Plate Tectonics 1 September 10, 13,700,002,008

The earth history of our planet for the last 4.2 Ga: slow cooling Slow cooling of the earth mantle temperatures have declined from 1580 degC to 1375 degC in the last 4.5 billion years

Sources of heat initial heat radioactive decay gravitational settling All sources are dying away …

Evolution of earth in last 4 Ga Is primarily consequence of the cooling process Key character of cooling is “Convection”

Convection cold

Conditions Necessary for Convection 1) Gravity. 2) A fluid that experiences changes in density as its temperature changes. 3) A heat source within or at the bottom of the fluid. cold

Results of Convection 1) Heat is transferred from bottom to top of fluid (flux of heat) 2) The fluid circulates 3) regions of upwelling & downwelling, divergence and convergence (i.e. flux of fluid) cold

Convection with Lithosphere 1) Lithosphere is cold, stiff, brittle region at top 2) Subduction zone: downwelling, convergence of plates 3) Ridge: upwelling, divergence of plates cold

Lithosphere in 3D 700 km

Comparison of Convection in Atmosphere and Solid Earth

Phenomenon Atmosphere Mantle Material Air Mg-Fe silicate rock “Peridotite” Driving force Bouyancy Bouyancy What changes TemperatureTemperature density & water content & depletion Heat sourceSunlight Radioactive decay Rate of heat1000 watts/m watts/m 2 transsport

Phenomenon Atmosphere Mantle Speed1 m/s 3x10 9 cm/year 1 cm/year Lapse rate6 deg-C/km 1 deg-C/km Coriollis important Yes No Phase change Water condenses Peridotite melts to make rainto make magma After phase change Dry air Depleted peridotite is heavieris lighter

Phenomenon Atmosphere Mantle Effects of flow Wind makesFlowing mantle ocean wavesmoves continents & sand dunes Small scale Tornadoes Mantle plumes Features Human viewpoint From the bottom From the top

Comparison of Convection in Atmosphere and Solid Earth

Magma is Solid Earth’s version of “rain”

Rain vs. magma rain … rising moist air adiabatically cools crosses vapor-liquid phase boundary water droplets form are negatively bouyant and fall down as rain Leaves dry air behind magma … rising undepleted mantle adiabatically cools crosses solid-liquid phase boundary liquid rock droplets form are positively bouyant and rise up as magma Leaves depleted mantle behind

Lithosphere – brittle top of part of the earth – is a key feature of our planet that makes its style of convection different than “bubbling soup” Lithosphere: about 100 km thick asthenopshere: about 600 km thick; vigorous convection Deeper mantle is more viscous and convects more slowly

Brittle Material cracks or faults instead of flows

Please memorize these fault configurations and names

Crustal Movements measured through GPS

Schematic earth Schematitic GPS velocities of the ground 10 mm per year

Schematic map “Plates” Really spherical caps Large sections of the lithosphere sliding over the lithosphere

Schematic map The plate boundaries are regions of active tectonism Plate 1 Plate 2 Plate boundary 2-3 boundary 1-2 boundary

Schematic map All motions are a type of rotation or spinning of the cap about a pole of rotation

Lithosphere is organized into plates that rigidly move Measured with GPS Inferred from geology

Four Plates in this Diagram

Divergent Plate Boundaries plates are moving apart Is a gap created as they move apart?

Convergent Plate Boundaries plates are colliding What happens to the excess material?

Transform Plate Boundaries plates are sliding past one another Is a gap created as they move apart?

Plate Motion in the Past Earth Scientists have worked out the history of the earth’s plate motions for the most recent a billion years or so of earth history but the farther back in time the more crudely the plate configuration is known

Why Melting? (The Mantle’s Rain)

Geological evidence for extension is very common at divergent plate boundaries on the sea floor at a divergent boundary

Geological evidence for volcanism is very common at divergent plate boundaries (e.g. this sea floor hot spring)

Divergent plate boundary in the ocean is called a mid-ocean ridge

Crest young & hot Flank old & cool 5000 km 0 Ma 200 Ma

Thermal Contraction Ridge Crest: Hot Lithosphere: 1300  C Ridge Flank: Cold Lithosphere: 800  C Change in temperature: 500  C Thickness of lithosphere 100 km = 10 5 m Thermal contraction coefficient of rock: 3  per  C Thermal contraction: 3   10 5  500 = 1500 m Ridge flank should be about 1500 meters below the crest

Depth-Age Relationship Shape of ridge is explained by cooling of initially hot lithosphere emplaced at ridge axis If you know the age of a patch of sea floor, you can predict its depth to amazing accuracy!

Ridge crests – young – little sediment Flank – old – thick sediment

Mid-Atlantic Ridge

Divergent Plate Boundary on a Continent