CHAPTER 8: EARTHQUAKES ∙ A tsunami may be just a broad, gentle swelling out at sea but grows as it approaches shore. o Tsunamis aDon't forget about the age old question of a firm produces 400 units of output at a total cost of $1,200. if fixed costs are $200,
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re caused by volcanic or seismic activity or by undersea landslides, can be up to 30 meters high, and can cross entire ocean basins. ∙ Earthquake prediction is not highly reliable, but geologists do know more earthquakes happen along plate boundaries than happen at intraplate locations. o Earthquakes can occur outside of seismic belts, though they are rare. o Accurate shortterm predictions are rare and are not based on recurrence intervals, which refer to the average time between successive quakes on a fault. o swarms may precede major earthquakes, but not always, and they are often identified after an earthquake, not before. ∙ Examining sedimentary bedding in a geologic study reveals disrupted layers formed 260, 820, 1,200, 2,100, and 2,300 years ago, what is the recurrence interval of the earthquakes that caused the disruption? o 510 years ∙ Friction is the force that resists sliding along a surface. o Friction, caused by bumps and snags along rock surfaces, is the force that resists sliding. ∙ Normal faults result from stretching the Earth's crust; thrust faults from squeezing and shortening it. ∙ Moment magnitude (MW) rating is considered the most accurate representation of an earthquake's magnitude. ∙ All earthquake magnitude scales are logarithmic, which means a difference of one unit in magnitude reading represents a 10fold difference in ground motion. ∙ The magnitude of an earthquake refers to the amount of energy released. Therefore, a given earthquake should only have one magnitude number. ∙ Seismic waves become smaller in amplitude with increasing distance from the epicenter. ∙ Earthquake magnitude is based on ground motion recorded by a seismograph; intensity is based on the amount of damage produced. ∙ Contour lines representing Mercalli values are used to delimit zones of quake intensity; the greater the quake, the higher the intensity values and the wider the zones. ∙ Hypocenters can be as deep as 660 km (about 400 miles). ∙ Longterm earthquake predictions o are based on the identification of seismic zones. o are based on the study of historic recurrence intervals. o involve looking for sand volcanoes and disrupted bedding in the area. o Longterm predictions range from a few decades to centuries. ∙ Seismometers o may be the mechanical type, consisting of a weight, spring, frame, pen, and revolving cylinder. o may be electronic, consisting of a heavy cylindrical magnet and a coil of wire which produces a signal that can be recorded digitally.o operate because of inertia; one part of the instrument remains motionless while the recording device moves in response to seismic waves. o A seismograph can detect ground motion down to a mere millionth of a millimeter. ∙ The Richter scale o measures the amplitude of the largest deflection on a seismogram in response to specifically defined seismic waves at a specifically defined distance and depth. o is today termed a local magnitude reading (ML). o works well only for shallow, nearby earthquakes. o The Richter scale measures quake size in terms of the ground motion it generates (its magnitude). The Modified Mercalli Intensity (MMI) scale measures the size of an earthquake in terms of the damage it does (its intensity) ∙ The tsunami event of December 26, 2004, in Indonesia o involved a monstrous magnitude 9.3 earthquake that lasted 9 minutes. o was first noticed as a withdrawal of the sea along the beach front. o consisted of nearfield tsunamis that affected the island of Sumatra and later far field tsunamis that struck all along the Indian Ocean coast. ∙ o block X is the hanging wall. o This is a normal fault because the hanging wall, block X, slipped down. The hanging wall, block X, is the rock mass above the sloping fault plane; the footwall is the rock mass below. A thrust fault would show a more shallowangled fault plane, and the hanging wall, X, would have moved up. A strikeslip fault produces horizontal motion, not vertical. ∙ The locations of major earthquakes o are usually along plate boundaries. ∙ o shows a displacement of 18 feet.o Normal faulting is vertical motion along the fault plane; this shows horizontal motion, which is strikeslip faulting. The recurrence interval is the average time between quakes. Fault scarps are vertical steps, which couldn't be shown by an aerial view. ∙ good technique for building earthquakeresistant structures o Bolt bridge spans to the top of support columns. o Wrap bridge supports with steel cables. o Use supports that are capable of holding more than the static (unmoving) weight of the building. ∙ the 2010 Haiti earthquake? o The event was so destructive because of poor construction standards. o Faulting occurred along a transform boundary between the Caribbean and North American plates. o The likelihood of an earthquake was great because stress had been building on the fault for over 200 years. ∙ Earthquakes in California are o shallow and occur in the upper 15 km of crust, even though the San Andreas fault cuts through the crust to deeper depths. o Quakes in California can be large or small and occur along hundreds of faults. Motion along the San Andreas fault is strike slip, and it's not causing the state to