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EESA06H3 (234)
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Chapter 4

EESA06 - Chapter Four

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Environmental Science
Nick Eyles

Chapter Four - Page 1 of 8 Chapter Four: The Earths Interior The only rocks that can be studied are those of the Earths crust Mantle rocks brought to the earths surface in basalt flow, in diamond bearing kimberlite pipes, and also by the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust Meteorites give clues about the possible composition of the core of the earth Geophysics evidence suggest that the earth is divided into 3 major layers: crust on the surface, rocky mantle beneath the crust, and the metallic core at the centre The crust and uppermost mantle can be divided into the brittle lithosphere and the plastic asthenosphere Kola peninsula - The deepest scientific well has reached 12km beneath the surface (sedimentary basins) in Russia. It penetrated ancient Precambrian basement rocks Earth has a radius of 6.370km Deep parts of the earth are studied indirectly through geophysics the application of physical laws and principles to a study of the earth; includes the study of seismic waevs and the earths magnetic field, gravity and heat Deep Drilling on Continents Structure and composition of most of the continental crust is unknown Continents are probably largely igneous and metamorphic rock (such as granite and gneiss, overlain by a veneer of sedimentary rocks Second deepest well drilled is the KTB hole in southeastern Germany, which reached 10km WHAT CAN WE LEARN FROM THE STUDY OF SEISMIC WAVES? Important way of learning about earths interior is via study of seismic reflection Seismic reflection: the return of some of the energy of seismic waves to the earths surface after the waves bounce off a rock boundary If two rock layers of differing densities are separated by a fairly sharp boundary, seismic waves reflect off that boundary just as light reflects off a mirror These reflected waves are recorded on a seismogram shows the amount of time the waves took to travel down to the boundary, reflect off it, and return to the surface allows calculation of depth of the boundary Canadian lithoprobe project applying seismic reflection techniques to map crustal structures at the base of the crust Another method used to locate rock boundaries seismic refraction the bending of seismic waves as they pass from one material to another. As a seismic waves strikes a rock boundary, much of the energy of the wave passes across the boundary. As the wave crosses from one rock layer to another, it changes direction. This change of direction (refraction) only occurs if the seismic waves velocity is different in each layer (happens if the rock layers differ in density/strength) Seismograph station 1 is receiving seismic waves that pass directly through the upper layer A Chapter Four - Page 2 of 8 Stations farther from the epicentre, such as station 2, receive seismic waves from 2 pathways: 1. A direct path through layer A 2. A refracted path through layer A to a higher-velocity layer B and back to layer A. Station 2 receives the same wave twice Seismograph stations close to station 1 receive only the direct wave or possibly two waves, the direct (upper) wave arriving before the refracted (lower) wave Stations near station 2 receive both the direct and the refracted waves Even though the refracted wave travels farther, it can arrive at a station first because most of its path is in the high velocity layer B The distance between this point of transformation and the epicentre of the earthquake is a function of the depth to the rock boundary between A and B Sharp rock boundary isnt necessary for the refraction of seismic waves Canadian Lithoprobe Project Investigating the composition and structure of the Canadian shield and surrounding organic belts Aim is to develop a comprehensive understanding of the geological evolution of north America The shield is made up of distinct geological terranes that were once separate land masses but were brought together by the forces of plate tectonics Will help answer how continental configuration came to be/what tectonics were involved Will also help evaluate earthquake risk across the shield and find oil/gas reservoirs Uses seismic reflection Uses large vibroseis trucks (dancing elephants) work together to stamp in unison WHAT IS INSIDE THE EARTH? Three main zones of the earths interior: crush, mantle and core Crust: outer layer of rock, which forms a thin skin on earths surface Below the crust lies the mantle a thick shell of rock that separates the crust above from the core below The core is the central zone of earth. It is probably metallic and the source of earths magnetic field The Crust Studies of seismic waves have shown: o The crust is thinner beneath the oceans than beneath the continents o Seismic waves travel faster in oceanic crust than in continental crust Its assumed that the two types of crust are made up of different rocks Seismic P waves travel through oceanic crust (and basalt and gabbro) at 7km/second Upper part of the oceanic crust is basalt; lower part is gabbro Oceanic crust thickness = 7kmChapter Four - Page 3 of 8 Seismic P waves travel slower through continental crust (and granite and gneiss) 6km/second Continental crust is also called granitic Continental crust consists of a crystalline basement composed of granite, other plutonic rocks, gneisses, and schists, all capped by a layer of sedimentary rocks Felsic rocks high in feldspar and silicon continental crust Mafic rocks high in magnesium and iron (ferric) oceanic crust Continental crust is thicker than oceanic crust 30 to 50 km Crust is thickest under young mountain ranges andes and Himalayas bulging downward as a mountain root into the mantle Continental crust is less dense than oceanic crust Mohorovicic discontinuity the boundary that separates the crust from the mantle beneath Mantle lies closer to the earths surface beneath the ocean than the continents Project Mohole use special ships to drill through the oceanic crust and obtain mantle samples Oceanic Crust basalt underlain by gabbro; 7km Continental Crust granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover); 20-70km The Mantle Assumed to be mostly made of solid rock P waves travel at 8km/s in the upper mantle suggests a different rock
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