Lecture 02

The Earth: Formation from Big Bang to present

Introduction to Earth ScienceEarth Sci 100

Outline

1. Origin of the Universe: the Big Bang

2. Evidence for the Big Bang

3. Earth’s Structure

4. Earth’s Magnetic Field

R. Williams (ST Scl)-NASA

Hubble telescope view of the “black void” of the night sky.

Each spot is a distant galaxy. The visible universe contains more than 100 billion galaxies.

Each galaxy is made up of up to 300 billion stars.

How did it all come to be?

1. Origin of the Universe: The Big Bang

• All matter and energy was packed into a single dense point

• The point exploded ~14 billion years ago => THE BIG BANG

• A model of the Universe’s evolution has been deduced

After the Big Bang

• First instant:– Hot (x1028 oC), only energy

• 3 minutes old:– Cooled and grew to 100 billion km– Nucleosynthesis

• 5 minutes old:

– chemical bond formation (H2)

After the Big Bang– cooling and expansion

– nebulae formation (gas cloud patches)

– centers of high gravity and began to grow, build heat, and spin forming protostars (200 my)

– star ignites (true star) (800 my) and stellar nucleosynthesis

– No fuel: star dies (supernova explosions)

– Next generation stars: incorporate stellar elements and form sequentially heavier elements (today 92 natural elements)

– Stream of atoms emitted from star is stellar wind

Fig. 1.08

Nebulae (Hubble telescope)

Gas clumps form nebulae (clouds). Stars are forming at the top of

the nebula. Already formed stars light up

the nebula from behind.

p.16-17a

A nebula forms from hydrogen and helium left from the big bang

The nebula condenses into a swirling disc, with a central ball surrounded by rings.

Formation of Solar System and Earth

The ball at the center grows dense and hot enough for fusion reactions to begin. It becomes the Sun.

Dust condenses in the rings.

Dust particles collide and stick together, forming planetesimals.

Gravity reshapes the proto-Earth into a sphere. The interior of the Earth differentiates

Planetesimals grow by continuous collisions, and an irregularly shaped proto-Earth develops. The interior heats up and becomes soft.

Soon after Earth forms, a small planet collides with it, blasting debris that forms a ring around the Earth.

The Moon forms from the ring of debris.

Eventually, the atmosphere develops from volcanic gases. When the Earth becomes cool enough, moisture condenses and rains to create the oceans.

Our Solar System

• Formed ~4.5 billion years ago • Planets orbit the sun• Moons orbit planets• Includes asteroid belt• 99.8% of solar system mass is in the sun• 99.5% of non-solar mass is in Jupiter

2. Evidence for the Big Bang:

How do we know the universe formed this way?

• Cosmic Background radiation– Predicted by Big Bang theory– Measured in the early 60’s

• Evidence for an expanding Universe… ALL galaxies are red-light shifted

Doppler effect (listen to animation)

Motion compresses waves– Higher frequency (pitch) as object moves towards you– Lower frequency (pitch) as the object moves away

Distant galaxies emit light…

So objects moving away from Earth are red-light shiftedAnd objects moving towards Earth are blue-light shifted

ALL distant galaxies are red-light shifted

Galaxies in the Press

http://www.nature.com/nature/journal/v443/n7108/full/443128a.html

Dr. Grottoli at CalTech observatory Mauna Loa, HI

13 telescopes on top of Mauna Loa, pointing skyward, searching the universe

EarthFormed

Prokaryotes(single celled)(no organelles)

Eukaryotes( single celled)(organelles)

Multi-celled(550-670 my)

Hominids (4-8 my)

DinosaurExtinction(65 my)

Mammals(200 my)(Pangea)

Present4.5 by 3.5 by 2.5 by 1.5 by 0.5 by

Time

Oldest Ocean Crustal plate (200 my)

Oldest Continental Crustal plate(3.9 by)

Geologic Time

Trilobite and Marine extinction (245my)

3. Earth’s Structure

Earth’s StructureStructure

Chemical Composition

Physical Composition

Crust Oceanic Basalt: O2, Si, Mg, Fe

Lithosphere

(cool, rigid)Continental Granite: O2, Si, Al

Mantle Uppermost

O2, Fe, Mg, SiAsthenosphereAsthenosphere (hot, flowing)

Mantle Mantle (hotter, denser)

Core

Outer

Fe, Ni

Outer (hottest, viscous liquid, 4x denser than crust)

InnerInner (hottest, solid, 6x denser than crust)

Earth’s Core ~5,500oC

Where does the heat within Earth’s layers come from?

• radioactive decay

• This heat travels in convection currents in the mantle (which creates flow and moves the crustal plates)

Earth’s Interior

• Oceanic Crust – 0.099% of Earth’s mass, 10 km (6 miles)

• Continental Crust– 0.374% of Earth’s mass, 70 km (44 miles)

• Mantle– 67% of Earth’s mass, 2880 km (1800 miles)

• Outer core– 30.8% of Earth’s mass, 2260 km (1413 miles)

• Inner core– 1.7% of Earth’s mass, 1220 km (762 miles)

(Total Radius of Earth = 6371 km or 3981 miles)

Determining the structure and composition of the Earth’s interior

i. Drill holes (12km)

ii. Density of Earth

iii. Meteorites

iv. Experiments with rocks

v. Deep rocks brought to the surface

vi. Seismic waves

ii. Density of Earth

– calculate MASS of Earth from its gravitational influence on other planets and moon

– Can calculate the VOLUME of Earth

– This gives a DENSITY (Mass/Volume) of

5.5 g/cm3

Water = 1 g/cm3 Gold = 19.3 g/cm3

iii. Meteorites

– Meteorites formed the Earth initially– Therefore meteorites and Earth should have

the same BULK COMPOSITION– Planetesimals differentiated too, just like the

Earth, so different types of meteorites correspond to different parts of the Earth

Stony Meteorites

=> Earth’s Crust

Stony-Iron Meteroites

=> Earth’s Mantle

Iron Meteorites

=> Earth’s Core

iv. Experiments with rocks

– Pressure and Temperature increase with depth in the Earth

– Different minerals are stable at different pressure and temperatures

– Properties at these pressures and temperatures (melting? flowing?) are used to make models

v. Deep rocks brought to the surface

– Let us see the upper mantle (we think) – but NOT the core or lower mantle

Kimberlite pipes in S. AfricaOphiolites (Cyprus)

vi. Seismic Waves

– Generated by Earthquakes

– Speed depends on properties of material the wave is passing through…

• Denser = faster• Less Dense = slower

– Waves diffract as density changes

Things to know about Seismic Waves

• Seismic Waves trace CURVED paths through the earth because of refraction (density increases with depth…)

Seismic Waves create an image of the Earth’s interior

Things to know about Seismic Waves

-Different types:

-P waves are

COMPRESSIONAL

-S waves cause

SHEARING

-S waves can’t go through liquid!!

P-wave paths through the Earth

S-wave paths through the Earth

S-wavesmore information

Q: Why do S-waves penetrate the Aesthenosphere if they do not go through liquids?

A: because the Aesthenosphere is soft (like warm wax) but not liquid (like water). In s-wave terms, this means that they penetrate the aesthenosphere, but not the outer core.

4. Earth’s magnetic field

• The Magnetic field could be generated electrically, by the circulation of liquid metal in the earth’s outer core

• Circulation driven by Coriolis effect

Earth’s magnetic field

The theory has this going for it:

• It predicts that the magnetic and geographic poles should be nearly the same

• The magnetic poles slowly drift, depending on specific details of convection

• N and S poles are arbitrary and can switch!!

Earth’s magnetic field