Chapter 12 Earth’s Interior. Probing Earth’s interior Probing Earth’s interior ► Most of our knowledge of Earth’s interior comes from the study of seismic.

Chapter 12Chapter 12 Earth’s InteriorEarth’s Interior

Probing Earth’s interiorProbing Earth’s interior

► Most of our knowledge of Earth’s Most of our knowledge of Earth’s interior comes from the study of interior comes from the study of seismic wavesseismic waves

► As P and S waves travel through the As P and S waves travel through the Earth their travel times will vary Earth their travel times will vary depending on the properties of the depending on the properties of the materialsmaterials

► Variations in the travel times Variations in the travel times

correspond to changes in the materials correspond to changes in the materials encounteredencountered

Probing Earth’s interiorProbing Earth’s interior

► The nature of seismic wavesThe nature of seismic waves ►Velocity depends on the density and elasticity of Velocity depends on the density and elasticity of

the intervening materialthe intervening material

►Within a given layer the speed generally Within a given layer the speed generally increases with depth due to pressure forming a increases with depth due to pressure forming a more compact elastic materialmore compact elastic material

►Compressional waves (P waves) are able to Compressional waves (P waves) are able to propagate through liquids as well as solidspropagate through liquids as well as solids

Probing Earth’s interiorProbing Earth’s interior

► The nature of seismic wavesThe nature of seismic waves► S waves cannot travel through liquidsS waves cannot travel through liquids

► In all materials, P waves travel faster than do S wavesIn all materials, P waves travel faster than do S waves

► When seismic waves pass from one material to another, the path When seismic waves pass from one material to another, the path of the wave is of the wave is refractedrefracted (bent) (bent)

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

► Changes in seismic-wave velocities and refraction of Changes in seismic-wave velocities and refraction of waves helped seismologists conclude that Earth must be waves helped seismologists conclude that Earth must be

composed of distinct shellscomposed of distinct shells

► Shells are defined by composition due to density sorting Shells are defined by composition due to density sorting during an early period of partial melting. during an early period of partial melting. (Differentiation)(Differentiation)

► Earth’s interior is not homogeneous Earth’s interior is not homogeneous

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

► Layers are defined by compositionLayers are defined by composition► Three principal compositional layersThree principal compositional layers

CrustCrust – the comparatively thin outer skin that ranges from 3 km (2 – the comparatively thin outer skin that ranges from 3 km (2 miles) at the oceanic ridges to 70 km (40 miles in some mountain belts)miles) at the oceanic ridges to 70 km (40 miles in some mountain belts)

MantleMantle – a solid rocky (silica-rich) shell that extends to a depth of about – a solid rocky (silica-rich) shell that extends to a depth of about 2900 km (1800 miles)2900 km (1800 miles)

CoreCore – – an iron-rich sphere having a radius of 3486 km (2161 miles)an iron-rich sphere having a radius of 3486 km (2161 miles)

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

► Layers defined by physical properties Layers defined by physical properties

► With increasing depth, Earth’s interior is With increasing depth, Earth’s interior is characterized by gradual increases in characterized by gradual increases in temperature, pressure, and densitytemperature, pressure, and density

► Depending on the temperature and depth, a Depending on the temperature and depth, a particular Earth material may behaveparticular Earth material may behave like like

a brittle solid a brittle solid deform in a plastic–like mannerdeform in a plastic–like manner or melt and become liquidor melt and become liquid

► Main layers of Earth’s interior are based on Main layers of Earth’s interior are based on physical properties and hence mechanical physical properties and hence mechanical strengthstrength

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

►Layers defined by physical propertiesLayers defined by physical properties►LithosphereLithosphere (sphere of rock) (sphere of rock)

Earth’s outermost layer Earth’s outermost layer Consists of the crust Consists of the crust andand uppermost mantle uppermost mantle Relatively cool, rigid shell Relatively cool, rigid shell Averages about 100 km in thickness, but may be 250 Averages about 100 km in thickness, but may be 250

km or more beneath the older portions of the km or more beneath the older portions of the continentscontinents

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

►AsthenosphereAsthenosphere (weak sphere) (weak sphere) Beneath the lithosphere, in the upper mantle to a depth Beneath the lithosphere, in the upper mantle to a depth

of about 600 km of about 600 km

Small amount of melting in the upper portion Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently below allowing the lithosphere to move independently of the asthenosphere of the asthenosphere

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

►MesosphereMesosphere or lower mantle or lower mantle Rigid layer between the depths of 660 km and 2900 Rigid layer between the depths of 660 km and 2900

kmkm Rocks are very hot and capable of very gradual flowRocks are very hot and capable of very gradual flow

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

►Outer coreOuter core Composed mostly of an iron-nickel alloy Composed mostly of an iron-nickel alloy Liquid layer Liquid layer 2270 km (1410 miles) thick 2270 km (1410 miles) thick Convective flow within generates Earth’s magnetic Convective flow within generates Earth’s magnetic

field field

Seismic waves and Seismic waves and Earth’s structureEarth’s structure

►Inner core Inner core Sphere with a radius of 3486 km (2161 miles) Sphere with a radius of 3486 km (2161 miles) Stronger than the outer core Stronger than the outer core Behaves like a solid Behaves like a solid

Discovering Earth’s Discovering Earth’s major boundariesmajor boundaries

► The MohoThe Moho (Mohorovicic (Mohorovicic discontinuity)discontinuity)

