Transcript Earth`s Interior

THE EARTHS INTERIOR
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Seismic Wave Review
1.
p-waves and s-waves
2.
velocity = Distance / time
3.
seismic waves travel at different speeds through
different materials
Geologic Discontinuities
1. seismic waves reflect and refract at discontinuities
or boundaries between geologic materials (ie.
rocks)
2. velocity relationships
Geologic Discontinuities
3.
Paths of Seismic Waves
i.
homogeneous composition = straight
ii.
non-homogeneous composition = curved
Composition and Structure of the Earths Interior
1.
Crust: Oceanic and Continental Crust
i.
ii.
Continental Crust
(1)
felsic (granitic): 6 km / sec
(2)
35 km thick
Oceanic Crust
(1)
mafic (basalt): 7 km / sec
(2)
5-10 km thick
(3)
denser than continental crust
Composition and Structure of the Earths Interior
2.
Isostasy
i.
continental crust floats on top of oceanic curst
3.
Mantle
i.
Andrija Mohorovi i (1909
(1)
discovered a discontinuity at a depth of about 30
km
(2)
Mohorovičić Discontinuity (Moho)
(3)
(a)
p-waves below travel at 8km / sec
(b)
p-waves above travel at 6.5km / sec
the Moho separates the crust from the mantle
3.
Mantle
ii.
low-velocity zone
(1)
100 - 250 km deep
(2)
p- and s-wave velocities decrease
(3)
corresponds to the asthenosphere
(a)
a layer in which the rocks are close to
there melting point and are less elastic
(b)
magmas are produced here(?)
(c)
discontinuous (?)
4. Core
i. 1906 R.D. Oldham
(1) discovered that seismic waves arrived later than
expected at seismic stations more than 130 degrees
from an earthquake focus
(2) he postulated the existence of a core that transmits
seismic waves at a slower rate than shallower depths
(3) only very weak p-waves between 103-143 degrees
from an earthquake focus: P-wave shadow zone
ii. Inge Lehman (1936)
(1) postulated that the Earth has a solid inner core
(2) explains the existence of weak p-waves in the shadow
zone
4.
Core
iii.
Harold Jeffreys (1926)
(1) discovered that s-waves were no just simply slowed by
the core but were blocked by it
(2) large and complete s-wave shadow at locations greater
than 103 degrees
Internal Heat and Heat Flow
1.
heat is due to meteor impact and radioactive decay
2.
earth cools by conduction and convection
i.
conduction
(1)
hot particles transfer heat to cool
particles
(2)
very inefficient and cannot explain the
solid mantle (must be another mechanism)
ii.
convection
(1)
heated liquid expands and rises since it
is less dense than cool material around it
Magnetic Field
1.
William Gilbert (1600) realized the earth is a big
magnet
2.
supports theory that the outer core is liquid
Magnetic Field
3. paleomagnetism
i. the earths magnetic field is recorded in cooling rocks
that contain material that will align to a magnetic field thermoremanent magnetism
Magnetic Field
3. paleomagnetism
iii. since rocks that are 3.5 billion years old record a
magnetism, the earth had a magnetic field at that time fluid core
iv. magnetic stratigraphy
(1)
about ½ of the rocks on earth contain a
magnetic field opposite to today's field
(2)
patterns have been recognized and we can use
it as a stratigraphy (a sequence) to tell approximate
age