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Crust • Outermost layer or shell of the Earth • Crust represents less than 0.1% of the Earth's total volume Mantle • The zone of the Earth below the crust and above the core • Divided into the upper mantle and the lower mantle, with a transition zone between Lithosphere • The solid portion of the Earth, as compared with the atmosphere and the hydrosphere • Includes the crust and part of the upper mantle and is of the order of 100 km in thickness Stress • Stress is a force that is capable of greatly deforming rocks, and may result in folding or faulting of rock, and even to the building of mountains Types of Stress • There are three types of stress – Compression – Tension – Shearing Compression • Opposing forces directed inward along a single line • Compression shortens an object along the axis of compression, and thickens it in the directions perpendicular to the stress direction Before After Tension • Tension is the result of divergence, pulling an object in opposite directions along a common axis • Tension lengthens an object along the axis of tension, and thins it in the perpendicular directions Before After Tension Crack Pictures • Nisqually Earthquake, 2/28/01, in Washington caused tension cracking. Shear • Opposing stress is created by two plates moving in opposite directions Deformation • • • • Rocks subjected to stress may: Fracture or crack Fold Fault, movement along break Why do some rocks fold/fault? Deformation Folding Faulting Very hot Cooler rock Ductile Brittle Stress applied slowly Stress applied quickly Anticline • If the fold is convex upward, it is called an anticline Anticlinal Fold • Rainbow Gap, Virginia • Photo: Henry Johnson Atlas Mountains Anticline • One of the best exposures of a complexly folded mountain belt anywhere occurs in the Atlas Mountain system of northwest Africa Syncline • If the fold is convex downward, it is called a syncline Syncline Photo • Photo: Duncan Heron • Synclinal fold exposed by roadcut Anticline-Syncline Pair • AnticlineSyncline pair in Devonian Old Red Sandstone. SW Wales, UK • Note the different fold shapes Domes • Domes are uplifted areas • Caused by magma pushing up on the crust Eroded Dome, Sinclair, Wyoming Faults • A fault is a fracture along which definite movement has occurred Fault Terminology • Foot Wall and Hanging Wall are borrowed from mining terminology • Ore veins are often deposited along faults Strike Slip Fault Photo: Arthur G. Sylvester. San Jacinto fault, Anza, Southern California Right-Lateral Strike Slip • Block is displaced to the right, looking across the fault Strike Slip Faults Right Lateral •Near Coos Bay, Oregon Left-Lateral Strike Slip • Block is displaced to the left, looking across the fault Strike Slip Faults - Left Lateral Near Lillooet, British Columbia Normal Fault •Normal faulting results from tensional forces •Hanging wall moves down relative to the footwall (here, to the right) •Places younger rocks on top of older Sevier Normal Fault Death Valley Normal Faults Reverse Fault • Reverse faulting results from compressional forces • Hanging wall moves up relative to the footwall (here, to the left) • Places older rocks on top of younger Reverse Fault • Reverse faults and associated fold Thrust Fault • Thrust faults are low angle reverse faults • They sometimes move large distances (tens of kilometers) Lewis Overthrust Explanation of Lewis Overthrust • Chief Mountain was moved about forty kilometers and isolated by erosion • Chief Mountain is much older (Precambrian) than the rock upon which it rests (Cretaceous) Chief Mountain Older rock above younger, typical of thrust faults Glacier National Park, Montana Rift valleys and Fault block mountains San Andreas Fault • Pacific plate, left • North America, right San Andreas Offsetting Fence Mountain building • Tectonic forces often create mountains, a process called orogeny • There are several types of mountains – Folded – Faulted – Upwarped – Volcanic Folded mountains • Plate collisions involving continental plates can produce high mountains – Examples: – Himalayas (India, Tibet, China) – Alps (Europe) – Urals (Europe/Asia boundary) – Appalachians Himalayan Mountains Mt. Everest High peaks in the Himalayas Owens Valley and the Sierra Nevada Range Volcanic Mountains Isostasy • The condition of equilibrium, comparable to floating, of the units of the lithosphere above the asthenosphere • Crustal loading, as by ice, water, sediments, or volcanic flows, leads to isostatic depression or downwarping • Crustal unloading, as by erosion, or melting of ice, to isostatic uplift or upwarping Isostasy Diagram Earthquake • A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated strain • Strain is a change in the shape or volume of a body as a result of stress Focus • The initial rupture point of an earthquake, where strain energy is first converted to elastic wave energy • The point within the Earth which is the center of an earthquake Epicenter • The point on the Earth's surface that is directly above the focus of an earthquake Seismograph • An instrument that detects, magnifies, and records vibrations of the Earth, especially earthquakes • The resulting record is a seismogram Principle of the Modern Seismograph Example Seismogram • Seismogram showing an earthquake - the three different traces represent vibrations in different directions • First peaks are P waves, the second peaks the S waves Types of seismic waves • 1. Primary • 2. Secondary • 3. Surface or L waves P-waves *Primary, pressure, push-pull *Travel fastest of the seismic waves *Travel through solids and liquids. