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Chapter 6 Lecture Outline Foundations of Earth Science Seventh Edition Restless Earth: Earthquakes, Geologic Structures, and Mountain Building Natalie Bursztyn Utah State University © 2014 Pearson Education, Inc. Focus Question 6.1 • What is the mechanism that generates large earthquakes? © 2014 Pearson Education, Inc. What Is an Earthquake? • The sudden movement of one block of rock slipping past another along a fault • Most faults are locked until a sudden slip • Seismic waves radiate out from the focus (hypocenter), where slip begins – Earth’s surface directly above the hypocenter is the epicenter © 2014 Pearson Education, Inc. What Is an Earthquake? © 2014 Pearson Education, Inc. What Is an Earthquake? • Weak earthquakes can be generated by – Volcanoes, landslides, and meteorite impacts • Destructive earthquakes occur because tectonic motion builds up stress – Friction keeps the fault from slipping – Slip initiates when stress overcomes friction – Elastic rebound causes deformed rock to spring back to undeformed position © 2014 Pearson Education, Inc. What Is an Earthquake? © 2014 Pearson Education, Inc. What Is an Earthquake? • Transform faults have many branches and fractures – Offset occurs in distinct segments • Some segments displace slowly with little shaking during fault creep • Some segments produce numerous small earthquakes • Some segments are locked for hundreds of years and rupture in large earthquakes – Earthquakes occur in repetitive cycles – Rupture propagates in a discrete time period © 2014 Pearson Education, Inc. What Is an Earthquake? © 2014 Pearson Education, Inc. What Is an Earthquake? • Convergent plate boundaries generate compressional forces – Mountain building and faulting associated with large earthquakes – Subducting plates form a megathrust fault • Produce the most powerful earthquakes • Vertical motion underneath the ocean generates tsunamis © 2014 Pearson Education, Inc. What Is an Earthquake? • Are there any local faults? If so, what type are they? • Have you ever experienced an earthquake? If so, describe it. © 2014 Pearson Education, Inc. Focus Question 6.1 • What is the mechanism that generates large earthquakes? – Stress (from movement of tectonic plates) accumulates in a block of crust until it overcomes frictional resistance and slip occurs – Seismic waves propagate out from the hypocenter © 2014 Pearson Education, Inc. Focus Question 6.2 • What are the different types of seismic waves? © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves • The study of earthquake waves is known as seismology • Waves are measured by seismometers – Intertia keeps a weighted arm from moving while ground motion moves the instrument – Amplifies ground motion – Generates seismograms © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves • Two main types of seismic waves generated by earthquakes: – Surface waves travel in rock layers just below Earth’s surface – Body waves travel through Earth’s interior © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves • Two types of body waves: – Primary or P waves • Push/pull rocks in direction that wave is traveling • Temporarily change volume of material • Travel through solids, liquids, and gasses – Secondary or S waves • Shake particles at right angles to direction that wave is traveling • Change shape of material • Do not travel through liquids or gasses © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves • Two types of surface waves: – Some travel along Earth’s surface like rolling ocean waves – Others move Earth materials from side to side • Most damaging type of ground motion © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves © 2014 Pearson Education, Inc. Seismology: The Study of Earthquake Waves • Speed of travel is very different for each type of wave © 2014 Pearson Education, Inc. Focus Question 6.2 • What are the different types of seismic waves? – Surface waves travel at Earth’s surface and body waves travel through the interior – Surface waves roll or move side to side – Body waves include P waves and S waves • P waves travel faster and push/pull material • S waves move material from side to side © 2014 Pearson Education, Inc. Focus Question 6.3 • How are seismographs used to locate an earthquake epicenter? © 2014 Pearson Education, Inc. Locating the Source of an Earthquake • P waves travel faster than S waves • Difference in arrival time is exaggerated by distance – Greater interval between P and S wave arrivals indicates greater distance to epicenter • Seismographs can be used to