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to discuss how and why upper layers of Earth are mobile,

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Presentation on theme: "to discuss how and why upper layers of Earth are mobile,"— Presentation transcript:

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

2 Mantle Pie Section and Seismic Velocities

3 Mantle Subdivisions

4 Mohorovicic discontinuity (Moho)
seismic boundary above: P wave velocities are 5-8 km/s below: P wave velocities are > 8 km/s below oceans, boundary is abrupt and likely represents change of lithology--gabbro (oceanic crust) to peridotite (upper mantle) beneath continents, boundary is less distinct and variable may reflect local intrusions (sills) of mafic material at base of crust (magmatic underplating) some places below continents, reflection in seismic data occurs need to state if you mean seismic discontinuity, reflector, or petrologic boundary for Moho

5 differentiation of Earth
assumed density increased smoothly with depth due to increase of pressure with depth estimate for center of Earth was 10,000-12,000 kg/m3 (not bad!) differentiation of Earth early in its history breakthrough came with idea that seismic waves generated by earthquakes could travel through the entire Earth and be recorded elsewhere on the surface: seismology

6 different behavior of P and S waves
earthquake seismic station different behavior of P and S waves led to idea of liquid layer in interior seismic station travel paths of seismic waves generated by earthquakes; directly through the earth or reflected by discontinuities S waves cannot travel through liquid creates S shadow zone from:

7 from:

8 by World War II, image of Earth as layered with layers
separated by discontinuities existed… layered Earth from: depths of layers and likely compositions from:

9 now have a good idea of seismic velocity with depth
seismic discontinuities define: crust; upper mantle; transition zone; lower mantle outer core; inner core indicate changes in physical properties with depth (mostly density and elastic modulii) still only a model to fit existing measurements…. debate continues on exact position of discontinuities from:

10 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 mafic mantle: ultramafic (peridotite) outer core: liquid iron alloy inner core: solid iron alloy crust/mantle: Mohorovicic discontinuity (Moho)--compositional mantle/core: Gutenberg discontinuity--compositional inner/outer core: phase (liquid to solid) 400 km discontinuity: phase (olivine to spinel structure) 670 km discontinuity: phase (spinel structure to perovskite)

11 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 melts exposed deep crust: occurs in mountain belts; 50 km originally geochemistry and elemental abundances: tell range of composition gravity anomalies: identifies density differences lithospheric flexure: constrains rheology magnetic anomalies: shows distribution of subsurface rocks mineral physics: measures seismic velocities in rock samples ophiolites: represents oceanic lithosphere xenoliths in volcanic rocks: represents upper mantle seismic reflection: identifies changes in lithology seismic refraction: defines velocities of seismic waves at depth seismic tomography: permits 3D visualization

12 obvious from space that Earth has two fundamentally different
crust obvious from space that Earth has two fundamentally different physiographic features: oceans (71%) and continents (29%) from: global topography

13 bimodal distribution of topography is best illustrated with
a hypsometric curve (cumulative frequency curve) from: 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: m

14 why bimodal distribution?
differences in composition and thickness of oceanic and continental crust oceanic crust: mafic; denser continental 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:

15 what controls differences in composition of oceanic and
continental 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:

16 fast-spreading: magma enters a large magma chamber in crust;
broad bulge exists at ridge slow-spreading: magma chamber freezes between pulses of spreading; axial rift valley occurs from:

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

18 oceanic crust forms both by intrusion and extrusion of magma…
layers (top to bottom) are similar everywhere: pillow basalt; gabbro dikes; olivine cumulates oceanic crust thickness: 6-10 km as seafloor spreading continues, old seafloor moves away from 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 sediments layer 2a: pillow basalt layer 2b: sheeted-dike (gabbro) complex layer 3: massive gabbro below: cumulates (base of crust/top of upper mantle)

19 ophiolites and seismic layering of oceanic crust
Kearey and Vine, 1972

20 Oman ophiolite harzburgite upper mantle
both from:

21 upper mantle mylonitized dunite layer 3 olivine gabbros
both from:

22 layer 2b: sheeted dikes layer 2a: pillows
both from:

23 dolerite dike intruding pillow basalt
from: Seth Stein from:

24 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

25 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

26 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’s interior during Archean ( 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 in composition from mafic to silicic

27 mantle general composition of peridotite from 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

28 convection in the mantle
models from: convection in the mantle observed heat flow warm: near ridges cold: over cratons from:

29 rheology of Earth lithosphere and asthenosphere distinguished by response to stress (their “strength”) ---not by seismic discontinuities thermal 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)

30 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?)

31 lithosphere “strong” asthenosphere “weak”

32 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.

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