Transcript Earth`s Internal Structure IV

Travel-time versus
Distance Curves
D
Distanced measured in degrees
from epicenter.
P-wave
Velocities
Mantle
8-13 km/sec
Outer Core
8-10 km/sec
Crust
6-7 km/sec
Inner Core
10-11 km/sec
VP = P-wave
velocity
VS = S-wave
velocity
D = density
Granite (and Average Continental Crust)
6.0 km/sec
2.5 km/sec
3.5 km/sec
3.0 km/sec
Differences in density and other physical properties affect
the velocity of seismic waves.
Density of Water
1 gm/cm3
Mantle
3.3-5.7 gm/cm3
Outer Core
10-12 gm/cm3
Crust
2.6-3.1 gm/cm3
Inner Core
13-14 gm/cm3

K  43 G
VP 
D
G
VS 
D
K = bulk modulus
= resistance of material
to squeezing
D = density
G = shear modulus
= resistance of material
to shearing
Seismological Results
K G
VP 
D
4
3
G
VS 
D
Values of K, G and D at a given point
in the Earth’s interior.
Search for materials with appropriate
values of K, G and D
Estimates of composition of different
parts of the Earth’s interior

K G
VP 
D
4
3
G
VS 
D
G=0
for liquids
Outer Core of Earth
is liquid
Basalt
Peridotite
Granite
Peridotite = a dense rock that is thought to be a major
component of the Earth’s Mantle.
Composition
of Earth’s
Interior
Crust:
Granite/Basalt
Mantle:
Peridotite
Outer Core:
Liquid Iron
Inner Core:
Solid Iron
Why have geophysicists
concluded that the Earth’s core
is primarily composed of iron?
Why have geophysicists concluded that the
Earth’s Core is primarily composed of iron?
 Iron is an abundant element in the Solar System
(meteorites).
 The Core has the right density for iron (10 to 13
gm/cm3).
 The Core has the right VP and VS for iron.
 Liquid iron in the Outer Core - Earth’s magnetic field.
Solar System
Meteorites
Meteor Crater, Arizona (about 0.75 miles across
and about 560 feet deep). The crater was created
approximately 50,000 years ago.
Meteorites
Meteorites are the remains of rocky and metallic
material that fall to the Earth from inter-planetary
space.
These meteorites are thought to be representative
samples of material that formed the early solar
system, and they are believed to have solidified at
about the same time as the Earth.
Origin
of the
Solar
System
Meteorites provide information about the types
of materials that were available to form the Earth
and the other planets when the Solar System
formed 4.6 billion years ago.
Thus, by combining information about meteorites,
with results of seismic studies, we can estimate the
composition of the Earth’s interior.
Types of Meteorites
Iron Meteorite
Stony Meteorite
Composition of Meteorites
Iron Meteorites – Composed of
iron and nickel.
Stony Meteorites – Composed
of dense rock similar to
“peridotite”.
The Earth’s crust contains
a much smaller % of iron
than meteorites do.
Heavy minerals (iron) sank
to the center of Earth
during the early history of
the Earth to form the
Core, and lighter minerals
floated to the top forming
the Crust.
Heavy minerals (iron) sank to the
center of Earth to form the Core, and
lighter minerals floated to the top
forming the Crust.
Therefore, the Core of the Earth is
believed to be mainly iron (similar to
iron meteorites).
and
The Mantle is believed to be mainly
peridotite (similar to stony meteorites).
What causes the Earth’s magnetic field?
Dynamo Theory
of Earth’s
Magnetic Field
Rapid motion of liquid iron
circulating in the outer Core
Venus, our nearest neighbor in space, may be
structurally similar to Earth, also having an iron
core.
Rock
Metallic Core
But, Venus is spinning very
slowly (once in 243 days).
So, one of the important
ingredients of the dynamo
theory is absent.
Thus, Venus would not be
expected to have a
magnetic field (and none
has been observed).
Composition
of Earth’s
Interior
Crust:
Granite/Basalt
Mantle:
Peridotite
Outer Core:
Liquid Iron
Inner Core:
Solid Iron
Variations in the seismic velocity are depicted by warm
(low velocity) or cool (high velocity) colors. Large scale
patterns correlate with features at Earth's surface.
Lower velocities are seen beneath mid-ocean ridges, high
velocities are detected beneath regions of past or
present subduction.
Computer simulation of P-waves from underground nuclear
explosion in Nevada, propagated westward under the
Sierra Nevada to stations in California.
Environmental
Geophysics
 Depth to Bedrock
 Subsurface Structure
 Subsurface Paths for Flow of Groundwater
VP
250 - 600 m/sec
300 - 900 m/sec
2000 - 6000 m/sec
Image of upper Mantle seismic structure at 100 km depth
beneath North America. Blue is high velocity Mantle and red
is low velocity Mantle.