Lecture 20

Earth's Interior and Geophysical Properties

Lecture Outline

I Earth's Internal StructureA) The CrustB) The MantleC) The Core

i. Compositionii. Core-Mantle Boundary

IIIsostasyIII Earth's GravityIV Earth's Magnetic Field

A) Magnetic ReversalsB) Magnetic Anomalies

V Heat Within the EarthVI Conclusions

Lecture 20i

Geophysics

photo by John Galetzka, USGS

geophysics: the branch of earthscience which employs physicalmeasurements and mathematicalmodels to explore and analyzethe structure and dynamics of thesolid Earth and other bodies andtheir fluid envelopes

Controls on Seismic Wave Velocity

pressure

state of matter

composition of matter

Controls on Seismic Wave VelocityPressure

Controls on Seismic Wave VelocityComposition

Material Vp (ms-1) Density (gcm-3)Salt 4.6 2.2Quartz 6.1 2.7 Calcite 6.7 2.7 Dolomite 7.4 2.9 Siderite 7.0 4.0Pyrite 8.4 5.0

Controls on Seismic Wave VelocityState of Matter

Earth's Internal Structure

crust

mantle

core

Earth's Internal StructureThe Crust

MohorovicicDiscontinuity

(Moho)

Earth's Internal StructureCrustal Thickness

Earth's Internal StructureCrustal Thickness (United States)

Earth's Internal StructureThe Composition of the Crust

Earth's Internal StructureThe Mantle

Earth's Internal StructureThe Mantle

Mantle is Solid (S waves propagate)

Mantle is Rigid (seismic waves propagate)

Mantle is Fluid (convection cells present)

Earth's Internal StructureThe Composition of the Mantle

Hofman Hart and Zindler Hutchinson Mason 1988 1986 1974 1966

SiO2 45.96 46.38 45.00 48.10

MgO 37.78 38.12 39.00 31.10

FeO 7.54 7.62 8.00 12.70

Al2O

3 4.06 4.10 3.50 3.10

CaO 3.21 3.24 3.25 2.30

Earth's Internal StructureThe Core

Size: radius 3480 km (1/2 Earth)volume 1/8 Earthmass 1/3 Earth

Properties:density 10-13 g/cm3

pressure 136-330 GPatemperature 5500±1500 Kstate liquid-solid

Earth's Internal StructureComposition of the Core

Iron

Nickel ~ 4%

Light Element (O or S) 10% by mass

Earth's Internal StructureMajor Discontinuities

crust/mantle - the MOHO

inside mantle (400 km and 670 km)

mantle/outer core (CMB)

outer/inner core (ICB)

Earth's Internal StructureMajor Discontinuities - The Moho

Earth's Internal StructureThe Core-Mantle BoundaryMAJOR Discontinuity

S-waves blocked by liquid outer core

Isostasy

the condition of equilibrium,comparable to floating, of theunits of the lithosphere abovethe asthenosphere.

Isostasy

Isostastic Adjustment

erosion and deposition

glacial rebound

crustal thickening

Isostastic AdjustmentErosion and Deposition

Isostastic AdjustmentGlacial Rebound

Isostastic AdjustmentCrustal Thickening

Gravity

gravity: the attraction between two masses,such as the Earth and an object on its surface. Commonly referred to as theacceleration of gravity (g).

Gravity

Gravity Anomaly

Gravity Anomaly

Gravity Anomaly

Gravity Anomaly

Earth's Magnetic Field

Earth's Magnetic Field

Rock Magnetism

Earth's Magnetic Field

Earth's Magnetic Field

Earth's Magnetic Field

Heat within the Earth

Heat within the Earth

Heat within the Earth

Sources of Earth's Heat

Pressure

Sunlight

Radioactive Decay

Primordial Heat

Redistribution of Earth's Heat

Radiation

Conduction

Convection

Redistribution of Earth's HeatConduction

Redistribution of Earth's HeatConvection