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MAS 603: Geological OceanographyMAS 603: Geological Oceanography

Lecture 2: The New Plate Tectonics: Lecture 2: The New Plate Tectonics: The Deep EarthThe Deep Earth

UNIVERSITY OF SOUTH ALABAMA Last TimeLast Time

A) Introductions• Syllabus (assessment etc.)• Additional subject material (student choice)• Introduction to geology and plate tectonics

TodayToday’’s Agendas Agenda TodayToday’’s Agendas Agenda

A) Seismic waves and the Earth's interiorB) Seismic TomographyC) Mineral phase changes, inner core nuclear

reactors, and outer core magnetism (weird stuff)

Internal “guts”of the Earth

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Internal “guts”of the Earth

Four Major “Geophysical” Layers1) The Crust2) The Mantle3) The Outer Core4) The Inner Core

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Internal “guts”of the Earth

The 4 layers of the Earth are distinguished on the basis of geophysics, specifically the way that seismic waves travel through the Earth.

Seismic Waves

Three major types of seismic waves are distinguished:

1) P-waves (Primary) travel by compression2) S-waves (Secondary) travel by shear

3) Surface waves (Long waves) travel along the surface

Seismic Waves

P and S-waves are called body waves because they travel through the Earth.

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

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.

This results in wave refraction.

If an earthquake is powerful enough, seismic waves can make it around the world.

Propagation of P-waves

Seismic Waves

But wave refraction results in the formation of “shadow zones” where P or S-waves do not occur.

P-wave shadow zones

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Seismic Waves

But wave refraction results in the formation of “shadow zones” where P or S-waves do not occur.

The S-wave shadow zone is wider than the P-wave shadow zone

P-wave shadow zones

Seismographs

Seismic waves are recorded using seismographs.

Seismograms Earthquake Magnitude

10,000,000Impossible unless you make bad movies10

1,000,000<1Rare great earthquake. Can cause major damage over a large region over 1000 km across.

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100,0001"Great" earthquake. Can cause serious damage and loss of life in areas several hundred kilometers across.

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10,00020"Major" earthquake. Can cause serious damage over larger areas.

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1000150Can 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

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1001300At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.

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1013,0004

--130,000Often felt, but rarely cause damage.3

-101,300,0002

-100?Not perceptible to people1

Strength compared to a Magnitude 3

quake

# per year (worldwide)

Source: USGS

Effects(source http://earthquakescanada.nrcan.gc.ca)

Magnitude

Seismology of the deep Earth refers to studying the Earth's lower mantle and core using various seismological techniques.

Seismology of the Deep EarthSeismology 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.

Seismology of the Deep Earth

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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.

Seismology of the Deep Earth Deep Seismic

Deep Seismic

The speed that S-and 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 S-and 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 S-and P- waves travel through the Earth varies an many places. The most dramatic changes are associated with the 4 major layers…

… but 5-50 km down, you get sharp drop in wave speed.

Deep Seismic

15- 50 km down: zone of partial melting (velocity drop) = “Moho”(Mohorovicic Disconformity)

5-35 km

Andrija Mohorovicic (1857-1936)

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Deep Seismic

15- 50 km down: zone of partial melting (velocity drop) = “Moho”(Mohorovicic Disconformity)

Andrija Mohorovicic (1857-1936)

Deep Seismic

Other changes are likely due to mineral phase changes (e.g., α→β quartz; coesite etc.)

200 km

Polymorphs of SiO2

Quartz has 6 polymorphs related to pressure and temperature

α-quartz

β-quartz

Quartz has 6 polymorphs related to pressure and temperature

β -quartz

α -quartz

If you heat “quartz” above 600 ºC it transforms to the β-polymorph (also known as high quartz). When the temperature falls below 600ºC it transforms back to the α- polymorph (also known as low quartz).

Polymorphs of SiO2

Mineral Phase Changes Deep Seismic

In the 1970’s, seismic techniques were refined by petroleum companies that started to reveal deep features not previously seen.

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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).

Deep Seismic

The preferred technique was VIBROSEIS which uses conga-lines of vehicles rather than more labor-intensive schemes.

Deep Seismic

The results were impressive

Deep Seismic

Really impressive.

COCORP lines are being run all around the world these days

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.

Seismic Tomography

Features like mid-oceanic ridge are easily identifiable using seismic tomography

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Seismic Tomography

So too are upwellings

Seismic Tomography

and descending lithosphericplates

Seismic Tomography Seismic Tomography

New computation power also permits large scale modeling of heat flow in the mantle

Seismic Tomography

Which is useful in heat exchange predictions and predicting the source of geochemical compounds (He, Ar and C)

So what’s next?

A combo of geophysical and mechanical

techniques to resolve geo-mysteries.

e.g., the diamond anvil

The Future

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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 field2) High heat flow3) High revolution rate4) The “D” layer

Next Time1. Divergent plate boundaries and evolution of ocean

basins (Wilson cycle)