Lecture19 nov20-bb

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The Earth is layered

knowledge of layering is recent (late 1800s); prior to that, only knew interior must be hot (volcanoes)

mean density ~ 5500 kg/m3, but surface rocks ~2300 kg/m3

therefore, density gradient exists

1000 oCCrust

The Crust• This is where we live!

• The Earth’s crust is made of:

Continental Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- young

1000 oCCrust

Mantle is essentially solid, but behaves like liquid.

LithosphereAsthenosphere

1000 oCCrust

Mantle is essentially solid, but behaves like liquid.

LithosphereAsthenosphere

4000 oC

2885 km

Outer core is liquid of iron and nickel.

2230 km

Inner core is solid of iron and nickel.

7000 oC1230 km

The lithosphere is

a. Continental crustb. Oceanic crustc. A combination of the crust and the uppermost mantled. Athenosphere

How do we know the Earth

is layered?

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Earthquakes: One way to learn about the Earth’s interior

Kump et al (2000)

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Seismic body waves: P and S waves

P-Waves

S-Waves

P-waves:-Primary waves-Series of compression and expansions in the direction of wave movement- much like a slinkyS-waves:- Secondary waves- Perpendicular replacement

Kump et al (2000)

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How to remember them

• P waves– Primary waves or pressure waves

• S waves– Secondary waves or shear waves

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Where do P and S waves propagate?

• P waves travel though both solids and liquids

• S waves travel only through solids

Fig. 5.04

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P-wave

S-wave

Mantle(solid)

Outer core(liquid)

Inner core(solid)

Andrija Mohorovicic (1857-1936)

Found seismic discontinuity at 30 km depth in the Kupa Valley (Croatia).

Mohorovicic discontinuity or ‘Moho’ 　

Boundary between crust and mantle

Moho

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Moho discontinuityBoundary Crust/Mantle

Boundary Mantle/outer core

Boundary Outer core/Inner core

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Photograph courtesy of the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Kump et al (2000)

the originator of the theory of continental

drift

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1. Supercontinent Pangaea started to break up about 200 million years ago.

2. Continents "drifted" to their present positions.

3. Continents "plowed" through the ocean crust.

Earth ~200 million years ago

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Wegener used land features, fossils, and climate as evidence to support his

hypothesis of continental drift.

Examples of land features land features such as mountain ranges in Africa and South America lined up; also coal fields on Europe matched up with coal fields in North America. Wegener noticed that fossilsfossils from reptiles that were found in places that are now separated by oceans. Since neither reptile could have swum great distances, Wegener inferred that these reptiles had once lived on a single landmass that split apart.

Continental Drift: EvidenceFossil critters and plants

Continental Drift: Evidence

Correlation of mountains with nearly

identical rocks and structures

Continental Drift: Evidence

Glacial features of the same age restore to atight polar distribution.

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Alfred Wegener published his book with title “The Origin of Continents

and Oceans” in 1915.

In his book, Dr. Wegener drew together evidence from various fields. Expanded editions during

the 1920s presented the accumulating evidence.

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His theory of continental drift proposed that centrifugal force moved the heavy continents toward the equator as the Earth spun. He thought that inertia, from centrifugal movement combined with tidal drag on the continents (caused by the gravitational pull of the sun and moon) would account for continental drift.

Alfred Wegener also came up with a theory to explain continental drift, although it was in error.

- Newton’s first law of motion: an object will not accelerate unless a force is applied.

- Of cause, our reference frame is accelerating (rotating)!

- Centrifugal force.

- Newton’s first law of motion: an object will not accelerate unless a force is applied.

- Of cause, our reference frame is accelerating (rotating)!

- Centrifugal force.

- What is difference between Centrifugal force and centripetal force?

- What is difference between Centrifugal force and centripetal force?

- Action and reaction- Action and reaction

- So the magnitude as what centripetal force is:- So the magnitude as what centripetal force is:

What are Centrifugal force and centripetal force?What are Centrifugal force and centripetal force?

- Action and reaction- Action and reaction

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Alfred Wegener’s explanation of continental drift was like “ice drift”

Centrifugal and tidal forces were far too weak to move continents!

Fig. 5.02a

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Alfred Wegener’s explanation of continental drift was like “ice drift”

Centrifugal and tidal forces were far too weak to move continents!

Another problem was that flaws in Wegener's original data caused him to make some incorrect: he suggested that North America and Europe were moving apart at over 250 cm per year (too fast, about a hundred times faster than the measured rate).

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While his ideas attracted a few early supporters from South Africa and Europe, the hypothesis was generally met with skepticism.

The one American edition of Wegener's work, published in 1924, was received so poorly that the American Association of Petroleum Geologists organized a symposium specifically in opposition to the continental drift hypothesis. Also its opponents could argue that the oceanic the oceanic crust was too firm for the continents to crust was too firm for the continents to "simply plow through"simply plow through".

By the 1930s, Wegener's geological work was almost universally dismissed by the scientific community and remained obscure for some thirty years.

ReactionReaction of Alfred Wegener’s continental drift theory

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•In the 1950s and 1960s, several developments in geology, notably the discoveries of seafloor spreading, led to the rapid resurrection of the continental drift hypothesis and its direct descendant, the theory of plate theory of plate tectonicstectonics.

•Alfred Wegener was quickly recognized as a founding father of one of the major scientific revolutions of the 20th century.

ResurrectionResurrection of Alfred Wegener’s continental drift theory

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Harry Hammond Hess Harry Hammond Hess (May 24, 1906 – August 25, 1969)

Considered one of the "founding fathers" of the unifying theory of plate tectonics,

The founding father of seafloor spreading theory.

Who resurrected the theory of Who resurrected the theory of continental drift?continental drift?

Rear Admiral Dr. Harry Hammond Hess was born in New York City, received B.S. from Yale University,

He received Ph.D. from Princeton

University, was a Professor of geology from

1934 to 1969 at Princeton

University

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Dr. Harry Hammond Hess Dr. Harry Hammond Hess He believe in many of the observations Wegener used in defending his theory of continental drift, but he had very different views about large-scale movements of the Earth, suggesting that the convection of the Earth's mantle was the driving force behind this process. This work provided a conceptual base for the development of the theory of plate tectonics.,

Who resurrected the theory of Who resurrected the theory of continental drift?continental drift?

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He was initially assigned to duty in New York City, where he was responsible for estimating the positions of enemy submarines in the North Atlantic. Hess was then assigned to active sea duty and eventually became commander of an attack transport ship. This vessel carried equipment for sounding the ocean floor, and Hess took full advantage of it. He mapped a large part of the Pacific Ocean, discovering in the process the underwater flat-topped seamounts that he named guyots, in honor of A.H. Guyot, the first professor of geology at Princeton.

Hess was in World War IIHess was in World War II

Sonar

• A device that bounces sound waves off underwater objects and then records the echoes of these sound waves.

• Sonar mapped mid-ocean ridges.