The Structure of the Earth

The Earth’s Interior: Methodology

Earth’s Interior

• How do we know what it is like today?– Drilling

• Most drilling includes the upper 7 km of crust• Deepest well: Soviet – 12 km deep

– 5 yrs, 7 km; 9 yrs, 5 km; then it got stuck

– Cost $100 million dollars

– Temperature 190oC

– Volcanic Activity– Laboratory experiments– Meteorites– Seismic Wave studies

Meteorites: Evidence about the Interior• Meteorites

– Fragments of asteroids and small early planets (planetesimals) that broke up as a result of collision with other solid bodies

• Composition of Meteorites– Divisions

• 1. Iron-nickel meteorites– From core of original bodies??

• 2. Stony meteorites– Original body area like our mantle

• 3. Stony-iron meteorites– 3 subclasses:

» Chrondrites» carbonaceous chondrites» achondrites

– From the cores and mantle of asteroids & planetismals

• . 90% iron

• 9% nickel

Iron-nickel meteorites

Stony Meteorite

Stony-iron meteorites

Seismology: Evidence of the Earth’s Interior

• Seismology– The scientific study of earthquake– The key to understanding the Earth’s

interior

Seismic Waves

• What are they?– Vibrations in the body of the Earth

• How are they generated?– Naturally

• Earthquakes• Volcanoes• Impacts

– Artificially• Explosions• Mechanical devices

Seismic Waves: Measuring

• Seismograph– Thousands of stations are set up

worldwide

– Used to locate earthquake

– Simple pendulum• When the ground shakes, the base

and frame of the instrument move with it

• Inertia keeps the pendulum in place• It will then appear to move; relative

to the shaking ground• As it moves, it records the pendulum

displacement• The tracing is called a seismogram

– Networks of seismograph stations are used to determine the location of the earthquake

• Seismology and the Science of Earthquakes – Waves – Magnitude – and the Richter Scale

• (3:22 minutes)

Seismic Waves: Terms• Earthquake

– Release of stress build up along a fault– Slipping and moving of rocks along a fault

• Focus– The origin of seismic waves– The energy of seismic waves is released from this point

• Epicenter– The point on the earth’s surface directly above the focus

• Fault– Crack or break in the earth’s crust– Surface along which rocks move

• Waves:– P waves– S waves– L waves

Seismic Waves

• 3 Main Types released by Earthquakes:1. P-waves (Primary / Compression waves)

• Longitudinal / compressional wave– Alternately push (compress) and pull (dilate) the rocks

through which they pass» Squeezes and stretches the material in the same

direction– Generated by the Focus

• Fastest of the waves• Propagates through solids, liquids and gases

2. S-waves (Secondary / Side-to-side waves)• Transverse waves

– Generated by the Focus

• Slower wave• Moves material at right angles to the wave

direction• Propagate only solids

Animation of P and S Waves

• http://www.classzone.com/books/earth_science/terc/content/investigations/es0402/es0402page05.cfm

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es1002/es1002page01.cfm?chapter_no=visualization

Visualization of an Earthquake

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

3. L-waves (Long waves or Love Waves)• Surface waves that cause horizontal shearing of the

ground– Travel on the surface of the earth and shake rocks

sideways as they move across the surface– Generated by the epicenter– Particles travel in a rolling motion

– Note:• Surface waves cause the most damage• Not used to determine Earth’s interior

Locating The Epicenter

• P and S waves travel at different rates– This helps determine the distance to the

epicenter

• P waves will always arrive at a seismic station first– The distance of the P wave ahead of the S

wave depends on how far away the earthquake is

• Key: The further away the epicenter, the wider the gap between the P and S waves.

Earthquake

• http://earthquake.usgs.gov/eqcenter/

• Predicting Earthquakes

• Video 2 minutes

• Earthquakes

• Video 3 minutes

Seismic Topography

Volcanoes: Evidence of the Earth’s Interior

• Referred to as “Windows into the Earth’s Interior.” Why?

• Volcanoes erupt material that is tens to hundreds of kilometers into the Earth– Deepest rock samples of the Earth’s

interior are volcanic rock

Importance of Earth Hot Interior

• Seismic activity – earthquake, volcano• Magnetic field (Geomagnetism)

– Generated by the movements of the molten iron and nickel layer in the outer core

– Convection current in this electrically conducting fluid act as a Dynamo*

• Fluid acts to generate and maintain the magnetic field• * Dynamo theory

– Process through which motion of a conductive body in the presence of a magnetic field acts to regenerate the magnetic field

– Note: Without this Dynamo effect, the magnetic field would die out within 10 000 years

Importance of Earth Hot Interior

• Interior heat powers convection current of the asthenosphere which in turn allows the lithosphere to move (plates moving)– Result:

• formation of mountain ranges• deep sea trenches• Volcanic belts• Earthquake zones

• May be linked to long-term climatic changes