101 Class 18 Spring 2014

GEOL 101CLASS 18SPRING 2014

Objectives Chapter 9• Explain the Nebula Theory of the creation of the

Earth and our solar system• List the resulting differences in the geology of

the planets of the solar system• Describe how the internal structure of the Earth

developed and the role of convection in the development

• Define why the formation of continental crust is less fully understood than oceanic crust

• List the probable sources of water on the Earth

Nebula Theory of Earth Formation

• Start with Big Bang• End – Space today (no worry about

contracting universe)

Big Bang Theory 13.7 billion years ago Creation of all matter Hydrogen and Helium the first More complex elements evolved through

time

2.7º K

Nebula Theory of Earth Formation

• Start with Big Bang• End – Space today (no worry about

contracting universe)• 9 billion years after BB, sun’s nebula forms

– Cloud of dust and a hearty “Hi Ho Silver!”– Really mostly hydrogen– Spins and contracts, spinning faster – Nebular

contraction – Temperature climbs in densifying center

A large gas cloud (nebula) begins to condenseMost of the mass is in the center, there is turbulence in the outer parts

The Nebular Hypothesis

Gravitational attraction causes the mass of gas and dust to slowly contract and it begins to rotateThe dust and matter slowly falls towards the center

The Nebular Hypothesis

Proto-sun

The Nebula Hypotheses

After perhaps100,000 years Center mass hits a million degrees C. Protostar born

After sufficient mass and density was achieved in the Sun, the temperature rose to one million °C, resulting in thermonuclear fusion.

H atom + H atom = He atom + energy

The Proto-Sun

Then: sun is formed, disk cleared

• Proto sun heats disk, driving gas and dust outward

• Moving into cooler regions, the dust and gases cool and condense

• Defined orbits of dust in flattened nebula.

Gas and dust particles collide and form planetesimals

Multiple collisions and accretion into first planetary embryos and then terrestrial planets

Nebula hypothesis: Planet formation

• Temperature differences with respect to distance from sun

• Closer, where temperatures are higher, iron and silicates condense

• Farther out is colder – hydrogen & water condense

• Material collides and accretes forming planetesimals (Mac-planets)

The Giant Planets: Jupiter and Saturn

• Primarily of H and He. • More like sun’s compostion• Small rocky core • Overlain by H and He layers moving from liquid

to gas going out)• First planets to form

Rocky Planets: Mercury, Venus, Earth, and Mars

• Rocky (silicate) outer parts (crust and mantle) and inner cores of Fe-Ni metal.

• Silicates (SiO2) and the oxides of other metals.

• Common silicates:– Olivine: (Mg,Fe)SiO4

– Pyroxene: (Mg,Ca,Fe)2Si2O6 – feldspar (e.g., (Na,K)AlSi3O8)– mica (e.g., biotite

K(Mg,Fe)3AlSi3O10(OH)).

• Venus, Mercury and Mars much like Earth

Geology and Composition of Inner planets (The Rocks) very different from Outer planets (The Gas Giants)

Our Solar System

Sorry Pluto Demoted!

Small and Large Planets

Saturnset on Titan?

Solar System Formation

Summary of Planet Formation• The mass Sun over 99% of the Solar System. • Solar nebula had a “solar composition”. • The “Gas Giant” planets formed early from

the H and He gases like the Sun. • The inner planets form after the gas had

dissipated from the inner solar system.– H, He, C, and N not major constituents of

planet as whole.– High temperatures in the inner solar

system delayed planet formation.

Observations from Meteorites

• Much of the evidence for how the solar system formed comes from meteorites.

• Most come from asteroids of the asteroid belt (e.g., Eros). A few from Mars and the Moon.

• Most meteorites give ages close to 4.56 Ga by most dating methods. (Ga = billion years)

• Small group of achondrites give much younger ages (e.g., 1.3 Ga). These meteorites are thought to be from Mars.

Bombardment From SpaceFor the first half billion years of its existence, the surface of the Earth was repeatedly pulverized by asteroids and comets of all sizesOne of these collisions formed the Moon

63,500YBP 100’ deep 550’ diameter

Leon County Marquez 56 MYBP 8 mile diameter

The Moon

• No other planet has such a relatively big moon• Moon has only a very small iron core• Moon has a bulk density about the same as the

Earth’s mantle (Highly depleted in gaseous elements)

• Has identical oxygen isotope composition to the Earth

• Bottom line: Earth and Moon probably have a shared history

Giant Impact Hypothesis• Idea in a nutshell:

– Body about 1/10 the mass of Earth struck the half or more accreted Earth with core had at least partially formed. About 60 million years after first creation

– Material, mainly from silicate mantle, is blasted into orbit around the Earth, eventually accreting to form the Moon.

Formation of the MoonThis collision had to be very spectacular!A considerable amount of material was blown off into space, but most fell back onto the Earth

Formation of the MoonPart of the material from the collision remained in orbit around the Earth By the process collision and accretion, this orbiting material coalesced into the MoonThe early Moon orbited very close to the Earth

Chelyabinsk, Russia February 15, 2013

Earth continues to gainmass by accretion,picking up 100,000 kgfrom meteors every day

Regional Meteor Craters

Evidence: Orion Nebula

• Third “star” down on Orion’s scabbard

• 100 light years across (1 light year equals 6 trillion miles)

• Reflection of dust and hydrogen

Infrared Image, Orion Nebula(NASA et. Al)

The Orion Nebula

California Nebula,NASA imageIn Milky Way,Orion Arm,1500 light years away 100 light years long

Dust in Orion NebulaCopyright Nicolas Villegas

Evidence: Collapsed nebulae discs found in the Orion nebula

• Gaseous disks are circling proto-suns.