triangulate the epicenter of an earthquake © 2014 Pearson Education, Inc. Locating the Source of an Earthquake © 2014 Pearson Education, Inc. Locating the Source of an Earthquake © 2014 Pearson Education, Inc. Locating the Source of an Earthquake © 2014 Pearson Education, Inc. Focus Question 6.3 • How are seismographs used to locate an earthquake epicenter? – Difference between P wave and S wave arrival time increases with distance from epicenter – Multiple seismographs are used to triangulate the epicenter © 2014 Pearson Education, Inc. Focus Question 6.4 • What scales are used to measure earthquakes? © 2014 Pearson Education, Inc. Determining the Size of an Earthquake • Intensity measures the amount of ground shaking based on property damage – Used for historical records – Modified Mercalli Intensity Scale developed in California in 1902 • Magnitude is a quantitative measure of energy released in an earthquake – Richter scale is related to the amplitude of the largest seismic wave – Moment magnitude measures total energy released based on amount of slide, area of rupture, and strength of faulted rock © 2014 Pearson Education, Inc. Determining the Size of an Earthquake © 2014 Pearson Education, Inc. Determining the Size of an Earthquake © 2014 Pearson Education, Inc. Focus Question 6.4 • What scales are used to measure earthquakes? – Intensity scales based on property damage • Modified Mercalli Intensity Scale – Magnitude scales based on measure of energy released • Richter scale • Moment magnitude © 2014 Pearson Education, Inc. Focus Question 6.5 • What destructive forces are triggered by earthquake vibrations? © 2014 Pearson Education, Inc. Earthquake Destruction • Magnitude and other factors determine degree of destruction • Area 20-50 km surrounding the epicenter experiences equal shaking • Ground motion diminishes rapidly outside of 50 km • Earthquakes in stable interiors are felt over a larger area © 2014 Pearson Education, Inc. Earthquake Destruction • Earthquake damage depends on: – – – – – Intensity Duration Nature of surface materials Nature of building materials Construction practices • Flexible wood and steel-reinforced buildings withstand vibrations better • Blocks and bricks generally sustain the most damage © 2014 Pearson Education, Inc. Earthquake Destruction © 2014 Pearson Education, Inc. Earthquake Destruction • Soft sediment amplifies vibrations • Vibrations cause loosely packed, waterlogged materials to behave like a fluid – During liquefaction stable soil becomes mobile and rises to the surface • Vibrations also trigger landslides, ground subsidence, and fires © 2014 Pearson Education, Inc. Earthquake Destruction © 2014 Pearson Education, Inc. Earthquake Destruction © 2014 Pearson Education, Inc. Earthquake Destruction • Megathrust displacement lifts large slabs of seafloor, displaces water, and generates a tsunami – Low amplitude wave travels at very high speed in open ocean – Amplitude can reach tens of meters in shallow coastal waters – Arrival on shore is preceded by a rapid withdrawal of water from beaches, followed by what appears as a rapid rise in sea level with a turbulent surface © 2014 Pearson Education, Inc. Earthquake Destruction © 2014 Pearson Education, Inc. Focus Question 6.5 • What destructive forces are triggered by earthquake vibrations? – – – – – – Ground motion Liquefaction Landslides Ground subsidence Fires Tsunamis © 2014 Pearson Education, Inc. Focus Question 6.6 • Where are the major earthquake belts? © 2014 Pearson Education, Inc. Earthquake Belts and Plate Boundaries • Circum-Pacific belt convergent boundaries experience 95% of earthquakes – Megathrust faults generate largest earthquakes • Earthquakes along Alpine-Himalayan belt because of continental collision • Weak earthquakes along oceanic ridge system because tension pulls plates apart • Transform and strike-slip faults generate large, cyclical earthquakes © 2014 Pearson Education, Inc. Earthquake Belts and Plate Boundaries © 2014 Pearson Education, Inc. Focus Question 6.6 • Where are the major earthquake belts? – Subduction zones around the Pacific ocean – Alpine-Himalaya belt – Oceanic ridge systems © 2014 Pearson Education, Inc. Focus Questions 6.7 • Why is Earth layered? • How are seismic waves used to describe Earth’s interior? © 2014 Pearson Education, Inc. Earth’s Interior • Earth has distinct layers: – Heaviest material at the center, lightest at top – Iron core, rocky mantle and crust, water ocean, gaseous atmosphere • Interior is dynamic – Mantle and crust are in motion – Material is recycled from surface to deep