Dynamic Earth

1
Dynamic Earth

• Class 12
• 16 February 2006

2
Volcanic Imagination(Chapter 4)Exploring the
Earths Interior
3
How do we know about the Earths Interior?

• By studying Meteorites
• Direct observation (rocks originating from depth)
• Experiments at high pressure
• By studying earthquake waves (Seismology)

4
Meteorites have struck the Earth in the past.
Many are probably pieces of proto-planets
similar in composition to Earth.
5
Meteorites Accumulate Daily
6
Meteorites
Stony meteorites are rich in olivine and
pyroxene Similar to Earths lithosphere
7
Meteorites
Iron meteorites are made of iron and
nickel Earths interior (core) is similar
8
Lafayette Meteorite
9
Types of Meteorites - I

• Stones
• Primarily silicates (like Earths crust and
  mantle)
• gt90 of all meteorites
• Irons
• Iron-nickel alloys
• Stony irons
• Combination of stony and iron meteorites

10
Types of Meteorites - I
11
Types of Meteorites - II

• Falls
• Meteorites observed falling to the ground
• Primarily stones (suggests they are more common)
• Finds
• Meteorites discovered on the ground
• Primarily irons (collected because they are
  unusual looking)

12
Composition of Meteorites
13
Chemical Composition of Earth
14
How do we know about the Earths Interior?

• By studying Meteorites
• Direct observation (rocks originating from depth)
• Experiments at high pressure
• By studying earthquake waves (Seismology)

15
Large Volcanic Eruptions

• Voluminous volcanic eruptions
• Sample significant part of mantle
• Can infer something about mantle composition

16
Kimberlites

• Rapidly injected rock
• Volatile-rich
• Often contain diamonds
• Known to form at high pressure - deep in mantle
  (gt400 km)
• Hosted by mantle rock

17
Kimberlites
18
Kimberlites Sample Mantle
Peridotite
19
How do we know about the Earths Interior?

• By studying Meteorites
• Direct observation (rocks originating from depth)
• Experiments at high pressure
• By studying earthquake waves (Seismology)

20
Diamond-anvil cell
21
Diamond-anvil cell
22
Multi-anvil Press
23
Multi-anvil Press
24
Seismology

• Study of the propagation of mechanical energy
  released by earthquakes.
• When energy is released, waves of motion (like
  the effect of a pebble tossed into a pond) are
  set up in the Earth.

25
Structure of the Earth

• Seismic velocity (how fast earthquake waves
  travel through rocks) depends on the composition
  of material and pressure.
• We can use the behavior of seismic waves to tell
  us about the interior of the Earth.

26
Seismic waves

• Waves are started because of initial tension or
  compression in the rock.
• Path of waves are curved because different rock
  types at different depths change speed at which
  waves travel

27
Most common types of earthquake waves

• P-waves and S-waves Body waves
• Primary waves travel the fastest in the crust and
  usually are the first waves to arrive
• Secondary (or Shear) waves are slower and
  therefore take longer to arrive

28
Three Main Types of Seismic Waves
P-waves travel faster than S-waves, so they
arrive at the recording station sooner
29
Types of Seismic Waves
30
Difference in travel- time for P and S waves
tells us how far away the earthquake is from the
recording station
Fig. 16.8
31
Seismic Travel-time Curve
32
Structure of the Earth

• Seismic velocity (how fast earthquake waves
  travel through rocks) depends on the composition
  of material and pressure.
• We can use the behavior of seismic waves to tell
  us about the interior of the Earth.

33
Changes in P- and S- wave Velocity Reveal
Earths Internal Layers
Velocities generally increase in each layer
34
Surface waves
Rayleigh waves
Love waves
35
Refraction and Reflection of a Beam of Light
Refraction
Reflection
36
Refraction and reflection of seismic body waves
37
P-and S-wave Pathways Through Earth
38
Travel paths for shallow seismic waves
39
P-wave Shadow Zone
40
S-wave Shadow Zone
41
S wave shadow zone
P wave shadow zone
42
Seismology and Earth structure
43
(No Transcript)
44
Layers of the Earth
45
Earths CORE

• Outer Core - Liquid Fe, 2200 km thick, No
  S-waves transmitted -gt S- P-wave Shadow Zones
• Inner Core - solid Fe (some Ni, Co, S, C), 2500
  km thick
• How do we know? Meteorites, Seismology, Magnetic
  field

46
Earths Geodynamo
47
Origin of Earths magnetic fieldthe geodynamo

• The basic idea an electric motor is a dynamo
• Motion of the liquid outer core -- a conductor --
  in a magnetic field generates current
• The current generates a stronger magnetic field

48
Origin of Earths magnetic fieldthe geodynamo
49
Modeled Geodynamo
50
Ocean crust records magnetic reversals
51
Magnetic Reversals in the Ocean
52
Magnetic Reversals
53
the inner core rotates faster than the mantle.
54
IsostasyAnother key to Earths Interior

• Buoyancy of low-density rock masses floating on
  high-density rocks accounts for roots of
  mountain belts
• First noted during a survey of India
• Himalayas seemed to affect plumb bob

55
The less dense crust floats on the less
buoyant, denser mantle
Mohorovicic Discontinuity (Moho)
56
Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
57
Uplift Formed byRemoval of Ice Sheet
58
Northern hemi-sphere during the last glacial age
59
Evidence of isostatic upliftafter melting of ice
sheet
Uplifted beach ridges
60
Earths internal heat

• Original heat
• Subsequent radioactive decay
• Conduction
• Convection

61
Earth Formation
62
Temperature vs. Depth