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. S - waves *Secondary, shaking, shear, side-to-side *Arrive at a given point after P-waves *Travel through solids. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Surface waves *Often referred to as L-waves or long waves. *Complex motion. Like waves on the ocean. * Slowest. *Causes damage to structures during an earthquake. Richter Scale • Numerical scale of earthquake magnitude • Devised in 1935 by the seismologist C.F. Richter • The magnitude is based on multiples of 10, i.e.,101, 102, 103 each number on the scale is ten times greater than the previous number. For example magnitude 3 is ten times greater than magnitude 2. Richter Scale Continued • Measures the vibrational amplitude of the earth in response to seismic waves • Does NOT measure the energy release Mercalli Scale • Arbitrary scale of earthquake intensity, ranging from I (detectable only instrumentally) to XII (causing almost total destruction) • Based on human perception of the earthquake, and damage observed after the earthquake is over Number of Earthquakes/Year Prince William Sound Earthquake Damage Earthquake Damage • Earthquakes can cause damage in a number of ways – – – – – – – Building Collapse Tsunami waves Seiche waves Landslides Liquefaction Fire Disease Tsunami • Gravitational sea wave produced by any large-scale, short-duration disturbance of the ocean floor • Disturbances caused principally by a shallow submarine earthquake, but also by submarine earth movement, subsidence, or volcanic eruption Tsunami Damage Seward, Alaska. After the “Good Friday” Earthquake in 1964. 9.2 on the Richter scale. Liquefaction • Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure • As a consequence of liquefaction, soft, young, water-saturated, well sorted, fine grain sands and silts behave as viscous fluids rather than solids Liquefaction Failure • Overturned building, Niigata Japan Earthquake. Magnitude 7.5. Volcano • A vent in the surface of the Earth through which magma and associated gases and ash erupt • Also, the form or structure, usually conical, that is produced by the ejected material • Plural: volcanoes • Etymology: the Roman deity of fire, Vulcan Pyroclastic Eruptions • Magma spews upward with great force through a central vent Left: Mt. St. Helens, 1980 Right: Kilauea, Hawaii Fissure Eruptions • Volcanic eruptions may occur much more quietly along long cracks in the ground Fissure Image • Eruptive fissure on southeast rim of Kilauea caldera, Hawaii Nahuku Lava Tube • Thurston (Nahuku) lava tube • Near summit caldera of Kilauea Volcano, Hawaii Volcanoes National Park Tephra • General term for fragments of volcanic rock and lava blasted into the air by an eruption. • Dust - smallest ,like flour • Ash - grain of rice • Cinders - golf ball size • Bombs - largest, as big as a car Volcanic Ash Fall • Mount Pinatubo (Philippines - 1991) Effect on Climate • Large volcanic eruptions can block a great deal of the sun’s energy from reaching the earth’s surface • This cools the climate until the tephra particles sink to the surface Stratovolcano • A volcano that is constructed of alternating layers of lava and pyroclastic deposits, along with abundant dikes and sills • Synonym: composite volcano; composite cone Mt. Fuji, Japan Vent and Steam Explosion • Mt. St. Helens Crater Lake, Oregon Crater Lake National Park, with Wizard Island Cinder Cone • Crater Lake, despite the name, is a caldera, formed after the eruption of ancient Mt. Mazama about 6600 y.b.p. Cinder Cone Crater Lake National Park, with Wizard Island Cinder Cone • Wizard Island, within Crater Lake, is a cinder cone, and one of the tallest in the world Shield Volcano • A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid basaltic lava or by rhyolitic ash flows • Shield volcanoes are the largest volcanoes on Earth that actually look like volcanoes (i.e. not counting flood basalt flows) Mauna Loa Shield Volcano - note the low slopes Kilauea Eruption •(Upper left) Explosive eruptions •(Center left) Gas bubbles splash lava over the edges (with sound) •(Lower left) Lava flowing after initial surge •(Right) Upwelling lava – note rapid change in color – yellow is hottest, then orange, and red is the coolest Pahoehoe Flow • Toes of a pahoehoe flow advance across a road in Kalapana on the east rift zone of Kilauea Volcano, Hawaii • Photograph by J.D. Griggs on 16 July 1990 Alfred Wegener, 1880-1930 Wrote The Origin of Continents and Oceans in 1915 Continental Drift • 550 MYBP 136 MYBP 220 MYBP 65 MYBP Fossil Plant Evidence Glossopteris • Extinct group of seed plants that arose during the Permian on the great southern continent of Gondwana How Can a Continent Move? • The biggest objection was the lack of a mechanism for moving continents • Wegener spent the rest of his life looking for evidence to support his ideas • He died in Greenland in 1930, while seeking more evidence Sea-floor Spreading • Concept came from oceanographic investigations • Uses convection cells, an idea Wegener would have been familiar with Hess-Dietz Hypothesis The Oceans • Mid-ocean