Chapter 12 Earth’s Interior Probing the Earth’s Interior • • • • • P waves are always faster than S waves Wave velocity increases with density and stiffness Passing from one material to another causes waves to refract (bend) P waves are able to propagate through liquids as well as solids. S waves cannot pass through liquids Discovering Earth’s Major Boundaries The core-mantle boundary • Discovered in 1914 by Beno Gutenberg • Based on the observation that P waves die out at 105 degrees from the earthquake and reappear at about 140 degrees • 35 degree wide belt is named the P-wave shadow zone • P-wave and S-wave shadows zones Discovery of the inner core • Predicted by Inge Lehmann in 1936 • P waves passing through the inner core show increased velocity suggesting that the inner core is solid The Moho (Mohorovicic discontinuity) • Discovered in 1909 by Andriaja Mohorovicic • Separates crustal materials from underlying mantle • Identified by a change in the velocity of P waves • Seismic waves and Earth’s structure Abrupt changes in seismic-wave velocities that occur at particular depths helped seismologists conclude that Earth must be composed of distinct shells Velocity increases at 410 km and 660 km depth are due to phases in the asthenosphere. Below 410 km, olivine becomes spinel, and below 660 km, spinel becomes perovskite. Crust Two parts • • Continental crust • Average rock density about 2.7 g/cm3 • Average composition of granodiorite • 30-70 km thick Oceanic crust • Density about 3.0 g/cm3 • Composed mainly of the igneous rock basalt • 8-10 km thick, except at spreading ridges, where it is very thin Mantle Contains 82% of Earth’s volume Solid, rocky layer Upper portion has the composition of the ultramafic rock peridotite Three parts • Mantle lithosphere (uppermost mantle) ~70-180 km thick • Asthenosphere (upper mantle) ~560 km thick • Mesosphere (lower mantle) ~2240 km thick Core Larger than the planet Mars Earth’s dense central sphere Two parts • Outer core - liquid outer layer about 2270 km thick • Inner core - solid inner sphere with a radius of 1216 km Density and composition • Average density is nearly 11 g/cm3 and at Earth’s center approaches 14 times the average density of water • Mostly iron, with 5% to 10% nickel and lesser amounts of lighter elements Earth’s magnetic field • Electrically conductive inner and outer core • Inner core rotates faster than the Earth’s surface and the axis of rotation is offset about 10 degrees from the Earth’s poles. Makes one extra rotation every 400 years. • Magnetic field is generated by a “self-perpetuating dynamo.” Electric current is generated when liquid iron circulating in the outer core moves through the magnetic field. The electric current in turn generates a magnetic field. The system is kept going by the circulation (convection) in the outer core. Origin of the layers: the “Great Catastrophe” Layers defined by physical properties Main layers of Earth’s interior are based on physical properties and hence mechanical strength Layers defined by physical properties Lithosphere (sphere of rock) Earth’s outermost layer Consists of the crust and uppermost mantle Relatively cool, rigid shell Averages about 100 km in thickness, but may be 250 km or more thick beneath the older portions of the continents Includes crust and upper mantle Asthenosphere (weak sphere) Beneath the lithosphere, in the upper mantle from depths of ~100 km to ~ 600 km Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere Mesosphere or lower mantle Rigid layer between the depths of 660 km and 2900 km Rocks are very hot and capable of very gradual flow Outer core Composed mostly of an iron-nickel alloy Liquid layer 2270 km (1410 miles) thick Convective flow within generates Earth’s magnetic field Inner core Sphere with a radius of 3486 km (2161 miles) Behaves like a solid Earth’s internal heat engine Earth’s temperature gradually increases with an increase in depth at a rate known as the geothermal gradient • Varies considerably from place to place • Averages between about 20C and 30C per km in the crust (rate of increase is much less in the mantle and core) • Estimated temperature at the earth’s center is ~6700 °C Major processes that have contributed to Earth’s internal heat • Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K). These elements are concentrated in the continental crust. • Heat released as iron crystallized to form the solid inner core • Heat released by colliding particles during the formation of Earth • Compression from increasing pressure during accretion Heat flow in the crust • Process called conduction • Rates of heat flow in the crust varies – more in the continental crust than in the oceanic crust. Mantle convection • Mantle must have an effective method of transmitting heat from the core outward • Probably limited to the asthenosphere; the mesosphere is very viscous and could only convect very slowly or not at all.