Formation of the Earth and Solar System

NEBULAR

HYPOTHESIS

b. Condensation of primordial

dust. Forms disk-shaped

nubular cloud rotating counter-

clockwise.

a. Supernova and formation of

primordial dust cloud.

c. Proto sun 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.

• 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.

Thin Section View

Moon: More Evidence from the Early Solar System

• Most of Earth’s early history

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

the Moon early in its history.

Why?

Number of Large Impacts

Gyr = gigayear = billion years

Terrestrial Planets:

• Close to sun:

inside the “frost

line.”

• small and rocky

Close to the sun, dense

Small rocky (silicate

minerals, metallic cores)

Jovian Planets

Far from the sun, low density

Large, gaseous (hydrogen, methane)

yet the primordial dust

cloud was mostly

comprised of hydrogen

gas.

Jovian Planets

• Far from the sun

• 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. Iron 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 Mars 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-Nickel Core

(outer core liquid)

(inner core solid)

2. Fe-Mg Silicate Mantle

3. Fe-Mg-Al Silicate Crust

(ocean and continental)

4. Oceans

5. Atmosphere

How is the earth

compositionally

zoned?

denser

lighter

Along a density

gradient

Differentiated Earth

1. Iron Core

(solid inner core)

(liquid outer core)

2. Fe-Mg Silicate

Mantle

3. Silicate Crust

(oceanic and continental)

4. Oceans

5. Atmosphere

.

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

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.

• 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.

Pallasite

CB Chondrite

How do we know what the composition of the core

and lower mantle is?

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.

Metallic meteorites

Iron-Nickel

Chondritic meteorites

Fe-Mg silicate (rocky)

Earth’s Internal Structure

• 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.

Compositional Boundaries:

Earth’s Internal Structure

• Lithosphere: Lithos =

rock. Lithosphere is

brittle (can produce

earthquakes)

• Asthenosphere:

asthenos = soft.

Asthenosphere is

ductile (bends instead

of breaking).

Behavioral Boundaries:

Lithosphere “floats” on a partially melted asthenosphere, similar to

a raft floating on water. The lithosphere is in isostatic equilibriium

with the asthenosphere.

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.

Earth’s Internal Structure

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.