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Transcript
Composition Physical Properties Summary The Earth is a layered planet The layers represent changes in composition and physical properties The compositional layers are the Crust, Mantle and Core The physical layers are the Lithosphere, Asthenosphere, Mesosphere, Outer Core and Inner Core Nearly all of this is known as the result of indirect observations, mostly through interpretation of seismic waves generated by earthquakes The Crust is divided into Oceanic crust, which is thinner, more dense, richer in iron and magnesium (Mafic) minerals and relatively young; And Continental crust, which is thicker, less dense, richer in silicon and aluminum (Felsic) minerals and relatively young; Summary (continued) The Mantle is composed of dense, mafic silicates (Peridotites) The Core is composed of iron There is a solid inner core and an outer liquid core that spins generating a magnetic field The speed and refraction of seismic waves is generally used to interpret the inner structure and composition of the Earth There are also a few samples of rocks from the mantle in volcanics, kimberlite pipes, and in oceanic crust Aside from seismic studies, we also can explore the subsurface using studies of gravitational and magnetic anomalies And finally, due to Isostasy continental mountains are inferred to have deep roots. The Interior of the Earth • The interior of the Earth must be studied indirectly – Examples of upper mantle fragments brought up by volcanic eruptions and kimberlite pipes, or scraped off onto continents by subducting oceanic plates – Deepest drillhole reached about 12 km, but did not reach the mantle • Geophysics is the branch of geology that studies the interior of the Earth http://www.youtube.com/watch?v=19fMs633Td4&feature=related http://www.youtube.com/watch?v=FW-TkpvKPl0&feature=related The Mohole Project. Finally abandoned after reaching 12 km Journey to the Center of the Earth, Jules Verne Earth’s Internal Structure • The solid Earth has a layered structure – Layers defined by composition and physical properties – Compositional layers • crust - mantle - core – Physical layers • lithosphere - asthenosphere - mesosphere outer core - inner core Composition Physical Properties Compositional Layers • Crust – Outermost compositional layer – Definite change in composition at the base of the crust – Crust may be divided into 2 types • Continental crust • Oceanic crust Compositional Layers • Crust – Continental crust • Thicker than oceanic crust - up to 90 km • Less dense - 2.7 g/cm3 • Strongly deformed • Contains portions that are very old; up to 3.8 Billion years Compositional Layers • Crust – Oceanic crust • Thinner than continental crust - about 8 km • More dense - 3.0 g/cm3 • Comparatively undeformed, i.e. no folded and faulted mountains • Much younger - < 200 million years old • Composed of basalt, (contains olivine) Crustal Properties Crust Density Composition Thickness continental ~2.8 g/cm3 Felsic Thick: 20-70 km ~3.2 g/cm3 Mafic Thin: 2-10 km oceanic Age Old: up to 4 Byrs Young: <200 Mys Compositional Layers • Mantle – Largest layer in the Earth • 2900 km thick • 82% by volume • 68% by mass – Composed of silicate rocks with abundant iron and magnesium: Mafic • Density ranges from 3.2 to 5 g/cm3 • Fragments found in some volcanic rocks, kimberlite pipes, and oceanic rocks scraped off onto the continent during subduction Compositional Layers • Core (inner and outer) – Central mass about 7000km in diameter – Average density of 10.8 g/cm3 – 16% by volume, 32% of mass – Composition • Density of Earth, composition of meteorites and the Earth’s magnetic field requires largely Metallic Iron plus other minor elements, e.g. Sulfur, Silicon, Nickel, etc. Physical Layers • Lithosphere – The rock layer – Crust + upper portion of the mantle – Solid & rigid – Thickness ranges from 10 km beneath oceans to 300 km in continental areas Physical Layers • Asthenosphere – Upper layer in the mantle – Temperature and pressure combine to allow rock to partially melt • Rocks are soft and plastic • They flow and are easily deformed • Results in a low velocity zone for seismic waves – Boundary with lithosphere is defined by mechanical properties, not composition Physical Layers • Mesosphere – The region between the asthenosphere and the core – Higher pressure offsets higher temperatures – Rocks gain rigidity and mechanical strength Physical Layers • Outer Core – ~2270 km thick – Liquid, flows – Flow creates magnetic field • Inner Core – ~1200 km thick – Solid The Earth’s Magnetic Field How you can use seismic waves to explore the interior of the Earth No refraction in homogeneous materials Fig. 11-4a, p. 341 