to discuss how and why upper layers of Earth are mobile,need to examine internal structure of Earth and plate tectonics

internal structure of EarthEarth is layered

knowledge of layering is recent (late 1800s);prior to that, only knew interior must be hot

(volcanoes)

astronomers calculated mass from radius and gravitational constant(known for 2,000 years)

mean density ~ 5500 kg/m3, but surface rocks ~2300 kg/m3

therefore, density gradient exists

Mantle Pie Section and Seismic Velocities

Mantle Subdivisions

assumed density increased smoothly with depth due to increaseof pressure with depth

estimate for center of Earth was 10,000-12,000 kg/m3 (not bad!)

breakthrough came with idea that seismic waves generated byearthquakes could travel through the entire Earth and

be recorded elsewhere on the surface: seismology

differentiation of Earthearly in its history

travel paths of seismic waves generated by earthquakes;directly through the earth or reflected by discontinuities

different behavior of P and S waves led to idea of liquid layer in interior

S waves cannot travelthrough liquid

creates S shadow zone

from: http://www.seismo.unr.edu/ftp/pub/louie/class/plate

seismic station

seismic station

earthquake

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

layered Earth

depths of layers and likely compositions

from: http://www.seismo.unr.edu/ftp/pub/louie/class/plate

by World War II, image of Earth as layered with layersseparated by discontinuities existed…

now have a good idea of seismic velocity with depth

seismic discontinuities define:crust; upper mantle; transition zone; lower mantle

outer core; inner core

still only a model to fit existing measurements….

debate continues on exact position of discontinuities

indicate changes in physical properties with depth(mostly density and elastic modulii)

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

why density variation in Earth?

• changes in chemical composition (compositional changes)• changes in mineral structures (phase changes)

what changes occur where is a large area of research………cannot make direct observation!draw from geochemistry, mineral physics, meteoritics,

igneous petrology, seismology

crust: felsic (shallow) to maficmantle: ultramafic (peridotite)outer core: liquid iron alloyinner core: solid iron alloy

crust/mantle: Mohorovicic discontinuity (Moho)--compositionalmantle/core: Gutenberg discontinuity--compositionalinner/outer core: phase (liquid to solid)400 km discontinuity: phase (olivine to spinel structure)670 km discontinuity: phase (spinel structure to perovskite)

crust and mantle(remember that they are distinguished on the basis of

their physical properties)

how do we know what is at depth?electrical conductivity: identifies partial meltsexposed deep crust: occurs in mountain belts; 50 km originallygeochemistry and elemental abundances: tell range of compositiongravity anomalies: identifies density differenceslithospheric flexure: constrains rheologymagnetic anomalies: shows distribution of subsurface rocksmineral physics: measures seismic velocities in rock samplesophiolites: represents oceanic lithospherexenoliths in volcanic rocks: represents upper mantleseismic reflection: identifies changes in lithologyseismic refraction: defines velocities of seismic waves at depthseismic tomography: permits 3D visualization

obvious from space that Earth has two fundamentally differentphysiographic features: oceans (71%) and continents (29%)

global topography

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

crust

bimodal distribution of topography is best illustrated with a hypsometric curve (cumulative frequency curve)

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

high mountains and deep trenches are only a small portion

two modes (left) or two plateaus (right) on curve with little transition

continental crust: 1000 m oceanic crust: -4000 m

why bimodal distribution?

differences in composition and thickness of oceanic and continental crust

oceanic crust: mafic; densercontinental crust: felsic; less dense

isostasy: columns of mass must be the same at a certain depth (compensation depth)

~ 50 km

continents have roots and stick-up

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

what controls differences in composition of oceanic andcontinental crust?

process of formation

oceanic crust: forms at mid-ocean ridges by seafloor spreading

partial melting of mantle peridotite (high Mg and Fe)mafic magma (basaltic composition)

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

fast-spreading: magma enters a large magma chamber in crust;broad bulge exists at ridge

slow-spreading: magma chamber freezes between pulses ofspreading; axial rift valley occurs

