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2nd Year Notes 2016 : Global Geophysics (Indian Institute of Technology (IIT) Bombay, Mumbai)

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Dipayan Mukherjee
Ramakrishna Mission Vidyalaya (RKMV), Narendrapur

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Global Geophysics GP401 Lecture The Dynamic Earth Books Fundamentals of Geophysics William Lowrie Geodynamics D L Turcotte and G Schubert Plate Tectonics and Crustal Evolution K C Condie The Blue Planet Skinner and Porter The Solid Earth C Fowler Geophysics The principles of physics applied to study the earth. Geophysical investigations of the interior of the Earth involve taking measurements at or near the Earth's surface. The measurements are influenced by the internal distribution of physical properties. Analysis of the measurements reveal how the physical properties of the Earth's interior vary vertically and laterally. By working at different scales, geophysical methods may be applied to a wide range of investigations from studies of the entire Earth (global geophysics) to exploration of a localized region of the upper crust for engineering or other economical purposes (exploration geophysics). Applied Geophysics - Methods Method Measured parameter Operative physical property Seismic Travel times of reflected /refracted seismic waves Density and elastic moduli, which determine the propagation velocity of seismic waves Gravity Spatial variations in the strength of the gravitational field of the Earth Density Magnetic Spatial variations in the Magnetic susceptibility and remanence strength of the geomagnetic field Electrical DC Resistivity Induced polarization Self-potential Electromagnetic Radar Polarization voltages or frequency Electrical conductivity dependent ground resistance Electrical capacitance Electrical conductivity Electrical potentials Response to electromagnetic radiation Travel times of reflected radar pulses Electrical conductivity Electrical conductivity and inductance Dielectric constant Global Geophysics Plate tectonics Gravity Geomagnetism Global Heat flow Mantle/Core dynamics Seismology Ocean-atmosphere interaction They all reveal that our Earth is dynamic. The Dynamic Earth The Earth is a dynamic planet. The surface is constantly being changing both externally and internally, throughout its geological history. The external forces of change are erosion and deposition (exogenic process) and volcanism and tectonism (internal origin). Regions covered with past ice-sheets are still rebounding vertically (rate several mm/yr). Tectonic forces are causing mountains to rise. Continents move relative to each other at speeds of up to several cm/yr. The Dynamic Earth The Earth s interior is also in motion. The mantle appears hard and solid to seismic waves, but flows at rates of several cm/yr. Still deeper, the Earth s liquid outer core flows at a rapid rate of a few tenths of a mm/sec (~10-4 mm/sec). At mid-oceanic ridges, new crust are always getting formed which ensures that the plates keep moving. As a process, continents move. The Dynamic Earth - Gondwana Dispersal Gondwana Dispersal The continents on our globe has been steadily moving with respect to each other over geological time. Today s familiar world map of continents and oceans is just a snapshot of a continuous process of movement that has been going on for thousands of millions of years. Rates of movement are typically only a few centimetres per year but these are frequently bought to our attention by the occurrence of the earthquakes and volcanoes that, for the most part, are confined to narrow marginal zones that separate the present large tectonic plates. During much of Phanerozoic time, i.e. from about 540 Ma (million years ago) when the Precambrian came to an end, the present southern continents made up a single stable supercontinent known as Gondwana. Gondwana Dispersal From about 180 Ma, however, this entity started to rift and fracture and new oceans began to form between the resulting fragments. These oceans the South Atlantic, Southern and Indian Oceans now occupy almost half of the world s surface area. It is only relatively recently (1960s) that earth scientists have realized that it is the growth and eventual consumption of such oceans that is central to the prolonged evolution of the earth s crust through global tectonics. Ocean crust itself is relatively short-lived; no substantial areas of ocean crust exist today that are much older than the start of Gondwana disruption. Gondwana dispersal The lighter, more buoyant continents, by contrast, contain the geological record that goes back to the oldest known rocks, in excess of 4000 million years (4 Ga) in age. It was only even more recently (late 1990s) that technology could map in detail the topography of the ocean floor, from which the precise pattern of its creation with time could be worked out in considerable detail (Smith and Sandwell, 1997). The Dynamic Earth simplified layered interior structure of Earth Duffy, 2011, Nature 479, 480 481 Earth Structure The study of seismic waves led to the revelation that Earth has layered structure, where boundaries (or discontinuities) are marked by abrupt changes in seismic velocity. Each layer is characterized by distinct physical properties determined by composition, pressure and temperature in the layer. The four main layers of the Earth are the crust, the mantle, the outer and the inner core. Earth Structure The seismic discontinuity between crust and mantle is called Mohorovicic discontinuity (A. Mohorovicic, 1909). The travel times of seismic compressional wave that traversed the body of the Earth were greater than expected. The delay was attributed to a fluid outer core (Gutenberg discontinuity, 1914). It was also observed that there is a shadow zone for seismic waves at epicentral distances > 1050. Lehmann (1936) observed the weak arrivals of compressional waves in the gap between 1050 and 1430. These are interpreted to be due to a solid inner core. Seismic ray path in the Earth Lithospheric plates Although interior of the Earth is spherically symmetric, this is not valid for crust and mantle. They show lateral variations. Lithosphere An outer shell of 100-150 km under continents and 70-100 km below oceans are rigid and hard called lithosphere. Asthenosphere A layer of 150 km thick beneath lithosphere shows lower seismic velocities, which suggests lower rigidity. It is thought to be partially molten (flows like viscous liquid/plastic solid) Lithospheric plates Hypothetical vertical cross-section through a Lithospheric plate from a spreading center to a subduction zone Plate tectonics lithospheric plates Plate tectonics epicenters of earthquakes Lithosphere and Asthenosphere The lithosphere is brittle which causes it to fracture when strongly stressed. The rupture produces an earthquake. Earthquake is a violent release of elastic energy due to sudden displacement on a fault plane. Earthquakes occur in a well-defined seismic zones that are associated with volcanic activity. The seismic zones subdivide the lithosphere laterally into tectonic plates. A plate can be as broad as 10,000 km (Pacific plate), or as small as a few 1000 km (Philippines plate).

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