Refraction in heterogeneous materials Fig. 11-4b, p. 341 Seismic waves in a homogeneous planet Seismic waves in a differentiated planet Fig. 11-4c, p. 341 Fig. 11-10, p. 346 Fig. 11-9c, p. 345 Fig. 11-9b, p. 345 Seismic Structure of the Earth • Seismic wave velocities vary with depth –Variation with depth is not smooth –Discontinuities at certain depths represent discrete changes in structure, chemistry and phase (liquid/solid) of minerals in the mantle Seismic Structure of the Earth • Mohorovicic Discontinuity (Moho) – First discovered by Andrija Mohorovicic – Occurs between 5 and 70 km deep – Represents the base of the crust, i.e. the crust/mantle boundary – Compositional change from feldspar rich to olivine rich rocks causes a significant increase in seismic velocities – Causes refracted seismic waves These waves travel are refracted here and here. Even though they travel farther, they arrive first. Therefore they have traveled faster Seismic Structure of the Earth • Low-velocity zone – Layer from ~100 to 250 km deep – Seismic velocities usually increase with depth – In the low velocity zone velocity slows by ~ 6% – Caused by partially molten mantle that slows seismic waves – May form a slippery layer that the overlying crust slides upon Internal structure of the Earth Movement in the Earth • In the core – 3-D models and magnetic field suggest flowing molten iron with likely internal convection resulting in occasional chaotic reversals • In the mantle – Investigations show a complex convection system occurring in the entire mantle system, including an active D” layer at the core/mantle boundary The ULVZ and the D” layer Convection in the Earth The Ultra Low Velocity Zone (ULVZ) and the D” layer; the stormy layer at the Core/Mantle boundary Source of mantle plumes and hot spots? The Core • Core composition inferred from its calculated density, physical and electro-magnetic properties, and composition of meteorites – Iron metal (liquid in outer core and solid in inner core) best fits observed properties – Iron is the only metal common in meteorites • Core-mantle boundary (D” layer) is marked by great changes in seismic velocity, density and temperature – Hot core may melt lowermost mantle or react chemically to form iron silicates in this seismic wave ultralow-velocity zone (ULVZ) Meteorites record the composition of the early solar system ~4.6 billion years old Three types of meteorites Iron (mostly iron, some nickel and other metals Stony (most common; silicate minerals: plagioclase, olivine, pyroxene) Stony-iron (mixed composition) One unusual type is a carbonaceous chondrite, which can contain up to 5% organic carbon, i.e. hydrocarbons, amino acids. Heat Within the Earth • Geothermal gradient - temperature increase with depth into the Earth – Tapers off sharply beneath lithosphere – Due to steady pressure increase with depth, increased temperatures produce little melt (mostly within asthenosphere) except in the outer core • Heat flow - the gradual loss of heat through Earth’s surface – Major heat sources include original heat (from accretion and compression as Earth formed) and radioactive decay – Locally higher where magma is near surface – Same magnitude, but with different sources, in the oceanic (from mantle) and continental crust (radioactive decay within the crust) Fig. 11-1a, p. 336 Fig. 11-1b, p. 336 Fig. 11-7, p. 343 Fig. 11-8a, p. 344 Fig. 11-8b, p. 344 Fig. 11-11, p. 347 Fig. 11-11a, p. 347 Fig. 11-11b, p. 347 Gravitational Anomalies Dense and therefore exerting more gravitational attraction Gravitational Anomalies Common way to explore for faults and therefore water in desert areas such as the southwest US Gravitational Anomalies Common way to explore for salt domes and therefore oil in the Gulf Coast area Fig. 11-14a, p. 350 Fig. 11-14b, p. 350 Fig. 11-14, p. 350 The mass of the volume of water displaced is equal to the total mass of the iceberg 10% of the mass 10% of the mass 90% of the mass 90% of the mass Fig. 11-15b, p. 350 Fig. 11-15a, p. 350 Fig. 11-16a, p. 351 Fig. 11-16b, p. 351 Fig. 11-16c, p. 351 Fig. 11-16, p. 351 Fig. 11-17, p. 351 Fig. 11-17a, p. 351 Fig. 11-17b, p. 351 Fig. 11-17c, p. 351 Fig. 11-17a, p. 351 Fig. 11-18, p. 352 Fig. 11-19, p. 352 Magnetic Anomalies Fig. 11-20a, p. 353 Magnetic Anomalies Fig. 11-20b, p. 353 Magnetic Anomalies Fig. 11-20c, p. 353 Earth’s Internal Structure • • • • Seismic waves have been used to determine the three main zones within the Earth: the crust, mantle and core The crust is the outer layer of rock that forms a thin skin on Earth’s surface The mantle is a thick shell of dense rock that separates the crust above from the core below The core is the metallic central zone of the Earth