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

cross-section through a slow spreading ridge(Mid-Atlantic Ridge)

oceanic crust forms both by intrusion and extrusion of magma…

layers (top to bottom) are similar everywhere: pillow basalt; gabbro dikes; olivine cumulates

as seafloor spreading continues, old seafloor moves awayfrom ridge axis and marine sediment is deposited on top

layered structure long recognized by seismologists

study of ophiolites (exposed oceanic crust) confirmed compositions:layer 1: marine sedimentslayer 2a: pillow basaltlayer 2b: sheeted-dike (gabbro) complexlayer 3: massive gabbrobelow: cumulates (base of crust/top of upper mantle)

oceanic crust thickness: 6-10 km

ophiolites and seismic layering of oceanic crust

Kearey and Vine, 1972

Oman ophiolite

harzburgite

upper mantle

both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html

mylonitized dunite

olivine gabbros

upper mantle

layer 3

both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html

layer 2a: pillows

layer 2b: sheeted dikes

both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html

dolerite dike intruding pillow basalt

from: Seth Steinfrom: http://www.bris.ac.uk/Depts/Geol/vft/oman.html

magnetic anomalies allow dating of oceanic crust… …for basalts intensity of remanent magnetism > induced

anomalies will vary with latitude and ridge orientation

if oceanic crust acquires its magnetism at high latitudes… magnetization vector dips steeply…

in northern latitudes……dips steeply north for normal…points steeply up and south for reversed

…closer to equator, magnetization vector not as steep…at equator, magnetization vector horizontal

negative anomaly coincides with normal blocks

as a consequence of seafloor spreading (and subduction),oceanic crust is < 200 Ma old (with exception of ophiolites)

note pattern of increasing age away from ridges

continental crust

• 5-10 times thicker than oceanic crust (40-70 km thick)• average chemical composition is similar to granodiorite• heterogeneous vertically and laterally• wide range of ages

most elements forming continental crust migrated from Earth’sinterior during Archean (3.8-2.5 Ga): differentiation

Earth was too hot to form permanent crust prior to 3.8 Ga;surface likely convecting ultramafic material

at ~ 3.8 Ga, interior of Earth cooled enough to allow a crust to form; only partial melting occurred (minerals melt at low temperature)

subsequent fractionation and crystallization led to variations incomposition from mafic to silicic

mantle

general composition of peridotitefrom seismic velocities, xenolith compositions

seismic tomography (similar to CAT scan of Earth)suggest inhomogeneous in 3 dimensions

--variations in composition? temperature? both?

composition: extraction of basaltic magma to produce oceanic crust

“depleted” (without basalt component) mantle“undepleted” (with basalt component) mantle

temperature:convection and mantle plumes

convection in the mantle

models

observed heat flowwarm: near ridgescold: over cratons

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

from: http://www-personal.umich.edu/~vdpluijm/gs205.html

rheology of Earth

lithosphere and asthenospheredistinguished by response to stress (their “strength”)---not by seismic discontinuitiesthermal boundary (more in a minute)

lithosphere first proposed to explain isostasy--response of Earth’s surface to vertical loads

(growths of glaciers, formation of islands)

upper most rheologic layer of Earth (lithos-rock):exhibits flexural rigidity on geological time scales

(resistance to bending)

steel: high flexural rigidity rubber: low flexural rigidity

over long time periods, lithosphere does flow (more later)

lithosphere moves as a coherent entity: plate• contains crust and uppermost mantle• base is the 1280°C isotherm (thermal boundary) at this temperature, peridotite weakens due

to easy deformation of olivine• base is not fixed depth; depth of 1280°C isotherm varies below ridges, temperatures high (lithosphere thin-few km) below cratons, temperatures low (lithosphere thick-150 km)

asthenosphere behaves like a viscous fluid on geological time scales

• layer of mantle below lithosphere• composed of predominantly solid, although, weak rock• low flexural rigidity• material flow (crystal plastic flow, diffusion): convection• low velocity zone exists in asthenosphere below oceans

(partial melting? rheology of olivine?)• base of asthenosphere problematic: 400 km; 670 km; core?

(layered convection? whole mantle convection?)

lithosphere “strong” asthenosphere “weak”

The layered Earth and ophiolites

websites from which images are drawn:

sources: Kearey, P. and F. Vine, 1996, Global tectonics, second edition, Blackwell

Scientific, 333 p.

http://www-personal.umich.edu/~vdpluijm/gs205.htmlhttp://www.seismo.unr.edu/ftp/pub/louie/class/platehttp://www.geo.lsa.umich.edu/~crlb/COURSES/270/http://www.bris.ac.uk/Depts/Geol/vft/oman.htmlhttp://pubs.usgs.gov/publications/text