Transcript L2 - School of Earth Sciences

Introduction to Earth Science
Earth Sci 100
Lecture 02
The Earth: Formation from Big
Bang to present
Outline
1. Origin of the Universe: the Big Bang
2. Evidence for the Big Bang
3. Earth’s Structure
4. Earth’s Magnetic Field
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?
R. Williams (ST Scl)-NASA
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
Nebulae
(Hubble telescope)
Fig. 1.08
Gas clumps form
nebulae (clouds). Stars
are forming at the top of
the nebula. Already
formed stars light up
the nebula from behind.
Formation of Solar System and Earth
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.
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.
Planetesimals grow by continuous
collisions, and an irregularly shaped
proto-Earth develops. The interior
heats up and becomes soft.
Gravity reshapes
the proto-Earth into
a sphere. The
interior of the Earth
differentiates
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 shifted
And 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
Geologic Time
Hominids
(4-8 my)
Dinosaur
Extinction
(65 my)
Trilobite and Marine
extinction (245my)
Earth
Formed
4.5 by
3.5 by
Oldest Continental
Crustal plate
(3.9 by)
Time
Eukaryotes
( single celled)
(organelles)
Prokaryotes
(single celled)
(no organelles)
2.5 by
1.5 by
Mammals
(200 my)
(Pangea)
Multi-celled
(550-670 my)
0.5 by
Present
Oldest Ocean
Crustal plate
(200 my)
3. Earth’s Structure
Earth’s Structure
Chemical
Composition
Structure
Crust
Mantle
Physical
Composition
Oceanic
Basalt: O2, Si, Mg, Fe
Continental
Granite: O2, Si, Al
Lithosphere
(cool, rigid)
O2, Fe, Mg, Si
Asthenosphere (hot,
flowing)
Uppermost
Asthenosphere
Mantle
Mantle (hotter, denser)
Outer
Outer (hottest, viscous
liquid, 4x denser than
crust)
Core
Fe, Ni
Inner
Earth’s Core ~5,500oC
Inner (hottest, solid, 6x
denser than crust)
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.
ii.
iii.
iv.
v.
vi.
Drill holes (12km)
Density of Earth
Meteorites
Experiments with rocks
Deep rocks brought to the surface
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. Africa
Ophiolites (Cyprus)
vi. Seismic Waves
–
Generated by Earthquakes
–
Speed depends on properties of material the
wave is passing through…
•
•
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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-waves
more 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