lecture20 nov25-bb

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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.

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Molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches.

Hypothesis of sea-floor spreadingHypothesis of sea-floor spreading

Sea-floorspreading

• Continental drift reexamined in 1960’s with new information

– Earthquakes

• New theory developed – Seafloor spreading

• Theory combining continental drift and seafloor spreading termed “Plate Tectonics”

– Volcanism– Age of seafloor

– Heat flow

– Paleomagnetism

• The submersible, Alvin, found strange rocks shaped like toothpaste squeezed from a tube. Such rocks can form only when molten material hardens quickly after erupting under water.

Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field

The earth’s magnetic field does not point exactly north

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The magnetic field is produced by the motion of electric charges, i.e., electric current

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The Earth's magnetic field appears to come from a giant bar magnet, but with its south pole located up near the Earth's north pole (near Canada).

The magnetic field lines come out of the Earth near Antarctica and enter near Canada.

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The liquid iron appears to produce electric currents in the opposite direction of the earth rotation. The currents create the earth magnetism.

The study of paleomagnetism has demonstrated that the Earth's magnetic field has changed over time.

The earth’s magnetic field direction

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The portion of the ocean crust nearest the mid-ocean ridge must have what type of magnetic polarity?

1. normal 63%

2. reverse 28%

3. none 9%

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We don’t know!!!

Can you find a solution to this tough scientific problem?

Scientists did a drilling sample and found rocks that the farther away from the ridge the older the rocks were. The younger ones were in the center of the ridge.

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ocean floors - 200 million yearsland - billions of yearsWhy is seafloor so young relative to continents?SUBDUCTIONSUBDUCTION

New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches

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SubductionSubduction

Ocean floor plunges into deep-ocean trenches.Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle

Theory of Plate

Tectonics

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The theory of plate tectonicsThe theory of plate tectonics

Earth’s lithosphere is broken into giant plates that move laterally on top of asthenosphere.

Most earthquakes and volcanic eruptions happen at plate boundaries.

Three types of relative motions between plates:

The plateplate refers to the pieces of the lithosphere.The plates of the lithosphere moves atop the

asthenosphere

Current Earth’s surface layers are divided into nine very large plates and several smaller ones

Earthquake distribution matches plate boundariesEarthquake distribution matches plate boundaries

Volcanoes match some plate boundaries; Volcanoes match some plate boundaries; some are hot spotssome are hot spots

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DivergentConvergent

Transform

• There are three styles of convergent plate boundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collision

Convergent Boundaries

• Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

HimalayasHimalayas

• Called SUBDUCTION

Continent-Oceanic Crust Collision

Earthquake depth indicates subduction zones

• When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.

• The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.

• The worlds deepest parts of the ocean are found along trenches.

– E.g. The Mariana Trench is 11 km deep!

Ocean-Ocean Plate Collision

Ocean-Ocean plate collision

Modern example: Japan

• Where plates slide past each other

Transform Boundaries

Above: View of the San Andreas transform fault

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San Andreas Fault San Andreas Fault near Gorman, California. The grey, metamorphic quartz monzonite on the left side of the fault are rocks of the Pacific Plate Pacific Plate and the brown sandstone and siltstone on the right of the fault are rocks of the North American PlateNorth American Plate. Photograph copyright by David Lynch.

• Plates move away from each other

• New crust is being formedNew crust is being formed

An example Atlantic Ocean

Divergent boundaries also can rip

apart (“rift”) continents

http://www2.nature.nps.gov/geology/usgsnps/animate/A14.gif

Continental Drift

1) Begins with view of Earth with continents in their present positions, 2) continents move back in time to reunite as Pangaea, 3) Pangaea label appears, 4) Locations of stratigraphic and fossil evidence that Wegener used to argue in favor of continental drift is added.

What are the driving forces of

plate motion?

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Rayleigh-Benard ConvectionRayleigh-Benard Convection

Fig. 5-11, p. 118

Atmospheric ConvectionAtmospheric Convection

Street CloudsStreet Clouds

Street Clouds over City College of New York taken by Professor Gedzelman

How does Earth work?How does Earth work?

Theory of Plate Tectonics

Two mantle convection cells

How does convection work? No one knows but they aren’t afraid to

propose models!

Complex convection

DivergentConvergentTransform

TransformConvergentDivergent