• 2-17 times larger than our solar system

• About 153 protoplanetary disks found in the Orion Nebula

Early Earth

• Homogenous• Very hot

The Early Earth Heats Up

1. Collisions (Transfer of kinetic energy into heat)

2. Compression 3. Radioactivity of

elements (e.g. uranium, potassium, or thorium)

Three major factors that caused heating and melting in the early Earth’s interior:

The CoreAbout 100 million years after initial accretion, temperatures at depths of 400 to 800 km below the Earth’s surface reach the melting point of iron

In a process called planetary differentiation, the heavier elements, including the melted iron, began to sink down into the core of the Earth, while the lighter elements such as oxygen and silica floated up towards the surface

Early Earth’s Magma Ocean

1000 km ?

Core formation from Magma Ocean

Heavier Fe and Ni begin to sink out ofthe homogeneous Magma Ocean toward Core

Planetary Differentiation

Separation by Gravity – like Salad dressing

Convection - materialmovement – heat transfer

Differentiation

• The process by which heavy materials sink towards center and lighter materials stay near the surface

Planetary DifferentiationPlanetary differentiation was completed by about 4.3 billion years ago, and the Earth had developed a inner and outer core, a mantle and crust

The layered structure of Earth• Atmosphere and

Hydrosphere

• Low Density Crust (6-35 km thick)

• Intermediate Density Mantle (~3000 km thick)

• High density core (~3000 km thick)– Liquid outer core– Solid inner core

Differentiation

• Explains relative densities of parts of Earth• Larger amounts of dense elements are

found in the Earth as a whole rather than in the crust

Earth’s LayersThickness

(km)Volume1027 cm3

Densityg/cc

Mass1027 kg

MassPercent

Atmosphere 0.000005 0.00009

Hydrosphere 3.80 0.00137 1.03 0.00141 0.024

Crust 17 0.008 2.8 0.024 0.4

Mantle 2883 0.899 4.5 4.016 67.2

Core 3471 0.175 11.0 1.936 32.4

All 6371 1.083 5.52 5.976 100.00

Objectives Chapter 9• Explain the Nebula Theory of the creation of the

Earth and our solar system• List the resulting differences in the geology of

the planets of the solar system• Describe how the internal structure of the Earth

developed • Define why the formation of continental crust is

less fully understood than oceanic crust

Formation of Crust

• Impacts of accretion, especially the one that lead to creation of Moon, early Earth covered with sea of hot, molten magma.

• Nuclear reactor grows as planet grows • Much more energetic then

Earth’s Nuclear Engine Slowing Down

Earliest Crust

• As Earth cooled, minerals and elements in magma became concentrated by density

• Early crust formed as soon as upper layer began to cool

• This crust was similar to the basaltic crust that is found under the oceans today– Pieces of early crust were recycled – no

survivors – However the process is the same

Age of the Oceanic Crust

Continental Crust

• Early crustal pieces carried water• Introduction of water was essential for

formation of first continental crust– Water reacted with mantle material and was

less dense than original crustal pieces

Continental Crust

• Requires more differentiation to get lighter granites, andesities, and rhyolites from remelted basalts

• How did plate tectonics get started if only oceanic crust?

• Hot spot plumes reheating?• Water also required for remelting• Do not know for sure how the first

continental crust reformed from oceanic

Continental Crust

• Do know from a grain of sand in Australia that felsic magma and thus Continental crust started to form 4.4 BYBP

• During Archean Eon “Cratons” the first continents form, now make up 10% of Earth’s continents and form the hearts of them• Granite-rich cores extend into the mantle as deep

as 200 km

Continental Crust Summary

• Thick (30-60km),• Old (250 - 4000 my),• Light ( = 2.75 g/cm3)• Silicic (dioritic to granitic in

composition).• Stable, ancient interiors called cratons.• Grows at active margins.• Does not bow to any plate, does

not subduct.

Difference Crust Formation• Oceanic crust - 60% of surface

– Young (Oldest 180 million years)– Dense remelted mantle (Peridotite)

• Continental crust– Older (To 4.4 billion years)– Intermediate and felsic rocks– Recipe more complex but best decribed as

reworked oceanic crust– Water required also– First continental crust 150 MMY after Oceanic

Objectives Chapter 9• Explain the Nebula Theory of the creation of the

Earth and our solar system• List the resulting differences in the geology of

the planets of the solar system• Describe how the internal structure of the Earth

developed • Define why the formation of continental crust is

less fully understood than oceanic crust• List the probable sources of water on the Earth

Source of Earth’s Water• Earth formed in water zone of solar nebula• Earth accreted in part from wet rocks• Water appears to be present 4.4 billion

years ago• Some water continues to arrive with

meteors• Dirty ice balls (comets) also deliver some• Recent comets tested have different

hydrogen isotope mix however (Hailey, Hale-Bopp, Hyakutake)

Comparison - Ratio water isotopes between comets, meteorites, and Earth

Astronomers also hypothesize that comets impacting the Earth were a major source of water that contributed to creation of the oceansRemember, that comets are best described as “dirty ice balls”

Creating the Oceans

It is hypothesized that water vapor escaping from the interior of the Earth via countless volcanic eruptions created the oceans (this took hundreds of millions of years)

Creating the Oceans

Creating the OceansThe earliest evidence of surface water on Earth dates back about 3.8 billion years

Objectives Chapter 9• Explain the Nebula Theory of the creation of the

Earth and our solar system• List the resulting differences in the geology of

the planets of the solar system• Describe how the internal structure of the Earth

developed • Define why the formation of continental crust is

less fully understood than oceanic crust• List the probable sources of water on the Earth

Objectives –Chapter 10

• Define convection.• Describe how convection works in the

mantle of the Earth and its effects • Explain the cause and result of motion in

the outer core of the Earth• Define magnetic field reversal