The Earth's Interior Revisited: Insights from Geophysics

The Earth's Interior Revisited: Insights from Geophysics
Interlude D

Basic Structure of Earth
EARTH5 Figure D.1 Basic Structure of Earth Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc.
Figure D.2a Seismic Waves Wave front: boundary between the rock through which a wave has passed and the rock through which it has not passed Seismic ray: changing position of an imaginary point on a wave front Copyright © 2016 by W. W. Norton & Company, Inc.

Seismic-Wave Propagation
Figure D.2b–d Seismic-Wave Propagation The ability of a seismic wave to travel through a material and its velocity depend on the characteristics of the material. Thus, seismic-wave velocity is affected by rock type; seismic waves usually travel faster through solids than liquids; and S-waves cannot travel through liquids. Copyright © 2016 by W. W. Norton & Company, Inc.

Reflection and Refraction
Figure D.3b Reflection and Refraction When a ray hits a boundary between two materials, the ray can undergo: Reflection: bounce off the boundary Refraction: bending as it passes through the boundary Copyright © 2016 by W. W. Norton & Company, Inc.

The Crust-Mantle Boundary (Moho)
Figure D.4a,b The Crust-Mantle Boundary (Moho) Discovered by Andrija Mohorovicic in 1919; based on average speed of seismic waves recorded at different distances from the epicenter Copyright © 2016 by W. W. Norton & Company, Inc.

Structure of the Mantle
Figure D.5a Structure of the Mantle Low velocity zone (LVZ) occurs beneath oceanic crust from 100 – 200 km. Below LVZ, under oceans, and throughout mantle under continents, seismic-wave velocity increases with depth. Upper mantle—above 660 km Lower mantle—below 660 km Transition zone—410 – 660 km Copyright © 2016 by W. W. Norton & Company, Inc.

The Core-Mantle Boundary
Figure D.6a The Core-Mantle Boundary Discovered by Beno Gutenburg in 1914; P-wave shadow zone: P-waves do not arrive at seismometers between 103° and 143° from the epicenter, due to refraction of waves entering a material that slows their velocity. Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.6b,c The Core Consists of liquid outer and solid inner core. S-wave shadow zone S-waves do not arrive at seismometers between 103° and 180°. S-waves do not travel through liquids. Solid inner core discovered by Inge Lehmann in 1936. P-waves reflect off a boundary within the core. Exact depth of inner core determined later by seismic waves created during nuclear explosions. Copyright © 2016 by W. W. Norton & Company, Inc.

Seismic Reflection Profiling
Figure D.9a,b Seismic Reflection Profiling This cross-sectional view of crust shows bedding, stratigraphy, and structures. After computer analysis, the data yield a seismic-reflection profile. Color bands represent horizons in the stratigraphic sequence. Copyright © 2016 by W. W. Norton & Company, Inc.

EARTH5 D.14 Gravity The crests of waves on a pond have higher potential energy than the troughs. Potential energy increases as you go up the hill. Boulder 1 has the most potential energy. Boulders 2 and 3 have the same potential energy. Boulder 4 has less. Gravitational potential energy: when gravity acts on an object but cannot move it Equipotential surface: a surface on which all points have the same potential energy When there are no waves, the surface of the water is an equipotential surface. Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.10, EARTH5 Figure D.15 The Geoid Reference geoid: imaginary equipotential surface of Earth’s gravity Inaccurate because gravity varies Geoid: irregular equipotential surface that best represents variations in Earth’s gravity Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.11 Gravity Anomalies Deviation of observed geoid from the reference geoid Positive anomaly: stronger gravitational pull Negative anomaly: weaker gravitational pull Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.12 Isostasy Archimedes’ principle: a floating object sinks into water until it has displaced a mass of water that equals the object’s mass. Isostatic equilibrium (isostasy): an object neither wants to sink deeper nor float higher. Lithosphere floats on the asthenosphere and thus can undergo isostatic adjustment. Copyright © 2016 by W. W. Norton & Company, Inc.

Earth’s Magnetic Field
Figure D.13a Earth’s Magnetic Field Dipole: an arrow representing the direction from the North Pole to South Pole Magnetic field lines form a continuous loop through dipole. Magnetic pole and geographic pole are not in the same location on Earth’s surface. Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.13b Self-Exciting Dynamo Flow of metal (within outer core) can produce an electric current. Early in Earth’s history, flow occurred in a magnetic field, which generated an electric current. Once the current existed, it began generating a magnetic field. Continued flow maintains both electric current and magnetic field. Copyright © 2016 by W. W. Norton & Company, Inc.

Figure D.14 Magnetic Anomalies Stronger or weaker than expected magnetic field at any given locality; caused by non-dipolar field and magnetization of rocks in the crust Copyright © 2016 by W. W. Norton & Company, Inc.

Think–Pair–Share Lake Mead is a large man-made lake that is held behind Hoover Dam just outside Las Vegas, Nevada. The lake was created in 1935. A 1950 survey of the region indicated that the elevation of the land around the lake had dropped as much as 17 cm (7 in) since the 1930s. Explain why this elevation change happened. The students should think about the fact that the weight of the water behind the dam is causing isostatic adjustment in the area. Copyright © 2016 by W. W. Norton & Company, Inc.

Think–Pair–Share You have been hired to search for volcanogenic massive sulfide deposits. These ore deposits have abundant pyrite, chalcopyrite, galena, and sphalerite. What geophysical methods will you use in your search? What data in particular would you look for when using these methods? If you were employed by an oil company searching for oil trapped by a salt dome, how would your methods and desired data be different? The VMS deposits are dense, so a positive gravity anomaly could indicate one is present. Some of the minerals are also magnetic, so magnetic surveys could be useful. Salt is very low density, so a negative gravity anomaly could indicate a salt dome. In addition, seismic surveys are routinely used in oil exploration. Copyright © 2016 by W. W. Norton & Company, Inc.

Isostasy Animation Isostasy Animation
Links are active only in Slideshow mode. Click the image to launch the animation (requires an active Internet connection).

Interlude D Photo Credits
1 USGS 13 (both): Matthew Fouch, Arizona State University 14 Adapted/Reproduced from Naliboff and Kellogg, "(2006). Dynamic effects of a step-wise increase in thermal conductivity and viscosity in the lowermost mantle", Geophysical Research Letters, 33. Copyright 2006 by the American Geophysical Union. 16 (left): Courtesy Gregory Mountain, Lamont-Doherty Earth Observatory; (center): Courtesy of Greg Moore, University of Hawaii and Nathan Bangs, University of Texas; (right): Courtesy Sercel and CGG Veritas 17 Stephen Marshak 18 (both): Figure provided courtesy F. Lemoine & J. Frawley, NASA Goddard Space Flight Center 19 USGS 23 USGS Copyright © 2016 by W. W. Norton & Company, Inc.

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