Ch 21: Earth’s interior 1.Probing Earth’s interior 2.Seismic waves and Earth’s interior 3.Discovering Earth’s major boundaries 4.Geodynamo 5.Earth’s internal

Ch 21: Ch 21: Earth’s interiorEarth’s interior

1. Probing Earth’s interior

2. Seismic waves and Earth’s interior

3. Discovering Earth’s major boundaries

4. Geodynamo

5. Earth’s internal heat engine

Ch 21: Ch 21: Earth’s interior - Study guideEarth’s interior - Study guide

Know Earth’s major layers (and depths) based on compositional and mechanical differences.

Properties of P-and S waves, and which ones move through solid, which through liquid (molten) rock?

Difference between refraction and reflection What is the Moho and how was it discovered? How was the core-mantle boundary discovered? Variations of S-and P wave velocities with depth. How can

you explain the changes in velocity inside the asthenosphere? What does it tell you about the composition of the mantle?

Why does earth have a magnetic field? What is the source of heat inside earth and how it is

transferred through earth (conduction and convection)?

earthquake happens (slip on a fault) seismic waves travel away from earthquake they carry info about material they travel through


1) Probing Earth’s interior1) Probing Earth’s interior


earthquakes make P-waves & S-waves seismologists use them to:

earthquakes make P-waves & S-waves seismologists use them to:

locate earthquakes

determine what deep Earth is made of

locate earthquakes

determine what deep Earth is made of

earlier lecture

today




The nature of seismic wavesThe nature of seismic waves

depend on material properties

are faster in more rigid materials

increase with increasing depth (higher pressure)

Seismic wave speeds:

P waves: compressional waves: are fastest

vibrate material back/forth in direction wave travels

S waves: shear waves: slower than P-waves

vibrate material side-to-side from direction wave travels

Don’t pass through liquids



P waves always faster than S-waves

“primary” “secondary”

wave paths are “bent” when crossing from one material into another



wave paths are “bent” when crossing from one wave paths are “bent” when crossing from one material into anothermaterial into another

reflections

refractions



wave paths are “bend” when going deeper in Earthwave paths are “bend” when going deeper in Earth

higher pressure = higher wave speedhigher pressure = higher wave speed

Compositional layers

5 Physical/mechanical layers

2) Seismic waves & Earth’s Interior2) Seismic waves & Earth’s Interior

crust 3-70 km thick

mantle down to 2900 km depth

core 2900-6370 km depth

lithosphere stiff/strong, 0-100 km

asthenosphere soft/weak, 100-660 km

layer depth

crust

mantle

core

3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries

boundariesbetween

layers

The Crust


Thickness: ~ 30 km (continents, 70km under mountains) 3-15 km (oceanic)

Composition: Continents: felsic (granite)

and mafic (gabbro) rocks

Oceani: Basalt, Gabbro

The “Moho”


Boundary between the crust and mantle

Discovered in 1909 by Andrija Mohorovicic

The Mantle


Over 82% of Earth’s volume,mainly peridotite (mineralsOlivine and pyroxene)upper mantle 0 - 660 kmlower mantle 660-2900 kmD” region 2600-2900 km

mantle

D”D”

upper mantle

lowermantle

400 & 660 km depth“phase transitions”

400660

Minerals suddenly compress toa more compact form (phase change)

See Fig. 21.7

“isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16

Insert: Isostacy and crustal uplift/subsidenceInsert: Isostacy and crustal uplift/subsidence

Less dense crust floats on top of the denser and deformable rocks of the mantle


“isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16

Isostatic rebound, adjustment: Readjustment of the isostatic equilibriumafter the ice-shield is removed, as happens still in Scandinaviaand Canada.


See 21.1: Isostacy and postglacial uplift

crust

mantle

core


about Mars sized

Nickel-iron alloy

Outer, liquid, spinning

Inner, solid

4 mio times atm. pressure at center

The core-mantle boundary


Boundary between the mantle and core

Discovered in 1914 by Beno Gutenberg

mantlemantle

corecore

Core-mantleCore-mantleboundaryboundary

…how?

P-wave shadow zone

Fig. 21.2

S-wave shadow zone

Core was discovered …from a “shadow zone”

The inner core


Boundary between the outer liquid and solid inner core

Discovered in 1936 by Inge Lehman

mantlemantle

corecore

Inner coreInner core

…how?

The inner core


She discovered reflections (‘echoes’) of seismic waves

‘LehmanDiscontinuity’

The Core

4) Earth’s magnetic field- Geodynamo4) Earth’s magnetic field- Geodynamo

Core behaves like adynamo and thus sustainsEarth’s magnetic field.

Polarity of Earth’s magneticfield reverses about every million years

4) Earth’s magnetic field- Geodynamo4) Earth’s magnetic field- Geodynamo

Geographic and magnetic poles do not coincide!Fig. 21.11

4) Earth’s magnetic field4) Earth’s magnetic field

See time-line of magnetic field reversals (paleomagnetic time-scale), Fig. 21.15 . Remember that magnetic reversals recorded in seafloor basalts were a major confirmation of seafloor spreading (Fig. Story 2.11).

3 reasons for internal heat:

5) Earth’s internal heat engine5) Earth’s internal heat engine

1) radioactive decay of uranium, thorium, potassium

2) heat released as inner core crystallized

3) from colliding particles during Earth formation

Ways to transfer heat:

1) conduction - molecular activity

2) convection - movement (circulation) of material

TSP 17.13


Fig. 17.14 Convective flow in the mantle


The geotherm: Increase of temperature with depth


What is the approximate distance from the surface to the center of the Earth?

A. 700 km B. 2900 kmC. 6400 kmD. 24,000 km


What type of seismic wave is depicted by the ray path in the diagram?

A. a P waveB. an S waveC. a surface waveD. all of the above

*earthquake

seismograph

4. The two kinks in the seismic wave path are examples of seismic __________.A. isostasyB. reflectionC. refractionD. tomographyAnswer = C (page 485)


The two kinks in the seismic wave path are examples of seismic __________.A. isostasyB. reflectionC. refractionD. tomography

*earthquake

seismograph


Continental crust beneath mountains can be up to ___ kilometers thick

A. 10B. 40C. 70D. 100


Which of the following statements is false?A. P waves travel slower in the crust than in the mantle.B. The crust is denser than the mantle.C. The crust-mantle boundary is called the Mohorovicic discontinuity.D. The oceanic crust consists of basalt and gabbro.Answer = B (page 487)


Which of the following regions in the Earth consists primarily of olivine and pyroxene?

A. the crustB. the upper mantleC. the lower mantleD. the inner core


Which of the following statements about the Earth’s core is true?A. The inner core and the outer core are both liquid.B. The inner core and the outer core are both solid.C. The inner core is liquid and the outer core is solid.D. The inner core is solid and the outer core is liquid.


What drives plate tectonics?

A. erosionB. solar energy C. thermal conductionD. thermal convection

Documents

Ch 21: Earth’s interior 1.Probing Earth’s interior 2.Seismic waves and Earth’s interior 3.Discovering Earth’s major boundaries 4.Geodynamo 5.Earth’s internal