STRUCTURE OF THE EARTH

Differentiation of Earth

• Earth is divided into layers based on density and composition

• Solid Layers– Core (iron-nickel)– Mantle (pyroxene, olivine)– Crust (feldspars)

• Liquid/Gas Layers– Hydrosphere (water)– Atmosphere (gases)

Fig 1.3

Differentiation of Earth

• Early Earth was mostly molten ("liquid rock")

• More dense material pulled by gravity to the center of Earth

• Less dense feldspar minerals formed solid crust

Fig 9.8

Earth's internal heat comes from:

• Residual heat from planetary accretion (about 20%)

• Heat produced through radioactive decay (80%). Radioactive isotopes in the Earth are:

• potassium-40• uranium-238• uranium-235• thorium-232

• At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa

INNER COREBased on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth's volume, the inner core is believed to be:• Solid• Composition - iron-nickle alloy -with very

small amounts of some other elements.

• 1220 km thick (radius)• Depth from crust: 5155 km

Temperature of the inner core• estimated using experimental and theoretical

constraints on the melting temperature of impure iron at the pressure (about 330 GPa)

• 5,700 K (5,430 °C; 9,800 °F). Pressure in Earth's inner core • 330 to 360 GPa (3,300,000 to 3,600,000 atm)• iron can only be solid at such high

temperatures because its melting temperature increases dramatically at these high pressures.

OUTER CORE• Liquid layer • 2,260 km thick • Composed of iron and nickel • Outer boundary lies 2,890 km (1,800 mi)

beneath the Earth's surface.

• Temperature of the outer core:– 4400 °C in the outer regions– 6100 °C near the inner core

• Eddy currents in the nickel iron fluid of the outer core are believed to influence the Earth's magnetic field.

• The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core.

• Sulfur and oxygen could also be present in the outer core.

• Highly viscous layer between crust and outer core

• ~2890 km thick (1800 miles)• Makes up 84% of Earth by volume• Structure known from seismology

(earthquake studies)

MANTLE

Convection Currents

• Plates move because heat is being released from deep inside the earth.

• Convection currents in the mantle cause hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge).

Lower Mantle (Mesosphere)

• Depth from surface: 600• Thickness: 2300 km• More rigid than asthenosphere• High temperature---from 500 to 4,000°C---

enough to melt the rocks but the pressure is so great that these rocks remain in the solid/plastic form

Asthenosphere (weak sphere)

• Below the lithosphere• Depth from surface: 100 km• Thickness: 100 km• Upper mantle• Slight melting due to increased

temperature/pressure (makes it weak)• Lithosphere “floats” over it

Lithosphere (sphere of rock)

• Crust and upper mantle: solid rock• Depth from surface: 10 km• 100 km thick (average) • 250 thick (mountains)• Few km thick (oceans)• Rigid• Cool

Plate

Mohorovicic Discontinuity• The “Moho”• Discovered in 1909• Seismic wave velocity increases below ~ 50 km• Boundary between crust and mantle

• covers mantle• solid outer shell• “floats” on mantle• upper part of the lithosphere• made of 2 parts: continental crust and

oceanic crust

THE CRUST

CONTINENTAL CRUST• crust is thickest below the continents• Thickness: 30-40 km• older than oceanic crust• rocks consist mainly of granites---light

colored, with large grains• Average density is 2.7 g/cm3

Oceanic crust• covers ocean floor• 6-11 km thick• made of a rock called basalt---dark, fine rock

made from lava that cools quickly• rocks contain iron and magnesium• grains of rock very small• average density is 3 g/cm3