Formation of the and Solar System a. Supernova and formation of primordial dust cloud. NEBULAR HYPOTHESIS

b. Condensation of primordial dust. Forms disk-shaped nubular cloud rotating counter- clockwise.

c. Proto and planets begin to form.

d. Accretion of planetesimals and differentiation of planets and moons (~4.6 billion years ago).

e. Existing solar system takes shape. Evidence to support the nebular hypothesis: 1. Planets and moons revolve in a counter-clockwise direction (not random). 2. Almost all planets and moons rotate on their axis in a counter-clockwise direction. 3. Planetary orbits are aligned along the sun’s equatorial plane (not randomly organized).

Meteorites: More Evidence from the Early Solar System • Chondrites are composed of undifferentiated, primordial matter that has remained nearly unchanged for about 4.6 billion years. These stony (not metallic) meteorites formed nearly simultaneously with the Sun. Thin Section View • It is thought that small droplets of magma crystallized into the minerals rich in Mg, Si and Fe from the hot solar nebula. These spheres are called chondrules. Moon: More Evidence from the Early Solar System • Most of Earth’s early history Number of Large Impacts has been wiped out by subduction or erosion. • The moon has remained virtually unchanged for the past 3-4 billion years. • The Moon’s cratered surface shows many craters, evidence of bombardment. • Earth was probably bombarded even more than Gyr = gigayear = billion years the Moon early in its history. Why?

Terrestrial Planets: • Close to sun: inside the “frost line.” • small and rocky Closeyet the to the primordial sun, dense dust cloud was mostly Smallcomprised rocky (silicate of hydrogen minerals, metallic cores) gas.

Jovian Planets Jovian Planets Far from the sun, low density • Far from the sun Large, gaseous (hydrogen, methane) • Large, gaseous How did the earth become compositionally zoned? 1. Accretion of planetesimals. 2. Heating due kinetic energy of colliding planetesimals and compression. 3. Heating from decay of radioactive elements. 4. catastrophe : Fe and Ni melt, and these heavy elements sink to core. Lighter materials are displaced outwards: silicate rock of mantle and crust, ocean waters and atmospheric gases, etc. 5. Earth become compositionally zoned based on density (Densest iron-nickel in core-least dense materials comprise the atmosphere). 6. Convective overturn in asthenosphere, mantle and outer core still occur today. 7. Most of the heat generated is still trapped—rock is a good insulator..

Iron catastrophe and differentiation of Earth. As iron “falls” towards center and stops, its kinetic energy transfers into the production of more heat, leading to a runaway process (positive feedback loop). Emissions from degassing of the Earth during its differentiation. Note that molecular H and He escape to space and that oxygenation of the atmosphere occurred later following evolution of marine algae and plants that use photosynthesis to convert

CO2 to O2 as a part of their life processes. Degassing occurred following the iron catastrophe and differentiation. Oceans and atmosphere formed during this period, though volatiles continue to escape today. Formation of Moon • After the formation of Earth’s core, it is believed an asteroid approximately the size of collided with Earth. • The collision re-melted Earth’s outer layers, and debris from the collision spun off into orbit • The two mostly molten bodies reformed spherical shapes • Evidence: Moon’s composition is similar to Earth’s mantle; isotopic dating of Moon rocks. • The moon cooled quickly due to its small size and has remained largely unchanged, except for meteorite impacts Differentiated Earth 1. Iron Core (solid1. Iron inner-Nickel core) Core (liquid(outer core outer liquid) core) (inner core solid) lighter 2. Fe-Mg Silicate 2. MantleFe-Mg Silicate Mantle 3. SilicateFe-Mg-Al Crust Silicate Crust (ocean and continental) (oceanic and continental) 4. Oceans denser 4. Oceans 5. Atmosphere 5. Atmosphere How is the earth compositionally. zoned? Along a density gradient Evidence of Earth’s Composition and Structure?

• Mining: down to ~3.6 km • Drilling: down to ~15 km • Volcanic Eruptions ~ (most geochemists think hot spot volcanoes such as Hawaii tap the deep mantle~2700 km, based on experimental evidence.) • Center of Earth: down to ~6400 km. Evidence? How do we know about the composition of the core and lower mantle?

Indirect Evidence: • Seismic waves Seismology • Seismic waves ARE sound waves.

• Fluids (liquids and gases) support only one type of sound wave: compressional (P-waves).

