the crust and the earth’s interior
DESCRIPTION
Propagation of seismic waves As seismic body waves travel through the Earth along various paths, their velocity varies as a function of the properties of the material they encounter. If all the Earth was made up of the same material, the velocity of body waves would change smoothly with depth as pressure and, in turn, the density and rigidity of the material increases readily predict arrival times, but ...TRANSCRIPT
The crust and the Earths interior
Most of the material making up the Earths interior is not available
for analysis. Some material is brought up to the surface by
volcanism and deformation from depths of several 100 kms but
represents a very small fraction of the Earth. Propagation of
seismic waves
As seismic body waves travel through the Earth along various paths,
their velocity varies as a function of the properties of the
material they encounter. If all the Earth was made up of the same
material, the velocity of body waves would change smoothly with
depth as pressure and, in turn, the density and rigidity of the
material increases readily predict arrival times, but ... Seismic
wave reflection and refraction
Much like light rays are reflected or bounce of the surface of
water and/or refracted (velocity and path is modified) upon
entering the water, seismic waves can be reflected (bounce off a
surface) or refracted (path and velocity are modified upon entering
a new medium) when they encounter the interface between phases of
different density. C_06.jpg 1909 Andrija Mohorivicic first
convincing evidence of layering. C_09.jpg P-wave travel paths
Core-mantle boundary
Because they are reflected and refracted at the core-mantle
boundary (CMB), none of the P-waves emerge at the surface between
103 and 143 from the epicenter. S-wave travel paths Core-mantle
boundary (CMB)
The core-mantle boundary casts an even more pronounced shadow for
the S-waves, between 103 and 180, from the epicenter. Reflection of
P-wave at core/mantle and outer/inner core boundaries
Just as a sound wave bounced off the bottom of a lake or a school
of fish can be used to determine its depth or the position of the
fish in the water column, the round-trip travel time for a
reflected P-wave can be used to determine the depth of various
boundaries CMB = 2900 km. Seismograph 5100 km 2900 km 1216 km The
presence of a solid inner core was first predicted in 1936 by the
discovery of weak reflections of P-waves from a boundary within the
core. Later, a Danish seismologist observed that P-waves accelerate
below a depth of about ~5100km, but it was not before the early
1960's that the actual size of the inner core was accurately
calculated after underground nuclear tests were conducted in
Nevada. Based on the velocity of seismic waves through the mantle,
we know that the density increases slowly from 3.3 g/cm3 to 5.5
g/cm3 from the top to the bottom of the mantle. We also know that
the mean density of the Earth is 5.5g/cm3. To make up for the
difference, the core must be composed of material with a density of
at least 10 to 11 g/cm3 iron. Mass of the Earth = 5.98 x 1024 kg
Density of the Earth = 5.52 g/cc Density of rock at the Earths
surface = ~2.67 g/cc Density of the ocean crust and upper mantle=
3.3 g/cc Velocity-versus-depth curve
From a composite of the data obtained from seismographic recordings
of earthquakes or man-made explosions and their analysis,
seismologists have constructed a map of the Earths interior and how
seismic waves travel through each layer. No trivial task
Tomographic images Subducting slab Whole Earth
In recent years, sophisticated algorithms have been used to compile
global seismic data and create a three-dimensional image of
seismic-wave velocities (reflecting temperature variations) within
the Earth. Subducting slab Whole Earth Convection in the
mantle
The upwelling regions, depicted in yellow, consist of rising hot
mantle, and the downwelling regions, depicted in blue, consist of
sinking cooler mantle. The red sphere inside is the surface of the
outer core. Convection in the mantle
Seismic tomography has allowed seismologists to better refine
conceptual models of the dynamics of Earths interior.
Velocity-versus-depth curve (Based on the velocity of P-waves in
the mantle and the analysis of the few rocks found near the
surface, believed to have originated from the mantle, the mantle
would be composed of rocks that are rich in dense minerals such as
olivine, pyroxene, and garnet.) Asthenosphere Mesosphere Boom
trucks for seismic surveys
Seismic techniques also allow us to fine-tune our image of the
crust and explore for mineral and energy resources. C_13d.jpg
Seismic surveys at sea
By using dynamite or releasing bursts of compressed air in the
ground (boom trucks) or at sea, geologists create artificial
seismic waves that propagate down into the earth and reflect off
the boundaries between different layers of rock in the crust.
Seismic-reflection profile (a cross-sectional view of the
crust)
This image defines the depths at which specific strata occur and
reveals the presence of subsurface features such as folds, faults,
mineral, gas and oil deposits.