Universe > galaxy > solar system Our solar system has 1 star (our sun); the galaxy has hundred’s of millions; the universe encompasses all the galaxies

Structure of Earth

Planets derived from material circling early sun (star)  Particles in solar nebula clumped to form planetesimals  Planetesimals collided to form larger planets by accretions  Fractionation of material among inner rocky and outer gassy planets

Age of planets  Oldest rocks on earth ~4.4 by old –Dating zircon  Planet is older ~4.6 by  Time since crystallization ~age of planets  Solar system formed 10 – 100 my earlier

Composition of Early Earth  Earth is layered –Liquid outer core and solid inner core – Fe & Ni  Mantle – silicate –Crust – continental & oceanic  Planetary formation –Initial accretion – homogeneous; non-gravitational; weak van der Waal’s binding (planetesimals R=1-10 km) –Then gravitational attraction – accretions (protoplanets) –Major and large collisions – major accretions –Really large collisions – melting (allowing Ni and Fe to separate); magma formation –Collisions also brought water and other volatiles

Chemical composition of Earth  Evidence of melting, chemical fractionation and separation –Assume composition of early Earth = composition of chondritic meteorite –Mantle depleted in Fe –Separation of Ni/Fe core during melting

Composition of the early Earth (prior to segregation) Separation of the Fe/Ni core during melting (mantle depleted in these elements)

Crustal formation

Formation of the crust  Continental versus oceanic crust  Continental crust –Repeated recycling and partial melting of mantle material and oceanic crust –Repeated heating, cooling, subsidence, burial, and melting leads to distillation/segregation of lighter granitic material from heavier oceanic crust & mantle; further chemical separation of elements –As old as 3.8 by  Oceanic crust –Young (100 my) – controlled by plate tectonics –More dense

Bowen's reaction series demonstrates how the cooling and crystallization of a primary magma of basaltic composition can change from basaltic to andesitic to rhyolitic, through reactions between mineral grains and magma followed by separation of mineral grains and magma.

~20-70 km Oceanic (basalt) crust (  = 2.8 g/cm 3 ) } ~2-5 km The Oceans (  = 1 g/cm 3 ) Mantle (  = 3.3 g/cm 3 ) Continental (granitic) crust (  = 2.7 g/cm 3 )

Segregation  Separation of Ni/Fe core during melting  Crust formed from partial melting of the mantle  Crustal material enriched in Na, Si, and Al  Depleted in Mg  Further fractionation formed continental (granite) and oceanic (basalt) crust

Period of major accretion (first my) Period of heavy bombardment  Major accretion –Once though to be 100 my –Recent thought is planet cooled quickly –Water begins to accumulate on Earth’s surface –Began forming crustal material  Heavy bombardment – my –Continued to bring material and volatiles (and water) to earth {

Importance of the moon  Tides  Gravitational attraction of moon & sun on earth’s bulge causes precession of earth’s orbit –Role in Milankovitch cycles (glacial cycles)  Tends to stabilize tilt of the earth –Earth’s axis at an angle relative to plane of earth’s orbit –Causes seasonality –Tilt of axis varies between 21.8 o and 24.4 o –Without moon, tilt would vary by a greater amount  Up to 85 o  Wreak havoc with climate due to extreme seasonality

Formation of the moon  Lots of theories – implausible or statistically unlikely –Capture –Fission – spinning of earth ejected moon –Binary accretion – Earth and moon formed side by side  Likely a collision (unlikely, but plausible) –Debris reassembled in orbit around earth –Analysis of moon rocks compared with earth rocks

Formation of the moon  Early in Earth’s history (>4 bybp) –Moon formed my after solar system  Formed during accretion –Impact with a nearly fully-formed Earth? –Impact led to termination of accretion?  Impact may have affected earth’s rotations –Caused the axial tilt? –Therefore contributed to seasonality and glacial cycles?

History of the moon  Before 4 bybp –Moon formed from hot debris after collision then solidified –Formation of small core  The next billion years –Volcanic activity formed the moon’s crust –Some similarity to earth

Maria (dark seas) basalt-like [ bybp] Highlands granite-like (anorthosite) [> 4 bybp] – more like continental crust

Moon structure  Both maria and highlands are old –Maria by; lava flows into giant impact craters –Highlands > 4 by  Little or no evidence of tectonic activity in the last 3 by –Small size allowed internal heat to escape w/o mantle convection –Moon’s surface pock-marked by comet and asteroid impacts –No evidence of plate tectonics or other landscape forming processes  Moon has no atmosphere or oceans –Size to small to gravitationally retain gas and volatiles

