Composition. Physical Properties

Transcription

1 Composition Physical Properties

2 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;

3 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.

4 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

5 Journey to the Center of the Earth, Jules Verne The Mohole Project. Finally abandoned after reaching 12 km

6 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

7 Composition Physical Properties

8 Crust Compositional Layers Outermost compositional layer Definite change in composition at the base of the crust Crust may be divided into 2 types Continental crust Oceanic crust

9 Crust Compositional Layers Continental crust Thicker than oceanic crust - up to 90 km Less dense g/cm 3 Strongly deformed Contains portions that are very old; up to 3.8 Billion years

10 Crust Oceanic crust Compositional Layers Thinner than continental crust - about 8 km More dense g/cm 3 Comparatively undeformed, i.e. no folded and faulted mountains Much younger - < 200 million years old Composed of basalt, (contains olivine)

11 Crustal Properties Crust Density Composition Thickness Age continental ~2.8 g/cm 3 Felsic Thick: km Old: up to 4 Byrs oceanic ~3.2 g/cm 3 Mafic Thin: 2-10 km Young: <200 Mys

12 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/cm 3 Fragments found in some volcanic rocks, kimberlite pipes, and oceanic rocks scraped off onto the continent during subduction

13 Compositional Layers Core (inner and outer) Central mass about 7000km in diameter Average density of 10.8 g/cm 3 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.

14 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

15 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

16 Physical Layers Mesosphere The region between the asthenosphere and the core Higher pressure offsets higher temperatures Rocks gain rigidity and mechanical strength

17 Physical Layers Outer Core ~2270 km thick Liquid, flows Flow creates magnetic field Inner Core ~1200 km thick Solid

18 The Earth s Magnetic Field

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20 How you can use seismic waves to explore the interior of the Earth

21 No refraction in homogeneous materials Fig. 11-4a, p. 341

22 Refraction in heterogeneous materials Fig. 11-4b, p. 341

23 Seismic waves in a homogeneous planet

24 Seismic waves in a differentiated planet

25 Fig. 11-4c, p. 341

26 Fig , p. 346

27 Fig. 11-9c, p. 345

28 Fig. 11-9b, p. 345

29 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

30 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

31 These waves travel are refracted here and here. Even though they travel farther, they arrive first. Therefore they have traveled faster

32 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

33 Internal structure of the Earth

34 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

35 Convection in the Earth The ULVZ and the D layer

36 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?

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38 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)

39 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.

40 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)

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42 Fig. 11-1a, p. 336

43 Fig. 11-1b, p. 336

44 Fig. 11-7, p. 343

45 Fig. 11-8a, p. 344

46 Fig. 11-8b, p. 344

47 Fig , p. 347

48 Fig a, p. 347

49 Fig b, p. 347

50 Gravitational Anomalies Dense and therefore exerting more gravitational attraction

51 Gravitational Anomalies Common way to explore for faults and therefore water in desert areas such as the southwest US

52 Gravitational Anomalies Common way to explore for salt domes and therefore oil in the Gulf Coast area

53 Fig a, p. 350

54 Fig b, p. 350

55 Fig , p. 350

56 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 b, p. 350

57 Fig a, p. 350

58 Fig a, p. 351

59 Fig b, p. 351

60 Fig c, p. 351

61 Fig , p. 351

62 Fig , p. 351

63 Fig a, p. 351

64 Fig b, p. 351

65 Fig c, p. 351

66 Fig a, p. 351

67 Fig , p. 352

68 Fig , p. 352

69 Magnetic Anomalies Fig a, p. 353

70 Magnetic Anomalies Fig b, p. 353

71 Magnetic Anomalies Fig c, p. 353

72 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