Plate tectonics 1. Intro-important concepts 2. Kinematics-plane view & on a sphere 3....

Plate tectonics

1. Intro-important concepts

2. Kinematics-plane view & on a sphere

3. Extensional tectonics

4. Compressional tectonics

5. Transform boundaries

Today’s lecture:1. Earth Structure2. What is lithosphere, asthenosphere?3. Describe the fundamental observations that led to PT4. Continents in motion5.The oceanic crust and lithosphere6.Transform faults7. Convergent margins, subduction and collision8. Making it all work.

Chondrites- bulk Earth

Iron meteorites=core-like

Heat engine- very efficientEarth differentiation- primarily by magmatism

Mantle convection-Mostly solid stateMelting shallow by adiabatic decompression

Lithosphere- the coldlid at the top

Earth Io

Convection hypotheses: two layers vs one layer

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Gravity highs (continents) and lows (oceans)Earthquales and magmatism- highly localized atcontinental margins and within oceans

Oceanic lithosphere- different from continental:Basaltic crust, relatively thin (5-7 km), leads to topographic lows

Mid-ocean ridges, transform faults

Young------- Old

CRUST REPLACEMENT Growth rate of 2 - 4 cm per year and a length of 60,000 km means that new crust is created at a rate of Crust - (2-4 cm per year) x 60,000 km = (2-3x10**[-5] km) x 60,000 km = 1.2-1.8 square kilometers per year How long does it take to replace all of the oceanic crust? The total surface area of the Earth is Area = 4 x pi x R**2 = 4 x pi x (6,400 km)**2 ===>

Area = 5.1x10**8 square km. The oceans cover 55 % of the Earth's crust and so the time required to replace all of the oceanic crust is time = 0.55 x 5.1 x 10**8 sq km / 1.2-1.8 sq km per year ===> time = 160,000,000-240,000,000 years

That is, the entire oceanic crust of the Earth is replaced every few hundred million years.

Is this prediction correct?

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Spreading at mid-ocean ridges must be compensated by subduction. In addition,there are transform faults in the oceans.

Transform faults accomodate obliqueness of spreading-that is spreading is perpendicular to the ridge axis and any curvature is taken up by transforms.

Oceanic transform faults are similar to lithospheric-scale strike-slip fault on continents, such as the San Andreas. There is a major difference- key to understanding plate tectonics.

Paleogeography

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Convergent margins- what happens there?There are two types- subduction and collision

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Continents carry the record of convergent margin events of the past

Continents also break- to form new oceans

IMPORTANT CONCEPTS:Lithosphere-asthenosphere, mantle convectionOceanic lithosphere, Mid-ocean ridgesTransform faults, magnetic anomalies,magmatismPaleogeography,, convergent margins- subduction and collision,

How does the future look like?

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