interior © 2014 Pearson Education, Inc. Earth’s Interior • Temperature increased as material accumulated to form Earth – Iron and nickel melted and sank to the center to produce iron-rich core – Buoyant rock rose to the surface and formed crust rich in O, Si, and Al (+ Ca, Na, K, Fe, and Mg) – Chemical segregation led to iron-rich core, primitive crust, and mantle © 2014 Pearson Education, Inc. Earth’s Interior • Seismic waves are the only way to “see” inside the interior – – – – Waves are reflected at boundaries Refracted through layers Diffracted around obstacles Velocity increases with depth as stiffness and compressibility of rock change • Can be used to interpret composition and temperature of rock © 2014 Pearson Education, Inc. Earth’s Interior © 2014 Pearson Education, Inc. Focus Questions 6.7 • Why is Earth layered? – Physical and chemical segregation of molten material when Earth formed • How are seismic waves used to describe Earth’s interior? – Reflection, refraction, diffraction, and velocity are used to interpret composition and temperature of rock © 2014 Pearson Education, Inc. Focus Question 6.8 • What is each of Earth’s layers like? © 2014 Pearson Education, Inc. Earth’s Layers • Earth is divided into three compositionally distinct layers: – Crust – Mantle – Core • Can be further subdivided into zones based on physical properties – Solid or liquid – Strength © 2014 Pearson Education, Inc. Earth’s Layers • Thin, rocky crust is divided into: • Oceanic crust – ~7 km thick – Composed of basalt – Density ~3.0 g/cm3 • Continental crust – ~35 – 40 (up to 70) km thick – Many rock types – Average density ~2.7 g/cm3 © 2014 Pearson Education, Inc. Earth’s Layers • Mantle – Solid layer extending to 2900 km – 82% of Earth’s volume • Chemical change at boundary between crust and mantle • Uppermost mantle (first 660 km) is peridotite – Stiff top of upper mantle (+ crust) is lithosphere • Cool, rigid outer shell • ~100 km thick • Weaker portion below is asthenosphere – Upper asthenosphere is partially melted – Lithosphere moves independently of asthenosphere © 2014 Pearson Education, Inc. Earth’s Layers • Core is an iron-nickel alloy – Density ~10 g/cm3 – Outer core is liquid • 2270 km thick • Generates Earth’s magnetic field – Inner core is solid sphere • 1216 km radius © 2014 Pearson Education, Inc. Earth’s Layers © 2014 Pearson Education, Inc. Focus Question 6.8 • What is each of Earth’s layers like? • Crust is thin, solid – Oceanic (thin, basaltic) and continental (thicker, granodiorite) • Mantle is solid • Crust + upper mantle = lithosphere • Core is divided into outer liquid and inner solid cores © 2014 Pearson Education, Inc. Focus Questions 6.9 • What is brittle deformation? • What is ductile deformation? © 2014 Pearson Education, Inc. Rock Deformation • Deformation – All changes in shape, position, or orientation of a rock mass – Bending and breaking occurs when stress exceeds strength © 2014 Pearson Education, Inc. Rock Deformation • Elastic deformation – Stress is gradually applied – Rocks return to original size and shape when stress is removed – Ductile or brittle deformation occurs when elastic limit is surpassed • Strength of a rock is influenced by temperature, confining pressure, rock type, and time © 2014 Pearson Education, Inc. Rock Deformation • Brittle deformation – Results in fractures – Common at/near surface • Ductile deformation – Solid-state flow at great depths – Produces a change in the size and shape of a rock – Some chemical bonds break and new ones form © 2014 Pearson Education, Inc. Focus Questions 6.9 • What is brittle deformation? – Fracturing of rock when the elastic limit of a rock is surpassed that occurs at or near the surface • What is ductile deformation? – Change in the size and shape of a rock that occurs at depth © 2014 Pearson Education, Inc. Focus Question 6.10 • What are the major types of folds? © 2014 Pearson Education, Inc. Folds: Structures Formed by Ductile Deformation • Folds are wavelike undulations that form when rocks bend under compression • Compressional forces result in shortening and thickening of the crust © 2014 Pearson Education, Inc. Folds: Structures Formed by Ductile Deformation • Anticlines – Upfolded or arched layers • Synclines – Associated downfolds or troughs • Symmetrical – Limbs are mirror images • Asymmetrical – Limbs are different • Overturned – One or both limbs are tilted beyond the vertical • Recumbant – Axis is horizontal © 2014 Pearson Education, Inc. Folds: Structures Formed by Ductile Deformation • Circular or elongated upwarping structures are called