sink. The San Andreas fault sits on a transform boundary, but because movement at depth is plastic, earthquakes usually occur in the upper 15 km. ∙ tsunami events o Tsunamis may be generated by underwater earthquakes, volcanic explosions, or submarine landslides. o Tsunami waves can be 100 to 1000 times wider than typical winddriven waves, when measured perpendicular to the wave. o Tsunamis can travel as fast as jet planes (several hundred miles per hour). ∙ Liquefaction o can cause sediment to turn into an unstable slurry incapable of supporting weight. o increases the pressure of the water that fills the pore space between sediment grains. o increases the pressure of the water that fills the pore space between sediment grains. o Shaking and displacement cause linear features, such as roads, fence lines, and pipes, to crack and separate across faults, not liquefaction. ∙ R and Lwaves are surface seismic waves. o Pwaves are compressional body waves that arrive first, followed by Swaves (shear body waves), and then by R and L (surface) waves. o Shallowfocus waves cause the most damage because they don't lose much energy before reaching the surface. ∙ Body waves pass through Earth's interior, whereas surface waves travel at the Earth's surface.o L and R are surface waves (not body waves); the epicenter is above the focus; and Swaves travel at 60% the speed of Pwaves. The correct choice describes the nature of body waves and surface waves. CHAPTER 10: DEEP TIME ∙ corpse with flesh intact, found in the Alps in 1991, was dated by the carbon14 method and showed a parentdaughter isotope ratio of approximately 1:1, with slightly more parent material than daughter material. The halflife of carbon14 is 5,730 years. o The age could logically be 5,300 years. o Since there was still more parent material than daughter, one halflife had not quite been reached. The carbon14 method is used on organic material not rock. ∙ A radioactive isotope of the element potassium decays to produce argon. If the ratio of argon to potassium is found to be 7:1, how many halflives have occurred? o 3 halflives ∙ an area of slightly dipping sedimentary rock layers has large inclusions and is intruded by an igneous dike. Apply the basic principles for determining relative ages o The sedimentary layers started out lying flat. o The oldest sedimentary layer is the bottom layer. o The igneous intrusion "baked" (metamorphosed) the sedimentary rock it touched ∙ Charcoal (burned wood) that was used to make prehistoric drawings on cave walls in France was scraped off and analyzed. The results were 4 mg carbon14 (parent isotope) and 60 mg nitrogen (daughter isotope). The halflife of carbon14 is 5,730 years. How old are the cave drawings? o 22,920 years o The age of the drawings should be close to four times the halflife (4 mg:60 mg = 1:15; this ratio means 4 halflives; 4 × 5,730 years = 22,920 years) ∙ listing of names to fit the following three descriptions Age of Mammals, Age of Dinosaurs, and longest geologic time period. o Cenozoic, Mesozoic, Precambrian o Mammals were the dominant lifeform during the Cenozoic, and dinosaurs were dominant during the Mesozoic; the Precambrian represents more than seven eighths of all Earth history. ∙ Geologists estimate that the age of the Earth is 4.54 billion years old, based on isotopic dating of meteorites thought to be from primitive solids of the early Solar System. o Geologists assume all objects in the Solar System developed at roughly the same time; therefore, the age of differentiated meteorites should represent the age at which the Earth formed. ∙ Radiometric dating o can begin only when the isotopes cool enough to lock into the crystal lattice. o of sedimentary rock dates the time of crystallization of the sedimentary minerals, not the time of sedimentary rock formation. o of metamorphic rock tells when the high temperatures of metamorphism cooled below the closure temperatures of the minerals involved.∙ William Smith's observations o recognized that groups of fossil species (fossil assemblages) occurred in limited intervals of strata. o were made around 1800 in fresh exposures of sedimentary bedrock in the English countryside. o lead to the principle of fossil succession. ∙ If you equate all Earth history to one calendar year, the history of our species (Homo sapiens) would occupy the last hour before midnight on New Year's Eve. ∙ In 1815, William Smith correlated strata from many locations and plotted it on paper to show the spatial distribution of rock units on Earth's surface. This document was the first modern geologic map. ∙ o On the figure, C is an angular unconformity and E is a disconformity—the entire Mesozoic is missing. ∙ The boundary surface between two stratigraphic formations is called a contact o The boundary surface between two stratigraphic formations is called a contact. A key bed (or marker bed) is a unique bed that helps in correlation of strata. A nonconformity can be a type of contact, but not all contacts are nonconformities ∙ o The reptile fossils are the secondyoungest fossils shown. o The ages of the fossils, oldest to youngest, are trilobite, shark, fern, reptile, ammonite, and mammal. Mammals and ammonites are both younger than reptiles and all the others. ∙ Uniformitarianismo is illustrated by scientists' seeing pillow lava form only underwater, then theorizing that pillow lava found high in the mountains today did nevertheless form underwater. o Features of the Earth are constantly changing, and