Plate Tectonics 1

September 10, 13,700,002,008

The earth history of our planet for the last 4.2 Ga:

slow coolingSlow 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 heatradioactive decaygravitational 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 cold cold cold coldcold

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 cold cold cold 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 cold cold cold 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 cold cold cold 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 Temperature Temperaturedensity & water content & depletion

Heat source Sunlight Radioactive decay

Rate of heat 1000 watts/m2 0.1 watts/m2

transsport

Phenomenon Atmosphere Mantle

Speed 1 m/s3x109 cm/year 1 cm/year

Lapse rate 6 deg-C/km 1 deg-C/km

Coriollis important Yes No

Phase change Water condenses Peridotite meltsto make rain to make magma

After phase change Dry air Depleted peridotiteis heavier is lighter

Phenomenon Atmosphere Mantle

Effects of flow Wind makes Flowing mantleocean waves moves continents& sand dunes

Small scale Tornadoes Mantle plumesFeatures

Human viewpoint From the bottom From the top

Comparison of Convection in Atmosphere and Solid Earth

Magma is Solid Earth’sversion 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 Materialcracks or faults instead of flows

Please memorize

these fault configurations and names

Crustal Movementsmeasured 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 3

1-3 boundary

2-3 boundary

1-2

boun

dary

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

http://www.ucmp.berkeley.edu/geology/anim1.html

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 timethe 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

Cre

styo

ung

& h

ot

Fla

nkol

d &

coo

l

Fla

nkol

d &

coo

l

5000 km

0 Ma 200 Ma

200 Ma

Thermal ContractionRidge Crest: Hot Lithosphere: 1300CRidge Flank: Cold Lithosphere: 800CChange in temperature: 500C

Thickness of lithosphere 100 km = 105 m

Thermal contraction coefficient of rock: 310-5 per C

Thermal contraction: 310-5 105 500 = 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 sedimentFlank – old – thick sediment

Mid-Atlantic Ridge

Divergent Plate Boundary on a Continent