Last Time
UNIVERSITY OF SOUTH ALABAMA
MAS 603: Geological Oceanography
A) Introductions
•
•
•
Syllabus (assessment etc.)
Additional subject material (student choice)
Introduction to geology and plate tectonics
Lecture 2: The New Plate Tectonics:
The Deep Earth
Today’s Agenda
Today’s Agenda
A) Seismic waves and the Earth's interior
B) Seismic Tomography
C) Mineral phase changes, inner core nuclear
reactors, and outer core magnetism (weird stuff)
Internal “guts”
of the Earth
1
Internal “guts”
of the Earth
1
Internal “guts”
of the Earth
2
The 4 layers of the Earth
are distinguished on the
basis of geophysics,
specifically the way that
seismic waves travel
through the Earth.
3
4
Four Major “Geophysical” Layers
1) The Crust
2) The Mantle
3) The Outer Core
4) The Inner Core
Seismic Waves
Three major types of seismic waves are distinguished:
Seismic Waves
P and S-waves are called body waves because they travel through
the Earth.
1) P-waves (Primary) travel by compression
2) S-waves (Secondary) travel by shear
3) Surface waves (Long waves) travel along the surface
P-waves travel through all media and are the fastest (4+ km/s)
S-waves cannot pass through liquids and are slower (3+ km/s)
Seismic Waves
Seismic Waves
As P and S-waves travel
through the Earth, they speed
up and slow down according to
the density of the materials
they pass through.
But wave refraction results in
the formation of “shadow
zones” where P or S-waves do
not occur.
This results in wave refraction.
If an earthquake is powerful
enough, seismic waves can
make it around the world.
Propagation of P-waves
P-wave shadow zones
2
Seismic Waves
But wave refraction results in
the formation of “shadow
zones” where P or S-waves do
not occur.
Seismographs
Seismic waves are recorded
using seismographs.
The S-wave shadow zone is wider
than the P-wave shadow zone
P-wave shadow zones
Earthquake Magnitude
Seismograms
Magnitude
1
Effects
(source http://earthquakescanada.nrcan.gc.ca)
Not perceptible to people
2
3
Often felt, but rarely cause damage.
4
# per year
(worldwide)
Source: USGS
Strength
compared to a
Magnitude 3
quake
?
-100
1,300,000
-10
130,000
--
13,000
10
5
At most slight damage to well-designed buildings. Can
cause major damage to poorly constructed buildings over
small regions.
1300
100
6
Can cause damage to poorly constructed buildings and
other structures in areas up to about 100 kilometers
across where people live. Substantial deaths, especially if
epicenter is in a major city
150
1000
7
"Major" earthquake. Can cause serious damage over
larger areas.
20
10,000
8
"Great" earthquake. Can cause serious damage and loss
of life in areas several hundred kilometers across.
1
100,000
9
Rare great earthquake. Can cause major damage over a
large region over 1000 km across.
<1
1,000,000
10
Impossible unless you make bad movies
10,000,000
Seismology of the Deep Earth
Seismology of the Deep Earth
Seismology of the deep Earth refers to studying the Earth's
lower mantle and core using various seismological
techniques.
Seismology of the deep Earth refers to studying the Earth's
lower mantle and core using various seismological
techniques.
This provides the best insight in the dynamic processes
that are happening thousands of km beneath the surface.
3
Deep Seismic
Seismology of the Deep Earth
Seismology of the deep Earth refers to studying the Earth's
lower mantle and core using various seismological
techniques.
This provides the best insight in the dynamic processes
that are happening thousands of km beneath the surface.
Of particular interest are investigating processes at the
core-mantle boundary, and heterogeneity/anisotropy in the
inner core using seismic body waves.
Deep Seismic
The speed that Sand P- waves travel
through the Earth
varies an many
places. The most
dramatic changes
are associated with
the 4 major layers…
Deep Seismic
The speed that Sand P- waves travel
through the Earth
varies an many
places. The most
dramatic changes
are associated with
the 4 major layers…
Deep Seismic
The speed that Sand P- waves travel
through the Earth
varies an many
places. The most
dramatic changes
are associated with
the 4 major layers…
Deep Seismic
15- 50 km down:
zone of partial
melting (velocity
drop) = “Moho”
5-35 km
(Mohorovicic Disconformity)
… but 5-50 km
down, you get sharp
drop in wave speed.
Andrija Mohorovicic (1857-1936)
4
Deep Seismic
15- 50 km down:
zone of partial
melting (velocity
drop) = “Moho”
Deep Seismic
Other changes are
likely due to
mineral phase
changes
(e.g., α→β quartz;
coesite etc.)
(Mohorovicic Disconformity)
200 km
Andrija Mohorovicic (1857-1936)
Polymorphs of SiO2
Quartz has 6 polymorphs related to pressure and temperature
β-quartz
α-quartz
Mineral Phase Changes
Polymorphs of SiO2
Quartz has 6 polymorphs related to pressure and temperature
β -quartz
α -quartz
If you heat “quartz” above 600
ºC it transforms to the βpolymorph (also known as high
quartz). When the temperature
falls below 600ºC it transforms
back to the α- polymorph (also
known as low quartz).
Deep Seismic
In the 1970’s, seismic techniques were refined by petroleum
companies that started to reveal deep features not previously
seen.
5
Deep Seismic
Deep Seismic
At the Society of Exploration Geophysicists Annual Meeting in
Mexico City in 1973, there was a special session that reviewed
deep seismic reflection experiments from around the world.
Subsequently proposals were made for the establishment and
funding of the Consortium for Continental Reflection Profiling
(COCORP) to the US National Academy of Sciences as part of
the US Program for the International Geodynamics Project
(COCORP Newsletter No. 1, Oct. 1976). These proposals were
accepted and funding was then forthcoming for the first three
COCORP projects from the National Science Foundation (NSF).
The preferred technique was VIBROSEIS which uses conga-lines
of vehicles rather than more labor-intensive schemes.
Deep Seismic
Deep Seismic
Really impressive.
COCORP lines are
being run all around the
world these days
The results were impressive
Seismic Tomography
Seismic Tomography
Seismic tomography is a method of using seismic waves
from earthquakes (plus some other data) to create 3D
images of the mantle. These studies pick out areas of fast or
slow mantle, which correspond to areas of high and low
temperature.
Features like mid-oceanic ridge are easily identifiable using
seismic tomography
6
Seismic Tomography
So too are upwellings
Seismic Tomography
Seismic Tomography
and descending lithospheric
plates
Seismic Tomography
New computation power also permits large scale
modeling of heat flow in the mantle
Seismic Tomography
The Future
So what’s next?
A combo of geophysical
and mechanical
techniques to resolve
geo-mysteries.
e.g., the diamond anvil
Which is useful in heat exchange predictions and predicting the
source of geochemical compounds (He, Ar and C)
7
Diamond Anvils
They let you model the
deep Earth where
temperatures exceed
2000 ºC and pressures
exceed 12,000
atmospheres.
Diamond Anvils
They let you model the
deep Earth where
temperatures exceed
2000 ºC and pressures
exceed 12,000
atmospheres.
Diamond Anvils
They let you model the
deep Earth where
temperatures exceed
2000 ºC and pressures
exceed 12,000
atmospheres.
The Core
There are 4 core mysteries
that are currently
intriguing geologists.
1) Magnetic field
2) High heat flow
3) High revolution rate
4) The “D” layer
Next Time
1. Divergent plate boundaries and evolution of ocean
basins (Wilson cycle)
8