ridges • Rift valleys • Heat flow • Age of ridge Magnetic Stripes • As magma rises, it hardens and its magnetic field matches the present field of the earth - after a polarity reversal, it will be aligned against the earth’s field Magnetic Stripe Creation • Picture from USGS animation of magnetic stripe creation • Dark gray area are have magnetic orientation pointing north, light gray to the south Age of Ocean Fossils • Continental fossils are at least 3.5 billion years old • Oldest marine fossils, found on the ocean floor, are about 180 million years • Since life is though to originate in the oceans, why aren’t ocean fossils older? Plate Movement • Plates move slowly (up to 15 cm/yr) • Plates may collide, move apart, or slide past each other • Friction during plate movement often generates earthquakes Plate Motions • Two plates move relative to each other – Convergent - Plates move toward each other, often a head-on collision – Divergent - Plates move away from each other – Sideways - Plates move past each other along transform faults Converging Plates • When two plates collide, the denser plate will sink (subside) beneath the less dense plate • Density differences as small as 1% are enough to cause subduction Subduction Zones The key to subduction is the density of the rock types involved Density = mass/unit volume Continental rock has a density of about 3 g/cc Oceanic rock is about 3.28 Plate Types • At any given point, a plate is either oceanic or continental • Interactions between plates are thus: – Ocean-ocean (O-O) – Ocean-continent (O-C) – Continent-continent (C-C) Plate Interactions Conv. Divg. Trans. O-O SID Quakes Oceanic Arc Volcanism Japan SI Quakes Fissure Volcanism MOR S Quakes No volcanism Oceanic transform faults O-C SID Quakes Stratovolcanic Chains Cascades, Andes Not known Not known C-C SI Quakes No Volcanism Himalayas SI Quakes Alkaline volcanism East Africa S Quakes No volcanism San Andreas Fault Hot Spot Diagram • Diagram showing creation of several Hawaiian Islands • Age of islands should be progressively older as they move away, and this is observed Map of Major Tectonic Plates Mid-ocean Ridge Map Geologic Time 4,600,000,000 Years - Estimated Age of the Earth Relative Dating • Relative Age is the answers to a question like, “Which is younger?” • Relative ages allow us to compare different geologic formations, and determine which is the oldest, next oldest, etc. Principle of Superposition • Layers on the bottom were deposited first, and are the oldest (A older than B, B older than C, etc.) - think of paint layers on a wall • In any unaltered sequence of rocks, the oldest is at the bottom, the youngest at the top Index Fossils - Short existence on earth helps to pinpoint the age of the rock in which they are found • Organisms with specific characteristics: – Short lived (geologically) – Widespread occurrence – Readily recognized Unconformity • Gaps in the rock record, unconformities mark boundaries between rocks of different ages • Can be produced by: • A. erosion & sedimentation, • B. faulting • C. intrusions & extrusions Unconfomity – intrusions (black basalt & white quartz) • A feature, such as a dike or fault, that cuts formations is younger than the formations it cuts Unconformity: cross-cutting by faults & intrusions/extrusions • The uncomformity is younger than the rock it offsets Unconformity in Volcanic Ash • Outcrop photo of volcanic ash layers in Japan • There is an erosional discontinuity (unconformity) that separates earlier folding in the lower half from folding (above) after later ash flows were deposited. Absolute Age • Determination of the absolute age is usually done using radiometric dating • Absolute ages are expressed in years, or millions or billions of years, before present Radiometric Dating • A parent isotope that is radioactive decays to yield a daughter isotope at a known rate – Example: – 14C 14N – Radioactive decay follows an exponential decay law Illustration of Radiometric Decay Half-life, t½ • The time necessary for half of the original atoms of the parent isotope to decay to the daughter isotope is the half life Parent and Daughter Isotopes • In the previous example, – 14C 14N • 14C is the parent, and 14N is the daughter • The half-life, t½, is 5730 years Isotope Systems Parent Daughter Half-life Dating Range Materials Dated Rb87 Sr87 47 billion 10 million to 4.6 billion IgneousMetamorphic U238 Pb206 4.5 billion 10 million to 4.6 billion IgneousMetamorphic U235 Pb208 713 million 10 million to 4.6 billion IgneousMetamorphic K40 Ar40 1.3 billion 100,000 to 4.6 billion IgneousMetamorphic C14 N14 5730 100 to 100,000 C- bearing material Geologic Time Scale • Eons - Largest divisions of time, beginning with the Hadean (4.6 to 3.8 billion years ago) – Eras - Largest subdivision, defined by dominant life forms • Periods - Divisions of eras, based on smaller scale changes » Epochs - Divisions of periods in the Cenozoic era, based on detailed, smaller scale changes Geologic Time Recent History of the Earth Earth’s recent history is divided into eras: Paleozoic – 570 to 245 million years before present (MYBP) Mesozoic – 245 to 65 MYBP Cenozoic – 65 MYBP to present Paleozoic Era • Trilobite fossil, early Paleozoic era Mesozoic Era • Dinosaurs were the dominant life forms Cenozoic Era • Kangaroos are marsupials, a type of mammal Geologic Time Line