domes – Upwarps in basement rocks deform overlying sedimentary strata • Downwarping structures are called basins © 2014 Pearson Education, Inc. Folds: Structures Formed by Ductile Deformation © 2014 Pearson Education, Inc. Focus Question 6.10 • What are the major types of folds? – Anticlines, synclines, overturned, recumbent © 2014 Pearson Education, Inc. Focus Question 6.11 • What is the relative motion on normal, reverse, and strike-slip faults? © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation • Faults – Fractures in the crust with appreciable displacement – Movement parallel to dip are dip-slip faults • Rock surface above the fault is the hanging wall block • Surface below the fault is the foot-wall block • Fault scarps – Long, low cliffs produced by vertical displacement © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation • Normal faults – Hanging wall moves down relative to footwall – Accommodate extension of crust • Fault-block mountains are associated with large normal faults – Uplifted blocks are elevated topography called horsts – Down-dropped blocks are basins called grabens © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation • Reverse faults – Hanging wall moves up relative to footwall – Thrust faults • Reverse faults with a dip of less than 45º – Result from compressional stress – Accommodate crustal shortening © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation • A strike-slip fault – Exhibits horizontal displacement – Parallel to strike © 2014 Pearson Education, Inc. Faults: Structures Formed by Brittle Deformation © 2014 Pearson Education, Inc. Focus Question 6.11 • What is the relative motion on normal, reverse, and strike-slip faults? – Hanging wall block down relative to footwall block = normal fault – Hanging wall block up relative to footwall block = reverse fault – Horizontal displacement = strike-slip fault © 2014 Pearson Education, Inc. Focus Question 6.12 • Where are Earth’s major mountain belts? © 2014 Pearson Education, Inc. Mountain Building • Orogenesis is the set of processes that forms a mountain belt • Older mountain chains are eroded and less topographically prominent • Compressional mountains – Large quantities of preexisting sedimentary and crystalline rocks that have been faulted and folded © 2014 Pearson Education, Inc. Mountain Building © 2014 Pearson Education, Inc. Focus Question 6.12 • Where are Earth’s major mountain belts? – Young mountain belts at convergent boundaries © 2014 Pearson Education, Inc. Focus Question 6.13 • What are the major features of an Andean-type mountain belt and how are they generated? © 2014 Pearson Education, Inc. Subduction and Mountain Building • Subduction of oceanic lithosphere is the driving force of orogenesis – Volcanic island arcs build mountains by volcanic activity, emplacement of plutons, and the accumulation of sediment from the subducting plate onto the upper plate © 2014 Pearson Education, Inc. Subduction and Mountain Building • Continental volcanic arcs form at Andeantype convergent zones – Before subduction, sediment accumulates on a passive continental margin – Becomes an active continental margin when a subduction zone forms and deformation begins – An accretionary wedge is an accumulation of sedimentary and metamorphic rocks scraped from the subducting plate © 2014 Pearson Education, Inc. Subduction and Mountain Building © 2014 Pearson Education, Inc. Focus Question 6.13 • What are the major features of an Andeantype mountain belt and how are they generated? – Continental volcanic arc – Accretionary wedge © 2014 Pearson Education, Inc. Focus Question 6.14 • What are the stages of development in a collisional mountain belt? © 2014 Pearson Education, Inc. Collisional Mountain Belts • Cordilleran-type mountain building occurs in Pacific-like ocean basins – Rapid seafloor spreading is balanced by rapid subduction – Island arcs and crustal fragments (terranes) collide with a continental margin • Some terranes may have been microcontinents • Small terranes are subducted • Larger terranes are thrust onto the continent © 2014 Pearson Education, Inc. Collisional Mountain Belts © 2014 Pearson Education, Inc. Collisional Mountain Belts • Alpine-type orogenies result from continental collisions – Himalayas – Collision between Indian and Eurasian plates © 2014 Pearson Education, Inc. Collisional Mountain Belts © 2014 Pearson Education, Inc. Focus Question 6.14 • What are the stages of development in a collisional mountain belt? – Large terranes are thrust onto a continental block during subduction © 2014 Pearson Education, Inc.