• Solids ALSO support a second type: shear (S-waves). Both types start together but travel at different speeds-- shear waves are always slower. Seismic wave evidence. Compression Waves (P-waves): Velocity: 6-7 km/sec within lithosphere. Propagate through all phases of matter. Seismic wave evidence: Shear waves (S wavevs): velocity 3-4 km/sec. Only propagate through solid phases of matter. Seismic waves refract (bend) because of velocity changes related to density changes within the earth. Seismic wave speed up with increasing density. Note the change in seismic wave velocity as the seismic waves propagate through the earth.

Note the decrease in seismic wave velocity at a depth of 100-350 km and at the mantle-core boundary.

Note that S-waves are only absorbed at the mantle-core boundary. What does that tell you about the physical property of the upper mantle (i.e., is it a complete liquid)?

animation • P-wave shadow zones. Note two shadow zones exist between 105°- 140° from the epicenter due refraction at outer core mantle boundary. • S-wave shadow zone. Note only one large shadow zones at an angle greater than 105° of the epicenter, due refraction at outer core mantle boundary and because S-waves are absorbed by the liquid outer core. animation How do we know what the composition of the core and lower mantle is?

Magnetic Field • The location of Magnetic North changes over time as convection currents shift and sometimes reverse The presence of the Earth’s magnetic field provides evidence that the Earth likely possesses a metallic core and that a component of this core must be liquid and convecting around the solid metallic portion of the core. How do we know what the composition of the core and lower mantle is?

Metallic Meteorites • About 5% of meteorite finds are metallic meteorites • Meteorite composition: mostly Fe with ~6-17% Ni; and small amounts of other metals • Widmanstatten Pattern: formed from slow cooling of metals—can only happen in cores of larger bodies How do we know what the composition of the core and lower mantle is?

Metallic Meteorites • Pallasites: olivine crystals in a metallic matrix. Believed to have been formed at the core-mantle boundary of a planetoid large enough to form a core. Pallasite

• CB Chondrite: origin unknown— probably from a parent body that was too small to form a metallic core. Also composed of Fe-Ni chondrules together with silicate (rocky) chondrules. CB Chondrite Metallic meteorites Iron-Nickel

Chondritic meteorites Fe-Mg silicate (rocky) Evidence of Earth’s core: since other planet-like bodies in our Solar System formed Fe-Ni cores and rocky mantles, and since Ni is a fairly common element, it is believed that Earth’s core is composed of Fe and Ni, as well as smaller amounts of other elements. Earth’s Internal Structure Compositional Boundaries: • Crust: 2-70 km thick. Oceanic crust is thinner (8-10km) and denser than continental crust (35 km on average). • Mantle: 2900 km thick. 80% of Earth’s volume but only 67% of its mass. Solid. • Core: Outer core 2200 km thick, liquid iron. Inner core radius 1200 km, solid iron. Earth’s Internal Structure Behavioral Boundaries: • Lithosphere: Lithos = rock. Lithosphere is brittle (can produce earthquakes)

• Asthenosphere: asthenos = soft. Asthenosphere is ductile (bends instead of breaking).

Lithosphere “floats” on a partially melted asthenosphere, similar to a raft floating on water. The lithosphere is in isostatic equilibriium with the asthenosphere. Earth’s Internal Structure Crust vs. Mantle is a compositional boundary. Both are made of silicates (oxygen, silicon, various metals), but the bulk chemistry is different.

Lithosphere vs. Asthenosphere is a behavioral boundary. Lithos = rock, asthenos = soft. Lithosphere is brittle (can produce earthquakes) and asthenosphere is ductile (bends instead of breaking).

Tectonic plates are LITHOSPHERE. P-wave velocity profile within the lithosphere (continental and ocean crust and uppermost solid mantle) and asthenosphere (upper ductile mantle).

Low velocity zone (100- 350 km) in the upper mantle is due to decreasing density. This low velocity zone defines the asthenosphere.

Why does the density decrease in this region of the upper mantle?

It is partially molten Internal Convection

• Convection in the liquid outer core produces the magnetic field.

• Convection in the asthenosphere moves the tectonic plates (pieces of lithosphere) around on the surface and is responsible for most geologic activity, such as volcanoes, earthquakes, etc. “Typical” picture of convection currents and plate tectonics

The actual story is a little more complicated Plate Boundaries

Divergent: Plates move apart, new oceanic crust is formed in between.

Convergent: Plates move together and either collide (continental-continental) or one is subducted (oceanic-continental or oceanic-oceanic). Continents stay on top.

Transform: Plates slide past each other.