3. Earth’s Internal Structure

Earth’s Internal Earth’s Internal StructureStructure

CVE 3205CVE 3205Engineering GeologyEngineering Geology

Wong Jee KhaiWong Jee Khai

Early Evolution of EarthEarly Evolution of Earth• Begins about 14 billion years ago with the

Bing Bang, an incomprehensibly large explosion that sent all matter of the universe flying outward at incredible speeds.

• The debris (H2 & He) began to cool and condense into the first star and galaxies.

Early Evolution of EarthEarly Evolution of Earth• Formation of the solar system according to

the nebular hypothesis which stated that our solar system evolved from an enormous rotating cloud called the solar nebula.

Early Evolution of EarthEarly Evolution of Earth1. Dust and gases (nebula) started to

gravitationally collapse.2. The nebula contracted into a rotating

disk that was heated by the conversion of gravitational energy into thermal energy.

3. Cooling of the nebular cloud caused rocky and metallic material to condense into tiny particles.

Early Evolution of EarthEarly Evolution of Earth4. Repeated collisions caused the dust-size

particles to gradually coalesce into asteroid-size bodies.

5. Within a few million years these bodies accreted into the planets.

Early Evolution of EarthEarly Evolution of Earth• As material accumulated to form earth, the

high-velocity impact of nebular debris and the decay of radioactive elements caused the temperature of earth to steadily increase.

• Iron and nickel began to melt produced liquid blobs of heavy metal that sank toward the centre of earth. (Earth’s dense iron-rich core)

Early Evolution of EarthEarly Evolution of Earth• Melting also formed buoyant masses of molten

rock that rose toward the surface and solidified to produce a primitive crust.

• Rocky materials were enriched in oxygen and “oxygen-seeking” elements (Si, Al, Ca, Na, K, Fe, Mg). Some heavy metals such as gold, lead and uranium that have low melting points were scavenged from earth’s interior and concentrated in the developing crust.

Early Evolution of EarthEarly Evolution of Earth• Large quantities of gaseous materials

were allowed to escape from earth’s interior, as happens today during volcanic eruptions.

Earth’s Internal StructureEarth’s Internal Structure• When a meteorite impacts a planet or

moon, its energy of motion (called kinetic energy) is transformed into heat energy.

• As Earth grew larger and larger from continual impacts, its temperature increased.

Earth’s Internal StructureEarth’s Internal Structure• Radioactive decay of materials like

uranium, thorium and potassium also added heat.

• Because Earth became partly fluid, less-dense molten materials (silicon, aluminum, sodium, and potassium) were freed to migrate toward the surface.

Earth’s Internal StructureEarth’s Internal Structure• Denser melted materials, such as molten

iron, sank toward the center of the planet.

• Planet Earth has three main parts:1. Crust.2. Mantle.3. Core (metallic iron, nickel)

Earth’s CrustEarth’s Crust• The crust is not uniform.• The oceanic crust on average is about 7

km thick and composed of the dark igneous rock basalt.

• The continental crust on average is about 35 to 40 km thick but may exceed 70 km in mountainous regions and consists of many rock types.

Earth’s MantleEarth’s Mantle• Contains > 82% of earth’s volume. • Nearly 2900 km.• Can be divided into two different parts, the

stiff lithosphere and the weaker asthenosphere.

Earth’s MantleEarth’s Mantle• The upper mantle extends from the crust-

mantle boundary down to 660 km depth.• Dominant rock type in the uppermost

mantle is peridotite, which is richer in metals magnesium and iron.

Earth’s MantleEarth’s Mantle• The lower mantle starts from 660 km

depth to the top of the core, at 2900 km depth.

• The rocks are very hot and capable of very gradual flow.

Earth’s CoreEarth’s Core• An iron-nickel alloy with minor amounts of

oxygen, silicon and sulfur.• Outer core is a liquid layer 2270 km thick.

It’s the movement of metallic iron within this zone that generates earth’s magnetic field.

• Inner core is a sphere having r = 1216 km and is solid due to the immense pressure.

Investigating Earth’s Interior Investigating Earth’s Interior How do we know anything about the

composition of the core and the mantle?• By measuring the time required for

earthquake waves to travel through Earth by different paths, we can determine the composition of the materials through which they move.

Investigating Earth’s InteriorInvestigating Earth’s Interior• Iron meteorites are believed to be

fragments from the core of a small terrestrial planet that was shattered by a gigantic impact.