Back to the Earth’s structure  Earth is layered  Heat did not escape  Recycling, reheating, remelting, recrystallization  Density stratified

Most simply The earth is layered & density stratified 1.Crust – cold, rigid, thin 2.Mantle – warmer, more dense; outer part rigid and inner part plastic (deformable) 3.Outer core – transition zone then thick liquid zone 4. Inner core – solid but warm, very dense, rich in magnetic materials (Ni, Fe)

How do we know this?  All we see is the crust!  Deepest drill-hole – 12,063 m (7.5 miles) –Still crustal  Deepest ocean drilling – 2 km (1.2 miles) –Still crustal  Studies of the earth’s orbit – gave an idea of mass –Surface rocks predicted lower total mass if the earth were homogeneous

Mohorovicic “Moho” discontinuity  Density discontinuity – P waves arrived at seismic station before they should have in an homogeneous earth  Boundary between the crust and mantle  Discovered by Croatian geophysicist based on observations of seismic waves generated by earthquakes.  Fun fact – there was an effort to drill a “Mohole” but failed due to lack of $$ and technology

Evidence for layering  Mainly we know depend on seismology  Seismic waves generated from earthquakes –“Primary” P-waves (compression waves; longitudnally propagated waves; oscillate in same direction as movement like sound waves) –“Secondary” S-waves (transverse waves; horizontally propagated; oscillate perpendicular to movement like water waves)  1900 – identified P & S waves on a seismograph (Oldham) –Waves were passing through the earth faster than predicted  Wave speed increases with increasing density! –Waves were being refracted (bent so they changed direction) –Hypothesized that there were areas of Earth with different densities  1906 – no S-waves passed through the earth –Shadow zone – no S-waves –P-waves took longer than expected

We can detect these waves independently They behave differently passing through different media Why are these waves important?

Prediction of earthquake waves passing through a homogeneous planet. Prediction of earthquake waves passing through a planet of regularly changing density. Point of origin of seismic source.

What S waves do around liquid outer core (do not penetrate). P-wave shadow zone P-wave shadow zone 142 o What P waves do in & around liquid outer core (bend) 142 o

Sharp increase in P-wave velocity at Moho

Seismology  Changes in travel time and path tell us about the earth’s structure –Refraction of waves led to discovery of earth’s core and Moho –Travel time of waves led to discovery of layers  Now we use changes in travel time and path tell us about location of disturbances (earthquakes or bombs)

Earth’s functional layers  Crust – we know most about it; continental crust is less dense  Moho – a density discontinuity that separates crust from the mantle –Depth varies under continents and oceans –First thought that this was layer where crust moved relative to earth’s interior BUT, outer layer of mantle moves with crust!  Lithosphere – crust plus rigid mantle (not totally rigid but, movements cause things like earthquakes and volcanoes  Asthenosphere – plastic layer of mantle; lithosphere floats on asthenosphere  Mantle includes part of lithosphere, asthenosphere and solid mesosphere

Chemical composition of layers: Crust – lightweight (0.4% mass/1% volume of earth) – ocean crust (basalt – O, Si, Mg & Fe) is denser than continental crust (granite – O, Si, Al) Mantle – denser (68% mass/83% volume of earth) - Si, O, Fe & Mg Core – densest (31.5% mass/16% volume of earth) - mainly Fe & Ni with some Si, S and heavy elements

TABLE I Typical Densities of Earth Materials SubstanceDensity* Sea Water 1.02 Limestone ** Granite ** Sandstone ** Slate ** Basalt ** Average Density of Continents 2.7 Average Density of SiMa (Mantle Material) 3.3 * Actual densities vary slightly, depending on chemical composition. (** Source: Handbook of Chemistry and Physics)

Physical responses Lower mantle Core2900 – 6370 km ~3400 Dense, viscous liquid Solid inner core

Classifying layers By composition

Isostatic equilibrium and rebound  This concept helps us understand the “floating” of lithosphere on asthenosphere

Isostacy  Ocean basins and continents “float” on asthenosphere at equilibrium so that total pressure at depth in mantle is everywhere the same.  Depending on density, things will float at a certain height and displace a different amount of water  Most mass is below the surface, what sticks out of the fluid is supported by bouyancy of displaced fluid below the surface  Examples – icebergs, ships, blocks of wood of different densities in water

What does this mean?  Mountains have roots that are deeper than surface expression  As erosion removes mass from the top of a mountain, the roots shrink upward or the asthenosphere “rebounds”  Example: younger (higher) Rockies have deeper roots than older Appalacians  Example: continental rebound from glaciers (Great Lakes & Long Island Sound examples); sea level decreases even though more water!

Next up  Mantle convection  Plate tectonics (Chapter 7)