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Lecture 13

Earth Sciences 2123A/B Lecture 13: Final Exam Study Notes ES 2123 (Lecture 13-20)

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Department
Earth Sciences
Course Code
Earth Sciences 2123A/B
Professor
Richard Secco

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Description
FINAL EXAM – ES2123B 1 Lecture 13: Earthquakes Earthquake: sudden, violent dislocation in the subsurface caused by stress buildup on a fault.  The EQ is the RELEASE of stress  > 90% occur near plate boundaries (correlated) Earthquake Physics:  Tectonic forces build up, &  stresses in crustal blocks  Stresses become too large for crustal rocks to sustain  Sudden rupture of crustal block (“earthquake”)  Crust rebounds to a new equilibrium position  When rocks slip suddenly, energy is released in waves that travel through Earth’s crust and cause shaking Time Sequence of Reid’s Elastic Rebound Theory: 1. Before EQ: stress accumulates by tectonic forces 2. During EQ: stress released by motion on a new/pre-existing rupture or fracture zone (fault) 3. Immediately after EQ: crust rebounds to an unstressed equilibrium position 4. Following equilibrium: stress re-accumulates (tectonism = continuous) Fault: sfc (plane) across which 2 blocks can move relative to one another  Allow blocks to move relative to each other  Rapid movement = EQ  Slow movement = creep Plate boundaries and associated Fault: 1. Divergent  Normal 2. Compressional  Thrust (Reverse) 3. Transform  Strike-slip Benioff-Wadati Zone  Dipping (subduction) zone of EQs are found as deep as 660km in the mantle  Occurs at convergent plate boundaries 2 Earthquake Terminology: Epicentre: point on the Earth’s sfc directly above the hypocentre Focus: location on a fault where EQ rupture initiates  Hypocentre: calculated position of an earthquake focus Magnitude: quantitative strength of the EQ; determined by seismographic observations (ground motion) Lecture 14: Measuring Earthquakes Seismometer Design:  Any movement of the ground moves the frame  The mass tends not to move  Geophones: measures ground motion (vertically/horizontally)  First “seismoscope” in China\ Locating Earthquakes:  S-P time and distance indicate earthquake  Locating epicentres requires S-P time from at least 3 stations (triangulation) “Size” of earthquakes determined by either: 1. Instrument (Magnitude) a. Richter magnitude b. Surface-wave magnitude c. Moment magnitude 2. Felt (Intensity) – shaking FINAL EXAM – ES2123B 3 How to measure the magnitude of an EQ:  The Richter Scale: logarithm of the amplitude of waves recorded by seismographs o each whole # increase corresponds to the release of about 32 X more energy o (Ex: M5.5 is about 32 times more powerful than M4.5) o  A = ground amp (microns)   = epicentre distance (degrees)  divide by 111 km  degrees  Moment Magnitude : measure of energy release o This is the instrument we currently use o Seismic Moment: o Moment Magnitude: o Increase in energy released = higher magnitude EQ Damage – Intensity Scale  Intensity: measure of the effects of an EQ  Magnitude = constant; Intensity changes with epicentral distance  Internet intensity maps that allow people to fill out a survey to estimate intensity Modified Mercalli Intensity Values: 1: Not felt 3: Felt indoors; hanging objects swing; duration estimated; may not be recognized as EQ 6: Moderate Shaking; objects fall; dishes and windows break 9: Violent Shaking; heavy damage (general damage to foundations like frame structures 12: Damage nearly total; large rock masses displaced (Increasing Intensity # = increase damage) Lecture 15: Volcanism – Intro and Magma Properties Volcanism: when less dense liquid rises to erupt at the surface as lava Source of heat for melting rock: 4  Magma’s heat sourced at the Earth’s core  Transported to the surface through convection and conduction Melting: can occur if enough energy is provided to break the atomic bonds of the solid Partial Melting: mix of melted and solid materials  Different rocks have different melting points (minerals have different melting temps)  Solidus: the line of temps when partial melt begins o Depth zone of partial melt: where geotherm intersects solidus  Liquidus: the line of temps and pressures when material = liquid (molten) Magma Differentiation:  As magma cools, mafic magma solidifies first and sinks  Felsic magma (still molten) may continue to rise Magma Viscosity:  Viscosity: measure of a fluid’s resistance to flow  Flow well = low viscosity  Resistive to flow = high viscosity Viscosity and Mobility:  Viscosity of magma  as temperature  o Low viscosity = high temp (becoming more liquid) o High viscosity = low temp (becoming more solid)  Low viscosity lavas flow easily and can cover thousands of (large areas)  High viscosity lavas flow slowly and cover small areas Silica and Viscosity:  combines with elements in the magma (ex: Fe, Mg, K) to form small framework structures in the magma causing the magma to be more viscous  Viscosity unit: poise Dissolved gas content, viscosity, and explosiveness of eruptions: FINAL EXAM – ES2123B 5  Dissolved gases () – trapped with increased pressure  As magma approaches sfc, and pressure , gas comes out of solution and forms bubbles which push the magma to the sfc  When magma reaches the sfc and becomes lava, the gases rapidly escape, causing an explosion  Gas can escape more easily from low viscosity material than high viscosity material  More violent eruptions occur when gases cannot escape easily  If it is diff for the gas to form bubbles, it will cause the material to fragment causing large explosive eruptions  Analogy: blowing air through a straw (water (low viscosity): will bubble readily and escape easily; milkshake (high viscosity): cannot bubble and lose its gas as readily, which allows gas pressure to build up resulting in a more violent “eruption” once the gas is finally released Lecture 16: Volcanism – Hot Spots and Plate Boundaries The degassing of magma through volcanism created the  Atmosphere  Hydrosphere 3 Ways that Earth Melts Rock: 1. Plume Heating (Intra-plate: Hot Spots) a. Arrival of hot mantle plume raises the geotherm enough to produce melting b. Plumes ae warmer than the surrounding mantle which causes them to rise c. New evidence: slow seismic velocities indicate that the plume is a region of hot molten that rises through the mantle bc of buoyancy d. Location doesn’t coincide with plate boundaries e. Effusive Eruptions (above hotspots) ie: Hawaii e.i. Lava flows (Mafic, low viscosity, basaltic: high Mg + Fe, low Q) e.ii. Pahoehoe Lava (ropy; sfc cools quick, but lava below continues to flow) e.iii. A’a Lava (blocky) 2. Decompression Melting (Divergent Boundaries) 6 a. Pressure decreases as magma rises quickly (temperature = constant) which causes the geotherm to reach the solidus b. Largest volume of new rock in the lithosphere generated at spreading ridges c. Mafic magma (created by decompression melting of Peridotite, and is partially melted into basalt) 3. Hydration Melting (Convergent Boundaries) a. Water significantly lowers rock melting temp b. Solidus shifts to a lower temperature (w/ presence of water) and reaches the geotherm c. Water comes from subducted oceanic crust d. Andesite (felsic) and Rhyolite (intermediate) Lecture 17: Earth’s Magnetic Field – How the field is generated History of Geomagnetism  Lodestone: mineral tha
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