Download The Earths interior structure - Lecture 1

The Earth!s
interior structure - Lecture 1
How do we know what’s inside the Earth?
•
samples from inside the Earth (not many)
•
indirect observations of the Earth’s interior
seismic, gravity, magnetic, and heat
flow measurements
•
today (mostly): seismic studies of Earth’s
interior
EARTH!S INTERIOR:
SAMPLES
Plummer 1st Cdn edition, Ch. 4
pp 107-109, plus Box 4.4
(page 117)
Kimberlites: samples of the upper mantle
Convecting Mantle
KTB borehole -- drilled to a depth of 10 km in Germany
Russians have drilled a 12-km-deep borehole
Kimberlites: contain samples of the upper
mantle (from >150 km deep)
Mantle xenoliths: samples of the upper
mantle from volcanoes
Box 4.4, page 118 (Plummer 1st Canadian edition)
http://www.ees.nmt.edu/condie/MantleXenolith_NM.jpg
http://www.calstatela.edu/faculty/acolvil/interior.html
Refracted and Reflected Waves
EVIDENCE FROM SEISMIC
WAVES
Plummer 1st Cdn. edition, Ch.
4 pp. 109-112
Seismic Reflection
Seismic Reflection
Fig. 4.01
Crustal Seismology: Seismic Reflection
Lithoprobe Seismic Reflection
November 2000
Vibroseis trucks
Stewart-Cassiar Hwy
Northwest BC
pp 110-111, Canadian edition of text
T53
Active source
reflection seismology:
Seismic Reflection
Petroleum industry’s
primary exploration
technique
• energy reflects off sharp boundaries
between rocks with strong contrasts in
density and seismic velocity
• reflection is widely used by oil industry
to find hydrocarbon traps in sedimentary
basins
Seismic Refraction
Fig. 4.3
Fig. 4.2
Effect of material change on paths of
Refracted and reflected waves
layer with slow v
layer with faster v
Velocity
gradually
increasing
with depth
Curvature
(refraction)
of the energy
seismic raypath bends
Slower
layer with even faster v
With no change in properties, no refractions, no reflections
Seismic Refraction
• energy is transmitted through layers and takes
a curved path back to the surface without a
reflection
• this happens because seismic velocity
increases gradually with depth
• seismic velocities in the Earth generally
increase with depth due to effects of pressure
Faster
EARTH!S INTERNAL
STRUCTURE
Plummer et al. Canadian
Edition Ch. 4 pp. 112-119
Earth!s Radial Structure
• Earth structure dominantly radial due to
– pressure: rearrange atoms to form denser
minerals, for example, in the mantle
– compositional differentiation (denser elements
sink to the center)
• main compositional layers are:
– crust (0 to 30 km, on average)
– mantle (30 to 2900 km)
– core (2900 to 6370 km)
Fig. 4.5
Earth!s Radial Structure
P-wave velocities in crust and upper mantle
MUCH lower
velocities near
Earth!s surface
• some important rheologic layers are:
–
–
–
–
solid lithosphere (0 to 150 km)
gooey asthenosphere (150 to 300 km)
liquid outer core (2900 to 5150 km)
solid inner core (5150 to 6370 km)
Figure 4.6
Table 4.1 in Canadian edition of text
P- and S- wave velocities inside the Earth
Fig. 4.7
Wait! How do we know this??
• upper mantle (30-660 km)
– 8 km/s : peridotite
– seismic velocity variations due to lithosphere,
asthenosphere (gooey), olivine phase changes
• lower mantle (660-2900 km)
– steady increase in velocity
– probably same composition as upper mantle
We measure the arrival
times of different
seismic waves, and
compare them with
what we would expect
from calculations for a
model Earth.