► Discovered in 1909 by Discovered in 1909 by Andriaja MohorovicicAndriaja Mohorovicic

► Separates crustal materials Separates crustal materials from underlying mantlefrom underlying mantle

► Identified by a change in Identified by a change in the velocity of P wavesthe velocity of P waves

Discovering Earth’s Discovering Earth’s major boundariesmajor boundaries

► The core-mantle boundaryThe core-mantle boundary ►observation that P waves observation that P waves

die out at 105 degrees from die out at 105 degrees from the earthquake and reappear the earthquake and reappear at about 140 degreesat about 140 degrees

► 35 degree wide belt is 35 degree wide belt is named the named the P-wave shadow P-wave shadow zonezone

Discovering Earth’s Discovering Earth’s major boundariesmajor boundaries

► The core-mantle boundaryThe core-mantle boundary► Characterized by bending Characterized by bending

(refracting) of the P waves (refracting) of the P waves

► The fact that S waves do not The fact that S waves do not travel through the core travel through the core provides evidence for the provides evidence for the existence of a liquid layer existence of a liquid layer beneath the rocky mantlebeneath the rocky mantle

Discovering Earth’s Discovering Earth’s major boundariesmajor boundaries

► Discovery of the inner coreDiscovery of the inner core ►P waves passing through the inner core show increased P waves passing through the inner core show increased

velocity suggesting that the inner core is solidvelocity suggesting that the inner core is solid

CrustCrust

► Thinnest of Earth’s divisionsThinnest of Earth’s divisions►Varies in thickness (exceeds 70 km under some Varies in thickness (exceeds 70 km under some

mountainous regions while oceanic crust ranges from 3 mountainous regions while oceanic crust ranges from 3 to 15 km thick)to 15 km thick)

► Two partsTwo parts►Continental crustContinental crust

Average rock density about 2.7 g/cmAverage rock density about 2.7 g/cm33 Composition comparable to the felsic igneous rock Composition comparable to the felsic igneous rock

granodioritegranodiorite

CrustCrust

►Two partsTwo parts►Continental crustContinental crust

Average rock density about 2.7 g/cmAverage rock density about 2.7 g/cm33 Composition comparable to the felsic igneous rock Composition comparable to the felsic igneous rock

granodioritegranodiorite

►Oceanic crustOceanic crust Density about 3.0 g/cmDensity about 3.0 g/cm33 Composed mainly of the igneous rock basaltComposed mainly of the igneous rock basalt

MantleMantle► Contains 82% of Earth’s volumeContains 82% of Earth’s volume

► Solid, rocky layerSolid, rocky layer

► Upper portion has the composition of the ultramafic Upper portion has the composition of the ultramafic rock peridotiterock peridotite

► Two partsTwo parts ►Mesosphere (lower mantle)Mesosphere (lower mantle) ►Asthenosphere or upper mantleAsthenosphere or upper mantle

CoreCore► Larger than the planet MarsLarger than the planet Mars ► Earth’s dense central sphereEarth’s dense central sphere► Two partsTwo parts

►Outer core - liquid outer layer about 2270 km thickOuter core - liquid outer layer about 2270 km thick ►Inner core - solid inner sphere with a radius of 1216 kmInner core - solid inner sphere with a radius of 1216 km

CoreCore►Density and compositionDensity and composition

►Average density is nearly 11 g/cmAverage density is nearly 11 g/cm33 and at and at Earth’s center approaches 14 times the average Earth’s center approaches 14 times the average density of waterdensity of water

►Mostly iron, with 5% to 10% nickel and lesser Mostly iron, with 5% to 10% nickel and lesser

amounts of lighter elementsamounts of lighter elements

CoreCore►OriginOrigin

►Most accepted explanation is that the core Most accepted explanation is that the core formed early in Earth’s historyformed early in Earth’s history

►As Earth began to cool, iron in the core began As Earth began to cool, iron in the core began to crystallize and the inner core began to formto crystallize and the inner core began to form

CoreCore

► Earth’s magnetic fieldEarth’s magnetic field ► The requirements for the core to The requirements for the core to

produce Earth’s magnetic field are produce Earth’s magnetic field are met in thatmet in that it is made of material it is made of material that conducts electricity and it is that conducts electricity and it is mobilemobile

► Inner core rotates faster than the Inner core rotates faster than the Earth’s surface and the axis of Earth’s surface and the axis of rotation is offset about 10 degrees rotation is offset about 10 degrees from the Earth’s poles from the Earth’s poles

Earth’ internal heat engineEarth’ internal heat engine

►Earth’s temperature gradually increases with Earth’s temperature gradually increases with depth at a rate known as thedepth at a rate known as the geothermal geothermal gradientgradient

►Varies considerably from place to placeVaries considerably from place to place ►Averages between about Averages between about 2020C and 30C and 30C per kmC per km

in the crust (rate of increase is much less in the in the crust (rate of increase is much less in the mantle and core)mantle and core)

Earth’ internal heat engineEarth’ internal heat engine

► Major processes that have contributed to Earth’s Major processes that have contributed to Earth’s internal heatinternal heat

►Heat emitted by radioactive decay of isotopes of Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K)uranium (U), thorium (Th), and potassium (K)

►Heat released as iron crystallized to form the solid Heat released as iron crystallized to form the solid inner coreinner core

►Heat released by colliding particles during the Heat released by colliding particles during the formation of Earthformation of Earth