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Earth’s Internal Structure

Earth’s internal layers defined by Chemical composition Physical properties Deduced from Seismographs of Earthquakes Meteorites lend support

Layers defined by composition Crust Mantle Core

Iron-Nickel Meteorite

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Evidence: Density of Earth’s Layers

“Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of much denser material. “

http://pubs.usgs.gov/gip/interior/

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The S-Wave Shadow ZoneThe S-Wave Shadow Zone

Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source.

http://en.wikipedia.org/wiki/Richard_Dixon_Oldham

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The P-Wave Shadow ZoneThe P-Wave Shadow Zone

Behavior of waves through center reveal Earth’s Interior

P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole

HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid.Inge Lehmann

http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html

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Earth’s internal structure Main layers of Earth are based on physical properties including mechanical strength

Outer layers mostly Silicate Minerals: Crust and Mantle Lithosphere (behaves like a brittle solid) Crust and uppermost mantle Asthenosphere “weak sphere” Rest of Upper Mantle Heat softened, plastic solid Lower Mantle Solid due High Pressures

• Inner Layers Core Iron and Nickel, outer above melting point - liquid, inner solid due to high pressures

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1_8

CRUST

MANTLE

(least dense)

CORE(most dense)

~2900 km

~5155 km

Upper mantle

Lower mantle

Outercore

Continental crust

Oceanic crust

0 km~100 km~350 km

Lithosphere

Asthenosphere

Innercore Earth’s center is 6371 kilometers

below the surface, 1 mi = 1.61 km. Equals ~ 3957 miles, or about4000 miles radius

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Harry Hess: Mid-ocean ridges are spreading apart due to flow in the mantle. Crust moves apart as if on conveyer belts.

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1_20

Mid-ocean

Ridge

• Mantle material is moved near the surface.• Lithosphere (Crust + Upper Mantle) bulges into a mid-ocean ridge. • It cracks, exposing the mantle to low pressures• Some of the Mantle minerals are unstable at atmospheric pressures• The unstable minerals melt forming lavas, and cool into basalt, the main rock of ocean lithosphere.

Origin of new Ocean FloorAt the Mid-Ocean Ridge

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How can we test the hypothesis?

Fred Vine: How about geomagnetic reversals?

Earth's magnetic field periodically reverses

polarity – north magnetic pole becomes south magnetic pole, and vice versa

Dates when polarity of Earth’s magnetism changed were determined from lava flows

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Magnetite crystals align with magnetic field

Away from equator and poles they dip toward the North or south poles

When the Earths polarity switches, new lavas adjacent older, point in opposite directions

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Paleomagnetic reversals recorded by new lava rock at mid-ocean ridges

This lava rock is called “Basalt”

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Test 1:

Princeton PostDoc Fred Vine

So, they checked. NOT FALSE

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Test 2: Oceanic Crust youngest at ridgesHess model prediction: youngest at ridges, oldest at trenches

Also NOT FALSE

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95% of energy released by earthquakes originates in narrow zones that wind around the Earth

These zones mark of edges of tectonic plates

Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates

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Earthquake depth from Trench to Arc

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Structure ofthree Plates

Three boundary types, divergent, convergent, and transform

Youngest at ridges, oldest at trenchesNOT FALSE

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Plate tectonics: The new paradigm

Earth’s major plates Associated with Earth's strong, rigid outer

layer Known as the lithosphere Consists of uppermost mantle and overlying

crust Overlies a weaker region in the mantle called

the asthenosphere. The Asthenosphere is hot and plastic, and sheds heat via convective currents.

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Mantle circulations are an example of convection, heat transfer by moving fluids

This example shows transfer of core heat to the upper mantle and crust

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1_15

NORTHAMERICANPLATE

CARIBBEANPLATE

PACIFICPLATE

COCOSPLATE

ANTARCTIC PLATEANTARCTIC PLATE

Mid-AtlanticRidge

Mid-AtlanticRidge

SOUTHAMERICANPLATE

AFRICANPLATE

ARABIANPLATE

INDIAN-AUSTRALIANPLATE

PHILIPPINEPLATE

PACIFICPLATE

FIJIPLATE

EURASIANPLATE

NAZCAPLATE

SCOTIAPLATE

90º90º 0º

0º

45º

45º

180º

0º

45º

45º

180º 180º90º 90º0º

180º

JUAN DEFUCAPLATE

Convergent plateboundaryDivergent plateboundary

Transform plateboundary

Seven major lithospheric platesSeven or so smaller ones.Plates are in motion and change in shape and sizeLargest plate is the Pacific plateSeveral plates include an entire continent plus a large area of seafloor

