Name_________________ Put your name on all pages NOW. ES 10 Geologic Principles; Test 2; October 25, 1994; Alfred Hochstaedter 1. a) (5 pts) What causes a stream to be either braided or meandering? Where applicable, be sure to include the concepts or roles of gradient, discharge, load, and the resistance to erosion of the river banks in your answer. Braided: high gradient, variable discharge, high load, easily erodable banks; Meandering: low load relative to discharge, low gradient, easily erodable banks. b) (5 pts) In order to continue your answer you draw a meandering stream, labelling the thalweg (i.e., the region of greatest depth and velocity), point bar deposits, and regions of greatest bank erosion. Also show how your meandering stream might evolve to form an oxbow lake. Thalweg stays near the outside of meanders; point bars form on the inside of meanders, greatest bank erosion occurs on the outside of meanders (near thalweg). Oxbow lakes form when the outside of two meanders meet each other due to erosion, and river takes shortest route to get downhill. c) (3 pts) How much of the EarthÕs total fresh water, not including that contained in the polar ice caps, is groundwater? Any answer that says almost all of it. 2. a) (4 pts) What does the coefficient of permeability represent? What are its units? What are some typical values? How does it influence discharge? Coeffeciant of permeability represents how easily water can flow through a given rock or sediment. Units are in length / time (velocity). Typical values range from 1 to 1X10-10cm/s. The greater the permeability, the greater the discharge, or amount of water flowing through the rock. b) (4 pts) What is porosity and how does it differ from permeability? Porosity measures the amount of pore space within a rock or sediment, while permeability measures the ease with which water may flow through a rock or sediment. Material may be porus without being permeable. c) (4 pts) What is the hydrologic gradient? What are its units? How would you determine a value in a real-life situation? How does the hydrologic gradient influence discharge? The hydrologic gradient is the difference in elevation per horizontal distance that the water travels. Dimensionless. In a real-life situation, one would measure horizontal distance as well as difference in elevation between the two points that one wishes to study. The greater the hydrologic gradient, the greater the discharge. d) (5 pts) Draw a cross section that relates geology to ground-water hydrology to explain the sort of setting your friends will need to find in order to narrow down the places they will look for an artisan spring or well. Using your drawing, explain where an artisan spring or well might be found and why. In addition to more obvious features, please be sure to include the potentiometric surface and recharge region in your drawing. Recharge region is where confined aquifer crops out. Artisan well or spring is located where potentiometric surface lies above ground surface. Partial credit for potentiometric surface above confining layer, but not above ground surface. 3. a) (3 pts) How do waves form in the first place? Offshore winds blowing across the water. b) (3 pts) Why do waves ÒbreakÓ? When they ÒfeelÓ the bottom, they start to slow down. The top goes faster than the bottom and hence they break. c) ( 4 pts) What causes wave refraction, and what are at least two examples of the geological effects? Shallow water causes refraction because it slows down the water in that specific area. Ex1: wave energy dissapated in pocket beaches, therefore sand deposition and nice beaches. Ex2: longshore drift due to waves not approaching shore in exactly perpendicular direction. d) (3 pts) How do waves cause longshore drift to occur? As waves enter shoreline in non-perpendicular direction, sand is washed up onto shore at an angle towards one direction or the other. Water washes back out to sea in exactly perpendicular direction due to gravity. This process, multiplied many times over, causes longshore drift. e) (5 pts) Beach morphology (i.e., shape) can often be described as the result of competing forces between tidal forces and longshore drift. This is especially true at the mouths of lagoons or rivers. Please give one example of this. Describe or draw a beach and explain which aspects of its morphology are due to longshore drift and which are due to tidal forces. Use a diagram if you like to make your answer more clear. Anything thoughtful here is acceptable. I was thinkng about spits forming at the mouths of rivers or estuaries: longshore drift wants to close the mouth, while tidal action keeps it open. Something about Barrier Islands would work well too. 4. a) (5 pts) What, exactly, do