catastrophic events still happen. The present is a key to the past and to the future, so since pillow lava forms underwater today, it must have done so in the past. ∙ What type of unconformity forms when sedimentary rocks overlie either igneous or metamorphic rocks? o Nonconformity o A disconformity is a surface between parallel sedimentary beds of significantly different ages; o an angular unconformity occurs when flat lying sedimentary layers overlie tilted layers; and baked contacts are formed when igneous intrusions create contact metamorphism (a baked zone) around the intruding magma. ∙ Which of the following accurately ranks the subdivisions of geologic time in order from largest to smallest? o eon, era, period, epoch ∙ a method to determine numerical age? o carbon14 dating o radioactive decay of uranium to lead o analyzing growth ring patterns in trees ∙ Specifying the age of one feature with respect to another is called its relative age. ∙ Relative age dating provides the age of a feature or event with respect to another feature or event in the same sequence. ∙ a radiometric dating technique o carbon dating ∙ Which of the following shows the four time divisions listed from oldest to youngest? o Precambrian, Paleozoic, Mesozoic, Cenozoic ∙ Carbon14 dating is used to date organisms, not minerals. ∙ Isotopic dating relies on the parentdaughter ratio. ∙ The "radiometric clock" starts when a radioactive mineral cools below its closure temperature. ∙ A formation is the name of a rock layer identified by such factors as rock type and approximate geologic age. ∙ Varieties of an element that differ only in the number of neutrons are called isotopes. ∙ The generally accepted age of Earth is 4.54 billion years. ∙ The principle of o superposition says in a sequence of sedimentary beds, the youngest is on top. o original continuity says sedimentary layers began as continuous expanses of sediment. o crosscutting relations says the feature doing the cutting is younger than the feature it cuts.o The principle of inclusions states that the inclusions are older than the surrounding rock. ∙ o intrusion (batholith G) o arranged from oldest to most recent, the events occurred in the following sequence: A, B, C, F, G, (erosion), D, H, E. CHAPTER 8 VISUALS ∙ the East African Rift vs. the Himalaya Mountains, respectively. o A Normal, B – Reverse o In the East African Rift (A), extension has led to many normal faults and crustal thinning. In the Himalayas (B), compression dominates with lots of reverse faults leading to uplift and mountain building.∙ Liquefaction o Sediment liquefaction occurs when shaking causes loose grains in sediment to settle together, forcing water that had occupied pore space upward, mixing with upper sediments to form a thick slurry. Buildings settle into this slurry and will tip or topple ∙ an earthquake o Once stress is applied to the rock unit, it begins to deform like the bent stick. The earthquake occurs when the rocks break, generating vibrations, or seismic waves. ∙ reverse fault o The fault in the image is a reverse fault. Reverse or thrust faults can initiate tsunamis by uplifting large columns of water from oceanfloor displacement. A tsunami is unlikely to be generated from a strikeslip fault because there is little or no vertical displacement. ∙ ∙ the approximate Richter magnitude of an earthquake that registered an amplitude of 10mm on a seismometer located 100km from the earthquake epicenter is 4 o The Richter magnitude is calculated by drawing a straight line between the amplitude value and the distance value on the chart (nomogram). A line drawn from 100km to 10mm will cross the magnitude line at approximately 4. ∙ ∙ a strikeslip fault o The figure shows rightlateral displacement of a fence. The motion on the fault trace is shearing resulting from a strikeslip fault. Dipslip faults, like normal or reverse faults, have vertical offset instead of the lateral motion shown here.∙ A shows P waves moving in a spring like motion ∙ B shows S waves that move in an up and down motion like a wave ∙ C – shows L waves that moves in a back and forth type motion ∙ D shows R waves which moves in a circular motion CHAPTER 10 VISUALS ∙ an anticline, syncline, and an angular unconformity ∙ The anticline is to the west of the syncline. Both have been eroded prior to the deposition of the newer sediments that are laid on top of them to the west. ∙ angular unconformity ∙ In an angular unconformity, the beds below the unconformity are folded or tilted at an angle to the horizontal layers deposited above the unconformity. In this case, folded beds were eroded and then new, flatlying sediments were deposited on top.∙ nonconformity ∙ In a nonconformity, an igneous pluton or other crystalline, nonbedded rock is eroded flat before new sediments are deposited on top of it. ∙ between the Unkar Group and the Tapeats Sandstone there is an angular unconformity o After the Unkar Group was deposited, there was likely uplift that tilted the beds, then, later, erosion. The Tapeats Sandstone was then deposited above the unconformity.∙ The principle of superposition stipulates that the oldest beds are at the bottom of a sequence, unless they have been overturned. Younger units, such as the dike and pluton, cut through older units. ∙ The crosscutting relationship is an important principle for figuring out the youngest event. The erosional surface cuts through all the rocks visible at the surface, making it younger than all of them.