of the block of clay. 7. On the answer sheet, draw the clay block with the impressions of circles and lines, and label your measurements in map view. 8
8. Beneath this drawing, draw the clay block in cross-sectional view and label the thickness with your measurements. 9. Make a hypothesis by drawing a diagram, predicting the deformation sequence that will result as you push the blocks together. (How do you think the clay will deform?). 10. Slowly push the blocks together about 3 cm. (The clay between the two blocks will touch the table.) Make observations of structures formed both in this location as well as in the clay on the wooden blocks. 11. Re-measure the length, width, and thickness of the clay block, as well as the long and short axes of each bottle cap imprint. 12. Draw in both map-view and cross-sectional view what you observe, with the measurement labeled. 13. In a short paragraph, write a conclusion. What did changes did you observe? You may have to use the back of the paper for more writing space. 14. Also, write how your conclusion differed from your original hypothesis. You may have to use the back of the paper for more writing space. Part 1c: Lateral motion 1. Using a rolling pin, smooth out the clay provided until the clay is about 0.5 cm in thickness. The clay must have a consistent thickness. 2. Place two wooden blocks with the attached wire mesh together, with the handles on the outside and facing each other. 3. Place the clay across both boards and gently using the rolling pin to attach the clay to the wire mesh. 4. Gently use the bottle cap to create at least 6 circular impressions (very lightly) across the clay. Make sure that some of the impressions lie across the clay overlying where the two boards meet. The impressions should not cut through the clay. 5. Gently use the ruler to create at least 3 linear impressions in the clay and overlying both boards. 6. Measure the length, width and thickness of the block of clay. 7. On the answer sheet provided, draw the clay block with the impressions of circles and lines, and label your measurements in map view. 8. Make a hypothesis by drawing a diagram predicting the deformation sequence that will result as you move the blocks past each other. (How do you think the clay will deform?). 9. Slowly push the blocks past each other about 5 cm. 10. Re-measure the length, width, and thickness of the clay block, as well as the long and short axes of each bottle cap imprint. 11. Draw in both map-view and cross-sectional view what you observe, with the measurement labeled. 12. In a short paragraph, write a conclusion. What changes did you observe? 13. Also, write how your conclusion differed from your original hypothesis. You may have to use the back of the paper for more writing space. (Extra credit) Part 1d: Combination 9
You choose: You must use all three types of deformation (extension, contraction, and lateral motion) but in an order of your choosing. Be sure to adjust the wooden blocks appropriately for each type of deformation. 1. Using a rolling pin, smooth out the clay provided until the clay is about 0.5 cm in thickness. The clay must have a consistent thickness. 2. Place two wooden blocks with the attached wire mesh together, with the handles on the outside and facing each other. 3. Place the clay across both boards and gently using the rolling pin to attach the clay to the wire mesh. 4. Gently use the bottle cap to create at least 6 circular impressions (very lightly) across the clay. Make sure that some of the impressions lie across the clay overlying where the two boards meet. The impressions should not cut through the clay. 5. Gently use the ruler to create at least 3 linear impressions in the clay and overlying both boards. 6. Measure the length, width and thickness of the block of clay. 7. On the answer sheet provided, draw the clay block with the impressions of circles and lines, and label your measurements in map view. 8. Beneath this drawing, draw the clay block in cross-sectional view and label the thickness with your measurements. 9. Make a hypothesis by drawing a diagram predicting the deformation sequence that will result after applying each type of deformation. (How do you think the clay will deform?). 10. Slowly displace the blocks in order to create extension, contraction, or lateral motion (you can only use each one once for this part). 11. Re-measure the length, width, and thickness of the clay block, as well as the long and short axes of each bottle cap imprint. 12. Draw in both map-view and cross-sectional view what you observe, with the measurement labeled. 13. In a short paragraph, write a conclusion. What did changes did you observe? 14. Also, write how your conclusion differed from your original hypothesis. You may have to use the back of the paper for more writing space. 10