GEOL 108Lg 1st Edition Lecture 1Outline of Current LectureI. SeismologyII. Seismic Wavesa. S waves and P wavesIII. Refraction and ReflectionIV. IsostasyV. Continent-Ocean DichotomyVI. Magnetic AnomaliesVII. Seafloor SpreadingCurrent LecturePlate Tectonics, Part OneEvolution of Our View of the Earth’s InteriorSeismology: a science particularly concerned with earthquakes – artificially creating vibrations in the earth; studying the way that waves propagate the earth. The arrival times of the waves depend on their distance from the core, as velocities increase with depth. Refraction: when a seismic ray changes orientation as velocity then increases. As to why velocity increases with depth: the deeper you are beneath the surface of the earth, the more pressure you are receiving from the rock above you. The further down you travel into the earth, the depth, density and temperature all increase. As we know, the earth was born out of heat, and there is still measurable radioactive decay.How do we know the physical structure of the earth?Seismic waves are any waves that travel through the earth – any kind of large or explosion causes them, sending energy through Earth itself. They are and were crucial to figuring outwhat the actual composition of the Earth (e.g. its layers, the material that composites the layers,etc.) There are two types of waves: Surface waves and Body waves ( also known as S waves and P waves). Body waves are the ones that are used to figure out the earth’s structure, as they are not simply traveling across the surface of a substance. P (Primary) Waves: shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving. They can travel through air, liquids, and solids. S (Secondary) Waves: or shear wave, shakes the ground back and forth perpendicular to the direction the wave is moving. S waves ONLY travel in solids. Refraction and Reflection, contd.Refraction of a seismic wave is essentially the deflection or bending of the ray’s course caused by its passage from one material to another that is composed of different elastic properties. Justas a mirror reflects light from its surface, wave reflection is the wave/energy created by an earthquake or disturbance inside the earth that has been reflected by a boundary between two different materials within the Earth.a. Refractionb.c. Reflection Scientists soon realized that a core was necessary to produce reflective seismic wave. P waves arrive first, then the S wave—their time difference is how far the source of the earthquake is away from the earth’s crust. S waves are not observed, as they do not go through the outer parts of the core, so the velocity of the shear wave has to go to zero (b/c the velocity of 0—no shear elastic resistance—this means that the outer core must be liquid, not solid.)We now know that the earth has an iron core, with an outer core that is liquid and a primarily silicone mantle. Crust--->mantle--->core.There are two types of crust, oceanic and continental. The oceanic crust layer is essentially 5km while the continental can range between 30-70km. Our Earth’s tomography is as a result of density change. Isostasy is the state of equilibrium that both continental and oceanic sections ofthe earth's lithosphere are believed to achieve when the vertical forces upon them are unchanged. The lithosphere (strong, upper 100 kilometers, and the more cool mantle stuff) floats upon the semi fluid asthenosphere (weak) below. If a section of lithosphere is loaded, for example with frozen ice, it will slowly subside to a different position to achieve equilibrium. If a section of the lithosphere is reduced in mass (caused by…take erosion, primarily) it will slowly rise to a new equilibrium position.Continent-Ocean DichotomyContinents have the thick curst—erosion is dependent on the material and climate. Isostasy, however, doesn’t account for the theory of Continental Drift. Pangea, the massive conglomerate of the yet to be separated continents, broke apart. 250 million years ago, they were a single mass—a super continent, if you will. Panthalassa was the name of the ocean. Applying isostasy to the theory of continental drift means that crustal rocks would shatter if pushed. Magnetic AnomaliesThe magnetic positioning of the earth (North/South poles) reverses every few million years. PaleomagnetismWhen magma turns to rock, magnetic minerals in the rock all orient towards the north pole. Magnetic poles show periods of time when N is S,and S is N. This creates the magnetic time scale that scientists reference today. Seafloor SpreadingThere is always new seafloor—it is continuously recycled in a process that is called seafloor spreading. This involves material moving to the side, becoming frozen, and making way for new material. This creates massive subduction zones, such as the mid ocean ridge. EX: Deep earthquakes->subduction zones->material being made->seafloor spreading->material converging->TRENCH in the earth<-subduction