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Geophysics

Be the first to like this. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings.

School of Earth and Environment, Geology and Geophysics

In Geophysics, principles of Physics are applied to study the "Interior" of the Earth. Depending on the problem under study, one has to decide which method should be applied. The gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide. Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases. The geopotential surface called the geoid is one definition of the shape of the Earth.

The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents such as with very narrow canals.

Looking into the Earth: An Introduction to Geological Geophysics - ProQuest

The Earth is cooling, and the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. Heat is mostly carried to the surface by thermal convection , although there are two thermal boundary layers — the core-mantle boundary and the lithosphere — in which heat is transported by conduction.

The heat flow at the Earth's surface is about 4. Seismic waves are vibrations that travel through the Earth's interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.

Recording of seismic waves from controlled sources provide information on the region that the waves travel through.

If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas. Earthquakes pose a risk to humans. Understanding their mechanisms, which depend on the type of earthquake e. Although we mainly notice electricity during thunderstorms , there is always a downward electric field near the surface that averages volts per meter.

A current of about amperes flows in the global circuit. The flow is manifested by lightning below the clouds and sprites above. A variety of electric methods are used in geophysical survey. Some measure spontaneous potential , a potential that arises in the ground because of man-made or natural disturbances.

Telluric currents flow in Earth and the oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies such as seawater across the Earth's permanent magnetic field. Geophysicists can also provide the electric current themselves see induced polarization and electrical resistivity tomography.

Electromagnetic waves occur in the ionosphere and magnetosphere as well as in Earth's outer core. Dawn chorus is believed to be caused by high-energy electrons that get caught in the Van Allen radiation belt. Whistlers are produced by lightning strikes. Hiss may be generated by both. Electromagnetic waves may also be generated by earthquakes see seismo-electromagnetics. In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic Rossby waves may be one source of geomagnetic secular variation.

Electromagnetic methods that are used for geophysical survey include transient electromagnetics , magnetotellurics , surface nuclear magnetic resonance and electromagnetic seabed logging. The Earth's magnetic field protects the Earth from the deadly solar wind and has long been used for navigation. It originates in the fluid motions of the outer core. The Earth's field is roughly like a tilted dipole , but it changes over time a phenomenon called geomagnetic secular variation.

Mostly the geomagnetic pole stays near the geographic pole , but at random intervals averaging , to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals , analyzed within a Geomagnetic Polarity Time Scale , contain polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the Laschamp event occurring 41, years ago during the last glacial period.

Event calendar

Geologists observed geomagnetic reversal recorded in volcanic rocks, through magnetostratigraphy correlation see natural remanent magnetization and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor spreading , a part of plate tectonics. They are the basis of magnetostratigraphy , which correlates magnetic reversals with other stratigraphies to construct geologic time scales.

Unstable isotopes decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras. Fluid motions occur in the magnetosphere, atmosphere , ocean, mantle and core.

Even the mantle, though it has an enormous viscosity , flows like a fluid over long time intervals. This flow is reflected in phenomena such as isostasy , post-glacial rebound and mantle plumes. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface.

Waves and other phenomena in the magnetosphere can be modeled using magnetohydrodynamics.

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The physical properties of minerals must be understood to infer the composition of the Earth's interior from seismology , the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals; their high-pressure phase diagrams , melting points and equations of state at high pressure; and the rheological properties of rocks, or their ability to flow. Deformation of rocks by creep make flow possible, although over short times the rocks are brittle.

The viscosity of rocks is affected by temperature and pressure, and in turn determines the rates at which tectonic plates move. Water is a very complex substance and its unique properties are essential for life. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere.

The many types of precipitation involve a complex mixture of processes such as coalescence , supercooling and supersaturation. Physical properties of water such as salinity have a large effect on its motion in the oceans. The many phases of ice form the cryosphere and come in forms like ice sheets , glaciers , sea ice , freshwater ice, snow, and frozen ground or permafrost.