stress and strain measure, and why do we plot them against each other? What are their units? Stress is the force / unit area that causes the deformation; it represents the thing that is doing the pushing or pulling. Strain is a dimensionless description of a change in shape or size of something. We plot them against each other to find out how much stress is needed to cause a certain amount of strain; to find out how much elastic and ductile strain can occur before rupture. b) (4 pts) Label which of the following diagrams refers to brittle deformation and which indicates ductile deformation. Also, list and briefly explain or provide examples of the four factors that control whether ductile or brittle deformation will occur. The more ductile deformation shown on the diagram, the more ductile (or less brittle) the material in question is. 1)Temperature 2)Pressure 3)Time and strain rate: slow rates or long times favor ductile deformation 4)Composition: clays, micas, calcite are ductile; qtz, f-spar, granite, SS are brittle. c) (5 pts) Complete the following table: kind of stress compressional extensional shear picture of typical fault in this environment name of this kind of faulting Thrust/reverse normal strike-slip d) (5 pts) The following is a map view of a plunging anticline. Label the limbs and axis. In what orientation would you expect to find axial plane cleavage? Which direction is the anticline plunging? Where are the youngest rocks? Where are the oldest rocks? Axial plane cleavage parallels the axial plane, which splits the limbs and conects the fold axes of all the different layers. 5. a) (5 pts) What causes EQs? What role does elastic rebound play? Motion between plates (or crustal blocks) cause EQs. Elastic rebound is the return of elastic strain to a more original position during an EQ; the release of stored elastic energy by slippage on faults. b) (4 pts) What is the difference between magnitude and intensity? What do each of these depend on? Magnitude is the energy released during an EQ, measured by the amplitude of seismic waves a certain distance away from the epicenter. Each EQ has only one magnitude, (measured by the Richter scale). Intensity is a measure of how much the ground shook at any particular place during an EQ. Each EQ can have several intensities. It depends on magnitude, distance from epicenter, and type of rock or sediment occuring where the measurement is made. c) (4 pts) Following is the list of possible building locations. Please list the specific seismic hazards (if any) that exist at each location with respect to the San Andreas and related faults along the Central Coast of California. In downtown Santa Cruz, equidistant between PergalesiÕs and the Java House. Danger of liquifaction and intensified shaking because of saturated sediments. In the Santa Cruz Mountains, on a steep slope just above the reach of the summer fog. Landslides, ground cracking, being close to the fault at a potentially dangerous time. On the UC Santa Cruz campus, using the Great Meadow Quartzite as a foundation. Pretty safe, on hard rock. Possible chimney damage from intense shaking. On the westside, very near EmilyÕs Bakery and on the Highway 1 Terrace. Pretty safe, reasonably hard rock just below thin vaneer of beach sands and sediments. Broken chimneys common. 6. a) (4 pts) Note the steep cliffs along this river that has a meandering shape. Starting with the deposition of sediments, what is the Earth history of this place? Incised meander. River came first, then uplift and consequent erosion down into the canyons. b) (4 pts) This is a strangely symmetrical spot. Starting with the deposition of sediments, what is the Earth history of this spot, and what is this structure called? After deposition, the lithification turned the sediments into rocks. Compressional stresses occurred and these rocks deformed ductily, or folded. This is an antilcine. Last, uplift and erosion. c) (4 pts) Never mind the rock types here, but why is this valley so U-shaped? What happened here? A glacier carved this U-shaped valley. d) (4 pts) Note the stepped regions of flat topography above the coast. What are these called? How do they form? Is this an emergent or submergent coastline? To what present-day feature -- of which nice examples occur at Natural Bridges -- can these be related? These are terraces. They can be related to wave-cut platforms -- flat areas at sea level cut by the waves. Terraces form on emergent coastlines as the wave-cut platforms are uplifted above sealevel. e) (4 pts) What is this linear feature? If it is a fault, specifically what kind is it and which way is the offset. This is a right-lateral strike-slip fault. It is also a beautiful example of an offset stream.