* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Evolution of Earth`s Atmosphere
Post-glacial rebound wikipedia , lookup
Physical oceanography wikipedia , lookup
Spherical Earth wikipedia , lookup
History of geomagnetism wikipedia , lookup
Schiehallion experiment wikipedia , lookup
Geochemistry wikipedia , lookup
Seismic anisotropy wikipedia , lookup
History of geology wikipedia , lookup
Seismic communication wikipedia , lookup
History of Earth wikipedia , lookup
Age of the Earth wikipedia , lookup
Plate tectonics wikipedia , lookup
Seismic inversion wikipedia , lookup
Earthquake engineering wikipedia , lookup
Reflection seismology wikipedia , lookup
Future of Earth wikipedia , lookup
Surface wave inversion wikipedia , lookup
Seismometer wikipedia , lookup
Large igneous province wikipedia , lookup
Evolution of Earth's Atmosphere Structure of the Earth The internal structure of the Earth is layered in spherical shells, like an onion (Fig. 1.2). These layers can be defined by either their chemical or their theological properties. Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of Earth's internal structure is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through Earth, measurements of the gravity field of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior. Fig 1.2: The internal structure of Earth consists of layers of different composition and physical properties. The structure of Earth can be defined in two ways: a. Mechanical properties such as theology (the study of religion and religious belief), or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. The interior of Earth is divided into 5 important layers. b. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core (Table 1.1). The geologic component layers of Earth are at the following depths below the surface: Table 1.1: Depth of various layers of earth’s interior Depth Kilometres 0–60 0–35 35–60 35–2,890 100–200 Layer Miles 0–37 0–22 22–37 22–1,790 62–125 Lithosphere (locally varies between 5 and 200 km) Crust (locally varies between 5 and 70 km) Uppermost part of mantle Mantle Asthenosphere Lecture Delivered by: Dr. Shahid-e-Murtaza Evolution of Earth's Atmosphere 35–660 660–2,890 2,890–5,150 5,150–6,360 22–410 410–1,790 1,790–3,160 3,160–3,954 Upper mesosphere (upper mantle) Lower mesosphere (lower mantle) Outer core Inner core a. Crust The crust ranges from 5–70 km (~3–44 miles) in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks, like granite. The rocks of the crust fall into two major categories – sial and sima (Suess,1831–1914). It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonizedharzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences. Many rocks now making up Earth's crust formed less than 100 million (1×108) years ago; however, the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that Earth has had a solid crust for at least that long. b. Mantle Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales. Convection of the mantle is expressed at the surface through the motions of tectonic plates. The melting point and viscosity of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 1021 and 1024Pa·s, depending on depth. In comparison, the viscosity of water is approximately 10−3Pa·s and that of pitch is 107Pa·s( Fig. 1.3). Lecture Delivered by: Dr. Shahid-e-Murtaza Evolution of Earth's Atmosphere Fig. 1.3: Internal structure of earth c. Core The average density of Earth is 5,515 kg/m3. Since the average density of surface material is only around 3,000 kg/m3, we must conclude that denser materials exist within Earth's core. Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ~1,220 km and a liquid outer core extending beyond it to a radius of ~3,400 km. The densities are between 9,900 and 12,200 kg/m3 in the outer core and 12,600–13,000 kg/m3 in the inner core. The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. It is not necessarily a solid, but, because it is able to deflect seismic waves, it must behave as a solid in some fashion. Experimental evidence has at times been critical of crystal models of the core. Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000K below those from shock laser (dynamic) studies. The laser studies create plasma, and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is plasma with the density of a solid. This is an area of active research. Evidences for Internal Earth Structure and Composition a). Seismic Waves When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior. Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have travelled through increasing depths in the Earth. Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass. Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves move more slowly through a liquid than a solid. Molten areas within the Earth slow down P Lecture Delivered by: Dr. Shahid-e-Murtaza Evolution of Earth's Atmosphere waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves. When seismic waves pass between geologic layers with contrasting seismic velocities (when any wave passes through media with distinctly differing velocities) reflections, refraction (bending), and the production of new wave phases (e.g., an S wave produced from a P wave) often result. Sudden jumps in seismic velocities across a boundary are known as seismic discontinuities. b). The Crust Mohorovic Seismic Discontinuity: Seismic stations within about 200 km of a continental earthquake (or other seismic disturbance such as a dynamite blast) report travel times that increase in a regular fashion with distance from the source. But beyond 200 km the seismic waves arrive sooner than expected, forming a break in the travel time vs. distance curve. Mohorovic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km from the earthquake source had passed through a lower layer with significantly higher seismic velocity. This seismic discontinuity is now known as the Moho (much easier than "Mohorovicic seismic discontinuity"). It is the boundary between the felsic/mafic crust with seismic velocity around 6 km/sec and the denser ultramafic mantle with seismic velocity around 8 km/sec. The depth to the Moho beneath the continents averages around 35 km but ranges from around 20 km to 70 km. The Moho beneath the oceans is usually about 7 km below the seafloor (i.e., ocean crust is about 7 km thick). c). The Mantle Low Velocity Zone: Seismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure, and therefore density, with depth. However, seismic waves recorded at distances corresponding to depths of around 100 km to 250 km arrive later than expected indicating a zone of low seismic wave velocity. Furthermore, while both the P and S waves travel more slowly, the S waves are attenuated or weakened. This is interpreted to be a zone that is partially molten, probably one percent or less (i.e., greater than 99 percent solid). Alternatively, it may simply represent a zone where the mantle is very close to its melting point for that depth and pressure that it is very "soft." Then this represents a zone of weakness in the upper mantle. This zone is called the asthenosphere or "weak sphere." d). The Core Gutenberg Seismic Discontinuity / Core-Mantle Boundary: Seismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle (exceptions: see Low Velocity Zone and 670 km Discontinuity above). However, at arc distances of between about 103° and 143° no P waves are recorded. Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as the result of a molten core beginning at a depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted (bent). Lehman Siesmic Discontinuity / The Inner Core, between 143° and 180° from an earthquake another refraction is recognized (Lehman, 1936) resulting from a sudden increase in P wave velocities at a depth of 5150 km. This velocity increase is consistent with a change from a molten outer core to a solid inner core. Lecture Delivered by: Dr. Shahid-e-Murtaza