Plates move relative to each other at a very slow but continuous rate Average about 5 centimeters (2 inches) per year

Cooler, denser slabs of oceanic lithosphere descend into the mantle

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1_22a

CONTINENTAL PLATE

CONTINENTAL PLATE

Oceanic lithospherebeing subducted

(a)

Concept caused revelation. Yes, revelation. Earth’s many features wereall caused by the same process.

Water driven out of ocean lithosphereWater hits mantle,which partially melts.Forms a deep basalticmagma

Plate Tectonics

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Plate boundariesEach plate bounded by combination of all three boundary types: divergent, convergent, transform Edges marked by Earthquakes

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Three main plate boundaries

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Divergent boundaries are located mainly along oceanic ridges

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The East African Rift

The rift valley collects river and lake sediments. Land animals are preserved as fossils instead of being eroded away

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Continental Rift into Ocean Basin

Rift Valleys and Oceans are the same thing

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Convergent plate boundaries

•On the other side of a plate, opposite the diverging margin, a converging margin is usual.

•Three different types, formed from pushing together of ocean floors and/or continental plate

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Types of Convergent BoundariesDefine:Density, Buoyancy,Gravity

Descending convective cell this side

Ocean-ContinentYields Continental Volcanic Arc

Ocean-OceanYields Volcanic Island Arc

Continent-ContinentYields Collision Mtns.Alps, Himalayas, Appalachians

Descending convective cell this side

Descending convective cell this side

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1_22a

CONTINENTAL PLATE

CONTINENTAL PLATE

Oceanic lithospherebeing subducted

(a)A Subduction Zone

A volcano forms as magmareaches the surface

As plate descends into the Subduction Zone, partial melting of mantle rock makes magmas(Molten Rock) These are buoyant, and rise.

Volcanic mountains associated with subduction are called volcanic arcs. Andes and Cascades mountains are continental volcanic arcs

1. Ocean - continent convergence

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Types of convergent boundaries:

2. Oceanic-oceanic convergence

When two oceanic slabs converge, one descends beneath the other. Often forms volcanoes on the ocean floor above the subduction zone.

If the volcanoes emerge as islands, a volcanic island

arc is formed (Japan, Aleutian islands, Tonga islands)

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1_22b

Rocks deformed in collision

Collisionalmountains

(b)

3. Continental-continental convergence

• Continued subduction brings continents together • Less dense, buoyant, thick continental lithosphere does not subduct •Result is a collision between two continental blocks. Process produces folded mountains (Himalayas, Alps, Appalachians)

Fault and Fold Mountains

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The collision of India and Asia produced the Himalayas

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Transform fault boundaries

Third type of plate boundary Plates slide past one another and no new lithosphere is

created or destroyed Transform faults

Most join two segments of a mid-ocean ridge (MOR) as parts of linear breaks in the oceanic crust known as fracture zones

Accommodate simultaneous movement of offset ridges

Source of weak (MOR) to fairly strong (San Andreas) earthquakes.

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Transform faults accommodate movement on offset ridge segments

Plates are moving in opposite directions

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NORTH AMERICAN PLATE

PACIFIC PLATE

JUAN DE FUCA PLATE

Mt. Redoubtvolcano Anchorage

AleutianIslands Mt. Saint

Helens volcano

Cascade Rangefrom dewatering

Ocean-to-continentsubduction

Ocean-to-oceansubduction

BeringSea

(a)

Mid-ocean ridge (divergent margin)Studied by Fred Vine and Drummond Matthews

Convergent and Divergent,Margins of Plates plus smalltransform margins betweenMOR segments

Small Transform Faults

Fault and Foldmountains from earlier Continent-Continent collisions

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Plate Tectonics Explains It All

We now understand mountains, volcanoes, and big earthquakes associated with, for example, the San Andres fault.

We understand rift valleys and how oceans form, deep ocean trenches, mid ocean ridges, why fossils and mountain ranges look alike across vast oceans.