Basic Structure of the Earth

How do we know what is inside the Earth?

• Based on the speed and path of seismic waves as they travel through the Earth.– Speed/path of the waves are affected by

different densities.• In general, density increases with depth.• S waves cannot travel through liquid.

• http://www.pbs.org/wnet/savageearth/animations/index.html

Layers (Inside – Out)

• Inner Core– Large amount of pressure from above layers

cause the inner core to be solid and dense.– Made mostly of iron with small amounts of

nickel, oxygen, silicon, and sulfur

Layers (Inside Out)

• Outer Core– Liquid

• S waves cannot travel through this layer.

– Made mostly of iron

Layers (Inside – Out)• Mantle

– Largest layer– Made mostly of silicon, oxygen, magnesium,

and iron.– Often divided into the upper and lower mantle

• based on changing speed of seismic waves

Layers (Inside – Out)

• Crust– Varies from less than 5 km to more than 60

km thick.– Contains more silicon and aluminum and less

magnesium and iron than the mantle.

Shadow Zone• Area between 105 and 140 degrees from

the earthquake in which no seismic waves are detected.– Due to the bending of primary waves and the

stopping of secondary waves at the boundary between the outer core and the mantle.

MOHO (Mohorovicic) Discontinuity

• Boundary between the crust and the upper mantle (causes seismic waves to speed up)